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INDUSTRIAL TRIBOLOGY
TRIBOLOGY SERIES
Advisory Editor: DOUGLAS SCOTT
Editorial Board
W.J. Bartz (Germany) R. Bassani (Italy) C.A. Brockley (Canada) H. Czichos (Germany) D. Dowson (UK) N. Gane (Australia) W.A. Glaeser (USA)
M. Goder (France) H.E. Hinterman (Switzerland) I.V. Kragelskii (USSR) K.C. Ludema (USA) G.W. Rowe (UK) T. Sakurai (Japan) J.P. Sharma (India)
Vol. 1 Tribology - A Systems Approach to the Science and Technology of Friction, Lubrication and Wear (Czichos)
Vol. 2 Impact Wear of Materials (Engel) Vol. 3 Tribology of Natural and Artificial Joints (Dumbleton) Vol. 4 Tribology of Thin Layers (Iliuc) Vol. 5 Surface Effects in Adhesion, Friction, Wear and Lubrication (Buckley) Vol. 6 Friction and Wear of Polymers (Bartenev and Lavrentev) Vol. 7 Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) Vol. 8 Industrial Tribology- The Practical Aspects of Friction, Lubrication and Wear (Jones
and Scott, Editors) Vol. 9 Mechanics and Chemistry in Lubrication (Dorinson and Ludema) Vol. 10 Microstructure and Wear of Materials (Zum Gahr) Vol. 11 Fluid Film Lubrication - Osborne Reynolds Centenary (Dowson et al., Editors) Vol. 12 Interface Dynamics (Dowson et al., Editors) Vol. 13 Tribology of Miniature Systems (Rymuza) Vol. 14 Tribological Design of Machine Elements (Dowson et al., Editors) Vol. 15 Encyclopedia of Tribology (Kajdas et al.) Vol. 16 Tribology of Plastic Materials (Yamaguchi) Vol. 17 Mechanics of Coatings (Dowson et al., Editors)
TRIBOLOGY SERIES, 8
INDUSTRIAL TRIBOLOGY The Practical Aspects of Friction, Lubrication and Wear
edited by
MERVIN H. JONES Department of Mechanical Engineering, University College of Swansea, Swansea, U. K.
and
DOUGLAS SCOTT Consultant, Editor of "Wear", Secretary of The Institution of Engineers and Shipbuilders in Scotland, Glasgow, U. K.
ELSEVIE R SCl ENTl F IC PUBLISHING COMPANY AMSTERDAM -OXFORD -NEW YORK
ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1,1000 AE Amsterdam, The Netherlands
Distributors for the USA and Canada:
ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas New York, NY 1001 0, USA
Library of Congress Cataloging in Publication Data Main en t ry under t i t l e :
Industrial tribology.
(Tribology s e r i e s ; 8 ) Includes b ib l iographica l re ferences and indexes. 1. Tribology. I. Jones, Mervin H. , 1939-
11. Scot t , Douglas, 1916- . 111. Ser ies . TJ1075.148 1983 621.8’9 82-24248 ISBN 0-444-42161-0 (U. S. )
ISBN 0444421614 (Vol. 8 ) ISBN 044441677-3 (Series)
First edition 1983 Second impression 1991
0 Elsevier Scientific Publishing Company, 1983 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or trans- mitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, wi- thout the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 21 1,1000 AE Amsterdam, The Netherlands.
Special regulations for readers in the USA. This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, MA 01 970, USA. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the USA. All other copy- right questions, including photocopying outside of the USA, should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.
No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, pro- ducts. instructions or ideas contained in the material herein.
Printed in The Netherlands
CONTENTS
ACKNOWLEDGEMENT
FOREWORD
V
X X I
XXIII
CHAPTER 1. TRIBOLDGY IN PERSPECTIVE, D. SCOTT, CONSULTANT,
EDITOR OF WEAR
1.1 I n t r o d u c t i o n
1.2 Impact o f T r ibo logy
1.3 Economic aspects o f t r i b o l o g y
1.4 Mechanisms o f wear
1.5 Surface s tud ies
1.6 L u b r i c a t i o n
1.7 Lub r i can ts
1.8 M a t e r i a l s
1.9 Surface t rea tments
1 .10 Computer a ided design
1.11 Machinery c o n d i t i o n mon i to r i ng
1.12 Conclusions
References
I
CHAPTER 2. WEAR, D. SCOTT, CONSULTANT, EDITOR OF WEAR
2.1 I n t r o d u c t i o n
2.2 The wear process
2.3 S c u f f i n g
2.4 Abras ive wear
2.5 F r e t t i n g
2.6 F l u i d and c a v i t a t i o n e ros ion
2.7 R o l l i n g con tac t f a t i g u e
2.8 Wear d e t e c t i o n and assessment
2.9 Conclusions
References
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7 8
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CHAPTER 3. SELECTION OF BEARINGS, M.J. NEALE, MICHAEL NEALE
AND ASSOCIATES LTD.
3.1 I n t r o d u c t i o n
3.2 Bear ing types
3.3 Performance of va r ious types o f bea r ing
3.3.1 Rubbing bear ings
3.3.2 R o l l e r bear ings
3.3.3 F l u i d f i l m bear ings
3.3.4 F l e x i b l e members
Se lec t i on o f a s u i t a b l e bea r ing
3.4.1 A p p l i c a t i o n s w i t h u n i d i r e c t i o n a l load
3.4
and cont inuous movement
3.4.2 A p p l i c a t i o n s w i t h o s c i l l a t i n g
movernen t
3.4.3 A p p l i c a t i o n s w i t h m u l t i d i r e c t i o n a l
load and cont inuous movement
CHAPTER 4. DESIGN OF PLAIN BEARINGS, USE OF BEARING DATA
D E S I G N CHARTS, F.A. MARTIN AND D.R. GARNER, THE
GLACIER METAL CO. LTD.
4.1 I n t r o d u c t i o n
4.1.1 No ta t i on
4.2 Journal bea r ing des ign l i m i t s
4.2.1 L i m i t s o f o p e r a t i o n
4.2.1.1 Th in o i l f i l m l i m i t
4.2.1.2 High bear ing temperature
4.2.1.3 O i l o x i d a t i o n l i m i t
4.2.1.4 O i l f i l m i n s t a b i l
4.2.2 Region o f sa fe ope ra t i on
4.3 C a l c u l a t i o n and des ign procedures
4.3.1 Dimensionless da ta - Stage
4.3.2 Design procedures w i t h hea
- Stage 11
4.3.3 Improvement i n des ign a i d s
1
ba 1 ance
- Stage 111
4.4 Design procedure f o r c y l i n d r i c a l bore bear ings
4.4.1 Method o f approach
4.4.2 Guidance f o r sa fe ope ra t i on
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4.4.2.1 O i l f i l m t h i ckness and
temperatures
4.4.2.2 O i l f i l m i n s t a b i l i t y
4.4.3 Performance p r e d i c t i o n
4.4.3.1 Minimum o i l f i l m t h i ckness
4.4.3.2 Misal ignment
4.4.3.3 Power l o s s and o i l f l o w
4.4.3.4 Temperatures
4.5 High speed a p p l i c a t i o n s
4.5.1 Non laminar ope ra t i on
4.5.2 P r o f i l e bore bear ings
Example o f use o f design a i d s
4.6.1 Problem
4.6.2 Procedure
4.6
4.6.2.1 Diametral c learance
4.6.2.2 Maximum s p e c i f i c l oad r a t i n g
4.6.2.3 Region o f sa fe ope ra t i on
4.6.2.4 P r e d i c t i o n o f minimum o i l
f i l m th ickness
4.6.2.5 Check for laminar o p e r a t i o n
4.6.2.6 P r e d i c t i o n o f power loss
4.6.2.7 P r e d i c t i o n o f o i l f l o w
4.6.2.8 P r e d i c t i o n o f maximum bear ing
temperature
4.6.2.9 P r e d i c t i o n o f o i l o u t l e t
temperature
Re f e r ence s
CHAPTER 5 . THE D I A G N O S I S OF PLAIN BEARING FAILURES, R.W. WILSON
AND E.B. SHONE, SHELL RESEARCH CENTRE.
5.1 I n t r o d u c t i o n
5.2 P r o p e r t i e s r e q u i r e d o f bear ing m a t e r i a l s
5.2.1 Fat igue res i s tance
5.2.2 Compressive s t r e n g t h
5.2.3 Confo rmab i l i t y
5.2.4 Embeddabi l i ty
5.2.5 Strength a t e leva ted temperatures
5 .2 .6 C o m p a t i b i l i t y
5.2.7 Corros ion res i s tance
5.2.8 Cost
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VIII
CHAPTER 6.
5.3 Type, c o n s t r u c t i o n and c h a r a c t e r i s t i c s o f p l a i n
bea r ing m a t e r i a l s
5.3.1 White meta ls (Babb i t t s )
5.3.2 Copper lead a l l o y s
5.3.3 Lead bronzes
5.3.4 Aluminium a l loys
5.3.5 Phosphor and s i l i c o n bronzes
5.3.6 S i l v e r
5.3.7 Porous and s e l f l u b r i c a t i n g bear ings
5.3.8 Discussion o f m e t a l l i c bear ing m a t e r i a l s
5.4 Bear ing f a i l u r e s
5.4.1 M e t a l l u r g i c a l d e f e c t s i n new bear ings
5.4.1.1 Bad bonding
5.4.1.2 Gas c a v i t i e s
5.4.1.3 Oversize cuboids
5.4.1.4 Excessive lead con ten t i n t i n
based w h i t e me ta l s
5.4.1.5 Uneven lead d i s t r i b u t i o n i n
copper- lead and lead-bronze a I Ioys
5.4.2 F a i l u r e s assoc ia ted w i t h design, f i t t i n g
o r ope ra t i ng environment
5.4.2.1 Bad f i t t i n g
5.4.2.2 Extraneous p a r t i c l e s
5.4.2.3 Cor ros ion
5.4.2.4 C a v i t a t i o n and e ros ion
5.4.2.5 E l e c t r i c a l d i scha rge damage
5.4.2.6 Wire wool f a i l u r e s
5.4.2.7 Fat igue
6.4.2.8 Thermal c y c l i n g damage
5.4.2.9 A l l o y i n g i n se rv i ce
5.4.2.10 Inadequate v i s c o s i t y and lack
o f l u b r i c a n t
5.5 Conclusions
Ref e rence s
ROLLING ELEMENT BEARINGS, D.G. HJERTZEN AND R.A. JARVIS,
SKF (UK) LTD.
6.1 I n t r o d u c t i o n
6.2 Bear ing s e l e c t i o n
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6.3
6.4
6.5 6.6
6.7 6.8
6.9
Bear ing types
6.3.1 S ing le row deep groove b a l l bear ing
6.3.2 Sel f -a1 i gn ing b a l l bear ings
6.3.3 Angular con tac t b a l l bea r ing
6.3.4 Double row angu lar con tac t b a l l bear ing
6.3.5 C y l i n d r i c a l r o l l e r bear ing
6.3.6 Double row c y l i n d r i c a l r o l l e r bear ings
6.3.7 Needle r o l l e r bear ings
6.3.8 Taper r o l l e r bear ings
6.3.9 Double row spher i ca l r o l l e r bear ings
6.3.10 Spher ica l r o l l e r t h r u s t bear ings
6.3.11 B a l l t h r u s t bear ings
6.3.12 Bear ings w i t h spher i ca l o u t s i d e surface
and extended inner r i n g w i d t h
Fat igue l i f e and load c a r r y i n g capac i t y
6.4.1 Her t z ian con tac t
6.4.2 Re la t i onsh ip between load and l i f e
6.4.3 Fu r the r development o f t he l i f e equat ion
Boundary dimensions
Usage
Speed l i m i t s
Fr i c t i o n
L u b r i c a t i o n
6.9.1 Greases
6.9.1.1 Temperature range
6.9.2 O i l l u b r i c a t i o n
6.10 Seals
6.11 Noise
6.12 A n t i c i p a t i n g bear ing damage
6.13 De tec t i on o f bea r ing damage by shock pu lse
mea su remen t
6.14 F i t s ( s h a f t and housing)
6.15 Cond i t ions o f r o t a t i o n
6.16 In f l uence o f l oad and temperature
6.17 Bear ing a p p l i c a t i o n s
6.18 Bear ing care
6.19 Bear ing mounting
6.20 D i m o u n t i n g bear ings
6.21 Mounting and dismount ing by o i l i n j e c t i o n
6.22 Cleaning o f bear ings
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6.23 Recogn i t ion o f bear ing f a i l u r e s
6.23.1 Wear
6.23.2 I n c o r r e c t mounting
6.23.3 Cage f a i l u r e s
6.23.4 Smearing
6.23.5 V i b r a t i o n s
6.23.6
6.23.7
Rust and o t h e r types o f c o r r o s i o n
Passage o f e l e c t r i c c u r r e n t through
bear ings
CHAPTER 7. PRACTICAL GEAR TRIBOLOGY, T . I . FOWLE, CONSULTANT
7.1 I n t r o d u c t i o n
7.2 Al ignment
7.3 Tooth a c t i o n
7.4 Tooth su r face d i s t r e s s
7.4.1 P i t t i n g
7.4.2 S c u f f i n g
7.4.3 Abras ive wear
7.4.4 Other forms o f gear wear
7.5 Problems i n l u b r i c a t i o n systems
References
Gear problems : causes and remedies
CHAPTER 8. MATERIALS FOR TRIBOLOGICAL APPLICATIONS, 0. SCOTT,
CONSULTANT, EDITOR OF WEAR
8.1 I n t r o d u c t i o n
8.2 Types o f m a t e r i a l s
8.3 M a t e r i a l s f o r s p e c i f i c a p p l i c a t i o n s
8.3.1 P l a i n bear ings
8.3.2 Gears
8.3.3 R o l l i n g bear ings
8.3.4 Wear r e s i s t a n t m a t e r i a l s
8.3.5 Tools
8.3.6 C u t t i n g t o o l s
8.3.7 P i s t o n r i n g s
8.3.8 Cams and tappets
8.3.9 F r i c t i o n m a t e r i a l s
8.3.10 P l a s t i c bear ings
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7-07 21 0
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XI
8.4 Surface t rea tments and coa t ings
8.5 Conclusions
References
CHAPTER 9. SELECTION OF LUBRICANTS, A.R. LANSDOWN, SWANSEA
TRIBOLOGY CENTRE
9.1 I n t r o d u c t i o n
9.2 Se lec t i ng the l u b r i c a n t type
9.3 P roper t i es o f minera l o i l s
9.3.1 V i s c o s i t y
9.3.2 V i s c o s i t y - temperature r e l a t i o n s h i p
9.3.3 V i s c o s i t y - pressure r e l a t i o n s h i p
9.3.4 Ant i -wear , extreme pressure and a n t i -
f r i c t i o n p r o p e r t i e s
9.3.5 S t a b i l i t y
9.3.6 Con tami nan t s
9.4 Choice o f base o i l
9.4.1 L i m i t a t i o n s o f minera l o i l s
9.4.2 High temperatures
9.4.3 F lammab i l i t y
9.4.4 Compat ib i l i t y
9.5 Greases
9.6 Sol i d l u b r i c a n t s
9.7 Gas l u b r i c a t i o n
CHAPTER 10. LUBRICANT ADDITIVES, THEIR APPLICATION, PERFORMANCE
AN0 LIMITATIONS, D.M. SOUL, LUBRIZOL INTERNATIONAL
LABORATORIES.
10.1 I n t r o d u c t i o n
10.2 Basic p r o p e r t i e s of l u b r i c a n t s
10.3 Lub r i can t a d d i t i v e s
10.3.1 Detergent and d i spe rsan t a d d i t i v e s
10.3.1.1 Detergent types
.1 Sulphonates
.2 Phosphonates and/or th iophosphonates
.3 Phenates
.4 A l k y l s u b s t i t u t e d sa l i c y l a t e s
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10.3.1.2 Mode o f a c t i o n o f detergent
add i t i ves
10.3.1.3 Dispersant a d d i t i v e s
. 1 Cop0 1 yme r s
.2 Subs t i t u ted Succinimides
.3 Amides
.4 Other chemicals
Mode o f a c t i o n o f d ispersant
a d d i t i v e s
10.3.1.4
10.3.2 An t iox idan ts and bear ing co r ros ion
i n h i b i t o r s
10.3.2.1 Di th iophosphates
10.3.2.2 Hindered phenol
10.3.2.3 Ni t rogen bases
10.3.2.4 Sulphur ised p o l y o l e f i n s
10.3.2.5 Mode o f a c t i o n o f a n t i o x i d a n t s
10.3.3 Corrosion i n h i b i t o r s
10.3.3.1 Di th iophosphates
10.3.3.2 Di th icarbamates
10.3.3.3 Sulphur products
10.3.3.4 Phosphorus - Sulphur products
10.3.3.5 T r iazo les and c h e l a t i n g agents
10.3.3.6 Dime rcap t o th i ad i o z o l e
d e r i v a t i v e s
10.3.3.7 Mode o f a c t i o n o f co r ros ion
i n h i b i t o r s
10.3.4 Rust i n h i b i t o r s
10.3.5 V i s c o s i t y improvers
10.3.5.1 Mode o f a c t i o n of V . I . improvers
10.3.6 Pour p o i n t depressants
10.3.6.1 Mode o f a c t i o n o f pour p o i n t
depressants
10.3.7 Extreme pressure a d d i t i v e s
10.3.7.1 Comrnerical E.P. a d d i t i v e s and
t h e i r a p p l i c a t i o n
10.3.7.2 Automotive E.P. gear o i l s
10.3.7.3 Ant iwear a d d i t i v e s
10.3.7.4 E.P. a d d i t i v e s f o r t u r b i n e o
10.3.7.5 E.P. a d d i t i v e s f o r c u t t i n g o
10.3.7.6 Mode o f a c t i o n o f E.P. a d d i t
Is
Is
ve s
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10.3.8 E m u l s i f i e r s
10.3.9 F r i c t i o n m o d i f i e r s
10.4 Conclusions
References
CHAPTER 11. CONSUMPTION AND CONSERVATION OF LUBRICANTS,
A.R. LANSDOWN, SWANSEA TRIBOLOGY CENTRE
11.1 Consumption
11.2 Reducing consumption
11.3 Reclamat ion and r e - r e f i n i n g
11.4 Economics
CHAPTER 12. HEALTH AND SAFETY ASPECTS OF LUBRICANTS,
A.R. EYRES, MOBIL EUROPE INC.,
12.1 I n t r o d u c t i o n
12.2 Composition o f l u b r i c a n t s
12.3 Minera l base o i l f a c t o r s
12.3.1 Acute t o x i c i t y
12.3.2 D e r m a t i t i s
12.3.3 O i l m i s t
12.3.4 O i l vapours
12.3.5 Skin cancer
12.3.6 Eye i r r i t a t i o n
12.4 A d d i t i v e f a c t o r s
12.4.1 Lead compounds
12.4.2 Orthophosphates
12.4.3 Ch lo r ina ted Naphthalenes
12.4.4 Sodium N i t r i t e and Amines
12.4.5 Sodium Mercaptobenzothiazole
12.4.6 T r i c h l o r o e t h y l e n e
12.5 Bac te r ia and b i o c i d e s
12.6 Syn the t i c l u b r i c a n t s
12.7
12.8 Hea l th and s a f e t y p recau t ions
Used and rec la imed o r r e - r e f i n e d o i l s
12.8.1 Supp l i e rs r e s p o n s i b i l i t i e s
12.8.2 Sk in p r o t e c t i o n
12.8.3 O i l m i s t and vapour
12.8.4 Sk in cancer
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XIV
12.8.5 Bac te r ia and b ioc ides
12.9 Conclusions
References
CHAPTER 13. EFFECTIVE CONTAMINATION CONTROL I N FLUID POWER
SYSTEMS, J.B. SPENCER, SPERRY VICKERS
13.1 I n t r o d u c t i o n
13.1.1 A systemat ic approach t o f i l t r a t i o n
13.2 D i r t Input - types and sources o f contaminat ion
13.2. I New oi 1
13.2.2 B u i l t i n contaminat ion
13.2.3 Environmental Contaminat ion
13.2.4 En t ry p o i n t s f o r environmental
contaminat ion
13.2.4.1 A i r breathers
13.2.4.2 Power u n i t access p l a t e s
13.2.4.3 Cyl inder seals
13.2.5 Generated contaminat ion
13.3 E f f e c t s o f types and s izes o f p a r t i c l e s
13.3.1 Catast rophic f a i l u r e
13.3.2 I n t e r m i t t e n t f a i l u r e
13.3.3 Degradat ion f a i l u r e
13.3.4 Pumps
13.3.5 Motors
13.3.6 D i r e c t i o n a l va lves
13.3.7 Pressure c o n t r o l s
13.3.8 Flow c o n t r o l s
13.3.9 Summary
13.4 Speci fy ing contaminat ion l e v e l s
13.5 Se lec t i ng the f i l t e r
13.5.1 Nominal r a t i n g
13.5.2 Absolute r a t i n g
13.5.3 The bubble t e s t
13.5.4 Mean f i l t r a t i n g r a t i n g
13.5.5 Mu l t i pass f i l t e r t e s t
13.5.6 Beta r a t i o
13.5.7 P r a c t i c a l c l a s s i f i c a t i o n of f i l t e r
performance
13.5.7.1 Pu lsa t i ng f l o w
13.5.7.2 Bypass ing
289
289
289
291
291
292
294
294
294
294
295
295
295
295
296
296
296
296
297
297
300
30 1
302
302
303
305
305
305
306
306
306
307
307
307
308
308
xv
13.5.8 F i l t e r s i z i n g
13.6 Loca t ing the f i l t e r
13.6.1 Pump i n l e t f i l t r a t i o n
13.6.2 Pressure 1 ine f i l t r a t i o n
13.6.3 Return 1 i ne f i 1 t r a t ion
13.6.4 O f f l i n e f i l t r a t i o n
13.7 Summary
CHAPTER 14. SEALS FOR FLUID POWER EQUIPMENT, PART ONE,
B.D. HALLIGAN, JAMES WALKER 6 CO. LTD.
14.1 I n t r o d u c t i o n
14.2 M a t e r i a l s
14.3 Seal designs
14.4 T r i b o l o g i c a l cons ide ra t i ons
14.4.1 F i l m c o n d i t i o n s
14.4.2 Surface f i n i s h
14.4.3 Seal f r i c t i o n
14.4.4 Type o f f l u i d
14.4.5 F i l t r a t i o n
14.4.6 A i r en t ra inment
14.5 Se lec t i on
14.6 Storage
14.6.1 Recommendations
14.6.1.1 Temperature
14.6.1.2 Humid i ty
14.6.1.3 L i g h t
14.6.1.4 Oxygen and ozone
14.6.1.5 Deformat ion
14.7 Assembly
CHAPTER 15. SEALS FOR FLUID POWER EQUIPMENT, PART TWO, ROTARY
L IP SEALS, B.D. HALLIGAN, JAMES WALKER E CO. LTD.
15.1 I n t r o d u c t i o n
15.2 Design
15.3 Ma te r ia l
15.4 Seal l u b r i c a t i o n
15.5 Shaf t sur faces
15.6 F r i c t i o n
15.7 Speeds
312
318
318
320
322
324
327
330
330
331
338
339
339
343
344
346
347
347
348
350
350
350
351
351
351
351
351
SHAFT
353
353
354
355
356
357
358
359
XVI
15.9 Pressure
15.10 Care and hand l i ng
15.10.1 Storage
15.10.2 Handl ing
15.10.3 F i t t i n g
15.11 Serv ice problems and t h e i r s o l u t i o n s
15.11.1 Unacceptable leakage
CHAPTER 16. SEALS FOR FLUID POWER EQUIPMENT, PART THREE, COMPRESSION
PACKINGS, B.D. HALLIGAN, JAMES WALKER t CO. LTD.
16.1 The packed g land
16.1.1 Pumps
16.1.2 Valves
16.2 Operat ing p r i n c i p l e s
16.3 Gland design
16.4 Packing c o n s t r u c t i o n and m a t e r i a l s
16.4.1 F i b r e m a t e r i a l
16.4.2 Lub r i can ts
16.4.3 Const ruc t i on
16.4.4 A d d i t i o n a l m a t e r i a l
16.4.5 Se lec t i on
16.4.6 F i t t i n g
16.4.7 F a u l t f i n d i n g
16.4.8 S tandard i za t i on
16.4.9 The f u t u r e
Appendix 1 Safety o f asbestos g land packings and gaskets
CHAPTER 17. CENTRALISED LUBRICATION SYSTEMS DESIGN, J.G. MERRETT,
ENGINEERING AN0 GENERAL EQUIPMENT LTD.
17.1 I n t r o d u c t i o n
17.2 Po in ts of l u b r i c a t i o n
17.3 Se lec t i ng the l u b r i c a n t - o i l or grease
17.3.1 O i l o r grease7 when t o use one and when
the o t h e r
L u b r i c a t i n g grease - the types and how
t o s e l e c t them
17.3.2
17.3.3 L u b r i c a t i n g o i l s - types and how t o
s e l e c t them
359
360
360
361
36 1
362
362
370
370
371
371
373
374
376
3 76
376
3 76
379
383
388
390
391
392
393
395
395
396
396
397
398
400
XVII
17.3.3.1 The p r o p e r t i e s o f t h e o i l 40 1
17.3.3.2 S p e c i a l i s t l u b r i c a n t s 402
17.3.4 Summarising l u b r i c a n t s e l e c t i o n 402
17.4 Pipe diameter vs f l o w c h a r a c t e r i s t i c s 402
17.5 L u b r i c a t i o n requirements f o r p l a i n bear ings 402
17.6 Se lec t i ng the r i g h t type o f l u b r i c a t i o n system 403
17.6.1 Grease l u b r i c a t i o n systems 403
17.6.1.1
17.6.1.2
17.6.1.3
17.6.1.4
17.6.1 - 5
17.6.1.6
D i r e c t f eed ing systems 403
I n d i r e c t o r l i n e systems 405
Comparison o f p a r a l l e l
systems - s i n g l e l i n e and
dual l i n e 412
Se lec t i ng grease l u b r i c a t i o n
systems 412
L u b r i c a t i o n o f p l a i n o r s leeve
bear ings 41 2
L u b r i c a t i o n o f a n t i f r i c t i o n
( b a l l and r o l l e r ) bear ings 412
17.6.2 O i l l u b r i c a t i o n systems 413
17.6.2.1
17.6.2.2
17.6.2.3
17.6.3 Mic ro- fog
17.6.3.1
17.6.3.2
17.6.3.3
17.6.3.4
17.6.3.5
17.6.4 Check l i s t
17.7 Summary
References
Tota l l o s s systems
Systems designed w i t h smal l
amount o f heat removal
Systems designed f o r l u b r i c a t i o n
coo l i ng
l u b r i c a t i o n systems
Working p r i n c i p l e s
O i l q u a l i t y
Compressed a i r
Systems des ign cons ide ra t i ons
Some t y p i c a l a p p l i c a t i o n s
CHAPTER 18. ON CONDITION MAINTENANCE, R.A. COLLACOTT, FAULT
DIAGNOSIS CENTRE
18.1 I n t r o d u c t i o n
18.2 Background
18.3
18.4
Management o f cond i t i o n mon i t o r i ng
F a i l u r e modes and e f f e c t s a n a l y s i s
413
416
418
41 9
419 421
421
421
421
423 424
425
427
427
427 428
428
XVIII
18.5 Monitoring technique se lect ion
18.5.1 V ib ra t i on monitoring
18.5.1.1 Proximity probes
18.5.1.2 Seismic v e l o c i t y transducers,
accelerometers, v i b r a t i o n
meters, spectrum analysers
18.5.1.3 Waveform analys is
18.5.2 Inspection, i n t e g r i t y su rve i l l ance
18.5.3 Contaminant analys is
18.5.4 Trends analys is
18.6 De te r io ra t i on l i m i t s
References
CHAPTER 19. THE TRIBOLOGY OF METAL CUTTING, E.M. TRENT, U N I V E R S I T Y
OF BIRMINGHAM
19.1 In t roduct ion
19.2 Metal c u t t i n g phenomena
19.3 Conditions a t the tool-work i n te r face
19.3.1 Tool forces and stresses
19.3.2 Cut t ing speed
19.3.3 Heat i n metal c u t t i n g
19.3.4 Seizure o f the tool-work i n te r face
19.3.5 Cut t ing too l temperatures
19.3.6 SI i d i ng a t the tool-work i n te r face
19.4 Cut t ing too l wear
19.4.1 Abrasion
19.4.2 Surface shearing
19.4.3 D i f f us ion and in te rac t i on
19.4.4 A t t r i t i o n
19.5 Coolants and lub r i can ts
References
CHAPTER 20. ROLE OF LABORATORY TEST MACHINES, F.T. BARWELL AND
M.H. JONES, UNIVERSITY COLLEGE O F SWANSEA
20.1 Experimental method
20.2 Lubricant type approval t e s t i n g machines
20.2.1 Engine t e s t s
20.2.2 Gear tes ts
20.2.3 Simulat ion o f i n d u s t r i a l s i t u a t i o n s
432
433
433
434
4 36
436
444
444
445
4 38
446
446
446
448
448
451
45 1
452
456
4 58
459
46 1
46 1
463
465
467
470
471
471
4 72
472
474
479
XIX
20 .3 Analys is i n t o system elements o f the bas i s f o r
s e l e c t i o n o f l abo ra to ry t e s t s
20 .4 Equipment f o r bas i c research
Ref e r enc e s
GLOSSARY
AUTHOR INDEX
480
488
492
493
502
SUBJECT INDEX 506
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XXI
ACKNOWLEDGEMENT
The e d i t o r s g r a t e f u l l y acknowledge the ass is tance o f M r . G. Wi l l iams and
M r . J . Thomas o f the l u b r i c a t i o n department o f B r i t i s h Steel Corporat ion,
Port Ta lbot and D r . G. Thomas o f the Ext ra Mural Department, U n i v e r s i t y
Col lege, Swansea, who have been a c t i v e l y assoc iated w i t h the annual
Seminars on i n d u s t r i a l T r i bo logy s ince t h e i r i ncep t ion .
Acknowledgement i s a l s o due t o v i s i t i n g l e c t u r e r s who generously gave t h e i r
t ime i n developing the Seminar t o i t s present successful format . These
inc lude R. Gronbech - Davey United, J. Bathgate - David Brown Gears Ltd. ,
P. Gadd - NAML Gosport, N.W. Mor r i s - Farvalube Ltd. , D. Hatton - Shel l
I n t e r n a t i o n a l , D r . E.T. Jagger - Angus Seals Ltd., and D r . D.J. Haines - B r i t i s h A i r c r a f t Corporat ion.
The e d i t o r s extend t h e i r g r a t i t u d e t o the t y p i n g e x p e r t i s e o f Mrs. M.A.
W i l l i ams and Mrs. P.T. Hancock who have so e x p e r t l y produced t h i s camera
ready copy.
This Page Intentionally Left Blank
XXIII
FOREWORD
Some eleven years ago M r . Ronald Dale, B r i t i s h Steel Corporat ion, Po r t Talbot,
v i s i t e d the Col lege t o propose the i n t r o d u c t i o n o f a course i n T r ibo logy f o r
Steelworks Design S t a f f . Th is proposal was r e a d i l y accepted because prepar-
a t i o n s f o r the establ ishment o f the T r ibo logy Centre were a l ready w e l l advanced.
The f i r s t course was r a t h e r ambi t ious i n s o f a r as i t was o f two weeks
du ra t i on , the f i r s t week being devoted t o f a i r l y heavy theory w h i l s t more
p r a c t i c a l m a t e r i a l was in t roduced du r ing the second week. A p a r t i c u l a r f e a t u r e
was the i n c l u s i o n o f p r o j e c t s based on contemporary problems a t the steelworks.
These p r o j e c t s were a l l o c a t e d t o small teams and, on the f i n a l day, the leader
o f each team had t o r e p o r t t o the whole course on the s o l u t i o n a r r i v e d a t by h i s
team.
The demand f o r repeat courses was s t rong and they have been run a t annual
i n t e r v a l s ever s ince.
year t o year as the r e s u l t o f quest ionnai res completed by course members and i n
response t o rep resen ta t i ons by employers. Thus the present course i s o f sho r te r
d u r a t i o n and i s o r i e n t a t e d towards p r a c t i c e from the ou tse t , theory o n l y be ing
in t roduced when requ i red t o p o i n t the way towards the s o l u t i o n s o f p r a c t i c a l
problems.
However, the p a t t e r n o f i n s t r u c t i o n has been mod i f i ed from
One o f t he o b j e c t s o f the f i r s t course, t o c o n t r i v e the maximum degree o f
interchange o f i n fo rma t ion between par t ic ipants ,has been re ta ined and developed,
p a r t i c u l a r l y s ince the course i s now r e c r u i t e d f rom a number o f i n d u s t r i e s
besides the s t e e l i ndus t r y and indeed from several coun t r i es .
The term ' T r i b o l o g y ' , de f i ned i n the Chambers Twent ie th Century D i c t i o n a r y
as fo l l ows : - a science and technology embracing a l l subjects invo lved when sur-
faces i n con tac t move i n r e l a t i o n t o each o t h e r (Greek t r i b e i n , t o rub, and
logos, speech, a d iscourse) had then been o n l y r e c e n t l y in t roduced t o emphasize
the m u l t i - d i s c i p l i n a r y na tu re o f the study o f bea r ing system. The term 'Lub-
r i c a t i o n ' which had been used p r e v i o u s l y was considered t o be inadequate because
i t focussed a t t e n t i o n on one element o n l y o f the bear ing system - no tab ly the
l u b r i c a n t t o the exc lus ion o f o t h e r f a c t o r s such as the ma te r ia l o f cons t ruc t i on
o f the i n t e r a c t i n g elements.
The need f o r the i n t r o d u c t i o n o f the new term was n o t necess i ta ted by any
f a i l u r e t o develop the sub jec t f rom the research and development p o i n t o f v iew
bu t r a t h e r t o draw a t t e n t i o n t o a body o f knowledge which was n o t thought t o be
s u f f i c i e n t l y a p p l i e d w i t h i n i ndus t r y a t t h a t p a r t i c u l a r t ime. The development
o f T r ibo logy has always been r e l a t e d t o advances i n the State o f the A r t o f
XXIV
Engineering. Thus, du r ing the n ine teen th century , progress was dominated by
the ra i lway and the steamship. The hydrodynamic theory o f l u b r i c a t i o n was
developed t o e x p l a i n Beachamp Towers experiments on bear ings used on the Metro-
p o l i t a n RaiIway,and M i c h e l l ' s t i l t i n g pad t h r u s t bear ings were considered t o be
very s u i t a b l e for sh ips ' p r o p e l l e r shaf ts . The t h i r t i e s t h i s cen tu ry were
devoted t o the a i r c r a f t and automobi le; the f o r t i e s t o the gas tu rb ine . Much
o f the e a r l y work o f N.E.L. and N.A.S.A. Cleveland was d i r e c t e d t o the problems
o f high-speed bear ings. Then came atomic energy.
i n s t i t u t i o n , The Na t iona l T r ibo logy Laboratory a t R is ley , can be a t t r i b u t e d t o
t h i s need and, more recent ly,space technology has in t roduced a whole range o f
new problems p a r t i c u l a r l y r e l a t e d t o ope ra t i on i n r a r i f i e d atmospheres.
The o r i g i n o f our s i s t e r
I f one has t o decide on today 's specia l character i t i s the urge f o r g rea te r
p r o d u c t i v i t y main ly achieved by automat ic and otherwise c a p i t a l - i n t e n s i v e equip-
ment. Th is has focussed a t t e n t i o n on the importance o f r e l i a b i l i t y and of
reduced maintenance,which c a l l f o r the utmost ref inement o f t r i b o l o g i c a l design.
Means f o r mon i to r i ng the c o n d i t i o n o f machines so as t o a n t i c i p a t e f a i l u r e and
t o r a t i o n a l i s e maintenance have now assumed the utmost importance. The t r i b o l o g y
o f the manufactur ing i ndus t r y w i l l p robably be the most important growth area
o f our subject dur ing the nex t decade.
Notwi thstanding the output o f present day research schools i n T r ibo logy , o f
which there a r e a growing number, the body o f knowledge which has been b u i l t up
dur ing the past century prov ides powerful t o o l s f o r any engineer who wishes t o
improve a product o r p r a c t i c e and i t i s hoped t h a t t h e f o l l o w i n g volume w i l l
p o i n t the way t o the a p p l i c a t i o n o f sound t r i b o l o g y i n many indus t r i es .
F.T. Barwel l U n i v e r s i t y Col lege o f Swansea
1
1 TRIBOLOGY IN PERSPECTIVE
D. SCOTT, Consul tant , E d i t o r o f Wear
1.1 INTRODUCTION
As our techno log ica l c i v i l i z a t i o n expands, ma te r ia l and energy conservat ion
i s becoming inc reas ing l y important. Wear i s a major cause o f ma te r ia l wastage,
so any reduc t i on o f wear can e f f e c t cons iderable savings. F r i c t i o n i s a
p r i n c i p a l cause o f energy d i s s i p a t i o n and considerable savinas a re poss ib le by
improved f r i c t i o n c o n t r o l . L u b r i c a t i o n i s the most e f f e c t i v e means o f c o n t r o l -
l i n g wear and reducing f r i c t i o n .
nology o f f r i c t i o n l u b r i c a t i o n and wear, i s o f cons iderable importance i n
m a t e r i a l and energy conservat ion.
which i s concerned w i t h problems t h a t have always presented man w i t h a chal lenge
has been recorded 111, and the fundamentals reviewed [2 ] .
Thus t r i b o l o g y , which i s the science and tech-
The h i s t o r y o f t h i s r e l a t i v e l y new science
1.2 IMPACT OF TRIBOLOGY
Since the p u b l i c a t i o n o f the L u b r i c a t i o n Report [ 3 ] t he re has been an i n -
creas ing awareness throughout i ndus t r y o f t he subject o f t r i b o l o g y .
the Nat ional Centre f o r T r ibo logy and I n d u s t r i a l U n i t s o f T r ibo logy have been
set up t o p rov ide adv ice t o i ndus t r y on the u t i l i s a t i o n o f e x i s t i n g knowledge.
These a r e now v i a b l e establ ishments ope ra t i ng as c o n t r a c t research o rgan isa t i ons
s e l l i n g t h e i r se rv i ces a t commercial ra tes . Over t h i r t y u n i v e r s i t i e s , po l y -
technics and techn ica l co l l eges have incorporated courses on va r ious aspects o f
t r i b o l o g y i n t o t h e i r sy l labuses. A bas ic t r i b o l o g y module [41 f o r undergraduate
mechanical engineer ing courses has been drawn up. T r ibo logy i s an e l e c t i v e sub-
j e c t f o r t he h ighe r n a t i o n a l c e r t i f i c a t e (H.N.C.) i n engineer ing i n the Uni ted
I n the UK
academic awa
leading t o h
c h a i r s i n t r
to indust ry .
T r i bo logy
Kingdom and a t r i b o l o g y content i s inc luded i n some committee f o r na t i ona l
ds (C.N.N.A.) courses. Post-graduate research i n t r i b o l o g y ,
gher degrees i s c a r r i e d o u t a t several u n i v e r s i t i e s ; t h ree have
bology. Various courses and t r a i n i n g programmes a re a l s o a v a i l a b
i s now recognized u n i v e r s a l l y and Pres ident Car ter o f U.S.A. 151 dec lared i t t o be a gener ic technology under l y ing many i n d u s t r i a l sect ions and
the prospectus f o r an I n d u s t r i a l T r i bo logy I n s t i t u t e a t Rensselaer Technolooy
Center has been presented [ 6 ] .
e
Numerous papers on t r i b o l o g y a r e publ ished annua l l y and many r e p o r t research
d i r e c t e d towards a b e t t e r under.standing o f the fundamental p r i n c i p l e s governinq
2
i n t e r a c t i n g surfaces. Un fo r tuna te l y , most of the i n fo rma t ion prov ided i s n o t
s u i t a b l e f o r d i r e c t use by designers and engineers as research workers genera l l y
f i n d i t more convenient t o express r e s u l t s i n terms o f non-dimensional para-
meters r a t h e r than as the s p e c i f i c data requ i red f o r design purposes. A t r i -
bology handbook [7] has been produced w i t h the o b j e c t o f p r o v i d i n g i n fo rma t ion
t o i ndus t r y i n a form t h a t i s r e a d i l y access ib le and understood by engineer ing
designers, draughtsmen and works engineers. A synopt ic j o u r n a l [8] has been
introduced t o reduce t ime spent i n l i t e r a t u r e perusal .
There i s a steady growth i n the format ion o f T r ibo logy Soc ie t i es on an
i n t e r n a t i o n a l scale.
1.3 ECONOMIC ASPECTS OF TRIBOLOGY
The L u b r i c a t i o n Report [ 3 ] est imated, w i t h i n an e r r o r o f twen ty - f i ve per
cent , t h a t an amount exceeding f i v e hundred m i l l i o n pounds per annum can be
saved i n the c i v i l i a n sec to r o f the UK economy by improvements i n educat ion and
research i n t r i b o l o g y . Such improvements a re s i g n i f i c a n t , n o t merely i n cos t
savings, bu t are c r u c i a l t o techno log ica l progress and have doubly s i g n i f i c a n t
imp l i ca t i ons f o r the economic wel l -be ing o f the n a t i o n and the r e p u t a t i o n o f i t s
engineer ing products.
The ASME Research Committee on L u b r i c a t i o n i n t h e i r "Strategy f o r Energy
Conservation through Tr ibo logy" [9 ] repor ted the magnitudes o f energy conserv-
a t i o n t h a t can p o t e n t i a l l y be obta ined i n the f o u r major areas o f road t ranspor t -
a t i o n , power generat ion, t u r b o machinery and i n d u s t r i a l processes through prog-
ress i n t r i b o l o g y . The est imated 1 1 per cen t t o t a l savings i n annual US energy
consumption i s equ iva len t t o some s i x teen b i l l i o n US d o l l a r s by an expendi ture
i n research and development o f an est imated twenty- four m i l l i o n d o l l a r s .
A techno-economic study [ l o ] concluded t h a t the a p p l i c a t i o n o f t r i b o l o g i c a l
p r i n c i p l e s and p r a c t i c e s can e f f e c t n a t i o n a l energy savings o f cons iderable
magnitude i n the Uni ted Kingdom, i n the areas covered which comprise the major
p a r t s o f 87% of energy consumption.
f700 m i l l ion per annum.
These savings a re est imated a t €468 t o
Erosion can be expensive and i t has been repor ted [ l l ] t h a t the i nges t i on of
dust c louds can reduce the l i v e s o f h e l i c o p t e r engines by as much as 90 per
cent; l o c a l s t a l l can be caused by removal o f as l i t t l e as 0 .05 mm o f ma te r ia l
from the leading edges o f compressor blades. I n pneumatic t r a n s p o r t a t i o n o f
m a t e r i a l through pipes, the e ros i ve wear a t bends can be up t o f i f t y t imes more
than t h a t i n s t r a i g h t sect ions. Even wood ch ips can cause such wear [ 1 2 ] .
Analyses of the f a i l u r e o f b o i l e r tubes i n d i c a t e t h a t about one t h i r d o f a l l
occurrences were due t o e ros ion [131.
3
Although ab ras i ve wear i s use fu l t o shape and p o l i s h eng ineer ing components,
i t s unwanted occurrence i s p robab ly the most se r ious i n d u s t r i a l wear problem.
In the a g r i c u l t u r a l i n d u s t r y as many as f o r t y per cen t o f the components
rep laced on equipment have f a i l e d by ab ras i ve wear [ 1 4 ] .
The wear o f t o o l s used f o r c u t t i n g meta ls i s o f cons ide rab le importance t o
the economics o f t he eng ineer ing i ndus t r y , I t was es t imated i n 1971 [ 1 5 ] t h a t
f o r t y b i l l i o n d o l l a r s was spent i n the USA on the machining o f metal pa r t s . I n
the UK about twenty m i l l i o n ca rb ide c u t t i n g t o o l s a r e used pe r year a t a c o s t o f
f i f t y m i l l i o n pounds.
Several es t ima tes have been made on the c o s t o f f r i c t i o n and wear. Jos t [16 ]
s ta ted t h a t f r i c t i o n and wear i n the USA accounted f o r an expend i tu re o f one
hundred b i l l i o n d o l l a r s pe r annum.
Technology of F.R.G. [17 ] es t imated t h a t f r i c t i o n and wear caused a n a t i o n a l
economic waste o f t en b i l l i o n OM per annum o f which about f i f t y per cen t i s due
t o ab ras i ve wear. Rabinowicz [18] has es t imated t h a t about ten per cen t o f a l l
energy generated by man i s d i s s i p a t e d i n f r i c t i o n processes.
A Committee o f t he M i n i s t r y o f Research and
T r i b o l o g i c a l f a i l u r e s a r e i n v a r i a b l y assoc ia ted w i t h bear ings and t o ill-
u s t r a t e the c o s t s which can be i nvo l ved i t has been repo r ted [19] t h a t a s imple
bear ing f a i l u r e i n a f u l l y i n t e g r a t e d s t e e l m i l l can l ead t o a t o t a l shut down
which a t f u l l ou tpu t r a t e may c o s t one hundred and f i f t y t o th ree hundred pounds
pe r minute. A s i m i l a r bea r ing f a i l u r e on a modern genera tor se t cou ld i nvo l ve
the Cent ra l E l e c t r i c i t y Generat ing Board i n a l o s s o f one t o twenty pounds
s t e r l i n g pe r minu te t i l l the se t was aga in o p e r a t i o n a l . A s i m i l a r bea r ing
f a i l u r e i n the USA has been quoted t o c o s t t w e n t y - f i v e thousand d o l l a r s per day
[ Z O ] . I t has been repo r ted [ Z l ] t h a t t h e t o t a l c o s t o f wear f o r a US naval
a i r c r a f t amounted t o two hundred and f o r t y th ree d o l l a r s pe r f l i g h t hour.
1.4 MECHANISMS OF WEAR
Progress i n wear c o n t r o l can be a ided by a b e t t e r unders tand ing o f t he
mechanisms by which i t occurs. Research workers have tended t o i s o l a t e and
study s p e c i f i c wear mechanisms such as adhesion, abras ion , e ros ion and f a t i g u e .
Such research has g e n e r a l l y been d i r e c t e d towards t h e study o f sur faces i n
r e l a t i v e mot ion , the changes brought about by t h e i r i n t e r a c t i o n and the e f f e c t s
o f the l u b r i c a n t and the environment p resent . L i t t l e a t t e n t i o n has been g iven
t o the produc ts o f wear, t h a t i s t o the d e b r i s generated. Recent ly, p a r t i c l e
t r i b o l o g y [ 2 2 ] has a l lowed p o s t u l a t i o n o f t he mechanisms o f t h e i r f o rma t ion
which toge the r w i t h r e f i n e d techniques o f sur face i n v e s t i g a t i o n and the study
of sub-surface changes a i d s the e l u c i d a t i o n of the wear process.
Advances i n unders tand ing emerge o n l y f rom a w i l l i n g n e s s t o ques t i on
accepted theo r ies . Ques t ion ing o f the t h e o r i e s o f wear a ided by r e f i n e d
4
i n v e s t i g a t i o n techniques a re now s t i m u l a t i n g the p ioneer ing s p i r i t . Suh's
de laminat ion theory o f wear [23] i s the t y p i c a l example o f recent progress.
Surface examinat ion and wear p a r t i c l e ana lys i s has l e d t o the hypothes is [24]
t h a t i n t e r a c t i o n po l i shes the surfaces and c rea tes a shear mix l a y e r o f sho r t
c r y s t a l l i n e order o f a lmost superduc t i l e ma te r ia l which spreads over the sur face
as f i r s t proposed by Be i l by [25]. mix l aye r t o f a t i g u e and c h a r a c t e r i s t i c p a r t i c l e s f l a k e o f f .
Repeated rubbing con tac t causes the shear
Fur ther work i s requ i red t o prov ide a more complete d e s c r i p t i o n o f the sur-
face behaviour o f m a t e r i a l s and the wider a p p l i c a t i o n o f new theo r ies must awa i t
the a d d i t i o n a l evidence. I t may thus be poss ib le t o p r e d i c t the wear r a t e s o f
m a t e r i a l s based on f i r s t p r i n c i p l e s and fundamental p r o p e r t i e s .
The a p p l i c a t i o n o f a system ana lys i s t o wear problems i s r e c e i v i n g consider-
ab le a t t e n t i o n [26]. The complex na tu re o f wear has delayed i t s i n v e s t i g a t i o n
bu t i t now appears t h a t the era when wear was considered a branch o f s tud ies i n
f r i c t i o n and l u b r i c a t i o n i s coming t o an end. The success o f the f i r s t I n t -
e rna t i ona l Conference on Wear o f Ma te r ia l s 1271 es tab l i shed wear as a subject o f
i n t e r n a t i o n a l importance i n i t s own r i g h t . A second [28 ] and a t h i r d I n t e r n a t -
i ona l Conference [29] have been h e l d and a f o u r t h i s planned. Microscopic
aspects o f wear a re r e c e i v i n g a t t e n t i o n [30] and c a l c u l a t i o n methods f o r
f r i c t i o n and wear have been reviewed [31]. A s t a t e o f the a r t rev iew o f wear i s
a v a i l a b l e [32].
1 . 5 SURFACE STUDIES
The f r i c t i o n a l and wear behaviour o f m a t e r i a l s i s g r e a t l y dependent upon the
sur face ma te r ia l and i t s topography. Surface i n t e r a c t i o n causes changes i n
these p r o p e r t i e s bu t d e t a i l e d knowledge o f happenings i n the i n t e r f a c e when wear
i s occu r r i ng i s d i f f i c u l t t o acqui re. I t has been usual t o study surfaces a t
va r ious stages o f wear t o p o s t u l a t e the sequence o f events. Besides t h i s pro-
cedure g rea te r a t t e n t i o n i s now being g iven t o the s i ze , morphology and
s t r u c t u r e o f wear p a r t i c l e s as w e l l as t o the l o c a l i s e d nature o f damage t o
surface, i n t e r f a c e and subsurface m a t e r i a l . Several new t o o l s a re a v a i l a b l e f o r
the study o f surfaces a t atomic l e v e l , no tab ly Auger e l e c t r o n spectroscopy,
x- ray photon e l e c t r o n spectroscopy, scanning ion spectroscopy and ion s c a t t e r i n g
spectroscopy which w i t h complementary i n fo rma t ion f rom x-ray energy a n a l y s i s i n
the scanning e l e c t r o n microscope and micro-probe a n a l y s i s a i d the t r i b o l o g i c a l
e l u c i d a t i o n o f sur face phenomena [33,34].
Advances have been made i n the a p p l i c a t i o n o f s t a t i s t i c a l techniques t o the
c h a r a c t e r i s a t i o n o f rough sur faces [35,36]. The e n t i r e s t a t i s t i c a l micro-
geometry o f c e r t a i n rough sur faces can now be complete ly descr ibed i n terms o f
the number o f peaks and mean l i n e cross ings counted on a s i n g l e p r o f i l e . These
5
techniques a r e now being a p p l i e d i n t r i b o l o g y and i t appears t h a t i n instrum-
e n t a t i o n three-dimensional mapping i s now w e l l es tab l i shed [ 3 7 ] . For the
measurement, assessment and c h a r a c t e r i s a t i o n o f very f i n e surfaces, a l ase r beam
technique o f l i g h t s c a t t e r i n g appears p o t e n t i a l l y a t t r a c t i v e f o r q u a l i t y c o n t r o l
purposes [38] .
1.6 LUBRICATION
Since Reynolds [39] produced h i s equat ion f o l l o w i n g the p ioneer ing work o f
Tower, (see [ 4 0 ] ) t he mathematical express ion o f the process o f f i l m format ion
between r e l a t i v e l y moving sur faces has been fundamental t o a l l l u b r i c a t i o n
theory. Equations have been de r i ved and a p p l i e d t o the s tudy o f the var ious
sur face c o n f i g u r a t i o n s used i n p r a c t i c e , and the i n t r o d u c t i o n o f t he high-speed
d i g i t a l computer a l lowed the simultaneous s o l u t i o n o f Reynolds equat ion together
w i t h equat ions f o r t he e l a s t i c deformat ion o f the surfaces. Op t i ca l s tud ies o f
e lastohydrodynamic l u b r i c a t i o n (EHL) f i l m s , i n f r a - r e d temperature measurements
and the e l u c i d a t i o n o f the response o f v iscous l i q u i d s t o h i g h frequency shear
have g r e a t l y improved the understanding o f e lastohydrodynamic contacts . I t i s
perhaps b e t t e r t o descr ibe the l u b r i c a n t i n a h i g h l y loaded EHL con tac t as an
e l a s t o - p l a s t i c s o l i d r a t h e r than a s a viscous f l u i d . Based on the new under-
standing, a theory o f EHL t r a c t i o n has been advanced [41 ] which may be app l i ed
to engineer ing components such as r o l l i n g bear ings and var iab le-speed d r i ves .
The elastohydrodynamics o f e l l i p t i c a l con tac ts has been a p p l i e d t o b a l l and
r o l l e r bea r ing l u b r i c a t i o n [42 ] .
Progress i n hydrodynamic l u b r i c a t i o n appears t o be centred on d e t a i l e d
developments r a t h e r than improved fundamental understanding. Work on boundary
l u b r i c a t i o n seems t o be o r i e n t e d towards s p e c i f i c problem areas such as
e leva ted temperatures and h o s t i l e environments. Two c e n t u r i e s o f study have
f a i l e d t o unravel complete ly the myster ies o f l u b r i c a t i o n problems most i m -
p o r t a n t t o mankind, the mechanism o f human j o i n t s . Fo l lowing the t e n t a t i v e
proposal o f squeeze f i l m s [43 ] and the emphasis on the p r o t e c t i v e mot ion o f
t he s imple squeeze f i l m [44] i t i s considered t h a t the prospect o f EHL i s good
bu t t h a t the promis ing mode i s squeeze f i l m and no t r o l l i n g s l i d i n g [451.
C o l l e c t i o n s o f i n fo rma t ion a re a v a i l a b l e on the t r i b o l o g y o f n a t u r a l and a r t -
i f i c i a l j o i n t s [46] and the mechanical p r o p e r t i e s o f b i o m a t e r i a l s [47 ] .
1.7 LUBRICANTS
When f a i l u r e i n se rv i ce occurs i t i s commercial ly more acceptable t o change
the l u b r i c a n t r a t h e r than the design. Thus research and development work i s
con t inuous ly d i r e c t e d towards improved l u b r i c a n t s , a d d i t i v e s t o impart o r
r e i n f o r c e d e s i r a b l e p r o p e r t i e s and syn the t i c l u b r i c a n t s w i t h unique p roper t i es .
The more recent major developments i n l u b r i c a n t f o r m u l a t i o n appear t o have
6
been on c u t t i n g f l u i d s , f i r e - r e s i s t a n t h y d r a u l i c f l u i d s and syn thes ised hydro-
carbon f l u i d s . Al though the l a t t e r may cos t more than m i n e r a l - o i l based
produc ts , exper ience i n d i c a t e s t h a t they may g i v e an o v e r a l l c o s t sav ing .
Compl icat ions caused by l u b r i c a n t s l ead t o c o n s i d e r a t i o n o f wear - res i s tan t
m a t e r i a l s w i t h good f r i c t i o n a l p r o p e r t i e s which can opera te w i t h o u t l u b r i c a t i o n .
A n t i - p o l l u t i o n and conserva t i on i s p l a c i n g emphasis on sealed, l u b r i c a t e d - f o r -
l i f e machinery us ing s o l i d l u b r i c a n t s and sur face t rea tments which l u b r i c a t e .
Under such c o n d i t i o n s i n t e r f a c i a l c o n d i t i o n s become impor tan t . P l a s t i c s
m a t e r i a l s a r e r e c e i v i n g increased a t t e n t i o n e s p e c i a l l y where chemical and
therma 1 i ne r tness a re requ i red.
the s tandard s o l i d l u b r i c a n t i n c ryogen ic a p p l i c a t i o n s . I t s tendency t o c o l d
f low has been c o n t r o l l e d by s u i t a b l e re in fo rcement .
i n c r e a s i n g l y used where h i g h thermal s t a b i l i t y i s requ i red . Metal f i l m l u b r i c -
an ts a r e now f i n d i n g use and p o t e n t i a l developments i n s o l i d l u b r i c a n t techno-
logy may a r i s e f rom composite s o l i d - l i q u i d l u b r i c a n t s t o use the s p e c i f i c
p r o p e r t i e s o f each.
Pol y t e t r a f 1 uoroe thy 1 ene ( P .T. F. E . ) has become
Newer polymers a r e be ing
1.8 MATERIALS
The emergence o f new design concepts i s a ma jor i n c e n t i v e f o r t he development
o f wear - res i s tan t m a t e r i a l s and t h e a c q u i s i t i o n o f m a t e r i a l s data. The thermal
and s t r e s s problems assoc ia ted w i t h advanced t r i bo -eng ineer ing r e q u i r e h igh -
s t reng th , l i g h t - w e i g h t m a t e r i a l s . Convent ional m a t e r i a l s have been improved by
or thodox methods almost t o the l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s so
t h a t new types o f m a t e r i a l s such as composites, s y n t h e t i c diamond and sapph i re ,
new g raph i te5 and carb ides , metal bo r ides and n i t r i d e s which approach the hard-
ness o f n a t u r a l diamond a r e be ing developed.
p e r t i e s new des ign concepts a r e requ i red as the s u b s t i t u t i o n o f such m a t e r i a l s
i n e x i s t i n g designs can lead t o problems and f a i l u r e s i n se rv i ce . Besides
r e p l a c i n g meta ls , ceramics may be used as coa t ings t o complement d e s i r a b l e metal
c h a r a c t e r i s t i c s w i t h r e f r a c t o r y p r o p e r t i e s , i n s u l a t i n g and e ros ion , wear,
o x i d a t i o n and co r ros ion res i s tance .
To u t i l i z e t h e i r s p e c i f i c p ro -
I n t h e f i e l d o f p l a i n bear ings no major development o f s o f t metal bear ings
appears l i k e l y i n the immediate f u t u r e as the p o s s i b l e a l l o y s o f a l l commercial ly
f e a s i b l e s o f t e r metals have been f u l l y e x p l o i t e d , A v a i l a b l e m a t e r i a l s come
c lose to u t i l i s i n g f u l l y t he p o t e n t i a l i t y o f p l a i n bear ings o f c u r r e n t designs
and l u b r i c a t i o n systems. The development o f p l a s t i c s bear ing m a t e r i a l s capable
o f be ing manufactured t o and m a i n t a i n i n g t h e c l o s e to le rances o f me ta l s c o u l d
cause something o f a r e v o l u t i o n i n the bear ing f i e l d .
P l a s t i c s and t h e i r composites dominate the d ry bear ing scene ma in l y due t o
the a v a i l a b i l i t y o f design and performance da ta [48,49].
i n fundamental understandinn o f t he wear o f p l a s t i c s composites has been the
A s i g n i f i c a n t advance
7
r e c o g n i t i o n o f t he dominant r o l e o f the coun te r face metal [50]. Vacuum
depos i t i on techniques such as s p u t t e r i n g , i on - imp lan ta t i on , i o n - p l a t i n g and
chemical vapour d e p o s i t i o n (C.V.D) appear p o t e n t i a l l y a t t r a c t i v e f o r s o l i d f i l m
l u b r i c a n t s o l u t i o n s t o a wide range o f d ry bea r ing problems.
I n the f i e l d o f r o l l i n g bear ing m a t e r i a l s , methods such as reduc t i on o f gas
conten t [ 5 1 ] and d e l e t e r i o u s ca rb ide segregat ion [ 5 2 ] a r e be ing developed t o
improve r o l l i n g con tac t f a t i g u e res i s tance . Developments i n h i g h speed t o o l
s t e e l bear ings have cen t red around a f i n e r d i s p e r s i o n o f ca rb ides [ 5 3 ] and
weight sav ing [ 5 4 ] . The use o f h ighe r than normal a d d i t i o n s o f a l l o y i n g
elements t o p rov ide m a r g i n a l l y improved p r o p e r t i e s may n o t j u s t i f y t he increased
cos t . There appears t o be a steady bu t unspec tacu la r development o f ceramic
m a t e r i a l s such as s i l i c o n n i t r i d e [ 5 5 ] .
Brakes and c l u t c h e s r e q u i r e t o d i s s i p a t e c o n t i n u a l l y g r e a t e r energ ies due
t o loads and speeds g e n e r a l l y i nc reas ing and improved m a t e r i a l s a re c o n s t a n t l y
demanded t o contend w i t h more arduous d u t i e s and h ighe r temperatures. Owing t o
the p o s s i b l e h e a l t h hazard the re i s cons ide rab le p ressure t o rep lace asbestos,
the most e f f e c t i v e f i l l e r m a t e r i a l f o r pheno l i c r e s i n s due t o i t s f i b r o u s na tu re
and heat res i s tance . S in te red metal ma t r i ces a r e now used f o r severe du ty
a p p l i c a t i o n s bu t a t tempts t o i n t roduce o t h e r o rgan ic and ino rgan ic m a t e r i a l s
have n o t y e t succeeded i n d i s p l a c i n g convent iona l m a t e r i a l s except i n h i g h l y
s p e c i a l i s e d f i e l d s . Concorde uses carbon composites aga ins t themselves and
these m a t e r i a l s w i t h cheaper f i b r e s and f i l l e r s may be a p romis ing method o f
approach t o the replacement o f a s b e s t o s - f i l l e d pheno l i c res ins .
1.9 SURFACE TREATMENTS
The m a t e r i a l o f eng ineer ing components must have s t r u c t u r a l c h a r a c t e r i s t i c s
t o s a t i s f y the design requirements and su r face c h a r a c t e r i s t i c s t o contend w i t h
wear, f a t i g u e and environmental e f f e c t s . Surface coa t ings o f f e r t he bes t com-
promise t o these requirements. Surface t rea tments a r e a l s o a t t r a c t i v e as an
a l t e r n a t i v e t o design o r l u b r i c a n t changes t o combat wear i n s e r v i c e [56,571.
Besides the convent iona l t rea tments r e c e n t l y developed, su r face t rea tments
i n v o l v i n g t h i n su r face f i l m s w i t h s p e c i f i c p r o p e r t i e s a r e now f i n d i n g i nc reas ing
use and p rov ing t o be advantageous as wear - res i s tan t coa t ings . The t rea tments
i nc lude phys i ca l and chemical vapour d e p o s i t i o n processes. The u s e o f T i c
coa t ings on s i n t e r e d ca rb ide c u t t i n g t o o l s i s a t y p i c a l example o f reduc ing t o o l
wear and c u t t i n g cos ts [58 ] . Low temperature CVD processes and c o n t r o l l e d
nuc lea r thermo-chemical d e p o s i t i o n a r e be ing developed t o produce equiaxed
g r a i n m a t e r i a l o f excep t iona l hardness. Ion p l a t i n g and i o n imp lan ta t i on a l s o
appear t o be f i n d i n g use f o r s p e c i f i c a p p l i c a t i o n s .
8
1.10 COMPUTER-AIDED DESIGN
I t i s on l y by the combinat ion o f improved s c i e n t i f i c undetstanding and i t s
speedy i n d u s t r i a l u t i l i s a t i o n t h a t r a p i d techno log ica l progress can be achieved.
In the past, a major d i f f i c u l t y has been the delay o r l ack o f feedback from
indust ry and thus the delay i n the t ime taken f rom the i ncep t ion o f a good idea
t o i t s f r u i t f u l a p p l i c a t i o n . To e l i m i n a t e such delays increas ing use i s be ing
made o f the computer i n design t o enable almost instantaneous feedback. I n
the f i e l d o f p l a i n bear ings, us ing a s u i t a b l e programme the designer need o n l y
t ransmit i n fo rma t ion on bear ing design t o be informed o f performance character-
i s t i c s . I n t h i s way he can have i n s t a n t feedback and make use o f the l a t e s t
research r e s u l t s w i thou t be ing an exper t i n the f i e l d s o f t r i b o l o g y , computat ion
o r programming. I n e f f e c t , he has a t hand what amounts t o a un i ve rsa l t e s t i n g
machine i n which he can p lan h i s design, t e s t i t s c h a r a c t e r i s t i c s , modi fy the
design and again measure i t s c h a r a c t e r i s t i c s , con t i nu ing the process u n t i l
s a t i s f i e d t h a t he has the optimum design be fo re commit t ing h imse l f t o f u l l - s c a l e
t e s t , product ion o r serv ice. M a t e r i a l s s e l e c t i o n f o r optimum performance by
computer i s now approaching r a p i d l y .
1.11 MACHINERY CONDITION MONITORING
Economic pressures a re causing the p r a c t i c e o f wi thdrawing equipment f rom
serv ice a t p e r i o d i c i n t e r v a l s f o r i nspec t i on and maintenance t o be rep laced by
f a i l u r e prevent ion maintenance. Thus, means have been developed t o determine
the c o n d i t i o n o f machinery w h i l s t i n se rv i ce and t o de tec t any d e t e r i o r a t i o n o f
performance so t h a t remedial a c t i o n can be taken be fo re the breakdown p o i n t i s
reached. The mon i to r i ng technique chosen depends upon the s p e c i f i c i n fo rma t ion
requi red and the cos t o f a c q u i r i n g the i n fo rma t ion compared w i t h the savings
such in format ion can e f f e c t .
There has been a gradual acceptance o f v i b r a t i o n a n a l y s i s a l though t h i s has
proved t o be n e i t h e r the s implest nor the most e f f e c t i v e method t o use.
problem o f data i n t e r p r e t a t i o n u s u a l l y c rea tes the need f o r expensive t rend
ana lys i s f rom a massive bu i l d -up o f data.
The
The h i s t o r y o f a wear process i s recorded i n the wear d e b r i s produced and
magnetic p lugs and spect rographic o i l a n a l y s i s (SOAP) a r e now e x t e n s i v e l y used
t o detect abnormal wear.
per year on o i l ana lys i s [ZO] t o p r e d i c t o n l y c e r t a i n types o f f a i l u r e i n one
power system, the a i r c r a f t gas tu rb ine , t o save tw ice t h i s f i g u r e i n terms o f
d i r e c t r e p a i r costs . Al though these techniques have proved e f f e c t i v e i n pro-
v i d i n g warning o f changes i n a system, they have some disadvantages. SOAP
prov ides a knowledge on ly o f the q u a n t i t y o f metal i n the l u b r i c a n t bu t no
i n fo rma t ion on the s i z e o r shape o f the wear p a r t i c l e s . Some damage has u s u a l l y
The US Defense Department spends f o r t y m i l l i o n d o l l a r s
9
occur red when t h e magnet ic p l u g p i c k s up d e b r i s l a r g e enough f o r obse rva t i on .
Ferrography [24,59] a convenient method f o r t he i s o l a t i o n and a n a l y s i s o f
wear p a r t i c l e s has opened up a new dimension i n wear d e t e c t i o n and assessment
i n the fo rm o f p a r t i c l e t r i b o l o g y , [ 2 2 ] . Non-meta l l i c p a r t i c l e s can a l s o be
i s o l a t e d f rom l u b r i c a n t s so t h a t l u b r i c a n t degenerat ion produc ts can be
i d e n t i f i e d t o assess c o n d i t i o n and performance. Recent developments [60 ] have
enabled the adop t ion o f fe r rography t o b io -eng ineer ing f o r t he study o f p ros-
t h e s i s j o i n t s , which should a s s i s t t he development o f improved m a t e r i a l s and
design o f a r t i f i c i a l imp lan ts . As a r t h r i t i c j o i n t s a r e sub jec ted t o wear the
a n a l y s i s o f a s p i r a t e d synov ia l f l u i d appears p o t e n t i a l l y a t t r a c t i v e f o r t he
s tudy o f wear ra tes , mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s .
Fer rograph ic synov ia l f l u i d a n a l y s i s should augment unders tand ing o f t he e t i -
o logy and pathogenesis o f degenera t ive a r t h r i t i s and p rov ide a method f o r the
d iagnos is , documentation, p r o g n o s t i c a t i o n and t rea tment o f t he disease. Some
f i f t e e n m i l l i o n USA c i t i z e n s a r e a f f l i c t e d w i t h o s t e o a r t h r i t i s [20 ] . The f i r s t
I n t e r n a t i o n a l Conference on Ferrography has been planned [611.
1 .12 CONCLUS I ONS
I n the sho r t t ime s ince t r i b o l o g y was launched as a concept on i t s own, i t
has been descr ibed as the w o r l d ' s f a s t e s t growing a p p l i e d science, as s t i l l i n
i t s i n fancy [62 ] and as a means o f n a t i o n a l wea l th c r e a t i o n w i thou t commensurate
c a p i t a l investment [ 6 3 ] . So t h a t t r i b o l o g y may q u i c k l y achieve m a t u r i t y s t i l l
g r e a t e r use must be made o f e x i s t i n g knowledge. Most i n d u s t r i a l t r i b o l o g i c a l
problems can be so lved s a t i s f a c t o r i l y by a l o g i c a l sys temat ic i n v e s t i g a t i o n o f
t he problem and the a p p l i c a t i o n o f e x i s t i n g knowledge. I t thus appears t h a t
increased e f f o r t i s requ i red t o d isseminate knowledge i n a r e a d i l y understood
fo rm t o e f f e c t g r e a t e r energy, m a t e r i a l s and manpower savings a t a minimum c o s t .
I t i s hoped t h a t the subsequent chapters go some way towards doing t h i s .
Fu ture t rends may be t o exper iment l e s s bu t t o measure and i n t e r p r e t more.
Research may be j u s t i f i e d o n l y i f i t can p rov ide i n fo rma t ion t o a l l o w indus t r y
t o so lve i t s immediate problems o r can produce s i g n i f i c a n t advances i n techno-
l o g i c a l progress. The env i ronmen ta l i s t s may in f l uence t r i b o l o g i s t s by
demanding reduced no ise l e v e l s o f mechanisms and the e l i m i n a t i o n o f p o l l u t i o n
and t o x i c i t y f rom l u b r i c a n t s . Lub r i can ts g e n e r a l l y do n o t wear ou t bu t become
contaminated and s o , f rom the p o i n t o f v iew o f t he environment and conserva t ion ,
rec lamat ion , which i s p r e s e n t l y o n l y p r a c t i s e d i f economical , w i l l tend t o
become o f major importance as we approach an e r a i n which q u a l i t y o f l i f e , sa fe ty
and a c lean environment may w e l l be the m o t i v a t i n g f o r c e behind techno log ica l
i nnova t ion .
10
REFERENCES
1 2
3
4 5
6
7 8 9 10
1 1 12
13 14
15 16
17
18
19 20
21
22 23 24 25 26
27 28 29 30
31
32
33 34
35
36
37 38
Dowson,D. H i s t o r y o f T r ibo logy , 1979, Longmans, London. Suh,N.P. and Saka,N. (Ed.), Fundaments o f T r ibo logy , 1980, M.I.T. Press, Cambridge, Mass., U.S.A. L u b r i c a t i o n (T r ibo logy ) - Educat ion and Research. A Report on the Present P o s i t i o n and I n d u s t r y ' s Needs, 1966, HM S ta t i one ry O f f i c e , London. A Basic T r ibo logy Module, 1973, Dept. Trade and Indus t r y , London. White House Fact Sheet - The P r e s i d e n t ' s I n d u s t r i a l Innovat ion A c t i v i t i e s , Oct. 1979, White House Press Secre tary , U.S.A. I n d u s t r i a l T r i bo logy I n s t i t u t e - Prospectus, Sept. 1981, Rensselaer Po ly techn ic , Troy, N.Y. Neale,M.J. (Ed.), T r i b o l o g y Handbook, 1973, But te rwor ths , London. Synopt ic Journa l , I n s t . Mech. Engrs., London. S t ra tegy f o r Energy Conservat ion Through T r ibo logy , 1978, ASME, N.Y. Jost,H.P. and Schof ie ld ,J . Energy Saving Through T r ibo logy - The James C lay ton Lec ture , Feb. 1981. I . Mech. Engrs., London. Ti l ly ,G.P., 8 t h I C A S Congress, Amsterdam, 1972. Lehrke,W.D. and Nonnen,F.A., 1st I n t . Conf. P r o t e c t i o n o f Pipes, Durham, 1975, Paper G2, BHRA, C r a n f i e l d . Raask,E., Wear, 1968, '3, 301. Richardson,R.C.D., Jone5,M.P. and Attwood,D.G., Proc. Ag r i c . Eng. Symp., 1967, Div. 2, Paper 26, I n s t . Ag r i c . Enors., London. Z la t i n ,L . , 1st I n t . Cemented Carbide Conf., Chicago, 1971, Paper 1071-918. Jost,H.P. i n Ha l l i ng ,J . (Ed.), P r i n c i p l e s o f T r ibo logy , 1975, X I I , Macmil lan, N.Y. T r i b o l o g i e Res. Rep., T76-35, 1976, M i n i s t r y o f Research and Technology, Z e n t r a l s t e l l e f u r L u f t und Raumfahrtdokumentation und In fo rmat ion , Munich. Rabinowicz,E., I n Chynoweth,A. and Walsh,Wm. (Eds). , M a t e r i a l s Technology, 1976, p. 165 (Amer. I n s t . Phys. Conf. Proc. No.32, N.Y.). Brai thwaite,E.R., I n d u s t r i a l L u b r i c a t i o n , 1969, 21, 241. Ling,F.F., Proc. o f t he T r ibo logy Workshop, 1974, 32, Nat iona l Science Foundation, U.S.A. Devine,M.J. (Ed.), Proc. o f a Workshop on Wear Cont ro l to A l l o w Product D u r a b i l i t y , 1977, Naval A i r Development Centre, Warminster, PA. Scott,D., Proc. I n s t . Mech. Engrs., 1975, l&, 623. Suh,N.P. Wear, 1977, 44, 1. Scott,D., Seifert,W.W. and Westcott,V.C., Sc i . Amer., 1974, 230, 88. Beilby,G., Aggregat ion and Flow i n So l i ds , 1921, Macmil lan, London. Czichos,H., T r i bo lony - A Systemat ic Approach t o the Science and Technology o f F r i c t i o n , L u b r i c a t i o n and Wear, 1979, E l s e v i e r , Amsterdam. Ludema,K. (Ed.), Wear o f M a t e r i a l s , 1977, ASME, N.Y. Ludema,K. (Ed.) , Wear o f M a t e r i a l s , 1979, ASME, N .Y . Ludema,K. (Ed.), Wear o f M a t e r i a l s , 1971, ASME, N.Y. Georges,J.M. (Ed.) , M ic roscop ic Aspects o f Adhesion and L u b r i c a t i o n , 1982, E l s e v i e r , T r i bo logy Ser ies 7, Amsterdam. Kragelsky, I .V. , Dobychin,M.N. and Kombalov,V.S., F r i c t i o n and Wear - C a l c u l a t i o n Methods, 1982, Pergamon, Oxford. Scott,D. (Ed.), T r e a t i s e on M a t e r i a l s Science and Technology, Vol.13, Wear, 1979, Academic Press, N.Y. Buckley,D.H., Wear, 1978, 5, 19. Buckley,D.H., Surface E f f e c t s i n Adhesion, F r i c t i o n and Wear, 1981, E l s e v i e r , T r i bo logy Ser ies 5, Amsterdam. P roper t i es and Metrology o f Surfaces, Proc. I n s t . Mech. Engrs., 1967/68, 182, 3 K , London. Thoma5,T.R. and King,M., Surface Topography i n Engineer ing - A Sta te o f the A r t Review and B ib l i og raphy , 1977, BHRA, C r a n f i e l d . Thomas,T.R., (Ed.), Rough Surfaces, 1982, Longman, London. Forsy th , l . , and Scott,D. Charac te r i sa t i on o f Micro-machined M i r r o r Surfaces, Wear, 1982, I n Press.
11
39 40 41 42
43 44 45 46
47
48
49 50
51 52. 53 54 55 56
57 58 59
60
61
62 63
Reynolds,O., P h i l . Trans. Roy. Soc., 1886, 177, 157. Cameron,A. Proc. I n s t . Mech. Engrs., 1979, 193, P r e p r i n t No.25, London. Johnson,K.L. and Tevazwark,J.L., Proc. Roy. SOC. A., 1977, 356, 215. Hancock,B.T., and Dowson,D., B a l l Bear ing L u b r i c a t i o n - The E las to - hydrodynamics o f E l l i p t i c a l Contacts, 1981, Wi ley & Sons, N.Y. Fein,R.S., Proc. I n s t . Mech. Engrs., London, 1967, 181, (3J) , 125. Dowson,D., Proc. I n s t . Mech. Engrs., London, 1967, TL (3J ) , 45. Higginson,G., Proc. I n s t . Mech. Engrs., London, 1977, 191, P r e p r i n t 33/77. Dumbleton,J.H., (Ed.), The T r ibo logy o f Na tu ra l and A r t i f i c a l J o i n t s , 1980, E l s e v i e r , T r i bo logy Ser ies , 3, Amsterdam. Hastings,G.W., and Will iams,D.F. Mechanical P r o p e r t i e s o f B iomate r ia l s , 1980, Wi ley & Sons, N.Y. A Guide t o the Design and Se lec t i on o f Dry Rubbing Bear ings, 1976, E.S.D.U. Data Item, 76029. Lancaster,J.K., T r i bo logy I n t . , 1973, 6, 219. Bartenev,G.M., and Lavrentev,V.V., F r i c t i o n and Wear o f Polymers, 1981, E l sev ie r , T r i bo logy Ser ies , 6 , Amsterdam. Scott,D. and McCullagh,P.J., Wear, 1973, 3, 339. Scott,D. and B lackwel l ,J., Wear, 1975, 46, 273. Scott,D. and B lackwel l , J . , Wear, 1978, 3, 149. Scott,D., T r i b o l o g y I n t . , 1976, 2, 261. Scott,D., Wear, 1977, 43, 71. Wilson,R.W., Proc. 1 s t T u r o . T r i b o l o g y Congress, 1975, p.165, I . Mech. Engrs., London. Scott,D., Wear, 1978, 5, 283. Hintermann,H.E., Wear, 1978, 5, 407. Bowen,R., Scott,D., Seifert,W.W. and Westcott,V.C., T r i bo logy I n t . , 1976, 2, 261. Scott ,D. and Westcott,V.C., Proc. I n s t . Mech. Engrs., London, 1978, P r e p r i n t C42/78, 123. F i r s t I n t e r n a t i o n a l Conf. on Ferrography, 1982 (Sept.) , Univ. Swansea ( I n Press). E u r o t r i b , ‘ 77 , 1977, Bundesrepubl ik, Deutschland, Dusse ldor f . Jost,H.P., T r i bo logy , 1978, lJ, 34.
12
9 WEAR
D. SCOTT, Consultant, Ed i to r of Wear
2.1 INTRODUCTION
Wear may be def ined as the undesired displacement o r removal o f surface mat-
e r i a l , although under some circumstances, the i n i t i a l stages o f wear o r m i l d
wear which tends t o smooth surfaces, may be bene f i c ia l f o r the running-in o f
mechanisms. The economic impl icat ions o f wear cause concern i n industry, as a
reasonable l i f e i s requi red o f mechanical equipment t o cover c a p i t a l and main-
tenance costs. Whi ls t , i n many instances, wear may no t place an absolute l i m i t
on the l i f e o f an investment, i t c e r t a i n l y causes a great deal o f expenditure
on maintenance t h a t must take place; such maintenance i s c o s t l y i n i t s e l f , but
a l so c o s t l y i n l o s t p r o d u c t i v i t y w h i l s t i t i s being c a r r i e d out .
Although wear has f o r long been a subject o f p r a c t i c a l i n te res t , fundamental
knowledge o f wear i s sparse. This i s due probably t o the i n t e r d i s c i p l i n a r y
nature o f wear making i t d i f f i c u l t t o e luc ida te and the f a c t t h a t wear has been
accepted as i nev i tab le and unavoidable and so mechanical p a r t replacement tech-
nology has dominated wear con t ro l technology.
Progress i n wear con t ro l and prevention can be made on ly a f t e r a b e t t e r
understanding o f the mechanisms by which i t occurs and o f the c o n t r o l l i n g
factors has been acquired.
2.2 THE WEAR PROCESS
Wear may take many forms depending upon surface topography, contact condi t ions
and environment but , genera l ly , there are two main types, mechanical and chemical.
Mechanical wear involves processes which may be associated w i t h f r i c t i o n ,
abrasion, erosion and fa t igue. Chemical wear a r i ses from surface a t tack by
reac t i ve compounds and the subsequent rubbing o r breaking away o f the react ion
products by mechanical act ion. The d i f f e r e n t types o f wear may occur s ing l y ,
sequent ia l ly o r simultaneously, but a l l wear phenomena centre on a common char-
a c e r t i s t i c , an overst ress ing o f the surface 111.
When two surfaces are in contact, the rea l area o f contact i s considerably
less than the apparent area o f contact being confined t o a number o f small areas
where opposing h igh spots touch. Pressure i n these areas w i l l be h igh and the
surface mater ia l deformed by the appl ied load u n t i l the contact area becomes
s u f f i c i e n t l y large t o support the load. According t o Bowden and Tabor 121, i n
13
the absence o f an e f f e c t i v e separat ing f i l m a j u n c t i o n may be formed between
the surfaces, and r e l a t i v e mot ion w i l l cause the j u n c t i o n t o be broken,
r e s u l t i n g i n the removal o f m a t e r i a l f rom one o r the o the r o f the surfaces.
Ming Feng [ 3 ] cons iders t h a t p l a s t i c f l o w occurs a t con tac t i ng a s p e r i t i e s so
t h a t mat ing sur faces conform in a p a t t e r n o f r i dges and grooves and g i v i n g r i s e
t o s t r a i n hardening. Mechanical i n t e r l o c k i n g prevents s l i p a t the i n t e r f a c e
du r ing r e l a t i v e mot ion and shear occurs i n the s o f t e r subsurface ma te r ia l un-
a f f e c t e d by s t r a i n hardening. The sheared o f f m a t e r i a l may be i n the form o f
deb r i s o r i f the shear ing process produces a s u f f i c i e n t r i s e i n temperature,
the wear products may become at tached t o one o f the surfaces.
The adhesive wear theory as descr ibed by Archard [4,5] pos tu la tes the
format ion o f wear p a r t i c l e s a t con tac t i ng a s p e r i t i e s which a re hemispher ical i n
shape. Rabinowicz [6 ] cons iders t h a t the r a t i o o f sur face energy t o ma te r ia l
hardness i s an impor tant f a c t o r i n wear and may have some e f f e c t on wear
p a r t i c l e s ize. Al though wear equat ions [4,6] de r i ved f rom these theo r ies a re
cons is ten t w i t h exper imental r e s u l t s , they do n o t account f o r the bas ic
me ta l l u rgy o f the m a t e r i a l s and a re based on severa l a r b i t r a r y assumptions.
S e i f e r t and Westcott [7,8] have demonstrated t h a t rubbing wear p a r t i c l e s take
the form o f t h i n f l a k e s o f metal w i t h h i g h l y po l i shed sur faces and a r e n o t hemi-
spher i ca l fragments genera l l y proposed by adhesion theory. Suh and o the rs
[9,10] have proposed a delaminat ion theory o f wear based on the behaviour o f
d i s l o c a t i o n s a t the rubbing surface, subsurface v o i d and crack format ion and
the subsequent j o i n i n g o f cracks by shear deformat ion o f the surface. The de-
l am ina t ion theory p r e d i c t s t h a t the wear p a r t i c l e shape i s t h i n f l a k e - l i k e
sheets as opposed t o the hemispher ical shape proposed by the adhesion theory
and t h a t the sur face l aye r can undergo la rge p l a s t i c deformat ion. Experimental
r e s u l t s showing the process o f wear sheet fo rma t ion by delaminat ion a re c la imed
t o subs tan t i a te the theory [ll]. Suh has repo r ted t h a t b u l k ma te r ia l hardness
i t s e l f i s n o t t he c o n t r o l l i n g f a c t o r on wear and t h a t the delaminat ion theory
s a t i s f i e s the thermodynamic requirements o f the f r i c t i o n a l and wear behaviour
o f meta ls [12] . Westcott and o the rs [8,13] suggest t h a t sur face i n t e r a c t i o n
po l i shes the sur faces and c rea tes a shear mix l aye r of sho r t c r y s t a l l i n e order
o f a lmost superduc t i l e m a t e r i a l which spreads over the sur face t o e f f e c t the
smooth na tu re o f run - in sur faces as f i r s t proposed by Be i l by [14] . Repeated
rubbing causes the shear mix l aye r t o become f a t i g u e d and rubbing wear p a r t i c l e s
f l a k e o f f [13] . Such a s t a t e o f e q u i l i b r i u m mainta ins benign wear, bu t d i s -
r u p t i o n o f t h i s s t a t e can cause i n i t i a t i o n o f a more severe mode o f wear [13] .
A rheo log i ca l mechanism of p e n e t r a t i v e wear [15 ] has been proposed f o r the
format ion o f p l a t e - l i k e deb r i s . The study o f subsurface e f f e c t s du r ing the
s l i d i n g o f meta ls supports the assumption o f de laminat ion 1161. Study o f the
14
wear behaviour o f u l t r a h igh molecular weight (UHMW) poly thene revealed a
p l a s t i c a l l y f lowed smooth sur face and subsurface c rack ing which separated
sheets o f polymer f rom the wear t r a c k i n the form o f smooth sur faced p l a t e l e t s
[17 ] . H i r s t [18], K r a g e l s k i i [19] and Scot t [20] have reviewed the sub jec t o f
wear. Bickerman 1211 has c r i t i c a l l y reviewed the t h e o r i e s of adhesion and
f r i c t i o n a l phenomena i n s l i d i n g con tac t and Barwel l [22] has reviewed the
theo r ies o f wear and t h e i r s i g n i f i c a n c e f o r engineer ing p r a c t i c e .
2.3 SCUFFING
Under s l i d i n g cond i t i ons , the c h i e f t ask o f a l u b r i c a n t i s t o a l l o w r e l a t i v e
This can be achieved mot ion between surfaces, w i t h low f r i c t i o n and no damage.
i f the l u b r i c a n t f i l m i s t h i c k enough t o keep the surfaces apar t and hydro-
dynamic c o n d i t i o n s p r e v a i l . I f however, i dea l cond i t i ons cannot be maintained,
the surfaces w i l l come i n t o con tac t and wear o r damage i n the form o f s c u f f i n g
w i l l occur, Fig.1. The m e t a l l o g r a p h i c a l l y changed, scuf fed m a t e r i a l , Fig.2,
i s cons iderably harder ( va ry ing f rom 300 t o 850HV) than the o r i g i n a l carbon
Fig.1. ( ~ 7 5 ) Scuffed s t e e l sur face . Fig.2. (H= x110, V= ~ 1 1 0 0 )
Taper sec t i on through a scu f fed surface.
s tee l (180HV). E lec t ron microscopica l i n v e s t i g a t i o n o f the scu f fed ma te r ia l
[23,241 revea ls t h a t i t i s mar tens i te and tempered mar tens i te o r t r o o t s t i t e ,
i n d i c a t i n g t h a t the ma te r ia l has been heated t o above the a u s t e n i t i s i n g temp-
e ra tu re and r a p i d l y cooled. Hydrocarbon l u b r i c a n t breakdown i n the con tac t
15
zone [ 1 5 ] can lead to increase o f carbon content and hardness due t o d i f f u s i o n
o f carbon and gases i n t o the heated deformed m a t e r i a l .
The complex mechanism o f s c u f f i n g i s d i f f i c u l t t o e l u c i d a t e as the process,
by cumulat ive a c t i o n , dest roys evidence o f i t s i n i t i a l stages. To study
s c u f f i n g and t o f o l l o w the development o f sur face f a i l u r e a cros,sed c y l i n d e r
machine has been used [26] i n which one c y l i n d e r i s r o t a t e d and a mat ing
c y l i n d e r , a t r i g h t angles t o i t , i s so t raversed t h a t the area o f con tac t moves
along the sur face o f bo th c y l i n d e r s .
t es ted c y l i n d e r reveals how sur face damage b u i l d s up w i t h t e s t d u r a t i o n and
load 1271. The bear ing t r a c k s on l u b r i c a t e d s t e e l c y l i n d e r s a re d i s t i ngu ished
by the increased o p t i c a l r e f l e c t i v i t y . As the load increases f a i l u r e i s
i n i t i a t e d by sho r t f i n e marks, f o l l owed by i n c i p i e n t s c u f f i n g and cont inuous
f a i 1 ure.
Examination o f the h e l i c a l t r a c k round a
Study o f taper sect ions has helped considerably i n the e l u c i d a t i o n o f the
e f f e c t s o f rubbing a c t i o n ; the i n i t i a l p o l i s h i n g seems t o be achieved by
smoothing o f the g r i n d i n g a s p e r i t i e s by p l a s t i c deformat ion as suggested by
Westcott [7,81 and Suh 191, Fig.3. Subsurface metal l og raph ic changes occur,
Fig.3. Heat produced by rubbing a c t i o n appears t o temper the hard s tee l .
Surface hardness may be preserved by r a p i d quenching from above the austen-
i t i s i n g temperature by l u b r i c a n t o r b u l k ma te r ia l . The whi te-etch ing, hard
sur face m a t e r i a l u s u a l l y conta ins cracks. Thermal so f ten ing may occur by con-
duc t i on o f heat i n the subsurface area away from l u b r i c a n t quenching ac t i on .
Local heat ing, and subsequent quenching by l u b r i c a n t o r c o l d b u l k metal may be
s u f f i c i e n t t o m e t a l l o g r a p h i c a l l y change and harden the sur face o f s o f t s t e e l ,
so t h a t bo th hard and s o f t s t e e l when scuf fed, and the scu f fed ma te r ia l ,
develop a s i m i l a r meta l lographic s t r u c t u r e and hardness. The e l e c t r o n micro-
scope has revealed the f i n e meta l lographic s t r u c t u r e o f rubbed m a t e r i a l , the
na tu re o f the o r i g i n a l ground sur face and the smooth run - in sur face and has
revealed t h a t the i n i t i a t i o n o f wear on a sub-microscopic sca le develops i n a
s i m i l a r manner t o t h a t observed on a l a r g e r scale. With l i g h t loads, the sur-
faces a re smoothed by p l a s t i c deformat ion o f the a s p e r i t i e s , m e t a l l u r g i c a l
t ransformat ions occur and s c u f f i n g appears t o i n i t i a t e f rom smal l scores.
W i th in the scores on hard s t e e l , f ea tu res a r e v i s i b l e suppor t ing the Bowden and
Tabor ( 2 ) mechanism o f f a i l u r e , Fig.4a. P l a s t i c deformat ion and roughening o f
s o f t s t e e l i n d i c a t e ove r -s t ress ing o f the sur face m a t e r i a l which i s n o t incon-
s i s t e n t w i t h the ideas o f Blok [ l ] and Ming-Feng 131, Fig.4b. Extreme
pressure (e.p.1 a d d i t i v e s a re used t o prevent metal t o metal con tac t between
h e a v i l y loaded moving surfaces. De ta i l ed examinat ion o f surfaces by the
e l e c t r o n microscope has shown the b u i l d up o f p r o t e c t i v e f i l m s and prov ides
exper imental evidence f o r the genera l l y agreed a c t i o n o f these a d d i t i v e s ;
reac t i on w i t h sur face metal t o form adherent sur face f i l m s w i t h good boundary
p r o p e r t i e s [27].
16
Fig.3a. (x80) F i n e l y qround Fig.3b. (~7500) F i n e l y ground
s u r f a c e a d j a c e n t t o r u n - i n sur face . sur face .
F i g . 3 ~ . ( ~ 7 5 0 0 ) Run-in sur face . Fig.3d. (H= x100, V= ~ 1 1 0 0 )
Taper s e c t i o n through a s c u f f e d
sur face .
17
Fig.4a. ( ~ 7 5 0 0 ) I n i t i a t i o n of
s l i d i n g wear on a hard s t e e l
surface.
Fig.4b. ( ~ 7 5 0 0 ) I n i t i a t i o n o f
s l i d i n g wear on a s o f t s tee l
surface,
2.4 ABRASIVE WEAR
From an economic p o i n t o f view, abras ive wear caused by p loughing o r gouging
o f a hard sur face, hard p a r t i c l e s o r debr is , aga ins t a r e l a t i v e l y s o f t e r mat ing
su r face i s probably the most ser ious s i n g l e cause o f wear i n engineer ing
p r a c t i c e . There a re i n d i c a t i o n s t h a t abras ion, p r i m a r i l y a crude machining
process, i s r e l a t e d t o i nden ta t i on hardness and hence t o s t a t i c y i e l d s t ress
[28,31]. Evidence o f ex tens i ve damage o r i g i n a t i n g f rom a small p a r t i c l e o f
d e b r i s has been repor ted [32,34], Fig.5. A p a r t i c l e o f hard b r i t t l e ma te r ia l
may cause damage i n a s i n g l e pass through the area o f minimum f i l m th ickness o f
a bear ing. However, i n so passing, i t may be rendered i n e f f e c t i v e due e i t h e r
t o breakdown i n t o smal ler p a r t i c l e s o f dimensions smal ler than the minimum o i l
f i l m th ickness o r by being completely embedded i n the s o f t e r o f the mat ing
ma te r ia l s . On the o t h e r hand a s o f t m a t e r i a l p a r t i c l e may work harden on
passage between r e l a t i v e l y moving surfaces. In gouging the s o f t e r bear ing
m a t e r i a l sur face, the p a r t i c l e may, i f the bear ing m a t e r i a l a l s o work hardens,
be o n l y p a r t i a l l y embedded i n an equa l l y hard sur face area and become a source
o f f u r t h e r damage t o the mat ing surface.
18
a. Large bear ing machined b. ( ~ 1 5 0 ) Sect ion through the
by hard p a r t i c l e p a r t i a l l y embedded hard p a r t i c l e
Fig.5. Wire-wool type bear ing f a i l u r e .
I f abras ive p a r t i c l e s a r e conveyed by a f l u i d stream the impact o f the
abrasive p a r t i c l e laden f l u i d w i l l g i v e r i s e t o e ros i ve wear of any in terposed
surface [35]. The ex ten t and type o f wear depends upon the impinging angle of
the p a r t i c l e s and the d u c t i l i t y o f the surface.
2.5 FRETTING
F r e t t i n g i s a s p e c i f i c form o f wear which occurs when the re i s s l i g h t
v i b r a t o r y movement between loaded surfaces i n con tac t and which mani fests i t s e l f
by p i t t i n g o f the surfaces and the accumulat ion o f o x i d i s e d debr is , Fig.6.
e l e c t r o n microscopical study of the i n i t i a t i o n o f f r e t t i n g [23] has prov ided
support f o r the suggestion t h a t a t t he o u t s e t i t i s no d i f f e r e n t f rom o t h e r
forms o f wear, but t h a t the f i n e d e b r i s produced by the i n i t i a l damage due t o
metal t o metal contact and r e l a t i v e mot ion prov ides the s t a r t i n g p o i n t f o r a
cumulat ive abras ive a c t i o n [36,38]. The debr i s , be ing l a r g e l y the ox ide o f the
meta ls involved, occupies a g rea te r volume than t h a t o f t he metal destroyed and
i n a l i m i t e d space, t h i s can lead t o a pressure b u i l d up and seizure. The form
and e x t e n t o f f r e t t i n g damage depends on the chemical na tu re o f the environment
and on whether or no t the debr i s can escape or i s b u i l t up between the surfaces.
An
19
I t has been suggested t h a t t he i n i t i a l deb r i s has a p l a t e
a process of delaminat ion [39].
The ac tua l r a t e o f wear may slow down i f the d e b r i s ac
i k e form produced by
s as a b u f f e r between
the two surfaces. Thus a process which i n i t i a t e s as adhesive wear may change
t o abras ion and then the wear r a t e may slow down due t o d e b r i s keeping the
sur faces apar t . The f i n a l f a i l u r e may then be by f a t i g u e f r a c t u r e , crack
i n i t i a t i o n being e f f e c t e d by the s t ress r a i s i n g r o l e o f f r e t t i n g p i t s .
Fig.6a. (~7500) I n i t i a t i o n
o f f r e t t i n g on hard s t e e l .
Fig.6b. (x10,ODO) F r e t t i n g
damage on a t i t a n i u m implant.
2.6 FLUID AND CAVITATION EROSION
These wear mechanisms a r i s e f rom the impact o f f l u i d s a t h i g h v e l o c i t i e s .
F l u i d e ros ion damage caused by smal l drops o f l i q u i d can occur i n steam t u r -
b ines and f a s t f l y i n g a i r c r a f t through the impact o f water d r o p l e t s causing
p l a s t i c depressions i n the surface. As the f l u i d f l ows f rom the deformed zone
i t can cause shear deformat ion i n pe r iphe ra l areas and repeated deformat ion
causes a f a t i g u e type o f damage by p i t t i n g and roughening o f the surface.
C a v i t a t i o n e ros ion damage is caused by impact from the co l l apse o f vapour or
gas bubbles formed i n con tac t w i th a r a p i d l y moving o r v i b r a t i n g surface.
phys i ca l damage t o meta ls i s cha rac te r i sed by p i t t i n g suggest ive o f a f a t i g u e
o r i g i n .
d i s s i p a t e d be fo re apprec iab le deformat ion and c rack ing occurs, appears t o be an
The
The u l t i m a t e r e s i l i e n c e o f m a t e r i a l , measured as the energy t h a t can be
20
important p roper t y o f metals i n c a v i t a t i o n res is tance.
2.7 ROLLING CONTACT FATIGUE
The use fu l l i f e o f r o l l i n g elements i s l i m i t e d by sur face d i s i n t e g r a t i o n
p i t s o r f r a c t u r e being caused by a f a t i g u e process dependent upon the pro-
p e r t i e s o f t he ma te r ia l , the na tu re o f the l u b r i c a n t and the environment
[ 4 0 , 4 2 ] . The phenomenon i s cha rac te r i sed by the sudden removal o f sur face
ma te r ia l or f r a c t u r e due t o repeated a l t e r n a t i n g stresses. The process has
th ree phases, p recond i t i on ing o f the m a t e r i a l p r i o r t o crack i n i t i a t i o n , crack
i n i t i a t i o n and crack propagat ion.
R o l l i n g con tac t f a t i g u e cracks i n i t i a t e e i t h e r a t the sur face and propagate
i n t o the m a t e r i a l , o r s t a r t below the sur face i n the area o f c a l c u l a t e d maximum
Hertz ian s t ress and propagate towards the sur face depending upon p r e v a i l i n g
circumstances. The propagat ion o f sur face cracks seems t o be c o n t r o l l e d by the
nature o f the l u b r i c a n t and the environment [43]. The cracks, t ransverse t o
the r o l l i n g d i r e c t i o n , Fig.7, propagate s t e a d i l y i n t o the m a t e r i a l a t an acute
Fig.7. ( ~ 1 5 , 0 0 0 ) R o l l ing
con tac t f a t i g u e cracks i n
En31 b a l l bear ing s tee l .
Fig.8. S ing le f a i l u r e p
a minera l o i l l u b r i c a t e d
bear ing b a l l .
angle t o the r o l l i n g d i r e c t i o n , then in f l uenced by the maximum sheai
t i n
ng
s t resses, propagate p a r a l l e l t o the sur face t o detach sur face ma te r ia l and form
a p i t , Fig.8. I f the environment i s de le te r i ous , f o r example, i f i t leads t o
hydrogen embr i t t lement [ 4 4 , 4 5 ] the cracks may propagate r a p i d l y , deep i n t o the
m a t e r i a l , Fig.9, so t h a t f r a c t u r e ensues, Fig.10.
21
Fig.9. ( ~ 7 5 ) Rapid crack Fig.10. Fractured non-
propagation i n a non-flammable flammable f l u i d l ub r i ca ted
f l u i d l ub r i ca ted bearing b a l l . bearing b a l l .
Subsurface cracks i n i t i a t e a t depths associated w i t h the region o f ca l cu l -
ated maximum Her t r i an s t ress and propagate p a r a l l e l t o the surface t o remove
surface ma te r ia l , Fig.11. Crack i n i t i a t i o n may be f a c i l i t a t e d by b r i t t l e , non-
m e t a l l i c inc lus ions i n the stressed region which crack, break the m e t a l l i c
c o n t i n u i t y and a c t as s t ress ra isers .
Owing t o r o l l i n g and s l i d i n g act ion, mechanical and metal lographic changes
occur i n the stressed surface and immediate subsurface mater ia l o f r o l l i n g
elements [41,42,46]. The s t ruc tu re o f conventional En 31 b a l l bearing s tee l
cons is ts o f f i n e l y dispersed carbide spheroids i n martensite, Fig.12. The
meta l lographica l ly changed mater ia l o f the surface layer i s devoid o f carbides
as a r e s u l t o f h igh contact stresses and loca l h igh temperature f lashes causing
so lu t i on o f the carbides fo l lowed by rap id quenching under pressure, Fig.13.
Absorption o f gases from l u b r i c a n t breakdown may con t r i bu te t o surface hardening
and crack i n i t i a t i o n . S t r i nge r type carbides may form i n the subsurface area
o f contact due t o a n n i h i l a t i o n o f the coarse carbides by p l a s t i c deformation,
Fig.14. Sections transverse t o the r o l l i n g d i r e c t i o n may reveal the presence
o f l oca l i sed areas o f tempered martensite; cracks develop i n such areas,
Fig.15. There i s a threshold s t ress l eve l above which metal lographic change
22
Fig.11. ( ~ 3 7 5 ) Subsurface c rack
i n an En31 s t e e l bea r ing b a l l .
Fig.13. (~12 ,000) Deformed meta l -
l o g r a p h i c a l l y changed, spher i ca l
ca rb ide f r e e , sur face m a t e r i a l o f a
used bear ing b a l l .
Fig.12. (~8000) S t r u c t u r e o f
En31 b a l l bea r ing s t e e l .
Fig.14. ( ~ 1 5 , 0 0 0 ) S t r i n g e r t ype
ca rb ides i n subsurface me ta l l og raph ic -
a l l y changed b a l l bea r ing s t e e l .
23
occurs, suggest ing t h a t the changes may be due t o a y i e l d i n g o r p l a s t i c f l o w
phenomenon r a t h e r than a tempering e f f e c t o r be i n d i c a t i v e o f over load o r a long
du ra t i on o f s t ress ing.
S i m i l a r subsurface changes and associated cracks a r e found i n sec t i ons o f
r o l l i n g elements p a r a l l e l t o the r o l l i n g d i r e c t i o n together w i t h associated
e longated wh i te e t c h i n g areas o f increased hardness, Fig.16. The extreme hard-
ness o f the wh i te e t c h i n g ma te r ia l may be due t o a f i n e c e l l s i z e o r the almost
c o l l o i d a l d i spe rs ion o f very f i n e carb ides formed poss ib l y by s t r a i n induced
p r e c i p i t a t i o n f o l l o w i n g s o l u t i o n o f coarse carb ides, Fig.17.
F i g
the
c r a
15. ( ~ 5 7 5 ) Tempered mar tens i te i n Fig.16 ( ~ 1 1 0 0 ) Subsurface cracks and
subsurface area o f r o l l i n g contact . assoc iated wh i te e tch ing m a t e r i a l .
t appears t h a t several d i f f e r e n t modes o f r o l l i n g con tac t f a t i g u e can cause
ks t o nuc lea te and propagate independent ly a t va r ious rates; thus the
phenomenon i s g r e a t l y i n f l uenced by h i g h l y l o c a l i s e d cond i t i ons . Wh i l s t the
general p r o p e r t i e s o f the b u l k m a t e r i a l a re important, s p e c i f i c aspects such as
the steelmaking process, gas content and c l e a n l i n e s s a r e a l s o e q u a l l y important
[48,52].
e f f e c t on f a i l u r e .
The na tu re of the l u b r i c a n t and the environment can have a dominant
Ma te r ia l combinat ion [53] and ma te r ia l l u b r i c a n t combinat ion r e q u i r e c a r e f u l
cons ide ra t i on t o ensure s a t i s f a c t o r y performance [54,55]. I n r o l l i n g con tac t i n the absence o f a l u b r i c a n t , f a i l u r e occurs, n o t by the
usual f a i l u r e mechanism bu t by excessive wear l i m i t i n g the use fu l l i f e due t o
v i b r a t i o n and no ise tS6,571.
24
Fig.17. ( ~ 7 0 0 0 ) Fine s t r u c t u r e i n
subsurface w h i t e e t c h i n g m a t e r i a l
o f En31 s tee l .
2.8 WEAR DETECTION AND ASSESSMENT
One o f the most d i f f i c u l t problems i n engineer ing design i s the p r e d i c t i o n
and assessment o f p o s s i b l e wear. An equa l l y impor tant and d i f f i c u l t t ask i s t he
de tec t i on o f wear du r ing the opera t i on o f machines.
measurement such as the determinat ion o f changes i n sur face topography by
s t y l u s measurement and the determinat ion of weight loss have disadvantages.
Machinery must be d ismant led f o r the measurements t o be made and inaccurac ies i n
weight may a r i s e due t o o x i d a t i o n and abso rp t i on o f l u b r i c a n t .
Simple methods o f wear
As the h i s t o r y o f t he wear process i s recorded i n the wear d e b r i s produced
[581 an a t t r a c t i v e method o f wear d e t e c t i o n and assessment i s contaminant
ana lys i s o f the l u b r i c a n t used. Lub r i can ts i n ope ra t i ng mechanisms may be con-
v e n i e n t l y checked f o r the d e t e c t i o n of wear by spect rographic a n a l y s i s a l though
the method has some disadvantages [59] be ing i n some instances r e l a t i v e l y b l i n d
t o l a r g e p a r t i c l e s . A s imple dev ice such as a magnetic p l u g i n an o i l sump can
c o l l e c t f e r r o u s debr i s and i n d i c a t e wear o f a moving p a r t 1601 b u t some ser ious
damage may have occurred before d e b r i s l a r g e enough t o be detected has been
co l l ec ted .
Wear p a r t i c l e s a r e unique, hav ing i n d i v i d u a l c h a r a c t e r i s t i c s which bear
evidence o f the c o n d i t i o n s under which they were formed [8,58,61]. Carefu l
25
examinat ion o f the morphology and determinat ion o f the composi t ion o f wear
p a r t i c l e s can thus y i e l d s p e c i f i c i n fo rma t ion concerning the surfaces from which
they were produced, the mechanism o f t h e i r f o rma t ion and the opera t i ve wear mode
i n the system f rom which they were ex t rac ted .
Ferrography [7,8,62,63] i s a technique developed t o separate wear deb r i s and
contaminant p a r t i c l e s convenient ly f rom a l u b r i c a n t f o r examinat ion and ana lys i s .
The duplex Ferrograph analyser c o n s i s t s o f two p a r t i c l e separators, a standard
analyser and a d i r e c t reading (DR) Ferrograph. The DR Ferrograph i s a s imple
instrument used t o determine the amount and s i z e d i s t r i b u t i o n o f wear p a r t i c l e s
i n a l u b r i c a n t sample f rom which s i g n i f i c a n t numerical data can be de r i ved [62 ,
641. When successive l u b r i c a n t samples y i e l d constant dens i t y readings i t may
be concluded t h a t the machine i s ope ra t i ng normal ly and producing benign wear
p a r t i c l e s a t a steady ra te . A r a p i d increase i n the q u a n t i t y o f p a r t i c l e s and
i n p a r t i c u l a r i n the r a t i o o f l a r g e t o small p a r t i c l e s i nd i ca tes the i n i t i a t i o n
o f a more severe wear process. The use o f a s imple equat ion prov ides a s i n g l e
f i g u r e f o r a comparat ive s e v e r i t y o f wear index. F u l l Ferrographic ana lys i s
us ing the b ichromat ic microscope [62,63], e l e c t r o n microscopy [62,651 and
hea t ing techniques [66] may be used t o supplement the i n fo rma t ion f rom p a r t i c l e s
p r e c i p i t a t e d accord ing t o s i z e on a prepared subs t ra te by the a n a l y t i c a l
Ferrograph.
P a r t i c l e s generated by d i f f e r e n t wear mechanisms have c h a r a c t e r i s t i c s which
may be i d e n t i f i e d w i t h the s p e c i f i c wear mechanism [8,61,62,67]. A wear par-
t i c l e a t l a s has been prepared [68]. Rubbing wear p a r t i c l e s found i n the lub-
r i c a n t o f most machines have the form o f p l a t e l e t s and i n d i c a t e normal permiss-
i b le 'wear , Fig.18.
loops and bent w i res s i m i l a r t o c u t t i n g s f rom a machining operat ion, Fig.19.
A concen t ra t i on o f such p a r t i c l e s i s i n d i c a t i v e o f a severe abras ive wear
process.
C u t t i n g wear p a r t i c l e s take the form o f m i n i a t u r e s p i r a l s ,
The o p e r a t i v e regimes o f s l i d i n g wear can be c l a s s i f i e d by the p a r t i c l e s
produced.
i d e n t i f i e d [ 1 3 ] . Free metal p a r t i c l e s a r e produced i n regimes 1,2 and 3 and
these regimes may be recognised by p a r t i c l e s ize, ranging f rom the smal l par-
t i c l e s o f regime 2 associated w i t h hydrodynamic l u b r i c a t i o n , Fig.18,to l a rge
m e t a l l i c p a r t i c l e s o f regime 3 , Fig.20, which may vary i n s i z e up t o 2 5 0 ~ 1
when the shear mixed sur face l a y e r becomes uns tab le and l o c a l i s e d adhesion
occurs.
S ix regimes which generate c h a r a c t e r i s t i c p a r t i c l e s have been
Three d i s t i n c t p a r t i c l e types, laminar, spher i ca l and chunks a r e associated
w i t h r o l l i n g bear ing fa t i gue . Laminar p a r t i c l e s a r e t h i n metal p a r t i c l e s up t o
50um i n major dimension con ta in ing holes formed i n passage through the r o l l i n g
contact . Such p a r t i c l e s a r e generated throughout the l i f e o f the bear ing.
The i r concen t ra t i on increases w i t h the onset o f s p a l l i n g . Spher ical p a r t i c l e s ,
26
Fig.18. ( ~ 7 5 0 ) Rubbing wear particles
and friction polymer.
( ~ 7 5 0 ) Optical micrograph ( ~ 4 5 0 ) Scanning electron micrograph
Fig.19. Cutting wear particles.
21
Fig.20. (~400) Large m e t a l l i c wear p a r t i c l e .
a. (~750) O p t i c a l micrograph b. (x3,000) Scanning e l e c t r o n m i c rog raph
Fig.21. S tee l spher i ca l p a r t i c l e s .
Fig.21, a r e generated w i t h i n a propagat ing f a t i g u e c rack and t h e i r d e t e c t i o n
g i v e s warning o f impending f a t i g u e f a i l u r e [69 ] . X-ray energy a n a l y s i s i n the
scanning e l e c t r o n microscope can e s t a b l i s h t h a t they a r e composed o f r o l l i n g
bear ing m a t e r i a l . Fa t i gue chunks c o n s t i t u t e the m a t e r i a l removed by sur face
d i s i n t e g r a t i o n and p i t fo rmat ion .
28
Fatigue p a r t i c l e s from a gear too th although o f s i m i l a r dimensions d i f f e r
from those from r o l l i n g bearings, are genera l ly o f i r r e g u l a r shape, f r e e from
holes and have a smooth surface. The number o f p a r t i c l e s increases as the
fa t i gue process progresses. Larger fa t i gue chunks o f gear mater ia l are ind-
i c a t i v e o f surface d e t e r i o r a t i o n by p i t t i n g .
Scuf f ing o f gears causes an increase i n the number o f wear p a r t i c l e s which
tend t o have a rough surface and an i r r e g u l a r shape. As the seve r i t y of
scu f f i ng increases, the l a rge r p a r t i c l e s produced have surface s t r i a t i o n s ind-
i c a t i v e o f s l i d i n g act ion. Owing t o the thermal e f f e c t s o f scu f f i ng , p a r t i c l e s
may be p a r t i a l l y ox id ised w i t h a range o f temper colours.
As both a r t h r i t i c and a r t i f i c i a l j o i n t s a re subjected t o wear, Ferrographic
analys is of synovial f l u i d i s a p o t e n t i a l l y a t t r a c t i v e method o f studying the
mechanisms and b i o l o g i c a l responses t o wear i n human j o i n t s 1701.
2.9 CONCLUSIONS
In the present and foreseeable f u t u r e world economic s i t u a t i o n , mater ia l and
energy conservation i s becoming increas ing ly important. As wear i s a major
cause o f mater ia l wastage, any reduction i n wear can e f f e c t considerable savings
i n mater ia l and the energy necessary f o r t h e i r production. Thus increasing
emphasis w i l l be given t o methods o f wear con t ro l and prevention.
The complex mechanisms o f wear, however, are no t e a s i l y e luc idated as the
process by cumulative ac t i on o b l i t e r a t e s evidence o f the important i n i t i a l
stages o f damage. Being an i n t e r d i s c i p l i n a r y subject, a m u l t i d i s c i p l i n a r y app-
roach i s requi red f o r the i nves t i ga t i on o f wear and the c o n t r o l l i n g fac to rs t o
enable the m s t su i tab le design t o be chosen embodying the best mater ia l and
the co r rec t l u b r i c a t i o n t o ensure minimum wear and sa t i s fac to ry serv ice per-
formance from moving mechanisms.
REFERENCES
1 Blok,H., Engineering, London, 1952, 173(4502)594.
2 Bowden,F.P. and Tabor,D., "The F r i c t i o n and Lubr i ca t i on o f Sol
3 Ming-Feng,l., J.Appl.Phys., 1952, 23(9)1011-1019. 4 Archard,J.F., J.Appl.Phys., 1953, 24(8)981-988. 5 Archard,J.F., Research, 1952, 5(8)395-396. 6 Rabinowicz,E., " F r i c t i o n and Wear o f Materials", 1966, J.Wiley 7 Seifert,W.W. and Westcott,V.C., Wear, 1972, 21,2742. 8 Scott,D., Seifert,W.W. and Westcott,V.C., Scient.Amer., 1974,
1950, Clarendon, Oxford.
230(5)88-97.
ds",
London.
. ..
9 Suh,N.P. and co-workers, Wear, 1977, 44,l-162. 10 Suh,N.P., Wear, 1973, 25,111-124. 1 1 Jahanmir,S., Suh,N.P. and Abrahamson,E.P., Wear, 1974, 28, 235-249. 12 Suh,N.P. and Sridharan,P., Wear, 1975, 34,291-299. 13 Reda,A.A., Bowen,E.R. and Westcott,V.C., Wear, 1975, 34,261-273. 14 Beilby,G., "Aggregation and Flow o f Solids", Macmillan, London, 1921. 15 Bates,T.R., Ludema,K.C. and Brainard,W.A., Wear, 1974, 30,365-375.
29
16 17 18 19 20
21 22
23
24 25 26 27
28 29
30 31 32
33
34 35 36 37 38 39 40
41 42
43
44 45 46
47 48
49 50
51 52 53
54 55 56 57
58 59
Kirk,J.A. and Swanson,T.D., Wear, 1975, 35, 63-67. Dumbleton,J.H. and Shen,C., Wear, 1976, 37, 279-289. Hirst,W., M e t a l l . Rev., 1965, 10, 145-172, I . Meta ls , London. Krage lsk i i , I .V . , " F r i c t i o n and Wear", 1955, But te rwor ths , London. Scott,D. (Ed.), "T rea t i se on M a t e r i a l s Science and Technology, 13, "Wear", 1979, Academic Press, N.Y. Bickerman,J.J., Wear, 1976, 39, 1-14. Barwe l l ,F.T., i n "T rea t i se on M a t e r i a l s Science and Technology, 13, "Wear", (Sco t t , D.Ed.) 1979, 1-83, Academic Press, N.Y. Scott,D. and Scott,H.M., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 609-612, I n s t . Mech. Engrs., London. Scott,D., Proc. I n s t . k c h . Engrs., 1967, 181 (3L) 39-51. Welsh,N.C., J . I ns t . Meta ls , 1959, 88, 103-111. Barwel1,F.T. and Milne,A.A., B r . Pat. No. 732, 447, 1955. Milne,A.A., Scott,D. and Macdonald,D., Proc. Conf. L u b r i c a t i o n and Wear, 1957, 735-741, I n s t . Mech. Engrs., London. Wright,K.H.R., Eng ineer ing , London, 1961, 191 (4956) 546-547. Kruschov,M.M. and Babichev, , Akad. Nauk. SSR, 1960, 66-76. NEL T r a n s l a t i o n 893, Nat iona l Eng ineer ing Labora tory , East K i l b r i d e , Glasgow. Kruschov,M.M., Wear, 1974, 28, 69-88. Moore,M.A., Wear, 1974, 28, 59-68. Barwel1,F.T. and Scott,D., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 277-297, I n s t . Mech. Engrs., London. Hother-Lushington,S., Proc. 4 t h L u b r i c a t i o n and Wear Convention, 1966, 243-252, I n s t . Mech. Engrs., London. Dawson,P.H. and F id le r ,F . , Proc. I n s t . Mech. Engrs., 1965/66, 180, 513-530. Engel ,P.A., "Impact Wear o f Ma te r ia l s " , 1976, E l s e v i e r , Amsterdam. Wright,K.H.R., Proc. I n s t . Mech. Engrs., 1952/53, lB(l1) 556-574. Godfrey,D. and Bailey,J.M., Lub r i c . Engng., 1954, 10, 155. Waterhouse,R.B., " F r e t t i n g Corrosion", 1972, Pergamon, Oxford. Waterhouse,R.B,, i n "Wear o f Metals", 1977, 55, ASME, N.Y. Scott,D., i n "Fa t igue i n R o l l i n g Contact", 1963, 103-115, I n s t . Mech. Engrs., London. Scott,D., i n "Low A l l o y Steels", 1965, 203-209, I .S . I., London. Scott,D., R o l l i n g Contact Fa t i gue in Wear, in (Sc0tt.D. Ed.) "T rea t i se on M a t e r i a l s Science and Technology", 1979, 13, 321-361, Academic Press, N.Y. Scott,D., Proc. I n s t . Mech. Engrs. Conf. L u b r i c a t i o n and Wear, 1957, 463-468. Grunberg,L. and Scott,D., J. I n s t . P e t r o l . , 1958, 44 (419), 406-410. Grunberg,L., Scott,D. and Jamieson,D.T., P h i l . Mag. 1963, 8(93) 1553-1568. Scott,D., Loy,B. and MilIs,G.H., Proc. l n s t . Mech. Engrs., 1967, 181 (31) 94-106. Scott,D. and McCullagh,P.J. Wear, 1973, 24, 235-242. Scott,D. and B lackwel l , J . , Proc. I n s t . Mech. Engrs., 1964, 178, (3N) 81-89. Scott,D. and B lackwel l ,J., Wear, 1978, 46, 273-279. Scott,D. and B lackwel l , J . , Proc. I n s t . Mech. Engrs., 1968, 182(3N) 239-242. Scott,D. and McCullagh,P.J., Wear, 1975, 34, 222-237. Scott,D. and McCullagh,P.J., Wear, 1973, 25, 339-344. Scott,D., i n " R o l l i n g Contact Fa t i gue and Performance o f Lubr ican ts" , (Tourret ,R. and Wright,E.P. Eds.), 1977, 3-17, Heydon and Sons, London. Scott,D. and Blackwel l ,J. , Proc. I n s t . Mech. Engrs., 1966, 180, (3K) 32-37. Scott,D., Proc. I n s t . Mech. Engrs., 1968, 182(3J) 116-123. Scott,D., Proc. I n s t . Mech. Engrs., 1967, 182(3A) 325-341. Scott,D. and Blackwel l ,J. , NEL Report 278, 1967, Nat iona l Engineer ing Labora tory , East K i 1 b r i de. Scott,D., Proc. I n s t . Mech. Engrs., 1976, 189/75, 623-633. Seifert,W.W. and Westcott,V.C., Wear, 1973, 23, 239-249.
30
60
61 62
63 64
65
66 67 68
69 70
Col lacott ,R.A., "Mechanical F a u l t Diagnos 1977, Chapman and H a l l , London. Scott,D., Wear, 1975, 34, 15-22. Bowen,E.R., Scott,D., Seifert,W.W. and Ve 1976, 9 ( 3 ) 109-115.
s and Cond i t i on Mon i to r ing" ,
t co t t ,V .C. , T r i bo logy I n t . ,
Scott,O. and Westcott,V.C., Proc. E u r o t r i b 77, 1977, Band 1, paper 70, 1-6 Westcott,V.C., Naval Research Reviews, 1977 (March) 1-8, O f f i c e o f Naval Research, Wash ing ton . Scott,D. and Mills,G.H., i n "Scanning E l e c t r o n Microscopy", 1974, Pa r t I V , 838-888, I.I.T., Chicago. Barwel l ,F.T. , Bowen,E.R. and Westcott,V.C., Wear, 1977, 44, 163-171. Ruff,W. Wear, 1977, 42, 49-62. Bowen,E.R. and Westcott,V.C., "Wear P a r t i c l e A t las" , 1976, Foxboro/Trans- o n i c s Inc. , Mass, U . S . A . Scott,D. and Mills,G.H., Wear, 1973, 24, 235-242. Mears,D.C., Wear, 1978, 50, 115-126.
31
'{ SELECTION OF BEARINGS
M.J. NEALE
Michael Neale and Assoc ia tes L td .
3 . 1 INTRODUCTION
The s e l e c t i o n o f an a p p r o p r i a t e type o f bear ing , f o r use i n a p a r t i c u l a r
a p p l i c a t i o n , i s a dec i s ion t h a t i s u s u a l l y made very e a r l y i n a design process.
A t t h a t stage, very d e t a i l e d i n fo rma t ion on bear ing performance i s n o t u s u a l l y
necessary, and what i s r e a l l y requ i red i s broad guidance on the impor tan t
a c t e r i s t i c s o f t he va r ious types. The in fo rma t ion presented i n t h i s sec t i on
ntended t o meet t h i s requirement.
_ L . .
3.2 B E A R I N G TYPES
The bas i c f u n c t i o n o f bear ings i s t o a l l o w a load t o be t r a n s m i t t e d between
two sur faces which a r e i n r e l a t i v e mot ion. There a re th ree main types o f
bea r ing as shown i n F igure 1, and these a r e p l a i n bear ings , r o l l i n g bear ings
and f l e x u r e s . I n p l a i n bear ings the l oad i s t r a n s m i t t e d over a cons iderab le
area, w h i l e i n r o l l i n g bear ings the area a c t u a l l y i n con tac t , and t r a n s m i t t i n g
the load, i s ve ry smal l . The t h i r d type depends on the use o f f l e x i b l e com-
ponents and i s o n l y s u i t a b l e f o r o s c i l l a t o r y movement.
I t can be seen f rom F igu re 1 t h a t t he re a r e f i v e bas i c p r i n c i p l e s beh ind the
opera t i on o f t he va r ious types and these are : -
( i ) To pe rm i t the two sur faces t o rub toge the r and t o a r range the sur face
p r o p e r t i e s so t h a t se i zu re o r excess f r i c t i o n does n o t occur and so t h a t
an acceptab le r a t e o f wear i s ob ta ined. I n p r a c t i c e t h i s i s u s u a l l y
achieved by the cho ice o f m a t e r i a l s w i t h s u i t a b l e b u l k p r o p e r t i e s o r
by the use of some fo rm o f sur face coa t ing , which may e i t h e r be
a p p l i e d i n advance o r a l lowed t o form i n s i t u .
( i i ) To keep the sur faces separated by a f i l m o f f l u i d , so t h a t t he r e l a t i v e
movement can occur w i t h i n the f i l m . To do t h i s the f l u i d must be
main ta ined a t a s u f f i c i e n t p ressure t o h o l d the sur faces a p a r t aga ins t
32
J ( ( ( ( ( ) , , ] , j
The basic problem L J Z h
Magnetic or electrostatic bearings
To transmit load between two surfaces in relative motior!
Plain rubbing bearings The surfaces are allowed tn 0 C
to rub together .- G I 1 Plain fluid film Pressure In the film keeps
bearings the surfaces apart ' L .-
Surfaces held apart by Q E mutual repulsion
tn m r circular cross section .- m
Rolling element Surfaces separated by bearings rolling elements of
L
Rocker pods s - 4 8
Q
a The pivot point moves as the bearing rocks
E .- I Knife edges The sharp edge gives the -
minimum pivot point movement
I
Rubber bearings Movement by elastic deflection of the rubber
Crossed strip Rotation about the crossover point by strip deflection
Cables or torsion rods
Movement permitted by elastic deflection
tn
3 X Q)
!!!
h
Fig.1 Var ious types o f bea r ing
the a p p l i e d load. Th is p ressure may be ob ta ined by feed ing i n f rom an
e x t e r n a l h igh pressure source, o r may be generated w i t h i n the f i l m by
r e l a t i v e movement o f s u i t a b l y shaped sur faces f l ooded w i t h a v iscous
f l u i d .
33
( i i i ) One sur face may be a l l owed t o r o l l on the o the r . Th is , however,
produces an i n t e r a c t i o n between the angu lar and t r a n s l a t i o n a l movements
o f t he ad jacen t components, which i s determined by the shape o f the
r o l l i n g sur faces , and i n p r a c t i c e may n o t always a l l o w the requ i red
degrees o f freedom. To overcome t h i s problem the r o l l i n g sur faces a r e
o f t e n p e r m i t t e d t o s l i d e as w e l l as r o l l , such as i n gear tee th , o r the
problem may be overcome by i n s e r t i n g a t h i r d element between the sur faces
o f t he two o r i g i n a l components as i n a r o l l i n g element bear ing .
( i v ) To produce a r e p u l s i v e f o r c e between the sur faces by magnetic o r
e l e c t r o s t a t i c means.
( v ) To p o s i t i o n a f l e x i b l e member between the two components which can
d e f l e c t t o a l l o w a r e l a t i v e o s c i l l a t i n g movement t o occur between them.
S u i t a b l e members can be formed by f i l l i n g most o f the space between the
sur faces w i t h an adequate th i ckness o f e las tomer i c m a t e r i a l o r by us ing
t h i n connect ing l igaments o f h ighe r s t r e n g t h m a t e r i a l s .
3.3 PERFORMANCE OF VARIOUS TYPES OF BEARING
Since the a b i l i t y t o t ransmi t a load w i t h r e l a t i v e movement i s t he bas i c
f u n c t i o n o f a bear ing , a study o f the r e l a t i o n s h i p between the a l l owab le load
and speed f o r bear ings o f va r ious s i zes and types should p rov ide a convenient
s tandard f o r the comparison o f t h e i r performance.
For the de te rm ina t ion o f load and speed c h a r a c t e r i s t i c s o f the va r ious
bear ing types , t he f i r s t i n fo rma t ion t h a t requ i res t o be e s t a b l i s h e d i s t h e ,
approximate shape o f t he r e l a t i o n s h i p , so t h a t a general comparison can be made
between the va r ious types. The bas ic forms o f these r e l a t i o n s h i p s can be
de r i ved f rom the phys i ca l p r i n c i p l e s which govern the opera t i on o f the va r ious
bear ing types. Th is i s discussed below and the r e s u l t i n g performance curves
a p p l i e d t o j o u r n a l bear ings compared i n Fig.2.
3.3.1 Rubbing Bear ings
I n a bea r ing which opera tes by p e r m i t t i n g the two sur faces t o rub toge the r ,
t he phys i ca l l i m i t a t i o n s on performance a r e the r i s k o f overheat ing and se i zu re
and the p o s s i b i l i t y o f excess ive wear.
The genera t ion o f heat a t the rubb ing sur faces a r i s e s f rom the movement
aga ins t t he f r i c t i o n a l res i s tance o f t he con tac t , and t h i s heat has t o be con-
ducted away a long hea t - f l ow paths, o f which the area w i l l be approx imate ly
p r o p o r t i o n a l t o the p r o j e c t e d bear ing area A.
The r i s k o f ove rhea t ing a t a s l i d i n g speed V i s t h e r e f o r e approx imate ly
p r o p o r t i o n a l t o - 'r o r 'PV
34
Pressure limit
PV limit
R P M N
(al Rubbing bearings
Fatigue limit
/-Available pressure limits
Fall d k t o effect 3 of speed on vixosity V
_1 8
R P M N
R P M N
(bl Rolling bearings
Fall due ta effect of
Film thickness limit
RPM N fcl Externally pressurized film (dl Pressure fed film
I r
Film thickness limit
RPM N (el Self- contained film
Pressure limit i0)i (rPV limit
8 limit
RPM N ( f l Porous metal
FIGURE 2 Performance of various types of journal bearing flog W plotted against log N )
35
When two sur faces a r e i n rubb ing con tac t , t he volume o f m a t e r i a l worn f rom
the rubb ing sur faces a f t e r s l i d i n g a d i s tance x w i t h a l oad W i s approx imate ly
p r o p o r t i o n a l t o Wx.
The wear volume i s , however, o f l i t t l e s i g n i f i c a n c e i n t h e performance o f a
bear ing , and the depth wear r a t e i s much more r e l e v a n t as a design f a c t o r , s ince
t h i s i s a measure o f t he r a t e a t which s lackness i n the assembly i s l i k e l y t o
be produced.
The depth wear r a t e w i l l be approx imate ly p r o p o r t i o n a l t o
wx - o r PV A t Thus bo th the phys i ca l l i m i t a t i o n s t o the performance o f rubb ing bear ings
i n d i c a t e t h a t ' t h e s e v e r i t y o f the bear ing opera t i on i s r e l a t e d t o PV (bear ing
pressure x s l i d i n g speed).
a rubb ing type o f j o u r n a l bear ing . Th is i s a composi te c h a r a c t e r i s t i c i n c l u d i n g
a PV l i m i t and a l s o a maximum a l l o w a b l e p ressure l i m i t assoc ia ted w i t h the r i s k
o f f a t i g u e o r e x t r u s i o n o f t he bear ing m a t e r i a l , which i s f r e q u e n t l y non-
metal 1 i c .
Fig.2a shows the shape o f t h e load-speed curve f o r
3 . 3 . 2 R o l l e r Bear ings
Owing t o the concent ra ted con tac t s t resses i n t h i s type o f bea r ing the
u l t i m a t e l i m i t on performance a r i s e s from the p o s s i b l e f a t i g u e o f t he r o l
elements o r races, a l t hough i n p r a c t i c e r o l l i n g bear ings o f t e n f a i l as we
lack o f p roper l u b r i c a t i o n , d i r t con taminat ion and u n s u i t a b l e load ing .
f a t i g u e i s taken as the performance l i m i t i t i s reasonable t o assume t h a t
l i f e o f a r o l l i n g con tac t bea r ing measured i n r e v o l u t i o n s i s p r o p o r t i o n a l
(l/W)3.
shown i n Fig.2b. As be fo re , t he a c t u a l maximum load i s a l s o l i m i t e d by the cap-
a c i t y o f t he bear ing t o c a r r y s t a t i c load which i n t h i s case i s l i m i t e d by the
Consequently f o r a g i ven l i f e W a N-1'3 and t h i s c h a r a c t e r i s t i c i s
'"9
1 f rom
f
the
t o
res i s tance o f t he races t o b r i n e l l i n g by the r o l l i n g elements.
3.3.3 F l u i d F i l m Bear ings
I f these a r e o f t he e x t e r n a l l y p ressu r i zed o r h y d r o s t a t i c type , t he load
c a p a c i t y i s p r i m a r i l y dependent on the a v a i l a b l e supply p ressure , and runn ing
speed has very l i t t l e e f f e c t , a l t hough w i t h l i q u i d l u b r i c a t i o n a s l i g h t f a l l i n
l oad c a p a c i t y may occur a t h ighe r speeds owing t o the reduc t i on i n l u b r i c a n t
v i s c o s i t y caused by h ighe r temperatures. The r e s u l t i n g load-speed c h a r a c t e r i s t i c
i s t h e r e f o r e as shown i n Fig.2c.
With the hydrodynamic type o f f l u i d f i l m bear ing , t he l oad capac i t y increases
w i t h speed p rov ided t h a t t he f i l m i s kep t adequately supp l i ed w i t h l u b r i c a n t ,
a l t hough the re i s no rma l l y a tendency w i t h l i q u i d l u b r i c a n t s f o r t h e l oad cap-
a c i t y t o f a l l away a t the h ighes t speeds owing t o the hea t ing o f t he l u b r i c a n t .
36
This g i ves a c h a r a c t e r i s t i c o f the general shape shown i n Fig.2d.
I n o rde r t o keep the f i l m f u l l y suppl ied w i t h l u b r i c a n t , t h i s has t o be f e d
under pressure, which in t u r n requ i res some form o f l u b r i c a n t supply system.
I t i s n o t always convenient o r economical t o h a v e t o inc lude such a supply
system and consequent ly many hydrodynamic f l u i d f i l m bear ings a re used w i t h
se l f - con ta ined l u b r i c a n t suppl ies. I n the l a r g e r s izes, t he o i l i s g e n e r a l l y
contained i n a sump below the s h a f t and l i f t e d by a r i n g o r d i s c , w h i l e i n the
smal ler s i zes i t may be f e d f rom an o i l - soaked pad o r r e t a i n e d i n the bear ing
s t r u c t u r e by making i t porous. These methods o f o i l feeding have a lower
e f f i c i e n c y than a pressure f e d arrangement and r e s u l t i n the bear ing opera t i ng
w i t h a l oad -ca r ry ing f i l m o f reduced c i r c u m f e r e n t i a l l eng th and t h e r e f o r e o f
reduced load capac i t y . I t i s a l s o u s u a l l y found w i t h these systems t h a t the
volume o f o i l d e l i v e r e d per r e v o l u t i o n of the s h a f t decreases w i t h speed and
consequently the l oad -ca r ry ing capac i t y a l s o decreases w i t h speed.
I n a d d i t i o n t o t h i s e f f e c t , t he absence o f a l u b r i c a t i o n system means t h a t
a l l the heat generated has t o be d i s s i p a t e d d i r e c t l y t o the surroundings, and
w i t h a maximum a l l owab le temperature f o r a long l i f e o f minera l o i l l u b r i c a n t s ,
p laces a l i m i t on the maximum a l l owab le speed.
T,hese va r ious e f f e c t s r e s u l t i n load-speed c h a r a c t e r i s t i c s f o r bear ings o f
t h i s type which a r e o f t he general shape shown i n Figs.2e and 2 f .
3.3.4 F l e x i b l e Members
Bear ings o f t h i s type, which use e i t h e r elastomers i n shear o r h igh t e n s i l e
l igaments i n bending t o a l l o w an o s c i l l a t i n g mot ion t o occur, a r e i n many cases
p h y s i c a l l y l i m i t e d i n performance by the f a t i g u e s t reng th o f the m a t e r i a l o r i t s
bonding on t o the adjacent components.
d e f l e c t i o n and f rom the a p p l i e d load and a re u s u a l l y a d d i t i v e a t some c r i t i c a l
reg ion o f the assembly, w i t h the r e s u l t t h a t t he a l l owab le loads tend t o dec-
rease w i t h the pe rm i t ted angle o f movement.
i n compression do n o t have t h i s a d d i t i v e s t r e s s c o n d i t i o n , b u t i n t h i s case the
a l l owab le d e f l e c t i o n i s l i m i t e d by the b lock th ickness, which i n t u r n l i m i t s
the maximum load i f the assembly i s t o be s t a b l e aga ins t excessive bu lg ing o r
buck l ing.
i n the elastomer arranged t o be p a r a l l e l t o the requ i red d i r e c t i o n o f mot ion,
b u t even when these a r e incorporated the maximum d e f l e c t i o n i s s t i l l l i m i t e d by
the amount t h a t the load can be pe rm i t ted t o be o f f s e t .
The s t resses a r i s e both f rom the
Blocks o f e lastomer ic ma te r ia l used
This can be overcome t o some ex ten t by the use o f s t i f f e n i n g p l a t e s
I t can the re fo re be g e n e r a l l y assumed t h a t the l oad -de f lec t i on c h a r a c t e r i s t i c
o f f l e x i b l e bear ings w i l l g e n e r a l l y show a decrease i n a l l owab le load as the
d e f l e c t i o n i s increased.
31
3.4 SELECTION OF A SUITABLE BEARING
The c h a r a c t e r i s t i c r e l a t i o n s h i p s between load and'movement f o r the va r ious
types o f bear ings can be compared w i t h the bear ing performance requ i red i n
va r ious a p p l i c a t i o n s and used as a guide t o the s e l e c t i o n o f a s u i t a b l e type o f
bear ing.
Any p o s s i b l e a p p l i c a t i o n f o r a bear ing i n a machine o r s t r u c t u r e w i l l have
some form o f c h a r a c t e r i s t i c r e l a t i o n s h i p between the type o f load t o be c a r r i e d
and the movement t o be al lowed. I n p r a c t i c e an important f ea tu re i s whether
the load and movement a r e nominal ly steady o r whether they vary i n some c y c l i c
manner. Th is can g i v e r i s e t o f o u r p o s s i b l e combinat ions o f load and movement
as i n d i c a t e d w i t h examples i n Table 3.1.
Probably the g rea tes t number o f bear ing a p p l i c a t i o n s a r e o f the u n i d i r e c t i o n a l
load and cont inuous movement type and i n t h i s category the way i n which the
bear ing l oad v a r i e s w i t h r o t a t i o n a l speed, d u r i n g the opera t i on o f the machine,
i s an impor tant f a c t o r i n bear ing s e l e c t i o n and design. For t h i s reason the
u n i d i r e c t i o n a l loads have been sub-div ided i n Table 3.1 t o draw a t t e n t i o n t o
t h i s s i t u a t i o n .
Table 3.1 Examples o f va r ious types o f load and movement p a t t e r n s
Type o f load Type o f movemen t
Examples
U n i d i r e c t i o n a l Cons tan t Continuous Turbine j o u r n a l bear ings
R is ing w i t h Continuous speed
F a l l i n g w i t h Continuous speed
Un i d i r e c t iona 1 Osc i 1 l a t o r y
M u l t i d i r e c t i o n a l Continuous
M u l t i d i r e c t i o n a l O s c i l l a t o r y
Marine gearbox p i n i o n bear ings
Hydrau l i c motor bear ings
Br idge support bear ings Gr ind ing machine t a b l e s
Piston-engine crankshaf t bear ings
Linkage bear ings
3.4.1 A p p l i c a t i o n s w i t h U n i d i r e c t i o n a l Load and Continuous Movement
Th is i s the f i r s t category o f bear ing a p p l i c a t i o n s l i s t e d i n Table 3.1 and
the va r ious types o f bear ing which cou ld be used a r e the rubbing, r o l l i n g
element and f l u i d - f i l m types.
have q u i t e d i f f e r e n t load-speed c h a r a c t e r i s t i c s .
Fig.2 shows t h a t these va r ious types o f bear ing
In f a c t , examples a r e g i ven in Table 3.1 o f va r ious a p p l i c a t i o n s which a l s o
have several forms o f r e l a t i o n s h i p between load and speed, and i d e a l l y a
bear ing should be se lected w i t h a matching c h a r a c t e r i s t i c .
roach t o the problem ind i ca tes t h a t t u r b i n e j o u r n a l bear ings should i d e a l l y be o f
This k i n d o f app-
38
the e x t e r n a l l y pressur ized o r h y d r o s t a t i c type, w h i l e an a p p l i c a t i o n such as
marine gearbox p i n i o n bear ing i s p a r t i c u l a r l y w e l l s u i t e d t o the s imple type o f
hydrodynamic bear ing. Whi le t h i s l a t t e r example i s i n l i n e w i t h c u r r e n t
p rac t i ce , t u rb ines a r e n o t a t present us ing the h y d r o s t a t i c type o f bea r ing
except i n those used t o d r i v e high-speed denta l d r i l l s . Th is may be an example
o f a design s i t u a t i o n where a p a r t i c u l a r type o f machine i s developed on the
bas i s o f a workable bu t n o t i dea l form o f bea r ing design and i t then requ i res
considerable commercial courage t o make the necessary change on a v i t a l com-
ponent i n a machine o f such h igh c a p i t a l value. From the economic p o i n t o f
view, however, h y d r o s t a t i c bear ings a r e p a r t i c u l a r l y a p p l i c a b l e i n s i t u a t i o n s
where a source of h i g h pressure f l u i d i s a l ready a v a i l a b l e , and t h i s i s i n f a c t
the case w i t h a steam tu rb ine . Problems o f e ros ion may occur i f the steam i s
al lowed t o condense i n the bear ing c learance b u t t h i s problem should n o t be
i n supe ra b 1 e . Hydraul ic motors were suggested as an example o f an a p p l i c a t i o n i n which the
load decreased w i t h i nc reas ing speed, a l though t h i s c h a r a c t e r i s t i c tends t o be
common t o any machine d r i v e n by a source o f approx imate ly constant power. For
such an a p p l i c a t i o n , a r o l l i n g con tac t bear ing would appear t o be i dea l i n t h a t
i t has a matching c h a r a c t e r i s t i c , as w e l l as a low s t a r t i n g f r i c t i o n which i s
u s u a l l y a l s o requ i red i n t h i s p a r t i c u l a r type o f a p p l i c a t i o n . A rubbing type o f
bear ing would also appear t o be poss ib le , b u t i n f a c t the loads and speeds
a l l owab le w i t h t h i s type o f bear ing a re considerably below the corresponding
values f o r the r o l l i n g - c o n t a c t type.
Al though t h i s technique o f matching the form o f the load-speed r e l a t i o n s h i p
o f the a p p l i c a t i o n w i t h t h a t o f va r ious bear ings i s a use fu l guide i n bear ing
se lec t i on , i t i s s t i l l necessary t o consider the ac tua l values o f load and
speed which can be c a r r i e d by d i f f e r e n t s i zes o f bear ings o f the va r ious types,
i n order t o be c e r t a i n t h a t they a re s u i t a b l e .
A convenient way o f doing t h i s i s t o p l o t the performance o f the va r ious
types o f bear ing on one diagram so t h a t comparisons can be made, and i f t h i s i s
done on l oga r i t hm ic axes, the whole span o f engineer ing loads and speeds can be
covered.
This technique can be used t o show the performance o f s t e a d i l y loaded j o u r n a l
bear ings w i t h cont inuously r o t a t i n g sha f t s , and Fig.3 shows the type o f diagram
which r e s u l t s .
Fig.3 i s o n l y intended t o g i v e broad guidance b u t i t does show the general
t rends q u i t e c l e a r l y . I t i nd i ca tes , f o r example, t h a t f l u i d - f i l m p l a i n bear ings
a re the best type t o use a t h i g h speeds and t h a t t h i s i s p a r t i c u l a r l y t r u e i n
the case o f a l l s h a f t s g rea te r than about 100 mm diameter. Th is f i g u r e a l s o
i nd i ca tes t h a t the rubbing type o f bear ing i s o n l y r e a l l y s u i t a b l e f o r low
0.01 0.1 1 10 100 1000 10000 100000
Shaft speed , n ( rev/s)
FIGURE 3 Indication of the performance of common types of journal bearings for shafts of various diameters
40
r o t a t i o n a l speeds, and a t
minu te a r o l l ing bear ing
3.4.2 A p p l i c a t i o n s w i t h
any th ing more than one o r two hundred r e v o l u t i o n s per
s u s u a l l y a b e t t e r s o l u t i o n .
O s c i l l a t i n g Movement
The rubb ing type o f bea r ing i s , however, p a r t i c u l a r l y good f o r a p p l i c a t i o n s
w i t h o s c i l l a t i n g movement, s ince i t does n o t need t o b u i l d up an o i l f i l m each
t ime i t s t a r t s t o move, and w i t h smal l ang les o f o s c i l l a t i o n i t cannot b r i n e l l
i n the same way as a r o l l i n g bear ing can do under these circumstances.
I t i s t h e r e f o r e use fu l t o cons ider the probab le performance o f the va r ious
types o f rubb ing bear ing under c o n d i t i o n s o f o s c i l l a t i n g movement. As i n the
case o f con t inuous r o t a t i o n , the performance w i l l be l i m i t e d by the maximum
a l l owab le load pressure and by the wear which w i l l occur as a r e s u l t o f t he
rubbing movement. A convenient method o f showing a comparison between the
va r ious types i s t o p l o t t h e i r a l l owab le bea r ing pressure and s l i d i n g speed and
t h i s been done i n Fig.4. The values g iven here a r e o n l y approximate b u t should
he lp t o g i v e an i n d i c a t i o n o f t he va r ious types o f m a t e r i a l which can be used
i n any p a r t i c u l a r a p p l i c a t i o n .
The f l e x i b l e member type o f bea r ing i s a l s o s u i t a b l e f o r a p p l i c a t i o n w i t h
o s c i l l a t i n g mot ion, and Fig.5 g i v e s an i n d i c a t i o n o f t he a l l owab le maximum
load ing on f l e x i b l e bear ings which can be f i t t e d w i t h i n a g i ven space.
bear ings a r e commonly used i n va r ious types o f l i n k a g e and the c ross -sec t i ona l
area a v a i l a b l e a t t he end o f a l e v e r which r e q u i r e s t o be connected t o another
component i s g e n e r a l l y t he l i m i t i n g f a c t o r on s i z e r a t h e r than the diameter of
any connect ing p i n which cou ld be regarded as the e q u i v a l e n t o f the s h a f t i n
the a p p l i c a t i o n s examined p r e v i o u s l y . For t h i s reason t h e l oad i n F ig .5 i s
p l o t t e d i n terms o f t he a l l owab le bea r ing pressure on the o u t s i d e diameter o f
rubber bushes and e q u i v a l e n t shaped crossed f l e x u r e p i v o t s . Th i s diagram i n -
d i c a t e s t h a t f l e x i b l e member bear ings a r e o n l y usable up t o an abso lu te maximum
equ iva len t p ressure o f 14 MN/m* 2000 I b / i n 2 w i t h very low angu lar movements,
and w i t h much lower loads up t o a maximum p o s s i b l e angu lar movement o f 30 degrees.
These
Outside these cond i t i ons , i t i s necessary t o r e v e r t t o the more convent iona l
pin-and-bush type o f bear ing design, f o r which the m a t e r i a l can then be se lec ted
as i n d i c a t e d by Fig.4.
I t may appear f rom Fig.5 t h a t rubber bushes a r e always l i k e l y t o be super io r
t o f l e x u r a l l igament bear ings . While t h i s i s t r u e i n terms o f compressive load
and a l l owab le d e f l e c t i o n , i t must be remembered t h a t f l e x u r a l l igaments can be
designed t o have a much lower s t i f f n e s s f o r a g iven load c a p a c i t y compared t o a
rubber bush, and they w i l l a l s o accept a much w ider range o f environmental
cond i t i ons .
f l e x u r e p i v o t s and i t i s impor tan t t o remember t h a t t he design can be s i m p l i f i e d
t o a much s imp ler s i n g l e l igament , u s u a l l y i n tens ion , i f the r o t a t i o n a l c e n t r e
The da ta presented f o r f l e x u r a l l igaments a r e based on crossed
41
Q, c
.- E! X
0.01 0 01 a
I I Hard steel on ,hard steel
0.1
Approximate 1
maximum 10 100
rubbing speed (m/sl
Fig.4 Indication of the performance of dry rubbing bearing with oscillating motion
42
.+ .- C 3
0 .I-
al
m 3 0
0 +
C 0
Maximum deflection each way (degrees)
Fig.5 I n d i c a t i o n o f t he performance o f f l e x i b l e member bear ings w i t h o s c i l l a t i n g mot ion
o f mot ion does no t have t o be kept under c l o s e c o n t r o l .
3.4.3 A p p l i c a t i o n s w i t h M u l t i d i r e c t i o n a l Load and Continuous Movement
I n a p p l i c a t i o n s o f t h i s k ind , o f which t y p i c a l examples a r e the c ranksha f t
bear ings o f p i s t o n engines and r e c i p r o c a t i n g compressors, t he bear ing pressures
used i n p r a c t i c e a r e cons ide rab ly g r e a t e r than those a p p l i e d t o u n i d i r e c t i o n a l l y
loaded bear ings . Th is i s p o s s i b l e because
( i ) t he l oad frequency changes i n d i r e c t i o n as w e l l as i n magnitude, and the
s h a f t does no t have t ime t o squeeze the o i l f rom the f i l m t o a s u f f i c i e n t
e x t e n t t o make metal- to-metal con tac t be fo re the l oad reverses and l i f t s
the s h a f t away again.
( i i ) s ince va r ious p o s i t i o n s o f t he bear ing and s h a f t sur face c a r r y the
minimum f i l m reg ion i n tu rn , t he l o c a l thermal c o n d i t i o n s a re n o t as
severe as i n a s t e a d i l y loaded bear ing where one reg ion o f t he bear in?
43
metal i s con t inuous ly sub jec ted t o l o c a l heat i npu t . As a r e s u l t lower
va lues o f f i l m t h i ckness may be pe rm i t ted f o r a g i ven r e v o l u t i o n speed
and s h a f t s i ze , p rov ided t h a t s h a f t sur face f i n i s h i s smooth enough.
The methods o f c a l c u l a t i n g the performance o f dynamica l l y loaded bear ings a re
e s s e n t i a l l y r a t h e r complex and the most comprehensive method o f assessing t h e i r
p robab le performance i s t o c a l c u l a t e the probab le l ocus o f t he s h a f t w i t h a
computer . From the p o i n t of v iew o f very genera l design guidance, however, i t can
probab ly be apprec ia ted t h a t s ince much g r e a t e r bea r ing pressures a r e a l l owab le ,
the s t r e n g t h o f t he bear ing meta ls becomes an impor tan t f a c t o r i n bear ing
s e l e c t i o n , t oge the r w i t h the method o f f eed ing o i l t o the bear ings i n o rder t o
ensure t h a t t he bear ing c learances a r e kep t as f u l l o f o i l as p o s s i b l e i n
readiness f o r c a r r y i n g the dynamic loads.
I t i s d i f f i c u l t t o quote p r e c i s e data f o r t he a l l owab le loads on va r ious
bear ing m a t e r i a l s s ince f a c t o r s such as s h a f t d e f l e c t i o n , e f f i c i e n c y o f l ub -
r i c a t i o n and the type o f l oad ing p a t t e r n have a cons ide rab le e f f e c t . Table 3.2
however, g i v e s some broad guidance on t h e type o f m a t e r i a l s t h a t a r e l i k e l y t o
be s u i t a b l e .
Table 3.2 Poss ib le m a t e r i a l s f o r p l a i n bear ings i n r e c i p r o c a t i n g machinery
Maximum bear ing pressure Poss ib le bea r ing m a t e r i a l l b / i n 2 MN/m2
Below 2500 17 Whitemetal
2500- 4000 17 - 28 Copper leads
2500- 5000 17 - 34 Aluminium t i n s
4000- 6000 28 - 41 Low t i n l ead bronzes
6000-10000 41 - 70 High t i n lead bronzes
10000-15000 70 - 100 Phosphor bronzes
The s t ronger m a t e r i a l s a r e harder than the weaker ones and i t i s t h e r e f o r e
d e s i r a b l e t o p i c k a m a t e r i a l which i s o n l y j u s t adequately s t rong enough i n
o rde r t o have the maximum p o s s i b l e c o n f o r m a b i l i t y w i t h the s h a f t and embedd-
a b i l i t y f o r d i r t . With harder bea r ing m a t e r i a l s i t i s a l s o necessary t o use a
hard shaft , p a r t l y t o ma in ta in an adequate hardness d i f f e r e n t i a l w i t h the
bear ing m a t e r i a l and p a r t l y because the h ighe r bear ing pressures which have
c a l l e d f o r t he harder m a t e r i a l a re a l s o l i k e l y t o r e s u l t i n lower f i l m t h i c k -
nesses and t h e r e f o r e g rea te r d i r t s e n s i t i v i t y .
As a very genera l guide, i t i s n o t u s u a l l y necessary t o harden the s h a f t a t
loads below 24 MN/m2 (3500 l b / i n Z ) bu t g e n e r a l l y d e s i r a b l e t o do so a t loads
44
above 31 MN/m2 (4.500 l b / i n 2 ) .
R o l l i n g element bear ings can a l s o be used f o r p i s t o n engine c ranksha f t
bear ings, a l though i n most cases they o f f e r no b e t t e r performance than a f l u i d -
f , i l m bear ing, a t the expense i n l a r g e r engines o f a r a t h e r more compl icated
design. R o l l i n g bear ings do have the p a r t i c u l a r m e r i t o f r e q u i r i n g much l e s s
o i l supply than f l u i d - f i l m bear ings and t h i s makes them a f i r s t choice f o r two-
s t roke engines o f the p e t r o i l type, and f o r o t h e r smal l cheap engines where
the o p p o r t u n i t y they o f f e r o f e l i m i n a t i n g the o i l supply system r e s u l t s i n a
more commercial design.
F igures were r e p r i n t e d by permiss ion o f the Counci l o f the I n s t i t u t i o n o f
Mechanical Engineers f rom the Proceedings o f 1967 I n t e r n a t i o n a l Conference on
L u b r i c a t i o n and \!ear.
45
4 DESIGN OF PLAIN BEARINGS Use of Bearing Data Design Charts
F.A. MARTIN and D.R. GARNER
Research 8 Development Organisat ion
The G lac ie r Metal Company L imi ted.
4.1 INTRODUCTION
The design o f hydrodynamic bear ings can be a daunt ing task f o r the average
machinery designer. With r o l l i n g element bear ings the design process i s very
much b iased towards bear ing s e l e c t i o n data, where manufacturers ' catalogues
g i ve tabu la ted i n fo rma t ion on s ize, load, speed and l i f e . For p l a i n bear ings
i t i s n o t so easy t o f i n d such ' p o t t e d ' in format ion, and i n the past the
designer had e i t h e r t o become involved i n the mathematical comp lex i t i es o f the
hydrodynamics, o r r e l y on the vagar ies o f ru le-of- thumb guides. With the con-
t i n u a l u p r a t i n g o f machinery, t h i s l a t t e r approach i s seldom adequate, but i t
can be d i f f i c u l t t o o b t a i n more d e t a i l e d i n fo rma t ion i n a r e a d i l y d i g e s t i b l e
form.
Var ious techniques f o r ana lys ing and checking the performance o f p l a i n
j o u r n a l bear ings a re presented here, so t h a t the designer has the means t o app-
r e c i a t e the e f f e c t s o f t he va r ious parameters on bear ing performance and
design, w i t h o u t the need f o r ex tens i ve mathematical manipulat ion.
4 . 1 . 1
b
'd
d
e
C
hmin H
M
N
Notat ion
= a x i a l bea r ing l eng th (m)
= bear ing d iametra l c learance (m)
= bear ing r a d i a l c learance (m)
= bear ing diameter (m)
= d i s tance between bear ing and s h a f t cen t res (m)
= minimum o i l f i l m th ickness (m)
= power loss (\I)
= e f f e c t i v e r o t o r mass a t bear ing (kg)
= s h a f t r o t a t i o n a l speed ( rev/s)
46
NL P
Q = o i l f l o w requirement (m3/s)
W = a p p l i e d load ( N )
6 = j o u r n a l misal ignment over bear ing l eng th (m)
I- = e f f e c t i v e f i l m v i s c o s i t y (Ns/m2)
ee = e f f e c t i v e temperature ( " C )
emax p = o i l dens i t y (kg/m3)
= r o t a t i o n a l speed o f load vec to r ( rev/s)
= bear ing s p e c i f i c l oad W/bd (Pa o r N/m2)
= maximum temperature ( " C )
t y ( rad/s)
any cons is ten t set o f u n i t s eg. as g i ven above)
w = angular ve loc
Dimensionless Terms ( i n
load v a r i a b l e
turbulence v a r i a b l e
e c c e n t r i c i t y r a t i o
W ' = & [?I2 d2N2p
= P [t] E = e/cr = 1 - hmin/cr
dimensionless c r i t i c a l mass c w 2 M/W
4.2 JOURNAL BEARING DESIGN LIMITS
The t o t a l design process f o r most mechanical components, i n c l u d i n g p l a i n
bear ings, invo lves many stages, w i t h f u n c t i o n a l , economic and perhaps a e s t h e t i c
aspects a l l needing t o be considered. I t i s the f i r s t o f these which no rma l l y
invo lves the designer in most e f f o r t and which i s t o be considered i n d e t a i l
here. For convenience we w i l l s p l i t t h i s f u n c t i o n a l design stage i n t o two:-
( i ) ensur ing t h a t the bear ing i s capable o f ope ra t i ng s a t i s f a c t o r i l y under
the imposed cond i t i ons , and t h a t i t i s n e i t h e r too c l o s e t o i t s l i m i t s o f
ope ra t i on t o endanger r e l i a b i l i t y nor so f a r away t h a t i t has p e n a l t i e s i n
over-design,
( i i ) p r e d i c t i n g the performance o f the component as i t a f f e c t s the design
o f i t s assoc iated p a r t s o r the o v e r a l l system.
i n i t i a l l y , t he re fo re , the l i m i t s o f ope ra t i on o f bear ings must be def ined.
4.2.1 L i m i t s o f Operat ion
Consider a bear ing o f some g i ven s i z e and geometry, w i t h a de f i ned l u b r i c a n t
grade and feeding cond i t i ons . The l i m i t i n g c o n d i t i o n s o f load and speed which
t h i s bear ing can success fu l l y accept a re shown i n Fig.1; these l i m i t s w i l l now
be considered i n more d e t a i l .
47
4.2.1.1 Thin O i l F i l m L i m i t
The danqer here i s o f metal- to-metal con tac t o f t he surfaces, w i t h con-
sequent severe wear (and perhaps overheat ing) l ead ing t o a breakdown i n bear ing
operat ion. There i s evidence t o show t h a t t h i s con tac t occurs a t a p red ic ted
f i l m th ickness which i s a f u n c t i o n o f the sur face roughness [l], which i n t u r n
i s dependent upon machining process and bear ing s ize.
i n some d e t a i l i n re ference [2 ] , and the conclus ion reached t h a t a r e a l i s t i c
' f a i l u r e ' va lue o f the f i l m th ickness i s g i ven by the peak- to-va l ley sur face
f i n i s h (Rmax) o f t he j o u r n a l , assuming t h a t the sur face f i n i s h o f the bear ing
i s o f t he same order .
Th i s has been considered
An approximate c o r r e l a t i o n between Ra and Rmax values i s a l s o g iven i n
re ference [2 ] , enab l i ng the i n fo rma t ion g i ven i n Fig.2 t o be presented. The
sur face f i n i s h e s shown here a re rep resen ta t i ve o f those which can be obta ined
by normal manufactur ing methods.
f a i l u r e values, some a d d i t i o n a l f a c t o r must be a p p l i e d i f we a r e t o spec i f y
safe values o f o i l f i l m th ickness. This f a c t o r has to a l l o w f o r s l i g h t un in-
t e n t i o n a l misal ignment which may take p lace between s h a f t and bear ing, and f o r
d i r t contaminat ion i n the o i l supply. The a l l owab le f i l m th ickness values shown
i n Fig.2 a r e a f a c t o r o f t h ree above the f a i l u r e values, and have been found by
exper ience t o be acceptable.
a f a c t o r o f two can be s a t i s f a c t o r y , b u t i t i s considered t h a t the values shown
Since the Rmax values have been thought o f as
For very h i g h standards o f b u i l d and operat ion
BEARIN( LOAD
JOURNAL SPEED
HIGH BEARING TEMPERATURE LIMIT danger of lining material wiping if load lies above this h e
OIL OXIDATION LIMIT danger of excessive oil oxidation i f speed lies beyond
/:OF SAFE OPERATION ) OIL this FILM line WHIRL LIMIT
THIN OIL FILM LIMIT danger of metal - to - metal contact if load lies above this line
/..
danger of unacceptable vibration i f speed lies beyond this line I
Fig.1 L i m i t s o f safe ope ra t i on f o r hydrodynamic j ou rna l bear ings
48
i n Fig.2 should be used as a general gu ide t o a v o i d work ing t o o c l o s e t o
f a i l u r e l i m i t s . I t must be emphasised t h a t t he r e l a t i o n s h i p between Ra and
Rmax i s o n l y approximate, as evidenced by the "spread o f r e s u l t s " band on the
f i g u r e , bu t i t i s adequate t o show the o rde r o f reduc t i on i n the a l l owab le f i l m
th ickness t h a t can be pe rm i t ted by improving sur face f i n i s h . To increase the
opera t i ng o i l f i l m th ickness , f o r g i ven c o n d i t i o n s o f l oad and speed, the
bear ing s i z e must be increased, o r a t h i c k e r l u b r i c a n t and/or a reduced i n l e t
temperature used (change i n o i l supply p ressure w i l l u s u a l l y have l i t t l e e f fec t ) .
An increase i n c learance may e i t h e r increase o r reduce the f i l m th ickness ,
depending upon the p r e c i s e opera t i ng cond i t i ons .
1 2 I 6 810 20 LO 1inI
JOURNAL DIAMETER
Fig.2 Guidance on s h a f t sur face f i n i s h and a l l owab le o i l f i l m t h i ckness
4.2.1.2 High Bear ing Temperature
The shear ing which takes p lace i n the bear ing c learance space i s l seen '
e x t e r n a l l y as power d i s s i p a t i o n , and a l s o as an inc rease i n the temperature o f
s h a f t and bear ing sur faces and o f t he l u b r i c a n t .
The l i m i t i n g acceptab le temperature obv ious l y depends upon the l i n i n g m a t e r i a l
used, bu t whatever the m a t e r i a l t he temperature must always be kep t w e l l below
i t s t h e o r e t i c a l m e l t i n g p o i n t . For example, i n t i n based w h i t e metal t he
m e l t i n g temperature o f 232OC cannot even be c l o s e l y approached due t o the
so f ten ing and subsequent p l a s t i c f l o w o f the m a t e r i a l which occurs a t temper-
49
atures we l l below 200°C under the in f luence o f hydrodynamic pressure. Booser
e t a l . [3 ] observed l i m i t i n g whitemetal temperatures i n j ou rna l bearings as low
as 130°C; however, since only ca lcu lated temperatures are ava i l ab le a t the
design stage, and current methods of est imat ion are known t o be f a r from
accurate, i t i s safer t o lower the l i m i t t o about 120'C. I n machines i n which
some dynamic loading can occur on top o f the normal steady load, and where
bearing f a t i g u e i s therefore a poss ib le problem, i t i s customary t o t r y and
keep the temperature below 100°C.
odated then the use o f an aluminium t i n o r a copper lead mater ia l must be con-
sidered.
temperatures up t o 15O-16O0C, and i s almost equal t o whitemetal i n i t s a b i l i t y
t o withstand seizure condi t ions and d i r t contamination. Copper lead i s much
less f o r g i v i n g i n t h i s respect, and i d e a l l y should have a so f t , t h i n over lay
p l a t e t o a i d i n bedding-in, but i t can be used a t temperatures o f 200°C plus.
It requi res a hardened sha f t (about 300 HV) and very good lub r i can t f i l t r a t i o n .
I f higher temperatures have t o be accomm-
The former o f these, when conta in ing about 40% t i n , can be used a t
It i s worth no t i ng tha t i f a h igh temperature cond i t i on i s present i n a
bearing there i s usua l l y noth ing tha t can be done t o the o i l feeding condi t ions
t o improve the s i t ua t i on , apar t from changing the o i l grade. The oft-used
' p a l l i a t i v e s ' o f increasing o i l feed pressure o r grooving area i n order t o
force more o i l through the bearing are usua l l y not successful since they only
reduce the bu lk temperature o f the o i l passing through the bearing (see section
4.2.1.3).The bearing mater ia l temperature i s c o n t r o l l e d by the amount o f lub-
r i c a n t passing through the a c t i v e p a r t o f the o i l f i l m , and t h i s i s very in-
sens i t i ve t o feeding condit ions, provided tha t the bearing i s no t grossly
starved. As a rough approximation, the maximum bearing temperature w i l l be
dropped by about a h a l f o f any decrease i n o i l i n l e t temperature, i e a 10°C
reduction i n i n l e t temperature may be expected t o decrease the maximum bearing
temperature by about 5°C.
creased by increasing the bearing s i ze o r clearance, o r by using a th inner
grade o f o i l .
The bearing temperature may a l so genera l ly be de-
One f u r t h e r p o i n t on bearing temperatures which i s sometimes ignored i s the
necess i ty o f considering the inf luence o f machine temperatures on the bearings.
If there can be appreciable heat soak along the shaf t , o r ( less o f ten ) through
the bearing housing, then the design must a l l ow f o r adequate o i l f l ow t o deal
w i t h it. Add i t i ona l l y , i n some machinery, the worst temperature condi t ions a t
the bearing can occur a f t e r shut down when heat soak ra ises temperatures wel l
above peak running values, and the choice o f mater ia l should then be d i c ta ted
by condi t ions which are o f t e n not drawn t o the a t t e n t i o n o f the bearing
designer.
50
4.2.1.3 O i l Oxidation L i m i t
S t ra igh t mineral o i l s i n a normal (oxygen conta in ing) atmosphere can be
rap id l y ox id ised a t the order o f temperatures tha t we have been discussing
above. There i s no prec ise 'go/no-go' l i m i t f o r t h i s process, ra ther the r a t e
o f degradation i s a func t i on o f temperature [4] . I n d u s t r i a l mineral o i l s
usual ly conta in ant i -ox idants which r e t a r d t h i s process, bu t f o r comnonly used
turb ine o i l s , f o r l i v e s i n the order o f thousands o f hours, i t i s necessary t o
r e s t r i c t bu lk temperatures o f o i l i n tanks, rese rvo i r s etc. t o about 75-80°C.
Thus a bu lk d ra in temperature from a bearing a t a higher value than t h i s i s
u n l i k e l y t o be acceptable.
As mentioned above, the d r a i n temperature can usua l l y be reduced by in-
creasing o i l supply pressure o r p rov id ing bleed grooves i n the bore, and thereby
help ing t o avoid the ox ida t i on l i m i t . I t i s unfor tunate t h a t the bulk o u t l e t
temperature i s o f t e n used t o judge bearing performance, presumably because i t
i s the eas iest temperature t o measure, since t h i s j u g g l i n g w i t h supply con-
d i t i o n s can r a d i c a l l y a l t e r d ra in temperatures wi thout appreciably a f f e c t i n g
mater ia l temperatures. The o u t l e t temperature can be used as a long-term
monitoring device ( i e a temperature which has been steady a t 70-75'C f o r months
should not suddenly r i s e t o 80-85°C) but i t i s no t su i tab le, and may indeed be
misleading, i f used for ' s e t t i n g up' o i l feed condi t ions on a new machine.
4.2.1.4 O i l F i l m I n s t a b i l i t y
Under c e r t a i n condi t ions, normally a t low load and/or h igh sha f t speeds, a
s e l f exc i ted and s e l f susta in ing motion can occur i n which the sha f t cent re
precesses around the bearing centre a t something s l i g h t l y less than h a l f sha f t
speed, t y p i c a l l y 0.42-0.47 o f sha f t speed. Under these condi t ions the hydro-
dynamic ac t i on o f the bearing i s a l l but l o s t , and metal-to-metal contact can
occur; i n pract ice, i f l e f t f o r long periods, a fa t i gue type damage i s pro-
duced due t o the h igh temperatures generated. There can a l s o be a large, and
perhaps unacceptable, v i b r a t i o n t ransmi t ted through the machine. Guidance on
the l i k e l i h o o d o f i n s t a b i l i t y i n c y l i n d r i c a l bore bearings i s given i n Fig.3.
This cha r t i s s t r i c t l y on l y appl icable t o r i g i d , simply supported shaf ts [ 5 ] ,
but experience has shown i t t o g i ve a f a i r guide f o r o ther systems, f o r example
overhung rotors. It can be seen tha t i f the operat ing e c c e n t r i c i t y r a t i o i s
greater than about 0.8 then the bearing i s stable. This i s the reason why
derat ing a bearing - adding ex t ra grooving i n the loaded region, using a
th inner grade o f o i l etc. - can sometimes cure h a l f speed w h i r l problems.
Add i t i ona l l y , whatever the operat ing e c c e n t r i c i t y r a t i o there i s some value o f
the dimensionless c r i t i c a l mass below which the bearing w i l l always be stable.
In heavi ly loaded appl icat ions, such as gearboxes, the appl ied load i s normally
51
- -
-
o f a much higher magnitude than the shaf t mass and i n s t a b i l i t y , a t any
r e a l i s t i c speed, i s n o t a problem. However there may well. be some p a r t load
condi t ion, f o r example a sp in tes t , a t which s t a b i l i t y needs t o be c a r e f u l l y
checked.
100 - 8 0
60
4 0 3 1 ~ V L
- 2 0 2
-10 g - 8 3 - 6 2
4 I
4: -I
w 4
- 4 v) z W E
- 2
1.0
Cr radial clearance 0 shaft angular velocity
M effective rotor mass at
W bearing load
With consistent units e.g. m, radls, kg. N.
1 bearing 1 length
diamotrr
UNSTABLE 0.1
. 0.5
. 1.0 1.5
t STABLE
I ~ ' " " ' " ' 0 0.2 0-4 0.6 0- 8
ECCENTRICITY RATIO
Fig.3 O i l f i l m w h i r l i n s t a b i l i t y o f c y l i n d r i c a l bore journal bearings - L i m i t i n g dimensionless mass
4.2.2 Region o f Safe Operation
The l i m i t s j u s t defined serve t o bound a region o f safe operation. The
simplest approach t o design i s merely t o ca l cu la te such operat ing parameters
as f i l m thickness and temperatures and t o compare them against the defined
l i m i t s . If both the l i m i t s and the design p o i n t are p lo t ted , i n a form s i m i l a r
t o Fig.1, then judgements on the degree o f safety can e a s i l y be made.
There are various methods o f t a c k l i n g th i s , and the most comnon w i l l now be
described;
est imates o f bearing operat ion t o be made w i t h minimal e f f o r t .
the f i n a l design cha r t s presented enable very rap id and accurate
52
4.3 CALCULATION AND DESIGN PROCEDURES
The ana lys i s o f o i l f i l m cond i t i ons w i t h i n a bear ing, desc ibed by the
Reynolds equat ion, i s a very invo lved mathematical exerc ise. The equat ion i s
decept ive ly s imple, b u t an a n a l y t i c a l s o l u t i o n cannot be obta ned f o r any b u t
the s implest o f arrangements.
techniques, u s u a l l y i n v o l v i n g computer so lu t i ons .
Instead recourse must be made t o numerical
A f u r t h e r comp l i ca t i on i n o i l l u b r i c a t e d bear ings i s the need t o make a
r e a l i s t i c es t ima t ion o f the o i l v i s c o s i t y w i t h i n the f i l m .
normal minera l o i l s is very dependent on temperature, and a r e l a t i v e l y accurate
assessment o f the l a t t e r must be made f o r r e a l i s t i c design. One method o f
doing t h i s i s t o so lve the Reynolds and energy equat ions s imultaneously.
cons iderably increases the computat ional e f f o r t and the r e s u l t i n g s o l u t i o n s a r e
somewhat r e s t r i c t e d i n t h e i r genera l i t y .
The v i s c o s i t y of
This
Instead i t i s u s u a l l y more convenient t o so l ve the Reynolds equat ion
assuming a s ing le , g loba l , e f f e c t i v e v i s c o s i t y value, which then can, and must,
be determined a t a l a t e r stage. This i s done by equat ing the work done i n
shear ing the o i l w i t h i n the bear ing c learance space t o the amount o f heat
c a r r i e d away, both by the o i l ( t he predominant mechanism) o r through the
bear ing and housing.
It i s , un fo r tuna te l y , n o t uncommon t o see design methods i n which t h i s heat
balance i s omi t ted, thereby assuming t h a t t rends i n dimensionless groups acc-
u r a t e l y p o r t r a y t rends i n r e a l performance. This can lead t o i n c o r r e c t con-
c lus ions ; f o r example an increased c learance may apparen t l y reduce f i l m
th ickness i f the v i s c o s i t y i s a r t i f i c i a l l y h e l d constant , b u t i n p r a c t i c e the
opera t i ng v i s c o s i t y may increase and p o s s i b l y reverse the trend.
Whatever c a l c u l a t i o n method i s chosen, the r e s u l t s must then be compared
against the est imated o r known l i m i t s o f s a t i s f a c t o r y operat ion.
Fig.4 i l l u s t r a t e s the va r ious stages i n producing design data:-
Stage 1
Stage I I
i s the s o l u t i o n of the Reynolds equat ion t o g i v e dimensionless data.
takes t h i s data and determines a r e a l i s t i c o i l v i s c o s i t y t o g i v e
s p e c i f i c answers.
Stage I l l uses the r e s u l t s f rom the p rev ious two stages t o produce
'easy-to-use' design char ts .
4.3.1 Dimensionless Data - Stage I
Dimensionless design data have been publ ished f o r t he most commonly used
bear ing c o n f i g u r a t i o n s [6,7,8,9] and Fig.5 shows a t y p i c a l c h a r t r e l a t i n g a
load v a r i a b l e t o a f i l m th ickness r a t i o f o r a c y l i n d r i c a l bore j o u r n a l bea r ing
w i t h a steady ( o r pure r o t a t i n g ) load. This may be used f o r bear ings w i t h two
a x i a l o i l feed grooves ( loaded midway between them), w i t h a s i n g l e feed groove
53
I 1 4 I Fundamental theory program e.g. solution to I I
StageI- Dimensionless groups StagelI-Specific answer for selected input Stagem- Infinite number of answers
(certain restrictions usually necessary)
Fig.4 Stages in design
54
i n the unloaded h a l f o r w i t h a c e n t r a l c i r c u m f e r e n t i a l groove;
the l eng th i s t h a t of one land and the load i s h a l f the t o t a l a p p l i e d load.
The c e n t r a l c i r c u m f e r e n t i a l l y grooved bear ing i s g e n e r a l l y used i n a p p l i c a t i o n s
where the load i s r o t a t i n g o r where an unknown load d i r e c t i o n has t o be catered
fo r . When a bear ing has t o c a r r y a r o t a t i n g load the load capac i t y i s pro-
p o r t i o n a l t o ( 1 - 2 N ~ / N l , as shown i n Fig.5 where NL i s t he angular r o t a t i o n a l
speed o f the load measured i n the same d i r e c t i o n as s h a f t r o t a t i o n (N) . Th is
i s f u r t h e r i l l u s t r a t e d i n Fig.6, which shows d iag ramna t i ca l l y the o i l f i l m
forma t ion.
w i t h the l a t t e r
FILM THICKNESS RATIO hmin/cr
Fig.5 Bear ing load capac i t y for steady and pure r o t a t i n g l oad cases
55
An examination of dimensionless data can also be instructive where gross
effects on performance are apparent. For instance the position of oil feed
grooves in a steadily loaded bearing is very important, since they can seriously
derate the load-carrying capability of the bearing.
clearly demonstrated in Fig.7.
Case (a) shows an uninterrupted coverging oil film which generates a
Such a derating effect is
substantial hydrodynamic pressure to support the load ( W ) .
Case (b) shows the same converging oil film as (a) but interrupted by an oil
groove. This will only support a very reduced load because of the
smaller integrated pressure.
Case (c) shows an uninterrupted film with the same load as (a) which needs a
smaller oil film thickness to generate sufficient pressure.
ongulor velocity of load N ongulor velocity of shaft mL
hmin + position oheod hmin position logs of pressure region I behind pressure region
I
Analogous
- DRAGGING OIL SQUEEZING PUSHING OIL
Fig.6 Film formation and relative load capacity under rotating load conditions.
56
~
I groove E; c
Cr
me min min
smaller load
I t W
(a) I (b)
Fig.7 The derating effect of grooving positioned in a converging oil film
- 0.1
Fig.8 Variation of load capacity with direction of load for a bearing with two axial grooves
67
The dimensionless load and f i l m th ickness c h a r t , Fig.8, shows more p r e c i s e l y
the v a r i a t i o n i n l oad capac i t y w i t h any load d i r e c t i o n r e l a t i v e to the grooves.
In t h i s f i g u r e a bea r ing w i t h a l eng th equal t o h a l f the diameter i s con-
s idered, hav ing two a x i a l grooves d i sp laced 180" apa r t , each groove extending
30" around the bear ing. These a r e t y p i c a l groove arrangements.
With the load d i r e c t i o n e x a c t l y between two grooves the load capac i t y
shown i n Fig.8 i s t he same as t h a t i n Fig.5 and v a r i a t i o n s i n the angle w i t h i n
f 20" cause l i t t l e d i f f e r e n c e . As the load l i n e ge ts near t o a groove p o s i t i o n
the load capac i t y d r a s t i c a l l y reduces as seen by the reduc t i on i n the mod i f i ed
load v a r i a b l e ( W ' / T ) f o r t he same f i l m th ickness. I t can be seen t h a t the load
v a r i a b l e i s ha lved when going from p o i n t (a) t o p o i n t (b) i n Fig.8, where
p o i n t s (a) , (b) and (c) can be i d e n t i f i e d w i t h cases (a), (b) and (c) i n Fig.7.
S i m i l a r l y f o r t he same value o f load parameter the f i l m th ickness f o r case (c)
i s much smal ler than case (a) .
I t should be noted t h a t t he c o n d i t i o n o f loading d i r e c t l y i n t o a groove does
no t normal ly produce the lowest o i l f i l m th ickness; instead t h i s occurs when
the load i s d i r e c t e d j u s t be fo re the groove. This i s because the peak o i l f i l m
pressure occurs j u s t beyond the load l i n e ( f o r a steady load c o n d i t i o n ) , and a
groove i n t h i s p o s i t i o n t h e r e f o r e more severe ly derates the bear ing.
4.3.2 Design Procedures w i t h Heat Balance - Stage I I
The heat balance procedure can be c a r r i e d o u t by hand c a l c u l a t i o n i n a
r e l a t i v e l y s t r a i g h t f o r w a r d manner, b u t i s i d e a l l y s u i t e d t o s imple computer
techniques.
c y l i n d r i c a l bore bear ings i s g iven i n re ference [9] which can form the bas i s o f
a computer programme [ l o ] . The procedure [9] enables the i n f l uence o f a very
wide range o f v a r i a b l e s t o be considered and i s an extremely va luable design
t o o l , e s p e c i a l l y when i n the format o f a conversat ional type computer programme.
A p a r t i c u l a r l y convenient and we1 1 documented procedure f o r
4 . 3 . 3 Improvement i n Design Aids - Stage I l l
Wh i l s t the above arrangement i s s u i t a b l e f o r p r e d i c t i n g bear ing performance
w i t h ' s i n g l e sho t ' answers, i t does n o t p rov ide a ' f e e l ' for how near a
p a r t i c u l a r design i s t o the va r ious safe ope ra t i ng l i m i t s . Therefore, t he re i s
a need f o r stage I l l where the 'computed answers' f o r bear ing cases a r e rep-
resented i n design c h a r t form. These enable the designer t o answer such
quest ions as:-
- W i l l t h i s bear ing operate s a f e l y ?
- How near i s i t t o the l i m i t s of ope ra t i on?
- What changes can be made t o improve the design?
58
When assessing safe l i m i t s o f operat ion many fac to rs are involved, f o r
example the bearing data considered must include diameter, length, o i l grade,
load and speed. The various permutati.ons,for a range o f these fac to rs r e s u l t s
i n many thousands o f cases t o be considered. I f such data were represented i n
conventional graphical form, a t h i c k volume o f unmanageable design a ids would
resu l t . The ' s l i d e c h a r t ' obviates the need f o r t h i s and gives a d i r e c t and
ins tan t f e e l f o r the e f f e c t o f changing any o f the var iab les, thus b r ing ing
the power o f the computer t o the designers' desk top.
4.4 DESIGN PROCEDURE FOR CYLINDRICAL BORE BEARINGS
4.4.1 Method o f Approach
The procedure d e t a i l e d here presents design a ids which a l l ow f o r l i m i t i n g
condi t ions o f operat ion and the p r e d i c t i o n o f bearing performance i n c y l i n d r i c a l
bore journal bearings which have two a x i a l grooves spaced 180" apar t . This
type o f bearing, w i t h a steady load midway between the grooves, i s one o f the
most common i n use. A l l the tedious heat balance ca l cu la t i ons have already
been c a r r i e d out by computer, based on the design procedure given i n ESDU Data
Item 66023 [ g ] , and are consequently inherent i n the design aids.
Comparison of experimental work w i t h r e s u l t s from t h i s theory ind icated tha t
w h i l s t c o r r e l a t i o n was good . fo r most var iab les, the predic ted maximum temp-
erature could, i n many circumstances, be considerably i n e r r o r . A new method
was evolved based on considerable experimental evidence which re la tes the
maximum bearing temperature t o the ca l cu la ted e f f e c t i v e temperature [ l l ] .
The r e s u l t i n g procedure was used t o ca l cu la te bearing performance over a
wide range o f operat ing condi t ions. However, t o keep the problem t o a manage-
able s ize i t was decided tha t c e r t a i n var iab les should be f i x e d throughout, t o
values commonly found i n current pract ice. These were:-
(i) o i l groove dimensions:
a x i a l length = 0.8 o f bearing length
c i rcumferent ia l width = 0.25 o f bearing diameter.
( i e 150" between the edges o f grooves,
i n top and bottom o f bearing).
Grooves w i t h 'square' ends
(ii) o i l feed condi t ions a t the bearing:
pressure 0.1 MPa (approx 15 Ib f / i nZ )
temperature 50°C
59
( i i i ) o i l grades:
i n o rde r t h a t the o i l grade cou ld be de f i ned s imply , a t r e n d i n
v i scos i t y - tempera tu re c h a r a c t e r i s t i c s t y p i c a l o f i n d u s t r i a l
minera l o i l s was assumed t o apply throughout.
an o i l grade can then be made by quo t ing one v i s c o s i t y a t a
c e r t a i n ( a r b i t r a r y ) temperature; 40°C was used ,here t o d e f i n e
the o i l grade, t h i s be ing the same basic re ference as f o r IS0
v i s c o s i t y grades.
The d e f i n i t i o n o f
Fig.9 shows the v i s c o s i t y t rends considered.
I n a design problem the bear ing diameter and opera t i ng c o n d i t i o n s o f speed
and load a r e g e n e r a l l y determined p r i o r t o the bear ing design stage, s ince they
u s u a l l y form p a r t o f an o v e r a l l system.
o i l grade t o be used i s o f t e n a l s o imposed leav ing c learance and bear ing length
as the two dimensions s t i l l t o be determined.
For s i m i l a r reasons the l u b r i c a t i n g
Fig.9 V i s c o s i t y % Temperature c h a r a c t e r i s t i c s f o r t he range o f o i l s considered i n the design c h a r t s
60
Several authors have considered optimising on clearance [12,13] to give the
largest possible oil film thickness in the bearing or the lowest power loss.
Unfortunately these two optima do not coincide and a compromise solution is
usually required.
exercise there is a danger of having excessively slack clearances for optimum
power loss and, in some instances, very tight clearances for optimum film thick-
ness, especially if the effect of change in clearance on viscosity is ignored.
Indeed when the problem is treated purely as a mathematical
4.4.2 Guidance for Safe Operation
4.4.2.1 Oil Film Thickness and Temperatures
It is appropriate to consider the tight clearance end of the tolerance range
These tight clearances may also when examining conditions at high temperature.
be used when making judgements on safe operation relating to small oil films
since the variation of clearance throughout m s t practical tolerance ranges
generally has little effect on oil film thickness.
(b) and (c), associated with both temperature and film thickness limits, have
been developed using the minimum clearance values shown in Fig.11. These
values relate to an extreme of the tolerance range; average manufactured
clearance will be larger.
with much 'feed-back' relating to the performance of practical designs.
The design aids, Figs.lO(a),
These clearances are based on many years' expe.rience
With clearances known it is possible, for any particular values of diameter,
length and lubricant grade, to plot limiting lines of load against speed, rep-
resenting film thickness limit and temperature limits (as in Fig.1).
aids, in 'slide chart' form, enable the designer to 'plot' these limiting lines
in a matter of seconds, not just for a discrete number of variable combinations,
but for an infinite number of cases. In fact the designer merely has to move
an already drawn curve to its correct position relative to the axes, the
bearing conditions dictating exactly where this position is.
The design
Fig.lO(a) is used to 'plot' the film thickness limit lines, and Fig.lO(b)
is used to 'plot' the temperature limit lines, both on the grid of load and
speed given in Fig.lO(c). A transparent version of the chart in question,
Fig.lO(a) or (b), should be placed on the backing sheet, Fig.lO(c) using the
guide lines on the top (transparent) sheet to keep the two sheets square.
transparency should then be moved to a position where a point in its grid,
defined by the diameter and oil grade values, is coincident with the cross on
the backing sheet. The relevant length/diameter line then shows the limit of
load against speed for the conditions considered.
The
By using the two transparencies the relative position of a design point
(defined by its speed and load values) to the limiting lines can be seen.
this point is within the limits then the bearing, under reasonable environmental
If
61
Fig. lO(a) Load capaci ty s l i d e c h a r t : Thin o i l f i l m l i m i t (transparency)
62
U
C
4
C L
z
5 e E 2
C
Y I
C
a
a
a
.- - K
C
.-
lest at LO'C I \
This chart may not be applicable at high speeds if non - laminar conditions prevail.
Ensure that the design point of speed and load lies to the le f t of the appropriate guidance curve, otherwise
full lines -there is a danger of EXCESSIVE OIL OXIDATION
broken lines - the BEARING MATERIAL may be TOO HOT
03' ) 0%
guide line -keep square with backing sheet
Fig. lO(b) Load c a p a c i t y s l i d e c h a r t : High temperature 1 i m i t s ( t ransparency)
63
such that appropriate DIAMETER and OIL GRADE are coincident with this point
Bearing load
length diameter
Ices
Journal weed Irev/minl
F i g . l O ( c ) Load c a p a c i t y s l i d e c h a r t : Load-speed g r i d (backing sheet)
64
cond i t i ons , should operate s a t i s f a c t o r i l y . I f i t i s c l o s e t o o r ou ts ide the
l i m i t s then the s l i d e c h a r t s p rov ide a qu i ck method o f determin ing which
v a r i a b l e o r va r iab les can be changed t o improve the design. For instance
increas ing the bear ing l eng th w i l l r a i s e the l i m i t l i n e f o r f i l m th ickness,
bu t may worsen a h igh temperature s i t u a t i o n . The l a t t e r e f f e c t was noted
exper imenta l ly by Brown and Newman [ 1 4 ] . A t these h i g h speeds the c h a r t s dem-
ons t ra te and q u a n t i f y the advantage o f t h inne r o i l s . I f the use o f t h i c k o i l s
i s unavoidable then an increased clearance, above the va lues g iven by Fig.11,
may he lp a t h igh speeds. The s l i d e c h a r t s a r e then n o t app l i cab le , b u t a
s u i t a b l e design method i s d e t a i l e d i n sec t i on 4.4.3.
F i n a l l y , i t i s adv isable, wherever poss ib le , t o work away from the ac tua l
l i m i t i n g l i n e s ; t h i s a l l ows an e x t r a sa fe ty margin f o r u n i n t e n t i o n a l adverse
cond i t i ons such as small misal ignment (see sec t i on 4 . 4 . 3 1 , contaminated o i l , etc.
I t i s a l s o adv i sab le t o work w e l l w i t h i n the l i m i t i n g l i n e s where the re i s a
chance o f having two modes o f f a i l u r e a t the same t ime ( i e . a t the d o t t e d apex
i n F ig .1) .
Current p r a c t i c e i s t o q u a n t i f y t h i s a d d i t i o n a l l i m i t i n terms o f s p e c i f i c
load ( l oad d i v i d e d by p r o j e c t e d area, W/bd). For hydrodynamic bear ings i t i s
usual t o keep the s p e c i f i c load below about 4 MPa, s t i l l checking t h a t f i l m
th ickness and temperatures are-acceptable.
s ta r t -up o r run down must be considerably lower, no more than 1 t o 1.3 MPa
otherwise h i g h pressure j a c k i n g o i l must be suppl ied.
However, the l oad on a bear ing a t
Fig.12 has been developed t o g i v e the maximum s p e c i f i c load r a t i n g based on
the t h i n o i l f i l m l i m i t ; t h i s assumes the same geometry and o i l feed cond i t i ons
as considered i n the s l i d e c h a r t .
4.4.2.2 O i l F i l m I n s t a b i l i t y
This i s o n l y l i k e l y t o be a problem i n l i g h t l y loaded bear ings a t h i g h speeds.
As a f i r s t check c a l c u l a t e the va lue o f t he dimensionless c r i t i c a l mass para-
meter shown i n Fig.3.
b/d curve, then the bear ing should be s tab le.
may s t i l l be p r e d i c t e d prov ided t h a t the e c c e n t r i c i t y r a t i o ( l -hmin/cr) i s
l a rge enough; sec t i on 4.4.3 g i ves a method for o b t a i n i n g t h i s value. The ,
res i s tance t o i n s t a b i l i t y i s ra i sed by any ex te rna l damping w i t h i n the system,
f o r example gear meshing, so t h a t the l i m i t s can be unduly pess im is t i c .
I f t h i s i s l ess than the minimum value on the p a r t i c u l a r
If no t , then s t a b l e ope ra t i on
I f i n s t a b i l i t y i s a problem the bore p r o f i l i n g can o f t e n p rov ide the so lu t i on ;
sec t i on 5. cons iders t h i s i n some d e t a i l .
MINIMUM
65
MINIMUM DIAMETRAL CLEARANCE
(in.]
0.02
0.01 6
0.012
0.008
0
- SPEED I rcv/min 1
Fig. 1 1 Recommended minimum c learance
4.4.3 Performance P r e d i c t i o n
Having determined t h a t a g i ven bear ing i s l i k e l y t o operate s a t i s f a c t o r i l y ,
the designer then o f t e n needs t o know the power loss and o i l f l o w requirement
o f the bear ing.
o f a machine i s concerned, these q u a n t i t i e s have a d i r e c t i n f l uence on the
l u b r i c a n t supply system - the s i z e of o i l pump and supply l i n e s , the need f o r
coo le rs e tc . Th is system must be capable o f adequately supply ing s u f f i c i e n t
cooled o i l (heat i s the t a n g i b l e form o f bea r ing power loss) f o r any bear ing
w i t h i n the spread o f t he manufactur ing to le rance on clearance.
the o i l pump capac i t y , t he bear ing f l o w a t maximum p o s s i b l e c learance ( w i t h i n
the to le rance range) should be considered s ince t h i s has the maximum f l o w
requirement.
the heat d i s s i p a t i o n from the o v e r a l l system. can occur anywhere w i t h i n the
to le rance range o f c learance.
v a r i a b l e i n the p r e d i c t i o n c h a r t s f o r power l oss , o i l f l o w and temperature.
However, the c learance range should p r e f e r a b l y s t i l l have a minimum value
corresponding w i t h Fig.11, s ince the s l i d e c h a r t s (which i nhe ren t l y con ta in
these c learances) can then be used t o check f o r safe ope ra t i on p r i o r t o pre-
d i c t i n g bear ing performance.
As w e l l as be ing o f i n t e r e s t as f a r as the o v e r a l l e f f i c i e n c y
When determin ing
Maximum power l o s s on the o t h e r hand, impor tant when cons ide r ing
The clearance has the re fo re been l e f t as a
66
Fig.12 Maximum
specific load
rating for a
two axial groove
bearing based
on thin oil
film limit
67
Under some c r i t i c a l c o n d i t i o n s the need t o c o n t r o l t he v a r i a t i o n s i n f i l m
th ickness , power loss o r o i l f l o w might r e g u l a t e t h e range o f the c learance
to le rances , b u t u s u a l l y p rede f ined manufac tur ing d e t a i l s w i l l impose a g i ven
c learance range which t h e des igner must c a t e r f o r adequately. The s i z e o f t h i s
to le rance range i s ma in l y a f u n c t i o n o f economic cons ide ra t i ons , and may there-
f o r e vary cons ide rab ly .
on d iamet ra l c lea rance l i e s w i t h i n the range (2.10
depending on t h e manufac tur ing process; ' d i a ' i s t h e nominal s h a f t d iameter i n
However, a p r a c t i c a l gu ide i s t h a t t he t o l e r a n c e (mm) -6 -6 1/3 x d i a ) l l 3 t o (5.10 xd ia )
mn.
The minimum f i l m t h i ckness r a t i o (hmin/cr) i s a s i g n i f i c a n t term when p re -
d i c t i n g the power loss, o i l f l o w and maximum bear ing temperature. Using t h i s
r a t i o , p r e d i c t i o n c h a r t s i n nomograph fo rm have been dev ised which pe rm i t qu i ck
and accu ra te de te rm ina t ion o f t h e va r ious parameters w i t h ve ry l i t t l e
c a l c u l a t i o n .
4.4.3.1 Minimum O i l F i l m Thickness
The f i l m t h i ckness r a t i o may be ob ta ined f rom Fig.13 f o r known o p e r a t i n g
cond i t i ons .
l i n k i n g these a long t h e a p p r o p r i a t e gu ide l i n e s , as i n d i c a t e d by t h e arrows on
t he c h a r t , a p o i n t i n the f o u r t h , 'answer ' g r i d i s ob ta ined.
B a s i c a l l y t h ree g r i d s a r e used t o d e f i n e t h e problem, and by
Th is c h a r t i s a l s o u s e f u l f o r p r e d i c t i n g the minimum f i l m t h i ckness i n a
bea r ing which has a d i f f e r e n t c learance t o the minimum va lue shown i n Fig.11,
i e a bea r ing which cannot be cons idered on t he s l i d e cha r t s . The a c c e p t a b i l i t y
o f any f i l m t h i ckness can be checked on Fig.2.
4.4.3.2 Misal ignment
Good a l ignment between s h a f t and bear ing can be c r i t i c a l l y impor tan t because
o f t he dramat ic reduc t i on i n o i l f i l m t h i ckness which misal ignment causes.
There a r e many ways i n which misal ignment can occur , f rom poor b u i l d o f t he
machine t o mechanical d i s t o r t i o n s due t o l oad o r temperature, and each o f them
produces d i f f e r e n t c o n d i t i o n s w i t h i n the o i l f i l m . However, a genera l guide t o
t h e d e r a t i n g e f f e c t o f misal ignment i s g iven i n Fig.14. Th is shows the re -
d u c t i o n i n o i l f i l m t h i ckness from the p e r f e c t l y a l i g n e d case (Fig.13) f o r a
g i ven misal ignment across the l e n g t h o f t he bear ing (6). The r e s u l t i n g minimum
f i l m th ickness , which occurs a t one a x i a l end o f t he bear ing , shou ld be checked
on Fig.2 f o r a c c e p t a b i l i t y .
4.4.3.3 Power Loss and O i l Flow
Power l o s s and o i l f l o w may be determined f rom Figs.15 and 16. The method
o f use i s shown on each c h a r t and i s f u r t h e r i l l u s t r a t e d by the example i n
sec t i on 4.6.The f l o w g i ven i s the bear ing requirement f l o w , and should be
68
Fig
.13
P
red
ictio
n o
f min
imu
m o
il film th
ick
ne
ss
L‘O
2’0
E‘O
7’0
so
9’0
L.
5 E
69
Fia.14 The d e r a t i n g e f f e c t o f misal ignment on f i l m th ickness
increased when determin ing pump capac i t y t o a l l o w f o r bea r ing wear, unequal f l o w
t o m u l t i - s u p p l i e d bear ings e tc . T y p i c a l l y the f i g u r e s should be increased by
20-25%, and a pump chosen which can adequately supply t h i s amount.
4.4.3.4 Temperatures
The va lue o f t he o i l o u t l e t temperature may be est imated f rom the power l oss
and o i l f l o w values, as read f rom the cha r t s , us ing the equat ion: -
Ou t le t temperature = i n l e t temperature + temperature r i s e - - 50 + A H/Q ( " C )
where A = 0 . 0 0 0 5 f o r H kW and Q m3/s
A = 5 (A = 6 f o r hp and US gal /min)
f o r H hp and Q gal /min.
The maximum bear ing temperature may be determined f rom Fig.17; again an
example o f use i s shown on the c h a r t and i s f u r t h e r a m p l i f i e d i n sec t i on 4.6.
I t should be noted t h a t t h i s temperature occurs a t t he sur face o f t he l i n i n g
ma te r ia l and a t some c i r c u m f e r e n t i a l p o s i t i o n which i s n o t w e l l def ined; i t
roughly co inc ides w i t h the p o s i t i o n o f t he minimum o i l f i l m th ickness.
I 0
70 : 1w:
200.
UUD IN1 1U1
POWER LOSS
Fig.15 P r e d i c t i o n of bear ing power loss
OM (mn 500 400 3w
2w /
7000
10000
20000
30000
L FLOW REQUIREMENTS
F ig .16 P r e d i c t i o n of bear ing o i l f l o w requirement
71
Fig .17
I here - - I
P r e d i c t i o n of maximum bear ing temperature
72
Unless a great deal o f thought and care i s taken i n determining the
p o s i t i o n o f temperature instrumentation then the recorded temperature w i l l be
less than the actual maximum value.
4.5 H I G H SPEED APPLICATIONS
The general t rend towards l a rge r sizes and f a s t e r operat ing speeds i n equip-
ment such as compressors and turb ines has caused d i f f i c u l t i e s w i t h both the
design and the operat ion o f conventional c y l i n d r i c a l bore bearings. The design
d i f f i c u l t y occurs when the o i l f l ow w i t h i n the bearing clearance space becomes
non-laminar, and the information presented so f a r ceases t o be v a l i d . The
operational d i f f i c u l t i e s concern poss ib le i n s t a b i l i t y o f e i t h e r the o i l f i l m
(see Sect ion 4.2.1.4) or of the complete r o t o r system.
4.5.1 Non-Laminar Operation
Co l l oqu ia l l y i t i s common t o r e f e r t o e i t h e r laminar o r t u rbu len t operat ion,
but i n p rac t i ce there i s a wide operat ing band separating the two regimes i n
which other forms o f f l u i d motion, eg Taylor vo r t i ces , occur,
methods d e t a i l e d i n the previous sections are f o r laminar operation on ly i t i s
necessary t o have some check on whether laminar o r non-laminar condi t ions apply.
Fig.18 i s a quick way o f performing t h i s check; note tha t i t uses the value o f
f i l m thickness r a t i o (hmin/cr) obtained from the laminar cha r t Fig.13. I f non-
laminar operation i s predic ted then recourse must be made t o a l t e r n a t i v e design
methods and procedures [ 1 5 ] .
Since the design
It should be emphasised t h a t turbulence per se i s not a problem as f a r as
safe bearing operation i s concerned, merely a d i f f i c u l t y ( a l b e i t a r e l a t i v e l y
major one) w i t h the design process. Turbulence i s increased by h igh speeds,
large clearances o r t h i n o i l s . Bearings operat ing i n the turbulent regime can
have s i g n i f i c a n t l y higher power losses and temperatures than predic ted by
I ami nar theory.
4.5.2 P r o f i l e Bore Bearings
A c y l i n d r i c a l bore bearing has a s ing le converging clearance space (which
may i n some circumstances be i n te r rup ted by a groove) i n which o i l f i l m pressure
i s generated t o support the external load.
bearing i s arranged so t h a t two o r more separate converging regions are present
under normal operation, each developing f i l m pressures which a c t i n various
d i r e c t i o n s around the bearing. The s t a b i l i t y o f such a bearing i s b e t t e r than a
c y l i n d r i c a l bore, but there are usua l l y pena l t i es t o be pa id i n terms o f reduced
load capaci ty o r increased power loss and lub r i can t f l ow requirement.
In contrast the bore o f a p r o f i l e
73
Fig.18 Guidance on the 1 i k e l ihood o f laminar o r non- laminar ope ra t i on
The range o f poss b l e p r o f i l e s i s almost i n f i n i t e bu t t h e more common forms
a re shown i n Fig.19, t oge the r w i t h an i n d i c a t i o n o f t h e i r r e l a t i v e m e r i t s i n
terms o f va r ious opera t i ng parameters.
approximate o n l y , s ince minor changes t o e i t h e r o f the two c learances invo lved
i n p r o f i l e bore bear ings ( t h e so c a l l e d 'shake c lea rance ' which i s a measure o f
p o s s i b l e sha f t movement and the hydrodynamic c learance which i s the d i f f e r e n c e
between r a d i i o f i n d i v i d u a l lobes and the s h a f t ) can s i g n i f i c a n t l y a l t e r per -
formance. Exper ience has shown t h a t as a general r u l e the 'shake c lea rance ' ,
as a r a t i o ( i e cd /d) , should be no t i g h t e r than 0.001 mm/mrn.
p o s s i b l e problems w i t h l oss o f c learance on s t a r t - u p due t o d i f f e r e n t i a l
O f necess i t y these comparisons a re
Th is then avo ids
74
thermal expansion o f the s h a f t r e l a t i v e
In fo rma t ion on s t i f f n e s s and damping
dynamics o f t he machine as a whole, w i t h
speeds o f t he r o t o r , and damping c o n t r o l
running through the c r i t i c a l s .
o the bear ing and housing.
s impor tan t when cons ide r ing the
s t i f f n e s s i n f l u e n c i n g the c r i t i c a l
ing the v i b r a t i o n ampl i tudes when
In o rde r t o g i v e a f e e l f o r i n s t a b i l i t y t h resho ld speeds f o r r e a l i s t i c
c learances, Fig.20, has been prepared. Th is shows the minimum speed a t which
o i l f i l m i n s t a b i l i t y can occur f o r a h o r i z o n t a l , we igh t loaded, r o t o r system
( i e where the bear ing load i s p u r e l y due t o the mass).
Fig.19 Comparison o f s t a t i c and dynamic c h a r a c t e r i s t i c s o f commonly used j o u r n a l bear ing types
15
Comprehensive and d e t a i l e d procedures f o r t h e d e s i g n o f p r o f i l e b o r e
b e a r i n g s have n o t been p u b l i s h e d , and c u r r e n t l y t h e d e s i g n e r must e i t h e r r e s o r t
t o a fundamenta l t h e o r e t i c a l s t u d y o r seek a d v i c e , u s u a l l y on a s p e c i f i c case
bas is , f r o m s p e c i a l i s t s [16 ] .
LOO.
- 300- -
W LL 0 m
4 200-
9 - LL
m
100-
OIL: IS0 VG L6 @, 50' INLET 1 MEDIUM TURBINE GRADE 1
b/d : 0.5
CLEARANCE VARIED
TO MAINTAIN : DRAIN TEMPERATURE < 75'C
MATERIAL TEMPERATURE < 100°C
LEMON BORE
0 10000 20000 30000 LOO00
F
4.6
4.6.
JOURNAL SPEED I revlmin 1
9.20 O i l f i l m w h i r l i n s t a b i l i t y o f v a r i o u s t y p e s o f j o u r n a l b e a r i n g s - L i m i t i n g speeds for h o r i z o n t a l r o t o r s
EXAMPLE OF USE OF DESIGN AIDS
Problem
A gearbox b e a r i n g 200 mm d i a m e t e r and 50 mm l o n g has t o c a r r y a l o a d o f
20 kN a t 3600 rev /min . The b e a r i n g has two a x i a l g rooves e x t e n d i n g 0.8 o f t h e
b e a r i n g l e n g t h , s i t u a t e d a t ? 90" t o t h e l o a d l i n e . The o i l i s w i t h i n t h e IS0
VG 32 s p e c i f i c a t i o n ( a c t u a l t y p i c a l v i s c o s i t y v a l u e s a r e g i v e n as 30 c S t a t
40°C and 5.2 c S t a t l O O " C , t h e o i l d e n s i t y b e i n g 880 kg/m3). The o i l f e e d
tempera ture i s 50°C and t h e o i l f e e d p r e s s u r e 1 b a r (0.1 MPa).
Check t h a t t h i s b e a r i n g w i l l o p e r a t e s a f e l y and e s t i m a t e o i l f i l m t h i c k n e s s ,
power loss , o i l f l o w r e q u i r e m e n t , maximum b e a r i n g t e m p e r a t u r e and o i l o u t l e t
tempera ture .
76
4.6.2 P rocedure
T h i s p a r t i c u l a r example has been i n c o r p o r a t e d on t h e d e s i g n a i d s a s dashed
1 i nes .
4.6.2.1 D i a m e t r a l C lea rance
From F i g . 1 1 , t h e minimum d i a m e t r a l c l e a r a n c e , f o r a 200 mm d i a m e t e r b e a r i n g
o p e r a t i n g a t 3600 rev /m in , i s 0.26 mm.
4.6.2.2 Maximum S p e c i f i c Load R a t i n g
From F ig .12 , t h e maximum s p e c i f i c l o a d r a t i n g (based on t h i n o i l f i l m
l i m i t ) , i s o b t a i n e d by f o l l o w i n g t h e g u i d e l i n e s , t h r o u g h t h e r e l e v a n t o i l g rade ,
l e n g t h , speed and d i a m e t e r . The a l l o w a b l e l i m i t i s seen t o be 4.3 MPa, w e l l
above t h e a c t u a l s p e c i f i c l o a d o f 2 MPa.
4.6.2.3 Region o f Sa fe O p e r a t i o n
Use F i g . l O ( a ) , t r a n s p a r e n t copy , w i t h F i g . l O ( b ) , b a c k i n g s h e e t . Mark p o i n t
on g r i d , F i g . l O ( a ) , c o r r e s p o n d i n g t o o i l grade (30 c S t a t 40°C) and d i a m e t e r
(200 mm). P l a c e t r a n s p a r e n c y on b a c k i n g s h e e t , l i n i n g up marked p o i n t w i t h
datum p o i n t (see F i g . 2 l ( a ) ) .
equa l t o 0 .25 , can t h e n e a s i l y be i n t e r p o l a t e d . The t e m p e r a t u r e l i m i t t r a n s -
pa rency , F i g . l O ( b ) , is p o s i t i o n e d i n an i d e n t i c a l manner (see F i g . 2 1 ( b ) ) .
The l i m i t i n g l i n e f o r s a f e o p e r a t i o n , w i t h b / d
BACKING SHEET TRANSPARE,NT OVERLAY
r------ --1
I I I I I I I < 1 ; i
' b" /
o'/\ THIN /' FILM LIMIT
__- - - - -
SPEED
I I I I I I I I I
J
I
$ 1
I HIGH A\ I TEMPERATURE
LIMITS
SI
I I I I I J
SPEED I
0
0 1 a
DESIGN POINT IS WITHIN LIMITS
\
3600 rei/rnin
I SPEED
( a 1 ( C )
F ig .21 Example use of s l i d e c h a r t s
These two combined g i v e gu idance t o a r e g i o n o f s a f e o p e r a t i o n on a l o a d ,
speed f ramework a s shown i n F i g . Z l ( c ) . The a c t u a l o p e r a t i n g c o n d i t i o n s a r e
w e l l w i t h i n t h i s l i m i t i n g r e g i o n , i n d i c a t i n g a r e l i a b l e d e s i g n .
4.6.2.4 P r e d i c t i o n o f Minimum O i l F i l m Th ickness
F ig .13 p r e d i c t s minimum o i l f i l m t h i c k n e s s and i s v e r y easy t o use, a l t h o u g h
l o o k i n g f a i r l y f o r m i d a b l e . F i r s t mark p o i n t s on t h e f o l l o w i n g t h r e e g r i d s .
1) C lea rance r a t i o and speed (0.0013 and 3600 r e v / m i n )
2 ) l e n g t h t o d i a m e t e r r a t i o and o i l
3) s p e c i f i c l o a d and o i l (2 MPa and 30 c S t a t 40°C)
(0 .25 and 30 c S t a t 40°C)
J o i n t h e s e p o i n t s a s shown, f o l l o w i n g t h e g u i d e l i n e s , and c o n t i n u e t o t h e
answer g r i d i n t h e t o p r i g h t hand c o r n e r ; a t b / d e q u a l t o 0 .25 t h e f i l m t h i c k -
ness r a t i o (hm in /c r ) i s r e a d o f f d i r e c t l y a s 0.16.
Minimum o i l f i l m t h i c k n e s s hmin = 0.16 x 0.13 = 0.021 mm
4.2.6.5 Check f o r Laminar O p e r a t i o n
F i g . 1 8 g i v e s gu idance on whe the r l a m i n a r , as p r e s e n t e d h e r e , o r non - lam ina r
d e s i g n methods [ l 5 ] s h o u l d be used.
I n c o n s i s t e n t u n i t s d = 0.2rn, N = 60 r e v / s , t i = 880 kg/m3,
P = 2.106 N/m2 and c i d = G . 0 0 1 3 - f r o m w h i c h Y = 48 .74 . d
I n F i g . 1 8 mark p o i n t where Y = g= 1.76 and b / d = 0.25 (on l ower g r i d ) .
P r o j e c t v e r t i c a l l y upwards a s i n d i c a t e d by dashed l i n e t o f i l m t h i c k n e s s r a t i o
o f 0.16. The r e s u l t i n g p o i n t appears i n t h e l a m i n a r r e g i o n , t h e r e f o r e t h e
d e s i g n a i d s p r e s e n t e d h e r e a r e s u i t a b l e .
4.6.2.6 P r e d i c t i o n o f Power Loss
On F ig .15 , u s i n g t h e v a l u e o f hmin /c r a l r e a d y o b t a i n e d ( 0 . 1 6 ) , j o i n up t h e
a p p r o p r i a t e p o i n t s on t h e s c a l e s a s shown.
Power loss = 5.3 kW
4.6.2.7 P r e d i c t i o n o f O i l F low
On F i g . 1 6 , s t a r t i n g a t t h e speed-c lea rance r a t i o g r i d , j o i n up t h e approp -
r i a t e p o i n t s a s shown.
O i l f l o w r e q u i r e m e n t = 0.00021 m 3 / s (0 .21 1 / 5 1
78
4.6.2.8 P r e d i c t i o n o f Maximum Bearing Temperature
From Fig.17, t he maximum bear ing temperature i s ob ta ined by f o l l o w i n g the
numbered steps s h o w n ~ i n the sketch i n the lower l e f t hand corner . l o 3
Ca 1 cu 1 a t e rd ] t (=o.oao47)
M u l t i p l y t h i s by the m o d i f i e d s p e c i f i c l oad P (=0.136)
S t a r t i n g a t hmin/cr equal t o 0.16 f o l l o w the gu ide l i n e s as shown, through
values o f b/d r a t i o , o i l grade, and the two terms c a l c u l a t e d above. A va lue of
e f f e c t i v e temperature ( O e ) , can be ob ta ined a t the end o f s tep 3 (=65"C), and a
va lue o f emax/ee (=1.55) f rom s tep 5 (which completes the pentagon).
Maximum temperature = l0 l 'C
4.6.2.9 P r e d i c t i o n o f O i l O u t l e t Temperature
From equat ion (1) - Sect ion 4 4.1.4.
H O i l o u t l e t temperature = 50 + 0.0005 - Q
0.0005 x 5.3 0.00021
= 50 +
= 63°C
REFERENCES
1
2
3
4 5
6
7 8
9
10
11
Elwel1,R.C. and Booser,E.R., 'Low Speed L i m i t o f L u b r i c a t i o n P a r t 1, What i s a "Too Slow" Bear ing? ' , Machine Design, 15 June, 1972. Martin,F.A., 'Minimum Al lowab le O i l F i l m Thickness i n S t e a d i l y Loaded Journal Bear ings ' , Proc. L u b r i c a t i o n and Wear Convention 1964 ( I n s t . Mech. Engrs. London), V01.178, P t . 3 N pp. 161-167. Booser,E.R., Ryman,F.D. and Linkinhoker,C.L. , 'Maximum Temperature f o r Hydrodynamic Bear ings under Steady Load ' , ASLE Trans. 1970, Vol.26, No.7. T r ibo logy Handbook, 1973, Sec t ion B1, But te rwor ths . Lund,J.W., ' S e l f - e x c i t e d S t a t i o n a r y Wh i r l O r b i t s o f a Journal i n a Sleeve Bear ing ' , Thesis, 1966, Rensselaer Po ly techn ic I n s t i t u t e , NY. Pinkus,O. and S te rn l i ch t ,B . , 'Theory o f Hydrodynamic L u b r i c a t i o n ' , 1961, Chapter 4, McGraw-Hi 11. Cameron ,A. ' P r i n c i p l e s o f L u b r i c a t i o n I , 1966, Chapter 13, Longmans. Lund,J.W. and Thomsen,K.K., 'A C a l c u l a t i o n Method and Data f o r t h e Dynamic C o e f f i c i e n t s o f O i l - L u b r i c a t e d Journa l Bear inas ' f rom 'TODiCS i n F l u i d F i l m Bear ings and Rotor Bear ing System Designland O p t i m i z a t i o n ' , 1978, ASME. ' C a l c u l a t i o n Methods f o r S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal Bear ings ' , Engineer ing Sciences Data U n i t I tem 66023, Sept. 1966 ( I n s t . Mech. Engrs., London). 'Computer Serv ice f o r P r e d i c t i o n o f Performance o f S t e a d i l y Loaded Pressure Fed Hydrodynamic Journal Bear ings ' , Engineer ing Sciences Data U n i t , I tem 69002, Sept. 1972, Amendment B. Martin,F.A. and Garner,D.R., ' P l a i n Journal Bear ings under Steady Loads Design Guidance f o r Safe Opera t i on ' , F i r s t European T r ibo logy Congress, 1973 ( I n s t . Mech. Engrs., London) paper C313/73.
79
12 Blok,H., D iscuss ion t o Conference. Proc. Conf. L u b r i c a t i o n and \.!ear,
13 Moes,H. and Bosma,R., 'Design Charts f o r Optimum Bearing Con f igu ra t i on 1 . 1957 ( I n s t . Mech. Engrs., London), pp. 745-747.
The F u l l Journal Bear ing ' A p r i l 1971, ASME Trans. Vol.93, Ser ies F, No.2, pp. 302-306.
Development', Proc. Conference on L u b r i c a t i o n and Wear, 1957 ( I n s t . Mech. Engrs), pp. 20-27.
Charts f o r Performance P r e d i c t i o n ' , J u l y 1977, ASLE Trans. V01.20, No.3, pp. 221-232.
16 Garner,O.R., Lee,C.S, and Martin,F.A., ' S t a b i l i t y o f P r o f i l e Bore Bear ings: I n f l uence o f Bear ing Type S e l e c t i o n ' , October 1980, T r ibo logy I n t e r n a t i o n a l , pp. 204-210.
14 Brown,T.W.F. and Newman,A.O., 'High-speed H igh ly Loaded Bear ings and The i r
15 Garner,D.R., Jone5,G.J. and Martin,F.A. 'Tu rbu len t Journal Bear ings Design
80
5 THE DIAGNOSIS OF PLAIN BEARING FAILURES
R . W . WILSON and E . B . SHONE
S h e l l Research L t d . , T h o r n t o n Research C e n t r e , P . O . Box 1, Ches te r CH1 3SH
5 . 1 I NTRODUCT I ON
A few y e a r s ago t h e casua l o b s e r v e r m i a h t have c o n c l u d e d t h a t b o t h t h e
m e t a l l u r g y o f b e a r i n g s and t h e u n d e r s t a n d i n g o f t h e i r f a i l u r e mechanisms had
reached a s e t t l e d s t a g e i n wh ich b e a r i n g m a t e r i a l s were a v a i l a b l e t o meet a l m o s t
e v e r y r e q u i r e m e n t and t h a t c h a r a c t e r i s t i c f a i l u r e s were we l l -documen ted . Thus,
most b e a r i n g f a i l u r e s c o u l d be d e s c r i b e d q u i t e a d e q u a t e l y f r o m a s t r i c t l y
m e t a l l u r g i c a l o r s t r i c t l y e n g i n e e r i n s v i e w p o i n t . The l u b r i c a n t was men t ioned
o n l y i n p a s s i n g ; as l o n g as i t had t h e r i q h t v i s c o s i t y , p r o v i d e d adequa te
c o o l i n g and d i d n o t c o r r o d e t h e b e a r i n o a l l o y , i t gave l i t t l e cause f o r conce rn .
O p e r a t o r s f r e q u e n t l y blamed l u b r i c a n t s f o r b e a r i n q f a i l u r e s , b u t b e a r i n g man-
u f a c t u r e r s acknowledged t h a t v e r y few f a i l u r e s c o u l d be a t t r i b u t e d t o i n f e r i o r
l u b r i c a n t s . There was no s p e c i a l p r e s s u r e o n l u b r i c a t i o n t e c h n o l o q i s t s t o
become e x p e r t s o n b e a r i n g f a i l u r e s ; i n any case , b e a r i n g m a n u f a c t u r e r s g e n e r a l l y
p r o v i d e d e x c e l l e n t t e c h n i c a l s e r v i c e t o t h e i r cus tomers . However, e n g i n e e r i n g
developments i n r e c e n t y e a r s have made i n c r e a s i n g l y seve re demands on p l a i n
b e a r i n g s and have g i v e n r i s e t o r a t h e r more c o m p l i c a t e d f a i l u r e mechanisms.
Many o f t hese f a i l u r e s can be d e s c r i b e d as t r i b o l o g i c a l f a i l u r e s ; t h e y canno t
be a s c r i b e d t o a p a r t i c u l a r d e f e c t i n t h e d e s i g n o r i n t h e m e t a l l u r g y o f t h e
b e a r i n g or t o a s h o r t c o m i n g i n t h e l u b r i c a n t - t h e y a r e t h e consequence of t h e
i n t e r a c t i o n o f a number o f f a c t o r s .
B e f o r e i t i s p o s s i b l e t o t a l k a b o u t p l a i n b e a r i n g f a i l u r e s i t i s e s s e n t i a l t o
have an u n d e r s t a n d i n g o f t h e m a t e r i a l s used, how t h e y a r e made and t h e r e q u i r e -
ments t h a t must be met. There a r e a number o f v e r y good p u b l i c a t i o n s d e s c r i b i n g
t h e c o n s t r u c t i o n and p r o p e r t i e s of p l a i n b e a r i n g s and we i n t e n d t o b r i e f l y
summarise these .
5 . 2 PROPERTIES R E Q U I R E D OF BEARING MATERIALS
I t i s e v i d e n t t h a t a b e a r i n g must w i t h s t a n d a v a r i e t y o f imposed c o n d i t i o n s .
No b e a r i n g i s e q u a l l y good w i t h r e s p e c t t o a l l r e q u i r e m e n t s , and t h e s e l e c t i o n
o f t h e most s u i t a b l e b e a r i n g m a t e r i a l f o r a p a r t i c u l a r s e t o f c i r c u m s t a n c e s
r e q u i r e s a c a r e f u l e v a l u a t i o n o f t h e most c r i t i c a l o p e r a t i n g f a c t o r s . Some o f
t h e f a c t o r s t h a t have t o be c o n s i d e r e d a r e o u t l i n e d i n S e c t i o n s 5.2.1 - 5.2.8.
81
5.2.1 Fatigue Resistance
This is the most important property in those applications where the load
varies. However, fatigue failure in bearings is frequently associated with some
other factor, such as corrosion, which reduces the strength of the bearing, or
wear, which reduces the load-carrying area. The fatigue strength of a fund-
amentally weak bearing alloy can be increased by making the bearing alloy thin
and bonding i t firmly to a bronze or steel backing.
5 . 2 . 2 Compressive Strength
This is the steady load that the bearing alloy can support without extruding.
There is little correlation between compressive strength and fatigue resistance
when a single class of bearing material is considered.
5.2.3 Conformability
This is the ability to compensate for misalignment that occurs as a con-
sequence of bad design or manufacture or that may develop in service.
5.2.4 Embeddabil ity
This i s the ability to tolerate and absorb foreign particles, thereby
avoiding scoring or wear.
5 . 2 . 5 Strength at Elevated Temperatures
High sliding speeds and heavy loads can generate considerable heat, even when
a bearing is operating hydrodynamically. One of the main functions of the lub-
ricant and the bearing alloy i s to conduct heat away from the sliding surfaces;
even so, bearings are often required to operate at elevated temperatures, and a
lack of high-temperature strength may result in extrusion of the bearing alloy
and/or fatigue failure.
5.2.6 Compa t i b i 1 i ty
All bearing assemblies experience some metal-to-metal contact at some stage
in their lives; the resistance of the bearing metal/journal combination to
seizure is therefore important. However some of the harder bearing alloys do
not function satisfactorily against unhardened steel journals.
5.2.7 Corrosion Resistance
Bearings may be exposed to weak organic acids formed as a result of the
oxidation of lubricating oils in service. They may also be subject to corrosion
by weak organic acids and strong mineral acids derived from fuel combustion pro-
ducts. Sometimes the operating environment is corrosive - for example. on many
chemical plants and marine installations.
a2
5.2.8 c o s t I t i s e s s e n t i a l t h a t bear ing a l l o y s a re cheap and easy t o manufacture.
5.3 TYPE, CONSTRUCTION AN0 CHARACTERISTICS OF PLAIN BEARING MATERIALS
Some o f the more w ide ly used bear ing a l l o y s a r e l i s t e d i n Table 5.1; t he
more commonly used ove r lays a r e descr ibed i n Table 5.2.
Sect ions 5.3.1 - 5.3.7 o u t l i n e the methods by which bear ings a r e u s u a l l y
manufactured f rom these a l l o y s and desc r ibe the c h a r a c t e r i s t i c s o f t he va r ious
bear ing m a t e r i a l s . The c h a r a c t e r i s t i c s o f the va r ious bear ing m a t e r i a l s a r e
summarised i n Table 5.3 and t h e genera l p r o p e r t i e s b r i e f l y discussed i n
Sect ion 5.3.8.
5.3.1 White Meta ls (Babb i ts )
These a r e t in -based o r lead-based a l l o y s , t he o r i g i n a l t in -based w h i t e metal
being invented by Isaac B a b b i t t i n 1839. Th is a l l o y i s w ide l y used today f o r
the manufacture o f bo th t h i c k , l a rge , i n d i v i d u a l l y manufactured bear ings and
th in -wa l l ed bear ings made f rom cont inuous s t r i p .
The coated s t r i p i s c u t i n t o sec t i ons o f a s u i t a b l e s i z e which a re press-
formed t o shape. Newer manufac tur ing techniques pe rm i t t he use of bea r ing
metal combinat ions and c o n t r o l l e d m i c r o s t r u c t u r e s t h a t cou ld n o t o the rw ise be
produced.
Another i nnova t ion i s t he three-component ( o r t r i - m e t a l ) bear ing , i n which
one o f t he harder bear ing m a t e r i a l s such as bronze, a l ready bonded t o a s t e e l
backing, i s covered w i t h a t h i n (20-200 urn), e lec t rodepos i ted o r c a s t o v e r l a y
o f wh i te meta l .
The t in -based a l l o y c o n s i s t s of a t i n - r i c h m a t r i x , w i t h some antimony and
copper i n s o l i d s o l u t i o n ; d ispersed i n the m a t r i x a r e cuboids o f SbSn and
needles o f Cu6Sn5.
bear ing a l l o y s must c o n s i s t o f hard c r y s t a l s d ispersed i n a s o f t m a t r i x . In
f a c t , t he i n t e r m e t a l l i c compounds i n w h i t e meta ls appear t o serve no purpose
o t h e r than t o s t rengthen the a l l o y s .
Th is m i c r o s t r u c t u r e l e d t o the erroneous b e l i e f t h a t
White meta ls a re o u t s t a n d i n g l y good bear ing a l l o y s i n many respec ts , t h e i r
main de fec t be ing l ack o f l oad -ca r ry ing capac i t y , p a r t i c u l a r l y a t e leva ted
temperatures. The c o r r o s i o n res i s tance o f lead-based w h i t e meta ls i s i n f e r i o r
t o t h a t o f t in -based w h i t e metals; never the less , lead-based a l l o y s a r e q u i t e
w ide ly used, p a r t i c u l a r l y i n the U.S.A.
TABLE 5.1
Some l u b r i c a t e d bear ing ma te r ia l s
Bear ing ma te r ia l Major a l l o y i n a elements Remainder Spec i a 1 f ea tu res
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Lead-based wh i te metal ( lead-Babb i t t )
Tin-based wh i te metal ( t i n Babb i t t )
S in te red copper- lead
Cast copper- lead
Lead-bronze
Aluminium-low t i n
Aluminium-high t i n
A l uminium-si 1 icon (over lay p la ted )
A l umin i um-Babb i t t
Phosphor-bronze (cas t )
S i l icon-bronze
Lead-bronzes (general l y s tee l -backed)
S i l v e r
8-16% antimony 5-11% t i n
7-14% an t imony 3- 9% copper
20-50% lead
20-30% lead
20-30% lead 3- 5% t i n
6% t i n , 1.5% s i l i c o n
20% t i n , 1.0% copper
11% s i l i c o n , 1 % copper
10% lead, 2% t i n
8-12% t i n , 0.2-1.0% phosphorous
1.5-4% s i l i c o n
5-10% t i n 8-12% lead
Porous bronze 8-12% t i n
Bronze-graphi te composites 5-40% g raph i te
Laminated res ins F ib re - re in fo rced phenol i c s o r epoxies
PTFE and f i 1 l e r i n PTFE and lead metal m a t r i x
PTFE mat r i x and metal Bronze and g raph i te f i l l e r s
Metal-backed thermo- Nylon o r po lyaceta l p l a s t i c s
Lead
T in
Copper
Copper
Copper
A l umi n i um
A l umi n i um
A l umin i um
A l umi n i um
Copper
Copper
Copper
1 1
Tin, bronze
Metal
PTFE
Genera l l y used w i t h 90% lead-10% t i n o r 96% lead-4% indium e lec t rodepos i ted over - l a y , 25 urn t h i c k ; lead bronzes can a l s o have th in -Babb i t t over lays
Hardened jou rna ls p r e f e r r e d
S t i l l under development
P r i ma r i 1 y bush i ng mate r i a 1
E lec t rop la ted , 250 -500~1 t h i c k , o f t e n used w i t h lead- indium ove r lay
S e l f - l u b r i c a t i n g , impregnated w i t h o i l
Can be used dry
Usua l ly wa te r - l ub r i ca ted
Good under cond i t i ons o f marginal l u b r i c a t i o n
W
Overlay type
TABLE 5.2
Commonly used overlays
Major alloying elements Special features ~~ ~
A lead-tin, electroplated
B Lead-tin-copper, electroplated
C Lead-indium, electroplated
D Lead-Babbitt, cast Tin-Babbitt, cast
8-12% tin
8-12% tin 1% copper
4-8% indium
Antimony-tin Antimony-copper
Simultaneously precision-plated, about 0.001 inch thick (25 urn)
Simultaneously precision-plated, about 0.001 inch thick (25 urn)
Precision-plated, first lead then indium, then alloyed by diffusion; about 0.001 inch thick (25 urn)
0.005-0.015 inch thick (125-375 thick)
E Satco alloy, cast Lead, with 2% tin and 0.5% calcium
TABLE 5.3 Charac te r i s t i cs o f bear ing rnater i a l s
Ma te r ia l (see Re la t i ve load-car ry ing Embeddabil i t y Seizure Maximum Tolerance fo r Corrosion res i s tance Spec i a 1 Table 1) capac i ty ( to le rance res is tance opera t i ng misal ignment Organic ac ids Mineral ac ids fea tures
I b / i n 2 MN/m2 f o r d i r t ) temp. " C
1 2
3 4
5 6
7 8 9
10
1 1 12
13
14
15
16
17 1 8
19
2000
2000 3000-5000
4000-6000 5000-8000
8000 5000
8000 5000
8000+
8000+
6000 8000+
2000-4000
Very v a r i a b l e
High
Comparat i v e l y poor; depends on opera t i ng cond i t i ons
14
14
20-35 28-41
35-55
55
35 55
35
55+
55+ 41
55+
14-28
Exce l l en t Very good 130 Very good
Exce l l en t Very good 130 Very good
Moderate with-Moderate w i th - 150 Good ou t over lay ; ou t over lay ; good w i t h good w i t h over I ay over I ay 170 F a i r
Poor F a i r 180 Poor
Poor Fa i r 170 Fa i r
Poor Moderate 180 Fa i r
Poor ? Moderate 160 Fa i r
Poor Moderate 220 Fa i r
160 Moderate
Poor Fa i r 220 Poor
Moderate Moderate 180 Moderate
Poor Poor 180 Poor
Moderate Good 130 Fa i r
Good Withstands F a i r occasional very h igh temp.
depending on r e s i n
Moderate Good 100-300 Moderate
F a i r Very Good 250 Good
Fa i r Very Good 250 Good
F a i r Good 100-350 Good depending on
Moderate
Excel l e n t
F a i r
Poor
Fa i r
Good
Good
Good
Poor ?
Fa i r
F a i r
Very good
Resistance t o o rgan ic ac ids g rea t 1 y improved by over lay Poor
Fa i r 1 Fa i r
Moderate
F a i r ?
Fa i r Exce l len t f o r h igh loads a t low speeds
Moderate Fa i r
Moderate Fa i r
Good except Moderate f o r sulphur
Poor Fa i r
Fa i r Fa i r
Some p l a s t i c s a re damaged by the so lvent a c t i o n of 1 ubr i can ts Good
Good Good I Good
Good Good
Good Very Good
86
5.3.2 Copper-lead A l l o y s
These can be regarded as the f i r s t o f t he modern a l l o y s . They a r e blanked
from con t inuous ly coated s t e e l s t r i p , the bear ing a l l o y be ing a p p l i e d e i t h e r by
c a s t i n g or by s i n t e r i n g . Lead and copper a re immisc ib le i n bo th the s o l i d and
the l i q u i d s t a t e , and, i n o rde r t o avo id complete segregat ion when c a s t i n g , the
moving s t e e l s t r i p i s c h i l l e d immediately a f t e r c a s t i n g by a water spray on the
underside. Th is r e s u l t s i n a pronounced d e n d r i t i c s t r u c t u r e , w i t h c r y s t a l s o f
copper and lead normal t o the s t e e l backing and the bear ing sur face .
I n the s i n t e r i n g process the copper- lead powder i s compacted a f t e r a p p l i c a t i o n
and then s in te red . Th is method o f manufacture g i v e s good c o n t r o l o f lead d i s t -
r i b u t i o n and r e s u l t s i n a homogeneous and e q u i a x i a l s t r u c t u r e .
Both types o f copper- lead bear ing a r e s a i d t o r e l y on a t h i n , ex t ruded
sur face f i l m o f lead f o r s a t i s f a c t o r y performance. In many ways they a r e
s i m i l a r , bu t the d i f f e r e n t methods o f manufacture g i v e r i s e t o impor tan t d i f f -
erences i n performance i n c e r t a i n respec ts . I n the cas t a l l o y the l ong copper
dendr i t es enhance the l oad -ca r ry ing c a p a c i t y and f a c i l i t a t e heat f l o w away from
the bear ing surface. I t i s , however, d i f f i c u l t t o o b t a i n a s a t i s f a c t o r y l ead
d i s t r i b u t i o n . On the o t h e r hand, i t i s much e a s i e r t o ensure a un i fo rm lead
d i s t r i b u t i o n by s i n t e r i n g techniques, and the s i n t e r e d a l l o y i s t h e r e f o r e less
suscep t ib le t o co r ros ion .
Copper-lead bear ings have g r e a t e r s t r e n g t h and b e t t e r h igh- tempera ture
performance than w h i t e meta ls , bu t i n most o t h e r respec ts they a r e i n f e r i o r .
I n p a r t i c u l a r , they a r e suscep t ib le t o co r ros ion , cannot t o l e r a t e as much d i r t
and g e n e r a l l y r e q u i r e hardened j o u r n a l s .
vantages, copper- lead bear ings a re g e n e r a l l y ove r lay -p la ted , a t h i n (% 25 urn)
e l e c t r o d e p o s i t o f a 90% lead - 1 0 % t i n o r a 95% lead - 5% indium a l l o y be ing
app l i ed t o the bear ing sur face . I n the case o f l e a d - t i n , t he a l l o y i s co-
p la ted ; i n t h e case o f lead- indium, the l ead i s p l a t e d f i r s t and then t h e
indium, which i s f i n a l l y d i f f u s e d i n t o the lead. I t i s o f t e n s t a t e d t h a t t h i s
ove r lay i s merely a runn ing - in a i d . Th is i s n o t t he case; the o v e r l a y i s
expected t o l a s t the l i f e o f t h e bear ing . I t p rov ides a s e i z u r e - r e s i s t a n t
surface, a l l ows s o f t s h a f t s t o be used, increases the a b i l i t y o f the bear ing t o
absorb d i r t and combats c o r r o s i o n o f t he pure lead i n the under l y ing copper-
lead. Since i t i s so t h i n i t de r i ves cons ide rab le suppor t f rom the under l y ing
bear ing a l l o y , and the f a t i g u e s t r e n g t h o f t he composi te i s h a r d l y impaired.
I n o rde r t o overcome these d i sad -
5.3.3 Lead Bronzes
The h igh- tempera ture performance and load -ca r ry ing c a p a c i t y o f copper- lead
a l l o y s can be improved by t i n a d d i t i o n s . The t i n d i sso l ves comple te ly i n t h e
copper, thereby s t rengthen ing the bear ing ; t he l ead remains una l l oyed and
sub jec t t o co r ros ion . L i k e copper- lead bear ings , lead-bronze bear ings can be
cas t o r s in te red , and show the same disadvantages. For t h i s reason they a r e
usua l l y ove r lay -p la ted .
o f t in-based wh i te metal cas t on them; such bear ings a r e sometimes c a l l e d
mic ro-Babb i t t o r t r i - m e t a l bear ings .
La rge r -s i ze bear ings may have a t h i n l a y e r (100-200 pin)
This mic ro-Babb i t t l aye r should always be separated from the lead-bronze by
a d i f f u s i o n b a r r i e r , o therw ise the bronze w i l l a l l o y w i t h the t i n - r i c h B a b b i t t
t o form a hard, i n t e r m e t a l l i c phase which can damage the s h a f t .
Lead-bronzes a r e o f t e n used f o r l i t t l e - e n d bushes i n p i s t o n s f o r t u rbo -
charger bear ings.
5.3.4 Aluminium A l l o y s
Two a l l o y s a r e i n widespread use, one c o n t a i n i n g about 6% t i n and the o t h e r
20% t i n .
unless i t i s ove r lay -p la ted , i t requ i res hardened j o u r n a l s i f wear i s t o be
kept w i t h i n acceptab le l i m i t s . The 20% t i n a l l o y i s a more recent development
and i t s method o f manufacture prov ides a good example o f t he advanced techno l -
ogy a p p l i e d t o bea r ing manufacture.
The 6% t i n a l l o y may be used i n massive fo rm o r bonded t o s t e e l , b u t ,
H i g h - t i n aluminium a l l o y s , as c a s t , have ve ry poor mechanical p r o p e r t i e s ,
because the t i n forms a cont inuous network enc los ing the pr imary aluminium
c r y s t a l s . By co ld -work ing and low-temperature heat t rea tment t h i s cont inuous
phase can be broken up t o produce an i n t e r l o c k i n g network s t r u c t u r e , t he so-
c a l l e d r e t i c u l a r s t r u c t u r e which has g r e a t l y improved mechanical p r o p e r t i e s .
However, the a l l o y i s s t i l l i n s t r i p form and must be bonded t o a t h i n , s t e e l
s t r i p backing. Th is i s achieved by a cont inuous pressure-we ld ing opera t i on
c a r r i e d o u t between r o l l e r s , t he bonding between the two s t r i p s be ing promoted
by a very t h i n sheet o f pure aluminium.
Th is type o f bea r ing has a h ighe r l oad -ca r ry ing c a p a c i t y than copper- lead
a l l o y s and y e t can be used i n con junc t i on w i t h s o f t j o u r n a l s . I t s t o le rance
f o r d i r t i s n o t good and i t i s o c c a s i o n a l l y ove r lay -p la ted .
The l a t e s t development i n aluminium bear ings i s t he a lumin ium-Babb i t t a l l o y .
A t l e a s t t h ree s u p p l i e r s a r e known t o be a c t i v e i n t h i s f i e l d , each us ing a
d i s t i n c t l y d i f f e r e n t manufac tur ing technique. I n one case the method o f manuf-
ac tu re i s n o t u n l i k e t h a t used f o r the 20% t i n -80% aluminium bear ings . I n
another, a cont inuous s i n t e r i n g process i s used. The B a b b i t t a l l o y i s about
90% lead-10% t i n , and s ince t i n i s o n l y s p a r i n g l y s o l u b l e i n aluminium, the t i n
remains i n s o l u t i o n i n the lead. Th is means t h a t the l e a d - r i c h phase ( the
B a b b i t t ) should remain c o r r o s i o n - r e s i s t a n t - i n c o n t r a s t t o the s i t u a t i o n w i t h
copper- lead and lead-bronze a l l o y s .
88
5.3.5 Phosphor and S i l i c o n Bronzes
These a l l o y s a re cas t as e i t h e r i n d i v i d u a l bear ings o r tubes from which
bear ings can be machined. I n recent years, c e n t r i f u g a l and semi-continuous
cas t i ng procedures have been used. Ma te r ia l s o f t h i s type a re used main ly f o r
bushes, p a r t i c u l a r l y l i t t l e - e n d bushes, where heavy loads and h i g h temperatures
a re encountered. The presence o f phosphorus a t about the 0.5% l e v e l has a
marked e f f e c t on res i s tance t o pounding, i.e. the a l l o y has outs tanding res-
istance t o wear when sub jec t t o heavy loads a t low s l i d i n g speeds.
5.3.6 S i l v e r
S i l v e r bear ings, sometimes p l a i n , sometimes w i t h a th in ove r lay of lead-
indium, have been adopted by the a i r c r a f t i ndus t r y and a re used on one w e l l -
known make o f d iese l engine. The s i l v e r i s e lec t rodepos i ted on a s t e e l backing
and i s about 0.5 mm t h i c k . E lect rodeposi ted s i l v e r i s much harder than c a s t
s i l v e r , and s i l v e r bear ings a re unequal led w i t h respect t o l oad -ca r ry ing cap-
a c i t y and f a t i g u e res is tance. Un fo r tuna te l y , they a r e prone t o se i zu re and a re
very s e n s i t i v e t o the na tu re o f the l u b r i c a n t and t o c e r t a i n l u b r i c a t i n g - o i l
add i t i ves .
5.3.7 Porous and S e l f - L u b r i c a t i n g Bearings
Most porous bear ings c o n s i s t o f s i n t e r e d bronze o r i r o n powders w i t h i n t e r -
connect ing pores. These pores may take up 10-30% o f the t o t a l volume and, i n
operat ion, l u b r i c a t i n g o i l i s s to red i n them and i s subsequently fed t o the
bear ing surface. Any o i l escaping from the loaded zone i s reabsorbed by cap-
i l l a r y ac t i on . A t y p i c a l bronze con ta ins 90% copper and 10% t in ; 1-4% g r a p h i t e
may be added t o the mix t o enhance the s e l f - l u b r i c a t i n g p r o p e r t i e s . High
p o r o s i t y and h igh l u b r i c a t i n g - o i l content a r e requ i red f o r high-speed, l i g h t -
load app l i ca t i ons , whereas a low-poros i ty ma te r ia l w i t h a h i g h g r a p h i t e content
i s b e t t e r f o r o s c i l l a t o r y and r e c i p r o c a t i n g movement, where i t i s d i f f i c u l t to
e s t a b l i s h an o i l f i l m .
5.3.8 Discussion on M e t a l l i c Bear ing M a t e r i a l s
No bear ing ma te r ia l combines a l l the d e s i r a b l e p r o p e r t i e s . However, the
important p r o p e r t i e s of bear ings can be grouped in two main categor ies. One
inc ludes sur face c h a r a c t e r i s t i c s , such as wear res i s tance , j o u r n a l compatabi l i ty ,
c o n f o r m a b i l i t y and embeddabi l i ty , t he o t h e r inc ludes mechanical p r o p e r t i e s ,
such a f a t i g u e s t reng th and load -ca r ry ing capac i t y . Since these two ca tegor ies
a re d ivergent , t o have optimum sur face c h a r a c t e r i s t i c s s t r e n g t h must be sac-
r i f i c e d and v i c e versa. However, by us ing modern manufactur ing methods, i t i s
genera l l y poss ib le t o a r r i v e a t a good compromise s o l u t i o n as, f o r example, i n
the case of t r i - m e t a l bear ings.
89
There i s no c l e a r understanding o f t he way i n which bear ing a l l o y s func t i on ,
and general t heo r ies regard ing t h e i r behaviour can almost always be d i s c r e d i t e d
by reference t o some p a r t i c u l a r bear ing. In the fo rmu la t i on o f bear ing a l l o y s ,
p r a c t i c e has always been ahead o f theory. This i s n o t t o say t h a t bear ing
design and manufacture i s a backward i ndus t r y ; i n f a c t , the manufactur ing
techniques descr ibed prove the opposi te , and bear ing manufacturers can prov ide
s a t i s f a c t o r y s o l u t i o n s t o almost any bear ing problem. Nevertheless, many
bear ings s t i l l f a i l prematurely i n se rv i ce and the re i s a g rea t need t o t e l l
designers about the bear ing m a t e r i a l s a l ready a v a i l a b l e o r under development.
5.4 BEARING FAILURES
Sect ions 5.1 - 5.3 have prov ided the reader w i t h a b r i e f i n t r o d u c t i o n t o the
subject , and i t i s now poss ib le t o descr ibe and d iscuss some o f the l i k e l y
causes o f bear ing f a i l u r e s , g i v i n g p a r t i c u l a r emphasis t o the i d e n t i f i c a t i o n o f
f ac to rs t h a t g i v e r i s e t o f a i l u r e s .
5.4.1 M e t a l l u r g i c a l Defects i n New Bearings
Modern p l a i n bear ings can be very complex cons t ruc t i ons and much may go wrong
dur ing t h e i r manufacture. Nevertheless, very few d e f e c t i v e bear ings en te r
serv ice, s ince the major bear ing manufacturers ma in ta in a h i g h standard o f
q u a l i t y c o n t r o l . Defects a r e more l i k e l y t o occur on i n d i v i d u a l l y manufactured
bear ings, e.g. l a rge , whi te-metal bear ings, than on mass-produced bear ings o f
the copper- lead o r a lumin ium-t in v a r i e t i e s .
described i n Sect ions 5.4.1.1 - 5.4.1.5.
Defects t h a t may be encountered a re
5.4.1.1 Bad Bonding
Bonding i s s t i l l a major problem on la rge , whi te-metal bear ings, a l though
poor bonds a r e extremely r a r e on o t h e r types o f bear ing. Bear ing s h e l l s f o r
white-metal bear ings must be t i nned before the w h i t e metal i s cas t i n place,
and the temperature o f both the s h e l l and the wh i te metal must be c a r e f u l l y con-
t r o l l e d . A m e t a l l u r g i c a l bond between the bear ing and the s h e l l strengthens
the wh i te metal and f a c i l i t a t e s the f l o w o f heat away from the bear ing surface.
In the past , some "white me ta l l e rs " have argued t h a t i f they machined d o v e t a i l
grooves i n the s h e l l , these h e l d the wh i te metal i n p lace and the re was no need
t o i n s i s t on good bonding. Experience proves t h a t t h i s i s an extremely sho r t -
s ighted p o l i c y ; d o v e t a i l grooves a r e no s u b s t i t u t e f o r good bonding and, i n
add i t i on , t he sharp edges a t the shoulders o f the grooves a c t as l o c a l i s e d
s t ress r a i s e r s and can i n i t i a t e cracks (F ig .1) .
A general v iew o f a copper- lead bear ing which f a i l e d due t o bad bonding, the
copper-lead bear ing a l l o y hav ing separated c l e a n l y f rom the s t e e l s h e l l , i s
shown i n Fig.2.
90
Fig.1 Dovetail in white-metal thrust pad bearing.
Fig.2 Bad bonding: copper-lead bearing.
91
Bad bonding on new bear ings can be detected r e a d i l y us ing u l t r a s o n i c o r
e l e c t r i c a l res i s tance measuring techniques, and the re a r e a number o f commerci-
a l l y a v a i l a b l e t e s t instruments. On a f a i l e d bear ing, bad bonding can be
d i s t i ngu ished from f a t i g u e f a i l u r e because w i t h t he former, the bear ing metal
detaches c l e a n l y f rom the s h e l l (Fig.3).
5.4.1.2 Gas C a v i t i e s
I n l a rge , whi te-metal bear ings, t oo low a c a s t i n g temperature o r i n s u f f i c i e n t
preheat ing o f t he s h e l l can g i v e r i s e to gas c a v i t i e s near the s h e l l sur face.
Such c a v i t i e s weaken the bear ing metal and adverse ly a f f e c t heat f l ow ; some-
times, the h y d r o s t a t i c pressure above the c a v i t i e s may cause t h e i r co l l apse
(Figs. 4 and 5) .
Cast copper- lead a l l o y s sometimes e x h i b i t c a s t i n g p o r o s i t y , which i n t h i s
instance i s genera l l y assumed t o be due t o hydrogen e v o l u t i o n f rom the copper.
This p o r o s i t y due t o out -gass ing must be d i s t i n g u i s h e d from co r ros ion o f the
lead phase by o i l - o x i d a t i o n products. Cast ing p o r o s i t y i s sub-surface, whereas
w i t h co r ros ion , t he l ead i n the sur face l a y e r s i s p r e f e r e n t i a l l y d i sso l ved
(Fig.6).
5.4.1.3 Oversize Cuboids
This problem i s s p e c i f i c t o whi te-metal bear ings. I n l a r g e r bear ings, slow
coo l i ng through the s o l i d i f i c a t i o n range can g i v e r i s e to ove rs i ze (> 0 . 1 mm)
t in-ant imony cuboids (Fig.7). Th is i n t e r m e t a l l i c compound i s b r i t t l e and the
l a rge cuboids can crack and break-up i n se rv i ce , causing scor ing o f the
j o u r n a l s and damage elsewhere i n the o i l system (F ig.8) . I n general , i f cuboids
are r e a d i l y v i s i b l e to the naked eye they a re t o o b i g .
5.4.1.4 Excessive Lead Content \ i n Tin-Based White Metals
The lead content o f t in-based wh i te metals should be below 0.5% t o prevent
the fo rma t ion o f a low-mel t ing, l e a d - t i n e u t e c t i c i n the g r a i n boundaries.
The e u t e c t i c weakens the a l l o y and makes i t more suscep t ib le to wip isg. For
s i m i l a r reasons, i t i s bad p r a c t i c e t o mix lead-based and t in-based white-metal
ha l f -bea r ings , as i s sometimes done t o save i n i t i a l costs , t i n be ing used on
the loaded h a l f and lead on the unloaded h a l f . Carry-over o f lead from one
ha l f -bea r ing t o the o t h e r can r e s u l t i n the fo rma t ion of the low-mel t ing-point
eu tec t i c .
5.4.1.5 Uneven Lead D i s t r i b u t i o n i n Copper-Lead and Lead-Bronze A l l o y s
Copper-lead and lead-bronze bear ing a l l o y s can be manufactured by m e l t i n g
and c a s t i n g o r by s i n t e r i n g techniques. Lead d i s t r i b u t i o n i s more r e a d i l y
c o n t r o l l e d by powder-metal lurgy techniques, so uneven d i s t r i b u t i o n i s main ly
92
Fig.3 Micro-sections.
93
Fiq.4 Cross-section o f White-metals Showing Gas Cavities
Fig.5 Collapsed surface o f white-metal thrust pad bearing.
94
F i g . 6 Cast ing p o r o s i t y i n c a s t copper-lead big-end bear ing.
F i q . 7 Oversize t in-ant imony cuboids i n white-metal t h r u s t pad from steam t u r b i n e
95
Fig.8 Single cuboids showing cracks.
F ig .9 Bad lead d i s t r i b u t i o n .
96
encountered on c a s t bear ings . Copper and lead a re immisc ib le and good lead
d i s t r i b u t i o n i n cas t a l l o y s depends on spec ia l manufac tur ing techniques, i n
p a r t i c u l a r on r a p i d c o o l i n g o f t he a l l o y a f t e r c a s t i n g . Th is r a p i d c o o l i n g
r e s u l t s i n a h i g h l y o r i e n t e d s t r u c t u r e w i t h long copper and lead dendr i t es
normal t o the sur face o f t he bear ing s h e l l . I f manufac tur ing c o n t r o l s a r e re -
laxed, t he lead can be unevenly d i s t r i b u t e d ; f o r example, i t may be p resen t i n
l a rge s lugs , normal o r p a r a l l e l t o the sur face (F ig .9 ) .
5 .4 .2 F a i l u r e s Assoc ia ted w i t h Design, F i t t i n g o r Opera t ing Environment
The f i n a l m a n i f e s t a t i o n o f f a i l u r e r e s u l t i n g f rom adverse o p e r a t i n g cond i -
t i o n s o f t h i s type i s g e n e r a l l y f a t i g u e . The load -ca r ry ing c a p a c i t i e s and tem-
pe ra tu re l i m i t a t i o n s o f t y p i c a l bear ing a l l o y s a r e i n d i c a t e d i n Table 5 . 2 .
However, these depend on a number o f f a c t o r s , such as the th i ckness o f the
bear ing a l l o y and the support i t ge ts f rom i t s subs t ra te o r s h e l l . A l s o , t he
s t reng th o f bea r ing a l l o y s decreases w i t h i nc reas ing temperature, and bear ing
sur face temperatures a r e g e n e r a l l y a t l e a s t 2OoC i n excess o f measured o i l
temperatures.
The design o f bear ings , p a r t i c u l a r l y t he l o c a t i o n o f o i l feeds and o i l
grooves, i s o u t s i d e the scope o f t h i s paper. Never the less , i t should be em-
phasised t h a t l oad -ca r ry ing c a p a c i t y i s r a r e l y enhanced by a p r o l i f e r a t i o n of
o i l grooves.
5.4 .2 .1 Bad F i t t i n g
Modern bear ings a r e manufactured t o very c l o s e to le rances and shou ld be ass-
embled w i t h g rea t care. I f they a r e n o t a c c u r a t e l y a l i gned , t h e l oad ing w i l l be
uneven and premature f a i l u r e due t o f a t i g u e may occur . I f t h e bear ing does no t
f i t p roper l y i n i t s housing, s l i g h t movement may occur , which can r e s u l t i n
severe f r e t t i n g on t h e back o f t he bear ing . Loose f i t t i n g and/or f r e t t i n g can
adverse ly a f f e c t heat t r a n s f e r and can g i v e r i s e t o excess ive bear ing-sur face
temperature (Fig.10).
D i r t p a r t i c l e s t rapped between a bear ing and i t s housing c o n s t i t u t e another
cause o f poor heat t r a n s f e r . Such p a r t i c l e s can a l s o cause h i g h spots on the
bear ing sur face , which may g i v e r i s e t o l o c a l i s e d w ip ing and f a t i g u e (Fig.11).
It w i l l be c l e a r f rom the above t h a t examinat ion o f t he back o f a f a i l e d
bear ing can prov ide impor tan t i n fo rma t ion rega rd ing i t s mode o f f a i l u r e . The
comment has been made t h a t t h e bes t way t o eva lua te "experts" on bear ing
f a t i g u e s i s t o no te whether they examine the backs o f bear ings w i t h as much care
as the sl i d i n g surface.
91
Fig.10 F r e t t i n g and d i s t o r t i o n due t o bad f i t t i n g .
5.4.2.2 Extraneous P a r t i c l e s
This i s probably the m s t common cause o f bear ing f a i l u r e . The more exper t
we become a t d e t e c t i n g and i d e n t i f y i n g f o r e i g n m a t e r i a l , t he more obvious i t i s
t ha t c l e a n l i n e s s on assembly and good l u b r i c a n t f i l t r a t i o n i n se rv i ce a re
essen t ia l f o r long, t r o u b l e - f r e e se rv i ce l i v e s . A l l t oo f requen t l y , engineer ing
components a re assembled w i thou t adequate c lean ing i n d i r t y environments, w i t h
the r e s u l t t h a t machining swarf , moulding sand and o t h e r k inds o f deb r i c c i r -
c u l a t e w i t h the l u b r i c a t i n g o i l .
o i l f i l t e r s , bu t medium-sized p a r t i c l e s may embed i n the bear ings and very small
p a r t i c l e s con t inue t o c i r c u l a t e w i t h the o i l .
debr is can a l s o accumulate du r ing se rv i ce , and very long o i l -change per iods,
now much i n favour , aggravate the s i t u a t i o n .
Large p a r t i c l e s a r e genera l l y removed by the
Fore ign p a r t i c l e s and wear
98
Fig.12 Shaft wear due t o embedded p a r t i c l e s i n bearinq.
99
Bearing a l l o y s , p a r t i c u l a r l y w h i t e metals, have a considerable capac i t y f o r
abras ive ma te r ia l s . When t h i s capac i t y i s exceeded, the bear ing sur face can a c t
as a lap, abrading away the j o u r n a l sur face, thereby i nc reas ing clearances and
leading t o f a t i g u e f a i l u r e . Th is lapping a c t i o n by embedded p a r t i c l e s i s the
prime cause o f j o u r n a l wear (Fig.12).
In most instances, t he bear ing sur face w i l l be i n r e l a t i v e l y good c o n d i t i o n
(unless complete f a i l u r e has occurred) , a l though the o v e r a l l th ickness o f the
bear ing can a c t u a l l y increase owing t o the amount o f embedded m a t e r i a l .
P a r t i c l e s embedded i n bear ing sur faces d i s p l a c e metal and t h e r e f o r e a r e o f t e n
surrounded by a ra ised, burnished h a l o (Fig.13).
I f the composi t ion o f embedded p a r t i c l e s can be determined, t h e i r o r i g i n may
be es tab l i shed and the approp r ia te s teps taken t o e l i m i n a t e them.
A v a r i e t y o f i d e n t i f i c a t i o n techniques a r e now a v a i l a b l e . For example, the
p a r t i c l e s may be e x t r a c t e d chemica l l y o r mechanica l ly f o r i d e n t i f i c a t i o n by
X-ray d i f f r a c t i o n o r X-ray spectrometry. They can be examined i n s i t u us ing an
electron-probe. A l t e r n a t i v e l y , chemical e x t r a c t i o n techniques have much t o
recommend them;
most p l a n t ope ra to rs (Fig.14).
t he amount o f d e b r i s g e n e r a l l y obta ined i s enough t o h o r r i f y
P a r t i c l e s i n e lec t rodepos i ted lead ove r lays can be e x t r a c t e d w i t h an a c e t i c
acid/hydrogen perox ide mixture, which d i sso l ves the ove r lay ; those i n aluminium
- t i n bear ings can be e x t r a c t e d w i t h c a u s t i c soda. Both m ix tu res can loosen
p a r t i c l e s i n t in-based wh i te metals.
Embedded fe r rous p a r t i c l e s ( the l a r g e s t s i n g l e group) can be q u i c k l y
i d e n t i f i e d by " i r o n p r i n t i n g " (Fig.15).
Th is i nvo l ves soaking an unglazed paper i n a 5 % ~ s o l u t i o n o f potassium f e r r i -
cyanide t o which a few drops of hydroch lo r i c a c i d and w e t t i n g agent have been
added.
t a c t w i t h the degreased surface o f the bear ing f o r about 30 seconds.
i s then removed, and b lue spots i n d i c a t e the presence o f f e r rous p a r t i c l e s .
p a r t i c l e s t h a t remain i n the bear ing sur face a r e a l s o s ta ined b lue. S im i la r
techniques a r e a v a i l a b l e f o r t he i d e n t i f i c a t i o n o f p a r t i c l e s o f most common non-
f e r rous metals.
The surp lus l i q u i d i s dra ined o f f t he paper, which i s then p laced i n con-
The paper
The
Since fe r rous p a r t i c l e s c o n s t i t u t e the most common type o f ab ras i ve wear pa r -
t i c l e , t he use o f magnetic f i l t e r s , i n a d d i t i o n t o o r d i n a r y f i l t e r s , has much t o
recommend i t.
Fine p a r t i c l e s , smal ler than normal bea r ing c learances, can c i r c u l a t e w i t h
the l u b r i c a n t and erode the bear ing su i face.
def ined channels, w h i l e s o f t p a r t i c l e s g i v e r i s e t o more general e ros ion (Fig.16),
p a r t i c u l a r l y on s o f t e lec t rodepos i ted ove r lays .
Hard p a r t i c l e s erode deep, w e l l -
Erosion by f i n e p a r t i c l e s i s most p reva len t on high-speed bear ings and may be
associated w i t h c a v i t a t i o n eros ion.
100
Fig .13 P a r t i c l e s embedded i n b e a r i n g s u r f a c e .
F ig .14 S tee l p a r t i c l e s e x t r a c t e d from b e a r i n g .
101
Fig.16 Erosion damage.
102
5.4.2.3 Cor ros ion
Lubr icants d e t e r i o r a t e i n se rv i ce i n two ways - they become contaminated and
they undergo phys i ca l and chemical changes due t o o x i d a t i o n . I n engines, the
common contaminants a re a i r b o r n e dust and wear products, unburnt f u e l , f u e l
combustion products and water.
e r i a l s and asphal tenes. Asphaltenes i n a s s o c i a t i o n w i t h f u e l contaminants form
sludges and lacquers.
l u b r i c a n t s a re g e n e r a l l y weak organic ac ids , a l though i n extreme cases s t rong
minera l ac ids may be produced. However, a lmost a l l s t rong minera l a c i d contami
n a t i o n i n l u b r i c a n t s i s de r i ved from f u e l combustion products , su lphu r i c a c i d
be ing a major contaminant i n d iese l engines and halogen ac ids i n gaso l i ne
eng i nes.
The o x i d a t i o n products a re main ly a c i d i c mat-
The a c i d i c m a t e r i a l s - r e s u l t i n g f r o m the o x i d a t i o n o f
Since bear ings a re o f complex cons t ruc t i on , the way i n which t h e i r s t r u c t u r e
and composi t ion a f f e c t s t h e i r c o r r o s i o n res i s tance w i l l be discussed w i t h r e f -
erence t o va r ious co r ros ion mechanisms.
It should be emphasised t h a t bea r ing f a i l u r e s t h a t a r e p r i m a r i l y due t o any
form o f c o r r o s i o n a re comparat ive ly r a r e and t h a t bear ing f a i l u r e s caused by
inherent d e f i c i e n c i e s on the p a r t o f t he l u b r i c a n t a r e extremely ra re . Never-
the less, c e r t a i n s p e c i f i c forms o f co r ros ion , such as t i n ox ide fo rma t ion on
t in-based white-metal bear ings and sulphur c o r r o s i o n o f phosphor-bronze a l l o y s ,
have a t t r a c t e d considerable a t t e n t i o n . A l s o , as w i l l be apparent i n Sect ion
5 .4 .2 .3 ( co r ros ion by weak o rgan ic ac ids ) and 5 .4 .2 .4 ( c a v i t a t i o n and e ros ion ) ,
i nc reas ing l y severe opera t i ng cond i t i ons , such as longer-oi l -change pe r iods ,
very h igh opera t i ng temperatures and increased speeds, can g i v e r i s e t o p a r t i c -
u l a r problems. These a r e problems i n which co r ros ion p lays a p a r t , b u t i n
which o t h e r f a c t o r s a re also opera t i ve .
( i ) Corrosion by weak organic a c i d s
Weak o rgan ic a c i d s a r i s e e i t h e r f r o m prolonged exposure o f the l u b r i c a n t
a t e leva ted temperatures o r by contaminat ion o f the l u b r i c a n t w i t h
p a r t i a l l y bu rn t combustion products. These a c i d s a t t a c k lead f a r more
r e a d i l y than o t h e r metals, and can d i sso l ve the lead phase i n copper- lead
and lead-bronze bear ings (Fig.17).
Some engine l u b r i c a t i n g o i l s p e c i f i c a t i o n s i nc lude t e s t s t o determine t h e
c o r r o s i v i t y o f l u b r i c a t i n g o i l s towards copper- lead bear ings. The loss
o f lead i s assessed by weighing the bear ing s h e l l s be fo re and a f t e r t e s t .
I n se rv i ce f a i l u r e s , where the weights o f new bear ings a re n o t a v a i l a b l e ,
lead co r ros ion can be detected by m e t a l l u r g i c a l sec t i on ing (Fig.18).
Copper-lead and lead-bronze bear ings may be manufactured by s i n t e r i n g or cas t i ng . I n s i n t e r e d a l l o y s i t i s poss ib le t o ensure t h a t the lead
103
Fig.17 Corroded copper-lead bear ing
F ig .18 Corroded s i n t e r e d copper-lead bear ing
104
phase i s w e l l d i s t r i b u t e d and d iscont inuous , so the c o r r o s i o n w i l l be
r e s t r i c t e d t o the sur face l aye rs . On the o t h e r hand, w i t h c a s t a l l o y s ,
s a t i s f a c t o r y l ead d i s t r i b u t i o n depends on h i g h l y s p e c i a l i s e d manufactu-
r i n g techniques and r a p i d c o o l i n g o f the a l l o y a f t e r c a s t i n g .
r a p i d c o o l i n g g i ves r i s e t o a h i g h l y o r i e n t e d s t r u c t u r e , w i t h long
copper and lead d e n d r i t e s normal t o t h e su r face o f t h e bear ing s h e l l .
The long lead dendr i t es p rov ide an easy pa th f o r t he p e n e t r a t i o n o f a
c o r r o s i v e l u b r i c a n t . I n these circumstances i t i s p o s s i b l e f o r a lmost
a l l o f the lead phase t o be leached o u t o f a c a s t copper- lead a l l o y
(F ig .19) .
Th i s
Fig.19 Corroded cas t copper- lead bear ing .
Most copper- lead bear ings now have a p r e c i s i o n e lec t rodepos i ted o v e r l a y
o f a l e a d - t i n o r lead- ind ium a l l o y . The indium o r t i n a d d i t i o n s improve
the c o r r o s i o n r e s i s t a n c e o f t he lead. We have e s t a b l i s h e d t h a t about 3% t i n o r 4 .8% indium i s requ i red t o render the o v e r l a y comple te ly r e s i s t a n t
t o c o r r o s i o n (Fig.20).
Bear ing manufacturers g e n e r a l l y p rov ide o v e r l a y s w i t h about 5% indium o r
10% t i n . However, a t t h e h i g h temperatures t h a t bear ings can exper ience
i n se rv i ce , t he indium o r t i n d i f f u s e s q u i t e r a p i d l y and m ig ra tes i n t o
105
Average wt loss m 9.
A Lead-tin alloys X Lead- indium alloys
200- Indium alloys
150-
- - - 4.0 5.0 6.0
Tin alloys 1 0 0 -
50 -
0 1.0 2.0 3.0 Percentage alloying element
Fig.20 Corrosion of l ead - t i n and lead-indium a l l o y s .
the under ly ing copper-lead.
on bearings removed from serv ice and on bearings tested i n the laboratory.
In the case o f t i n , the t i n a l l o y s w i t h the copper t o form copper- t in
i n t e r m e t a l l i c compounds.
jacent t o the copper, but we have no evidence o f compound formation.
S im i la r e f f e c t s can be observed on s i l v e r bearings w i t h lead-r ich e lec t ro -
deposited overlays.
This migrat ion o f indium and t i n from the surface layers o f the over lay
leaves them suscept ib le t o corros ion (Figs.21-23). I t i s sa id tha t one
way t o prevent t h i s d i f f u s i o n i s t o interpose a dam between the overlay
and the under ly ing copper-lead (Fig.24).
Very t h i n n i c k e l , i r o n o r brass layers have been used as dams.
many bearings users are u n w i l l i n g t o face the increased cost associated
w i t h the use o f dams and, i n any case, t h e i r e f fect iveness i s being
questioned.
There i s another way i n which the indium i n lead-indium overlays may be
depleted.
ox ida t i on o f the indium can occur i n the g r a i n boundaries (Fig.25). The
indium oxide formed i n t h i s way e m b r i t t l e s the a l l o y and renders i t very
susceptible t o fa t i gue f a i l u r e as we l l as t o corros ive a t tack (Fig.26).
This d i f f us ion can r e a d i l y be observed both
I n the case of indium, enrichment occurs ad-
However,
I f the indium content f a l l s t o below about 3%, i n te rna l
106
Fig.21 Corrosion o f bear ing o v e r l a y .
F ig .22 Electron probe micrographs showing indium d i s t r i b u t i o n .
107
A f t e r 13,000 h rs . A f t e r 200 h r s . a t f u l l load.
Fig.23 ln te rmeta l 1 i c compound fo rma t ion i n o v e r l a y bear ings (copper - lead) .
Fig.24 Overlay bear ing w i t h dam.
108
F i g . 2 5 Internal oxidation of lead indium overlay.
F i g . 2 6 Corrosion of lead indium overlay.
109
Another consequence o f t he d i f f u s i o n o f t i n f rom l e a d - r i c h ove r lays
i s t h e roughening of t h e ove r lay su r face (Fig.27). Al though copper
has a much h ighe r a f f i n i t y f o r t i n than has lead, t i n d i f f u s e s much
more r a p i d l y i n l ead than i n copper. Th is means t h a t where the lead
phase i n copper- lead a l l o y i s cont inuous w i t h the ove r lay , the t i n
can pene t ra te deeply i n t o the a l l o y i n the lead phase (Fig.28) and
cause s i n k i n g o f t he ove r lay . Th is movement o f t i n away f rom the
o v e r l a y can g i v e the o v e r l a y a roughened appearance, which may be
mistaken f o r co r ros ion .
( i i ) Cor ros ion by s t rong minera l ac ids
The main source o f s t rong minera l a c i d s i n l u b r i c a n t s i s con taminat ion
by f u e l combustion produc ts . D iese l f u e l s , p a r t i c u l a r l y marine
d iese l f u e l s , c o n t a i n s i g n i f i c a n t q u a n t i t i e s o f su lphur ; t h i s su lphur
i s t h e source o f s u l p h u r i c a c i d , which may f i n d i t s way i n t o the
l u b r i c a n t . Gasol ines, on the o t h e r hand, c o n t a i n very l i t t l e su lphur ,
bu t do c o n t a i n c h l o r i n e and bromide compounds which a re added t o
scavenge the l ead an t i knock compounds. Thus, l ead h a l i d e compounds
may accumulate i n the l u b r i c a t i n g o i l i n gaso l i ne engines and, i n
c e r t a i n c i rcumstances, halogen ac ids can form.
The s t rong a c i d s g e n e r a l l y a t t a c k bare s t e e l sur faces r a t h e r than
bear ing a l l o y s . Th is u s u a l l y r e s u l t s i n j o u r n a l s be ing roughened by
co r ros ion , and bear ings then f a i l e i t h e r because o f t h i s o r as a
r e s u l t o f damage by c o r r o s i o n produc ts ( r u s t ) .
I n the presence o f a luminium and mois tu re , l ead h a l i d e s can de l iquesce,
g i v i n g r i s e t o halogen ac ids , which cor rode aluminium. Th is co r ros ion
r e a c t i o n was f i r s t observed on a lumin ium-a l loy p i s t o n s and can r e a d i l y
be d u p l i c a t e d i n the l abo ra to ry . Fo r tuna te l y , examples o f t h i s type
o f a t t a c k on a lum in ium- t i n bear ings i n engines a r e ex t remely ra re ,
a l t hough bear ings a r e suscep t ib le t o h a l i d e c o r r o s i o n a f t e r removal
f rom engines.
Halogen a t t a c k has a l s o been observed on l ead o v e r l a y bear ings
opera ted a t very h i g h engine temperatures, when mixed bromide/ch lo r ide
l a y e r s can fo rm on t h e o v e r l a y su r face and cause b lacken ing (F igs .
29 and 30).
i t con ta ins indium a t t he same concen t ra t i on as i n the bear ing ,
i n d i c a t i n g t h a t t h e l ead h a l i d e s have been formed i n s i t u as a
r e s u l t o f c o r r o s i o n and have no t been de r i ved d i r e c t l y f rom the f u e l .
E lec t ron-probe s tud ies o f t he r e a c t i o n l a y e r show t h a t
( i i i ) Sulphur c o r r o s i o n
Th is i s a general d e s c r i p t i o n which i s a p p l i e d t o most forms of
c o r r o s i o n encountered on s i l v e r - r i c h o r copper - r i ch bear ing a l l o y s .
110
Fig.27 Roughening o f overlay due to d i f f u s i o n .
F ig .28 ' S i n k i n g ' o f overlay.
111
F i g . 2 9 Blackened big-end bear ings .
F ig .30 Lead h a l i d e l a y e r .
112
There i s no doubt tha t sulphur compounds i n l u b r i c a t i n g o i l s can promote
the corrosion of these p a r t i c u l a r a l l oys . On the other hand, both
n a t u r a l l y occurr ing sulphur compounds i n l ub r i can ts and sulphur-containing
add i t i ves (e. g. z inc d i a l k y l d i t h iophosphates) confer bene f i c ia l proper t ies
on lubr icants . Thus, the dithiophosphates show considerable a,nt ioxidant
and anti-wear e f fec ts . With regard t o n a t u r a l l y occurr ing sulphur com-
pounds, modern r e f i n i n g techniques can remove them completely, but ex-
perience shows tha t t h i s i s most unwise, since some o f these compounds
p lay a large p a r t i n i n h i b i t i n g the corros ion o f many metals, p a r t i c u l a r l y
lead.
On s i l v e r bearings the problem, i n theory, i s f a i r l y simple. S i l v e r has
a h igh a f f i n i t y f o r sulphur, and dithiophosphate-type add i t i ves cannot be
used. However, a s m a l l amount o f e.p. (extreme-pressure) a c t i v i t y on the
p a r t o f the l ub r i can t i s necessary, and the natura l sulphur can sometimes
meet t h i s requirement. A l t e r n a t i v e l y , a ch lor ine-conta in ing a d d i t i v e may
be used.
c e r t a i n amount o f d i sco lo ra t i on due t o the formation o f s i l v e r sulphide
o r s i l v e r ch lor ide, which have a bene f i c ia l e f f e c t on the f r i c t i o n a l
cha rac te r i s t i cs . However, too much reac t i ve sulphur can cause severe
corrosion. It can be concluded tha t the formulat ion o f o i l s t o l u b r i c a t e
s i l v e r bearings requi res the exper t ise o f an o i l chemist.
The corros ion problems o f copper a l l o y s are more complex because the
a l l o y s themselves are complex.
phase MY be attacked by sulphur, but t h i s i s a comparatively ra re
occurrence, the problem o f sulphur corros ion being much more acute on
phosphor-bronze a1 loys (Fig.31). This i s because phosphor-bronze a1 loys,
which are very popular f o r l i t t l e - e n d bushes i n d iesel engines, are
expected t o operate a t considerably higher temperatures than copper-lead
bearings. I n any case, there are very few bare copper-lead bearings i n
use today.
There i s no general agreement about the corros ion mechanism. Some
engine manufacturers and users ho ld dithiophosphate add i t i ves e n t i r e l y
responsible, but t h i s op in ion cannot be substantiated, f o r severe
corros ion can occur when o i l s conta in ing on ly na tu ra l sulphur compounds
are used.
TWO important f ac to rs i n f l uenc ing the seve r i t y o f corros ion are the
amount o f a l l o y i n g element i n so lu t i on i n the copper-r ich phase and the
po ros i t y o f the a l l o y . In phosphor bronzes, i f the amount o f t i n i n
so lu t i on can be increased by special cast ing techniques, such as con-
tinuous cast ing, o r by so lu t i on treatment a f t e r manufacture, the
resistance t o corros ion i s g r e a t l y increased. The presence o f z inc
A s i l v e r bearing a f t e r serv ice would be expected t o show a
On simple copper-lead a l l o y s the copper
113
and/or s i l i c o n as a l l o y i n g elements i n copper a l l o y s a l s o increases the
res i s tance o f these a l l o y s t o sulphur co r ros ion . However, i t i s d i f f i -
c u l t t o make sound s i l i con -b ronze cast ings.
I f the a l l o y i s porous, the l u b r i c a n t i s drawn i n t o the pores, where i t
stagnates and, i f ope ra t i ng temperatures are h igh, can become very
c o r r o s i v e (Fig.32). The p a r t i c u l a r temperature a t which co r ros ion be-
comes severe depends on the type o f d i th iophosphate used; very a c t i v e
v a r i e t i e s can s t a r t t o corrode a t about 1 3 O o C , whereas o the r v a r i e t i e s
may be comparat ively s t a b l e up t o 180°C.
l i t t l e t r o u b l e below 1 7 O O C . Most phosphor-bronze cas t i ngs a re micro-
porous, and the g rea te r t he p o r o s i t y the g rea te r the r i s k o f corros ion.
Cases a re known where o n l y c e r t a i n bushes corroded i n a p a r t i c u l a r
engine, and m e t a l l u r g i c a l examinat ion showed t h a t the bushes which cor-
roded were porous. Cont inuous-cast ing techniques g i v e sounder a l l o y s
than o t h e r c a s t i n g techniques and, i n a d d i t i o n , a g rea te r p r o p o r t i o n o f
t i n s tays i n s o l u t i o n , thereby improving the i n t r i n s i c co r ros ion
res is tance. However, i f the problem i s t o be c o n t r o l l e d , the o n l y sa t -
i s f a c t o r y s o l u t i o n i s t o use an a l l o y t h a t i s r e s i s t a n t t o sulphur
corros ion.
tending over ten years have shown t h a t a l l o y s o f the gun-metal type, i .e.
copper - t i n a l l o y s w i t h 2-4% z inc , are completely immune f rm sulphur
co r ros ion . These gun-metal a l l o y s are r a t h e r s o f t e r than the t r a d i t i o n a l
phosphor bronzes, and n i c k e l may a l s o be added t o compensate f o r the
reduced hardness.
The importance o f m ic ros t ruc tu re i n t h i s type o f co r ros ion was r e c e n t l y
demonstrated by some phosphor-bronze bushes which corroded i n r e g u l a r l y
spaced bands, despi te the f a c t t h a t they were f r e e f rom p o r o s i t y and had
been operated on a l u b r i c a n t con ta in ing o n l y a small amount o f a very
s tab le di th iophosphate. M e t a l l u r g i c a l examinat ion showed t h a t they had
been manufactured by a semi-continuous c a s t i n g process, which gave r i s e
t o marked segregat ion i n the a l l o y , making i t very suscep t ib le t o co r -
ros ion i n c e r t a i n areas (Fig.33).
Recent ly some cases o f what i s c la imed t o be sulphur co r ros ion have been
encountered on t in-based white-metal bear ing a l l o y s . I t appears t h a t very
a c t i v e sulphur compounds can s e l e c t i v e l y a t t a c k the copper- r ich, copper-
t i n i n t e r m e t a l l i c compounds i n the wh i te metal , and t h a t t he r e s u l t i n g
co r ros ion products , r i c h i n Cu2S, can spread over the bear ing surface.
The darkening caused by these co r ros ion products should no t be confused
w i t h the darkening due t o t i n ox ide format ion.
Natura l sulphur compounds g i v e
Laboratory t e s t s fo l l owed by extens ive f i e l d exper ience ex-
114
Fig.31 Corrosion o f phosphor bronze.
Fig .32 Subsurface a t tack on phosphor-bronze.
115
( i v ) Corrosion o f t in-based wh i te meta ls : t i n ox ide format ion
I n recent years the format ion o f hard, b lack l aye rs on the sur face o f t i n -
based white-metal bear ings i n s h i p s ' t u rb ines has a t t r a c t e d a l o t o f
a t t e n t i o n . Yet t h i s i s by no means a new problem; i t has been encoun-
te red i n the main d iese l engines o f sh ips f o r many years. Some tu rb ine
b u i l d e r s and sh ip owners have blamed the co r ros ion on the increas ing use
o f a d d i t i v e s i n l u b r i c a t i n g o i l s , bu t the f a c t t h a t d iese l engine bear ings
opera t i ng on base o i l s have su f fe red f r o m t h i s c o r r o s i o n f o r many years
throws some doubt on t h e i r assumptions.
The c h a r a c t e r i s t i c f ea tu res o f the co r ros ion a r e as fo l l ows . A hard,
b lack l aye r forms on the sur face o f t he bear ings, sometimes o n l y on the
working sur faces b u t g e n e r a l l y on a l l t he exposed sur faces (Fig.34).
The presence o f t h i s l aye r i s extremely harmful f o r two reasons: f i r s t l y ,
i t i s ve ry hard and may damage the adjacent s t e e l surfaces, and secondly,
i t s format ion decreases normal bear ing c learances. M e t a l l u r g i c a l sect ions
f rom f a i l e d bear ings show t h a t the hard l aye r i s formed from the t i n - r i c h
m a t r i x o f the bear ing a l l o y (Fig.35). The copper - t i n and ant imony- t in
i n t e r m e t a l l i c compounds are complete ly unattacked. Microhardness t e s t s
show t h a t the hardness o f t he b lack m a t r i x i s between 200 and 600 DPN,
and u s u a l l y between 400 and 500 DPN.
m a t r i x i s about 25 OPN.
X-ray d i f f r a c t i o n examinat ion shows t h a t the b lack l a y e r cons is t s main ly
o f c a s s i t e r i t e , Sn02, u s u a l l y w i t h a l i t t l e stannous ox ide, SnO, assoc-
i a t e d w i t h i t . Dn t h r u s t pads, where bear ing c learances a r e large, the
b lack l a y e r can grow t o a considerable th ickness, bu t even tua l l y i t d i s -
i n t e g r a t e s and the hard d e b r i s c i r c u l a t e s w i t h the l u b r i c a n t .
Some years ago, Bryce and Roehner discussed t h i s problem i n d e t a i l , bu t
f a i l e d t o a r r i v e a t a s a t i s f a c t o r y explanat ion. However, i t has been
es tab l i shed t h a t t he co r ros ion occurs o n l y when aqueous e l e c t r o l y t e s a re
present i n the l u b r i c a n t , which suggests t h a t t he t i n d iox ide i s formed
by an e lect rochemical mechanism. Some i n v e s t i g a t o r s c l a i m t o have re -
produced the co r ros ion i n the l abo ra to ry by making a p iece o f wh i te metal
the anode i n sea-water bu t , a l though t h i s produces a b lack l a y e r , the
l aye r i s s o f t and amorphous. We have found t h a t the n a t u r a l c u r r e n t
between p ieces o f wh i te metal
p a r t i a l l y immersed i n f u l l - s t r e n g t h o r d i l u t e d syn the t i c sea-water under
o i l a t 60"C, w i l l cause the wh i te metal t o corrode, forming t h i c k , hard,
adherent s tann ic ox ide.
The b lack l aye rs formed i n t h i s way have the c r y s t a l l o g r a p h i c and metal-
l og raph ic c h a r a c t e r i s t i c s o f t he l aye rs found i n engines. With t h i s
s imple set-up i t has been poss ib le t o i n v e s t i g a t e the co r ros ion mechanism
The hardness o f the untransformed
( t in-based t o B.S.S. 3332/2) and copper,
116
Fig.33 Effect o f micro-structure on corrosion.
f i g . 3 4 T in oxide on thrust bearing
117
and t o s tudy t h e i n f l u e n c e o f va r ious o i l a d d i t i v e s on the c o r r o s i o n r a t e .
Some a d d i t i v e s reduced o r p revented t i n d i o x i d e fo rma t ion i n the t e s t
c e l l s ; u n f o r t u n a t e l y , a lmost a l l t he a d d i t i v e s t h a t behaved i n t h i s way
would have an adverse e f f e c t on o t h e r components i n engines.
one a d d i t i v e t h a t comple te ly prevented c o r r o s i o n o f the t i n was c o r r o s i v e
t o copper a l l o y s .
The mechanism o f t i n ox ide fo rma t ion on bear ings i s s t i l l no t f u l l y
understood. However, i t has been d e f i n i t e l y e s t a b l i s h e d t h a t co r ros ion
occurs o n l y when e l e c t r o l y t e s a r e p resent i n the l u b r i c a n t and when the re
i s a r e s t r i c t e d supply o f oxygen. The f a c t t h a t bear ings on bronze
supports appear t o exper ience more t r o u b l e than bear ings on s t e e l supports
suggests t h a t t h i s i s p r i m a r i l y ga l van ic co r ros ion . Th is i s no t a problem
t h a t can be e a s i l y overcome by changes i n l u b r i c a t i n g o i l compos i t ion o r
changes i n a l l o y composi t ion. The bes t remedy i s t o keep e l e c t r o l y t e s ou t
o f the system.
For example,
(v ) Cor ros ion o f copper - lead a l l o y s by water p resent i n the o i l
Th i s fo rm o f c o r r o s i o n i n uncommon and i n d i c a t e s t h a t cons ide rab le
q u a n t i t i e s o f water a r e p resent i n the l u b r i c a t i n g o i l system. Ana lys is
may i n d i c a t e t h a t the o i l i s i n e x c e l l e n t c o n d i t i o n and t h a t t he water
conten t i s no t abnormal. However, t he water conten t o f a sample taken f o r
a n a l y s i s can depend very much on when and where i t was taken. I f sub-
s t a n t i a l amounts o f water a r e p resent i t cou ld be e x e m p l i f i e d by the need
t o change f i l t e r s f r e q u e n t l y - as they swe l l and b l o c k when i n con tac t
w i t h water.
The damage assoc ia ted w i t h the presence o f water i n the o i l takes the
fo rm o f removal o f t h e lead-based o v e r l a y m a t e r i a l and severe l o c a l i s e d
c o r r o s i o n o f t he lead phase i n the copper - lead bear ing a l l o y (Figs. 36
and 37 ) . The o v e r l a y i s even removed f rom the unloaded shoulders o f the
bear ing , and the i n t e n s i v e l o c a l i s e d na tu re o f t h e a t t a c k i s i n d i c a t i v e
o f ga l van ic co r ros ion , t h e less-nob le (anod ic ) l ead being p r e f e r e n t i a l l y
a t tacked .
5.4 .2 .4 C a v i t a t i o n and Eros ion
C a v i t a t i o n and e ros ion a r e i n c r e a s i n g l y a cause o f f a i l u r e on bear ings , owing
t o i n c r e a s i n g l y h i g h and v a r i a b l e loads and speeds. I t i s impor tan t t o d e f i n e
these terms. Two types o f c a v i t i e s can form i n l u b r i c a n t s , vaporous c a v i t i e s
and gaseous c a v i t i e s .
l u b r i c a n t , can fo rm and c o l l a p s e o n l y s l o w l y and cannot cause any mechanical
damage d i r e c t l y . However, t h e i r presence reduces the l oad -ca r ry ing capac i t y o f
the l u b r i c a n t f i l m , thereby promot ing o t h e r k inds o f f a i l u r e , such as f a t i g u e .
Gaseous c a v i t i e s , which a r e formed by ou tgass ing o f the
118
Tig .35 General ox idat ion o f white-metal.
Fig.36 Damage associated wi th the presence o f water i n o i l .
119
Vaporous c a v i t i e s , which c o n t a i n vapour o f t he
no gas, can fo rm and c o l l a p s e ex t remely r a p i d l y
assoc ia ted w i t h t h e i r c o l l a p s e can cause mechan
surfaces. Both types o f c a v i t y can be generate
assoc ia ted w i t h the f l o w o f l u b r i c a n t through a
i q u i d concerned, and l i t t l e o r
and t h e very h i g h pressure
c a l ( impact ) damage on metal
by the pressure f l u c t u a t i o n s
bear ing and the f l u c t u a t i n g
loads imposed on i t . Vaporous c a v i t a t i o n can a l s o be caused by the v i b r a t i o n
o f a metal sur face i n con tac t w i t h a l u b r i c a n t , as i n an u l t r a s o n i c c leaner .
The damage assoc ia ted w i t h bo th types o f c a v i t a t i o n may be aggravated by the
presence o f f i n e p a r t i c l e s i n the l u b r i c a n t ; i t i s i n such circumstances t h a t
i t may be j u s t i f i e d t o t a l k o f e ros ion damage and c a v i t a t i o n e ros ion .
i t should be emphasised t h a t vaporous c a v i t a t i o n can cause severe damage even i n
the absence o f sol i d p a r t i c l e s .
However,
Un l i ke o t h e r types o f damage, vaporous c a v i t a t i o n damage i s g e n e r a l l y encoun-
te red on the unloaded areas o f bear ings , where o i l - f i l m pressures a r e low, and
t h i s p rov ides a u s e f u l means o f i d e n t i f i c a t i o n (F ig .38) . M ic rosec t ions o f
damaged areas show s igns o f l o c a l work-hardening and f a t i g u e c rack ing . When the
damage i s due s o l e l y t o c a v i t a t i o n , t he t e x t u r e i n the damaged areas i s rough
(Fig.39); when p a r t i c l e s a r e p resent ( c a v i t a t i o n e r o s i o n ) , the damaged surfaces
are smooth (Fig.40).
Vaporous c a v i t a t i o n can remove p r o t e c t i v e f i l m s , such as ox ides , f rom meta ls
and i n i t i a t e co r ros ion . I n a d d i t i o n , t he ve ry h i g h l o c a l pressures and temp-
e ra tu res assoc ia ted w i t h the f i n a l stage o f c a v i t y c o l l a p s e can induce chemical
reac t i ons which would no t no rma l l y take p lace . There i s some evidence t h a t
c e r t a i n o i l a d d i t i v e s a r e uns tab le under c a v i t a t i n g c o n d i t i o n s and t h a t the
decomposition produc ts can be co r ros i ve . I n such circumstances, bear ing sur -
faces can be sub jec ted t o the combined e f f e c t s o f c a v i t a t i o n and co r ros ion .
Work c a r r i e d o u t a t Thornton Research Centre i n d i c a t e s t h a t t he re i s ve ry
l i t t l e one can do t o a commercial l u b r i c a n t t o e l i m i n a t e the e f f e c t s o f c a v i -
t a t i o n , and c a v i t a t i o n must be regarded p r i m a r i l y as a des ign problem.
5.4.2.5 E l e c t r i c a l Discharge Damage
On e l e c t r i c a l machinery, and o c c a s i o n a l l y on o t h e r types o f machinery,
p o t e n t i a l d i f f e r e n c e s can be b u i l t up and e l e c t r i c a l d ischarges may occur across
the bear ing sur faces .
Each d ischarge g i ves r i s e t o a small p i t , and a l a r g e number o f d ischarges
can e v e n t u a l l y cause damage o f t he type i l l u s t r a t e d i n Fig.41. Th is type o f
damage i s cha rac te r i sed by the f a c t t h a t t he p i t t i n g occurs on bo th bear ing and
jou rna l surfaces.
120
F i g . 3 7 Section showing the intensive l o c a l i s e d nature o f the a t t a c k .
Fig.38 Cavi ta t ion damage.
121
Fig.39 C a v i t a t i o n of w h i t e meta l .
Fig.40 C a v i t a t i o n erosion o f white meta l .
122
Fig.41 E l e c t r i c a l p i t t i n g i n l ead base b a b b i t t bear ing.
5.4.2.6 Wire-wool F a i l u r e s
This i s the name g iven t o a ca tas t roph ic type o f f a i l u r e t h a t has been en-
countered du r ing the l a s t 10 years on t in-based j o u r n a l and t h r u s t bear ings on
bo th land-based and marine t u r b i n e i n s t a l l a t i o n s . A hard, b lack scab (Fig.421,
which i n some cases has been shown t o be an amorphous m ix tu re o f i r o n and t i n
con ta in ing d ispersed i r o n carb ides and p o s s i b l y n i t r i d e s , forms on the surfaces
o f t he t in-based white-metal bear ings and proceeds t o machine-away the adjacent
s t e e l surfaces. Steel s l i v e r s a re u s u a l l y found i n and around the bear ing,
sometimes i n s u f f i c i e n t q u a n t i t y t o g i v e the appearance o f wire-wool. The
f a i l u r e s almost always occur w i t h i n a few hours o f s ta r t -up , e i t h e r f rom new o r
a f t e r an overhaul , and a r e cha rac te r i sed by a very h igh wear r a t e (sometimes
cent imetres o f s t e e l may be l o s t ) w i thou t h igh c o e f f i c i e n t s o f f r i c t i o n o r much
temperature r i s e . The f a i l u r e s have been a t t r i b u t e d bo th t o the use o f chrom-
ium-steel r o t o r sha f t s and t o e . ~ . o i l s . There i s no c l e a r understanding o f t he
sequence o f events l ead ing t o f a i l u r e , and i n p a r t i c u l a r o f the r o l e s p layed by
metal surfaces, the base o i l and the a d d i t i v e s present i n the base o i l . A l l t h a t
can be s a i d i s t t ia t t h i s i s indeed a t r i b o l o g i c a l f a i l u r e . However, va r ious
i n v e s t i g a t o r s have es tab l i shed t h a t t he t r i g g e r i n g agent responsib le f o r w i re -
wool f a i l u r e s i s a d i r t p a r t i c l e , p a r t i a l l y embedded i n a whi te-metal bear ing.
I t i s n o t c e r t a i n what causes p a r t i c u l a r p a r t i c l e s t o t r i g g e r the sequence o f
events leading t o f a i l u r e , b u t whether o r n o t a b lack scab "machine t o o l "
develops depends very much on the na tu re o f t he l u b r i c a n t and the composi t ion o f
123
the r o t o r s t e e l . O i l a d d i t i v e s (e.g. su lphur-conta in ing e.p. a d d i t i v e s ) t h a t
can prevent b lack scab format ion w i t h chromium-containing-steel r o t o r s may
a c t u a l l y promote scab fo rma t ion when used w i t h o t h e r r o t o r s t e e l s such as m i l d
s tee l and 4% molybdenum s t e e l . With these l a t t e r s t e e l s i t has been claimed
t h a t ch lo r i ne -con ta in ing e.p. a d d i t i v e s a l s o increase the s u s c e p t i b i l i t y t o
f a i l u r e , bu t t h i s i s n o t supported by our i n v e s t i g a t i o n s a t Thornton Research
Centre. What i s more probable i s t h a t some l u b r i c a n t s a re more l i k e l y t o a l l o w
f a i l u r e when t h e i r s p e c i f i c an t i - rus t /an t i -wear agent becomes depleted.
The bes t way t o a v o i d wire-wool f a i l u r e s i s t o avo id d i r t . The system from
the o u t s e t must be as c lean as poss ib le . The o i l should be con t inuous ly f i l t e r e d
and the oil pressure i n the t h r u s t housing under running cond i t i ons should always
be a t l e a s t 7 I b f / i n 2 (48 kN/m2).
Now t h a t b lack scab f a i l u r e s have been w ide ly pub l i c i sed , i t appears t h a t they
may have been encountered p r e v i o u s l y b u t passed unrecognised.
severe wear o f n i t r i d e d s t a i n l e s s - s t e e l s h a f t j o u r n a l s on power-recovery t u r -
b ines o f a i r c r a f t p i s t o n engines has been observed from t ime t o t ime and these
appear t o have s u f f e r e d severe machining-type wear. I t i s p a r t i c u l a r l y s i g n i f -
i c a n t t h a t t h e bear ings i n con tac t w i t h these f a i l e d j o u r n a l s a r e i n genera l l y
q u i t e good c o n d i t i o n , showing much less damage than would be expected from the
s t a t e o f the j o u r n a l s .
However, t he a i r c r a f t bear ings were s i l v e r w i t h lead- indium over lays. I f these
f a i l u r e s a re o f t he b lack scab type, then b lack scab i s no t p e c u l i a r t o whi te-
metal bear ings.
For example,
This i s a l s o a c h a r a c t e r i s t i c o f b lack scab f a i l u r e s .
5.4.2.7 Fat igue
A p l a i n bear ing may f a i l by f a t i g u e when i t has achieved i t s designed l i f e
expectancy; however, i f f a i l u r e occurs prematurely, t h i s w i l l be because e i t h e r
an i n c o r r e c t bear ing ma te r ia l has been used o r t he bear ing has been i n c o r r e c t l y
f i t t e d .
I n f a t i g u e f a i l u r e s the cracks s t a r t a t t he bear ing sur face, propagate nor-
mal t o the su r face u n t i l they approach the s h e l l , then t u r n through 90" and
extend p a r a l l e l t o the bond between the bear ing metal and the s h e l l l eav ing a
t h i n l a y e r o f bea r ing metal a t tached t o the s h e l l (Fig.43).
Bad bonding and f a t i g u e a r e s u p e r f i c i a l l y s i m i l a r . However, w i t h bad bonding
the bear ing metal separates c l e a n l y f rom the s h e l l , whereas w i t h f a t i g u e the
cracks s t a r t a t the bear ing sur face, propagate normal t o the bear ing sur face
u n t i l they approach the s h e l l and then extend p a r a l l e l t o the bond between the
bear ing metal and the s h e l l , always l eav ing a l aye r o f bear ing metal a t tached
t o the s h e l l . For a comparison o f the two types o f f a i l u r e see Fig.44.
124
Fig .42 Black scab formation on a thrust bearing.
F i 9 . 4 3 Fatigue of lead bronze bearing.
125
Fig.44 White metal bear ings
5.4.2.8 Thermal Cyc l i ng Damage
This i s o n l y a problem on t i n - r i c h bear ing a l l o y s . The thermal expansion
o f t i n c r y s t a l s i s a n i s o t r o p i c , i .e . t h e l i n e a r c o e f f i c i e n t o f thermal expansion
i s d i f f e r e n t a long the th ree p r i n c i p a l c r y s t a l l o g r a p h i c axes. Consequently,
t in-based white-metal bear ings exposed t o considerable thermal c y c l i n g can ex-
per ience grain-boundary d i s t o r t i o n and c rack ing (Fig.45). Bad bonding f a c i l -
i t a t e s t h i s type o f f a i l u r e .
5.4.2.9 A l l o y i n g i n Serv ice
This can be a problem on e l e c t r o p l a t e d l e a d - a l l o y ove r lay bear ings and on
micro-Babbi t t bear ings. I n both cases the t i n i n the sur face l a y e r migrates
towards and a l l o y s w i t h the copper - r i ch phase o r s i l v e r phase i n the under lay.
Hard, i n t e r m e t a l l i c compounds a r e formed which, i f they a r e exposed, can score
the j o u r n a l sur faces. This problem i s most acute on m ic ro -Babb i t t bear ings,
where complete a l l o y i n g o f the 200-pm t h i c k , t i n - r i c h sur face has been observed
i n s e r v i c e (F ig .46) . Obviously, the a l l o y i n g r e a c t i o n i s dependent on both
t ime and temperature; exper iments show t h a t i t can proceed very r a p i d l y a t
150°C (Fig.47).
126
Fig.45 D i s t o r t i o n s i n t i n due t o thermal c y c l i n g .
5.4.2.10 Inadequate V i s c o s i t y and Lack o f Lub r i can t
Should the supply o f l u b r i c a n t t o a bear ing be i n t e r r u p t e d , even f o r a mat ter
o f seconds, c a t a s t r o p h i c f a i l u r e can occur. The damage u s u a l l y takes the form
o f complete se i zu re and ex tens i ve m e l t i n g o f the bear ing a l l o y . I f the f l o w o f
a l u b r i c a n t i s l i a b l e t o i n t e r r u p t i o n , some degree o f temporary p r o t e c t i o n can
be prov ided by the use o f spec ia l sur face t reatments on the s t e e l surfaces, by
us ing p l a s t i c o r p last ic - impregnated bear ings and by the use o f g r a p h i t e o r
molybdenum d isu lph ide i f the l a t t e r i s c o r r e c t l y app l i ed .
Some f a i l u r e s due t o o i l s t a r v a t i o n a r i s e because o f l a c k o f l u b r i c a n t on
s ta r t -up , and on many items o f heavy machinery i t i s e s s e n t i a l t o have a means
of c i r c u l a t i n g the o i l p r i o r t o s t a r t - u p t o p rov ide l u b r i c a t i o n and coo l i ng .
127
Fig.46 I n t e r m e t a l l i c compound fo rma t ion on m ic ro -Babb i t t bear ing .
D i l u t i o n o f t he l u b r i c a n t by f u e l can occur i n bo th gaso l i ne and d i e s e l
engines. I t i s most f r e q u e n t l y encountered on l i g h t l y loaded, coo l - runn ing
engines, e s p e c i a l l y those w i t h poor combustion c h a r a c t e r i s t i c s . The presence o f
f u e l in t h e l u b r i c a n t lowers i t s v i s c o s i t y , thereby reduc ing i t s l oad -ca r ry ing
capac i t y . I f t he reduc t i on i n v i s c o s i t y i s marg ina l , premature f a i l u r e by
f a t i g u e may occur ; however, i f the reduc t i on i s severe, then sudden, c a t a s t -
roph ic f a i l u r e may take p lace .
5.5 CONCLUS IONS
I n the preced ing pages we have at tempted t o desc r ibe the m e t a l l u r g i c a l
f ea tu res o f some o f the bear ing f a i l u r e s t h a t we have inves t i ga ted i n recent
years. Bear ing f a i l u r e mechanisms a r e becoming i n c r e a s i n g l y complex and i t i s
no longer always p o s s i b l e t o g i v e r e l i a b l e , on-the-spot d iagnos is . M e t a l l o -
g raph ic examinat ion o f sec t i ons f rom f a i l e d bear ings w i l l o f t e n p rov ide the
necessary i n fo rma t ion t o e s t a b l i s h the cause o f the t roub le . For the more
d i f f i c u l t and complex cases the re i s a wide v a r i e t y o f i n v e s t i g a t i o n a l tech-
niques t h a t can be a p p l i e d t o the problem. To a s s i s t o the rs i n the d iagnos is
o f p l a i n bear ing f a i l u r e s we have compi led Table 5.4, i n which the fea tu res o f
va r ious f a i l u r e s a r e descr ibed. W h i l s t t h i s i s n o t a s u b s t i t u t e f o r exper ience,
we b e l i e v e t h a t , coup led w i t h the accompanying b i b l i o g r a p h y , i t may be an a i d t o
e s t a b l i s h i n g the cause o f f a i l u r e s .
128
Electro deposited t i n 25pm on lead overlay -
After 2 hrs. a t 170°C
After 2 h r s at 130'C
A f t e r 2 h r s . a t 150°C
Fig.47 Electron-probe micrograph showing rate of diffusion of tin.
129
TABLE 5.4 Cause o f f a i 1 ure Typical features
Extraneous matter c i r c u l a t i n g i n 1 ubr i cant
Extraneous matter embedded i n bearing
Fatigue
Bad bonding
The rma 1 cyc 1 i ng
Surface p i t t i n g o f t in-based white metals
Hard, black surface layers a l l over tin-based white-metal bearings
Sudden catastrophic wear o f a s ing le tu rb ine journal o r th rus t bearing sho r t l y a f te r s ta r t -up
E lec t r i ca l p i t t i n g
Cav i ta t ion i n lubr ican t f i l m
Corrosion by weak organic acids
Corrosion by strong acids
"Sulphur" a t tack
Corrosion by water i n the o i l
Excessive operat ing temperatures
Inadequate v i scos i t y of lubr ican t
Lack o f lubr ican t
Bad f i t t i n g
Misalignment
Manufacturing defects, e.g. poor lead d i s t r i b u t i o n i n copper-lead a l l oys
Surface hardening o f t i n Babb i t t over lay
In te rna l ox i dat ion o f lead- ind i um over I ays
Soft pa r t i c l es , e.g. carbonaceous matter, can erode white metals and overlays. Hard pa r t i c l es , e.g. me ta l l i c wear debr is cu ts wel l -def ined channels.
Burnished spots round embedded par- t i c l e s . Wear of journal - can lead t o f a t igue.
Cracks, i n i t i a t e d a t bearing surface, propagate normal t o surface; then, near backing, t u rn and run pa ra l l e l w i th bearing surface.
Bearing a l l o y l i f t s c lea r l y away from backing - no evidence o f a l l oy ing . Comnon on large, white-metal bearings.
Can cause roughening o f t in-based white- metal bearing surfaces
Cracking o f oversize tin-antimony cuboids
T in - r i ch mat r ix o f a l l o y transformed t o hard t i n oxides. Antimony-tin and copper- t in phases unchanged.
Black scab formation i n white-metal bearing, which "machines" shaft , producing wire-wool.
Fine p i t s on both bearing surface and journa l .
Local ised metal removal, general ly i n unloaded areas o f bearing.
Surface roughening and f i l l i n g . In copper-lead and lead-bronze, pure lead phase i s leached out and surface may d is in tegra te . Lead overlays de f i c ien t i n t i n o r indium may be corroded.
Takes a va r ie t y o f forms; bearing metal o r journal may be attached. See tex t .
D isco lo ra t ion and corrosion o f copper and s i l v e r - r i c h a l l oys . Can be very severe i n phosphor bronzes.
Takes the form o f removal of the over lay and loca l i sed in tens ive a t tack o f the lead phase o f copper-lead bearing a l l oys .
\ t ip ing o f surface layers. Fatigue fa i l u re .
If marginal, may g ive r i s e t o premature f a i l u r e by fat igue. If severe, f o r example due t o excessive fue l d i l u t i o n , can g ive sudden catastrophic fa i l u re .
Complete seizure. Extensive mel t ing o f bearing a l l oy .
F re t t i ng o f backs o f bearing. Lack of thermal contact w i th housing may cause bearing t o bow.
Uneven contact and wear across bearing surface.
Requires expert. meta l lu rg ica l examinat ion.
Cracking o f over lay, wear of journa l .
Corrosion and fat igue of over lay.
130
REFERENCES
1 2 3
4
5
6
7
8
9
10
11
12
13 14 15
16
17
18
19
20
21
22
Sleeve Bear ing M a t e r i a l s , 1949, ASM Cleveland, Ohio. Metals Handbook, 1961, 8 t h e d i t i o n , Vol.1, p.843-863, ASM Cleveland, Ohio. L u b r i c a t i o n , 1953, 8, ( 3 ) , p.29, "The Modern Bus and Truck - Fuels and Lubr icants". L u b r i c a t i o n , 1953, 8, ( 5 ) , p.57, "Some Problems Assoc ia ted w i t h L u b r i c a t i o n o f Large Engines". L u b r i c a t i o n , 1953, 8, I ( l l ) , p.45, "Automotive Engine Bearings". L u b r i c a t i o n , 1955, 10, ( 4 ) , p.37, " I n d u s t r i a l Bear ing Lubr i ca t i on " . L u b r i c a t i o n , 1957, E, (8 ) , p.85, "Abrasives and Wear". L u b r i c a t i o n , 1958, 13, (9 ) , p.245, "Petroleum Labora tory Inves t i ga t i ons " . L u b r i c a t i o n , 1963, 5, (6 ) , p.81, "Diesel Power Plants". L u b r i c a t i o n , 1964, z, (7 ) , p.77, "P la in Bear ing Fa i l u res " . Fo r res te r , P.G., Modern M a t e r i a l s , 1964, 4 , p.173, Academic Press, New York and London, "Ma te r ia l s f o r P l a i n BeaTings". Hunter,M.S., C h u r c h i l l , J.R. and Mear, R.B. , "E lec t ro -graph ic Methods o f Surface Ana lys is " , Metal Progress 1942, 42, p.1070. Crooks, C.S. and Eastham, D.R. " P l a t i n g z r Bearing App l i ca t i ons " . Trans. I n s t . o f Metal F i n i s h i n g 1982 v01.60. Raf ique, S.O., I n s t . Mech. Eng. L u b r i c a t i o n and Wear, Second Convention 1964, p.180. Love,P.P., Forrester,P.G. and Burke,A.E., I n s t . Mech. Eng. Auto. D iv . Proc., 1953-54, 2, p.29. "Funct ion o f M a t e r i a l s i n Bear ing Operat ion". Lub r i can ts and L u b r i c a t i o n ( e d i t o r Bra i thwate , E.R.) E l s e v i e r , London, 1967. Mor r i s , J.A., Ch.7. p.310, " M e t a l l i c Bear ing Ma te r ia l s " . P r a t t , G . C . , Ch.8, p.377, "P las t ic -based Bearings".
Pratt,G.C. and Perkins,C.A., " S i l i c o n Aluminium Bearings f o r High-speed Diesels", D iese l and Gas Turb ine Worldwide Vol . X I I I , No.10, p.76-78. Davis,T.A., "P la in bear ing wear i n IC Engines", Automot ive Engineer, Aug./Sept. 1981. P r i n c i p l e s of L u b r i c a t i o n ( e d i t o r A. Cameron) Longmans Green and Co., London, 1966. Holl igan,P.T., Ch.25, p.511. "P la in Bear ings - Bear ing M a t e r i a l s and Diagnosis o f Bear ing Fa i 1 ures". Engineer ing, 1967, 20, p.260. "Bond St rength o f White Me ta l l i ng " . Rose, A. Trans. l n s t . Mar. Eng. 1967, 79, p.233, "Marine Bearings". Wilson,R.W. and Shone,E.B., J o i n t Course on T r ibo logy , I n s t i t u t i o n o f M e t a l l u r g i s t s , London, 1968, Paper 4, " M e t a l l u r g i c a l S tud ies o f Bear ing Fa i lu res" . Wilson,R.W. and Shone,E.B., An t i -Cor ros ion Methods and M a t e r i a l s , 1970, 17, p.9. "The Cor ros ion o f Lead Over lay Bearings". Quayle, J.P., Copper, 1969, 3, (5 ) , p.12. "The Science o f T r i b o l o g y - Par t 3Il. Bryce,J.B. and Roehner,T.G., Trans. I n s t . Mar. Engs., 1961, 73, p.377, "The Cor ros ion o f Tin-Base B a b b i t t Bear ings i n Marine Steam Turbines". Lloyd,K.A. and Wilson,R.W., I n s t . Mech. Engs., T r i bo logy Convention 1969, Paper 10, p.76. "Formation o f T i n Oxides on White Metal Bearinqs". H i l e y , R.W., "Corrosion o f T i n Base B a b b i t t Bear ings t o fo rm T i n Oxides", Trans l n s t . Mar. Eng., 1979, 91, (2) p. 52-66. Dawson, P.H. and F i d l e r , F., I n s t . Mech. Engs. ( L u b r i c a t i o n and Wear F i f t h Convent ion), 1967, 1811, p. 207, "Wire-wool Type F a i l u r e s ; The e f f e c t o f S tee l Composition, S t r u c t u r e and Hardness". Dowson, D., Godet, M. and Tay lo r , C.M., "Cav i ta t i on and Re la ted Phenomena i n Lub r i ca t i on " , U n i v e r s i t y o f Leeds, Yorks, England. Leeds-Lyon Symposium on T r ibo logy , l s t , Prog. Pap and Discuss., U n i v e r s i t y o f Leeds, Yorks, England, Sept. 1974, Publ. by Mech. Eng. Publ. Ltd. , New York, NY, 1974, 248. Publ. f o r I n s t i t u t e o f T r ibo logy , Leeds U n i v e r s i t y , Yorks, England.
"Fa i l u res o f P l a i n Bear ings and t h e i r Causes".
131
23 James, R.D., " C a v i t a t i o n Damage i n P l a i n Bearings". T r i b o l o g y I n s t . Feb. 1978, 1 1 , (1 ) p. 22-23.
24 Garner, D.R., James, R.D. and Warr iner , J.F., " C a v i t a t i o n Erosion i n Engine Bear ings - Theory and Prac t i ce " , 1 3 t h CIMAC Conf. Vienna 1979.
25 For res te r , P.G., "Bear ing Ma te r ia l s " , M e t a l l u r g i c a l Reviews, 1960, 5, p. 507.
26 F o r r e s t e r , P.G., "E lec t rodepos i t i on i n P l a i n Bear ing Manufacture", Trans. I n s t . Met. F i n i s h i n g , 1961, 38 , p.52.
27 Booser, E.R., "P la in Bear ing Ma te r ia l s " , Machine Design, 1970, 42 ,
28 Standard Handbook o f L u b r i c a t i o n Eng ineer ing , ASLE, McGraw-Hi l l , 1968.
29 T r i b o l o g y Handbook ( e d i t o r M.J. Neale), Bu t te rwor ths , London 1973.
p. 14.
Chapter 18, "S1 i d i n g Bearings".
132
(; ROLLING ELEMENT BEARINGS
D . G . HJERTZEN and R . A . J A R V I S , SKF (U.K.)LTD.
6.1 INTRODUCTION
The present development o f r o l l i n g bear ings i s cha rac te r i sed by numerous
apparen t l y smal l i n t e r n a l improvements i n t h e bear ings and t o m a t e r i a l q u a l i t y
r a t h e r than the i n t r o d u c t i o n o f r a d i c a l new designs. There i s cons iderab le
techn ica l research i n t o r o l l i n g bear ing technology and impor tan t progress i s
be ing made i n manufac tur ing and inspec t i on methods, improved l u b r i c a n t s and
l u b r i c a t i o n equipment, e t c .
I t i s p o s s i b l y s u r p r i s i n g , b u t t rue , t h a t a l t hough r o l l i n g bear ings a r e
e x t e n s i v e l y used, t h e r e a r e many misconcept ions and the methods o f s e l e c t i n g
the most s u i t a b l e bear ings a r e n o t always f u l l y understood o r app l i ed .
has been known f o r some un fo r tuna te exper ience w i t h a c e r t a i n bea r ing type t o
r e s u l t i n l o s s of conf idence; consequent ly, t he c h a r a c t e r i s t i c s o f t h a t p a r t i -
c u l a r
I t
bea r ing a r e o f t e n n o t f u l l y u t i l i s e d .
6 . 2 BEARING SELECTION
The major f u n c t i o n o f bear ings i s t o t ransmi t loads between machine p a r t s i n
r e l a t i v e mot ion, b u t a d d i t i o n a l l y t he re may be spec ia l performance o r env i ron-
ment requirements a f f e c t i n g cho ice o f bear ings .
I t i s e s s e n t i a l f o r t h e des igner t o s e l e c t f rom the wide range o f bear ings
a v a i l a b l e the bear ing design s u i t a b l e t o c a r r y the loads i nvo l ved under the
va r ious opera t i ng c o n d i t i o n s and t o s a t i s f y the requirements o f r o t a t i o n a l
speed, temperature v a r i a t i o n s , bea r ing housing misal ignment, and r i g i d i t y e t c .
I t i s a l s o impor tan t t o ensure t h a t c o r r e c t f i t s a r e used between the bear ing
i nne r r i n g t o s h a f t and o u t e r r i n g t o housing.
e s s e n t i a l l y on the work ing c o n d i t i o n s , b u t t h e r e a r e o t h e r cons ide ra t i ons such
as bear ing type and s i ze , bea r ing i n t e r n a l c learance and method o f assembly,
e t c .
The cho ice o f f i t depends
133
6.3 BEARING TYPES
6.3.1 S ing le Row Deep Groove B a l l Bear ing (Fig.1)
The depth o f the b a l l t r acks coupled w i t h a r e l a t i v e l y l a rge b a l l s i z e and
h igh degree of con fo rm i t y ( r a t i o o f t r a c k rad ius t o b a l l r ad ius ) g ives t h i s
bear ing considerable a x i a l - c a r r y i n g capac i t y i n a d d i t i o n t o r a d i a l capaci ty ,
even a t h i g h speeds.
mounted d i r e c t on to the s h a f t . For l o c a t i o n purposes, a groove can be prov ided
i n the o u t e r r i n g f o r a snap r i n g which can be used f o r a x i a l l o c a t i o n .
These bear ings normal ly have a c y l i n d r i c a l bore and a re
These bear ings can be f i t t e d w i t h s h i e l d s o r seals . The s h i e l d s a re intended
as grease r e t a i n e r s and t o keep o u t a c e r t a i n amount o f f o r e i g n ma t te r , bu t the
seals a re a c t u a l l y rubbing seals and designed f o r dusty environments, e t c .
Bear ings w i t h two s h i e l d s o r seals a re i n i t i a l l y charged w i t h the c o r r e c t quan-
t i t y o f grease and, consequent ly, do n o t r e q u i r e r e l u b r i c a t i o n . These bear ings
a r e f r e q u e n t l y c a l l e d l u b r i c a t e d - f o r - l i f e bear ings.
F ig .1 S ing le Row Deep Groove B a l l Fig.2 Sel f -A Bear ing.
6.3.2 S e l f - a l i gn ing B a l l Bear ings (Fig.2)
These have two rows o f b a l l s each i n i t s own groove on
i gn ing B a l l Bear ing.
the i nne r r i n g , b u t
i n a common sphered t r a c k i n the o u t e r r i n g . The inne r r i n g and b a l l s form a
u n i t which can a l i g n f r e e l y about the bear ing centre. Th is f e a t u r e i s an advan-
tage i n cases where i t i s d i f f i c u l t t o o b t a i n accurate p a r a l l e l i s m between the
s h a f t and housing bore, o r where t h e r e i s a d e f l e c t i o n o f t he s h a f t . Due t o
the sphered o u t e r r i n g t r a c k , t he bear ing does n o t have h i g h l oad -ca r ry ing
134
capac i t y and t h e a x i a l - c a r r y i n g capac i t y i s l i m i t e d .
The degree o f mis-al ignment o f t he s h a f t o r housing i s l i m i t e d by the b a l l s
con tac t i ng the edges o f t he o u t e r r i n g and p e r m i s s i b l e mis -a l ignment no rma l l y
v a r i e s from 2' - 3'.
These bear ings a r e manufactured w i t h c y l i n d r i c a l o r taper bore and the l a t t e r
i s u s u a l l y mounted on a s p l i t s leeve.
6 .3 .3 Angular Contact B a l l Bear ing [F ig .3 )
The d i r e c t i o n o f load through the b a l l s i s a t an ang le t o the bear ing a x i s
which makes these bear ings p a r t i c u l a r l y s u i t a b l e f o r c a r r y i n g combined r a d i a l
and a x i a l loads. A r a d i a l load imposed on a s i n g l e row angu lar con tac t b a l l
bear ing g i ves r i s e t o an induced a x i a l l oad which must be counterac ted ; there-
f o r e these bear ings need t o be arranged so t h a t they can be ad jus ted aga ins t a
second bear ing .
F ig .3 Angular Contact B a l l Bear ing . F ig .4 Double Row Angular Contact B a l l Bear ing.
These bear ings a r e f r e q u e n t l y mounted i n p a i r s i n face- to - face , back-to-back
o r tandem format ion , and i n o rde r t o achieve t h i s the s i d e faces o f t he bear ing
r i n g s a r e ground t o achieve t h e c o r r e c t r e s i d u a l i n t e r n a l c learance.
The contac t ang le can vary between 15' - 25' - 30' - 40'.
6.3.4 Double Row Angular Contact B a l l Bear ing (F ig .4 )
Th is bear ing has s i m i l a r c h a r a c t e r i s t i c s t o two s i n g l e row angu lar con tac t
b a l l bear ings mounted back-to-back; consequent ly, t h e l i n e s o f p ressure o r
135
con tac t through the b a l l s a re d i r e c t e d outwards, thereby g i v i n g increased
r i g i d i t y . These bear ings have very l i t t l e a x i a l c learance, g i v i n g c lose a x i a l
l o c a t i o n o f t he s h a f t and e l i m i n a t i n g the necess i t y f o r a x i a l adjustment.
These bear ings can be supp l i ed t o g i v e a p re - load c o n d i t i o n when mounted so
t h a t even when sub jec ted t o a x i a l load, t h e a x i a l d isplacement o f the s h a f t i s
very smal l .
6.3.5 C y l i n d r i c a l R o l l e r Bear ings (F ig .5 )
The r o l l e r s i n these bear ings a r e gu ided between i n t e g r a l f langes on the
o u t e r o r i nne r r i n g , thereby a l l o w i n g the r i n g s t o move a x i a l l y r e l a t i v e t o
each o t h e r , which i s an advantage when the s h a f t expansion i s g r e a t e r than the
housing expansion i n an a x i a l p lane. The f l anged r i n g and r o l l e r s a r e h e l d
toge the r by a cage t o form an assembly which can be removed from the o t h e r r i n g .
Th is separab le f e a t u r e i s o f t e n u t i l i s e d t o ease assembly problems.
i n g has h i g h r a d i a l l oad -ca r ry ing capac i t y w i t h accura te g u i d i n g o f t he r o l l e r s ,
r e s u l t i n g i n a c l o s e approach t o t r u e r o l l i n g . Consequently, t he low f r i c t i o n
pe rm i t s h i g h speeds.
The bear-
F ig .5 C y l i n d r i c a l R o l l e r Bear ing. F ig .6 Double Row C y l i n d r i c a l R o l l e r Bear ing .
Var ious designs a r e made w i t h d i f f e r e n t f l ange arrangements and a d d i t i o n a l
loose f langes can be i nco rpo ra ted t o a x i a l l y l o c a t e the r o l l e r s i n bo th d i r e c -
t i o n s . The r o l l e r ends and r i n g f langes a r e capable o f s u s t a i n i n g r e l a t i v e l y
h i g h a x i a l loads.
136
6.3.6 Double Row C y l i n d r i c a l R o l l e r Bear ings (F ig .6)
These bear ings a re o f separable design which a l l ows a f langed r i n g w i t h
r o l l e r and cage assembly and f l ange less r i n g t o be f i t t e d independent ly, thus
f a c i l i t a t i n g mounting and dismount ing.
Low cross-sect ional h e i g h t and h i g h l oad -ca r ry ing capac i t y g i v e r i g i d bear-
i ng arrangements f o r the accommodation o f heavy r a d i a l loads. They are, there-
fore, main ly used f o r machine t o o l and r o l l i n g m i l l a p p l i c a t i o n s . The inne r
r i n g normal ly has a tapered
j o u r n a l t o achieve a g i ven r a d i a l c learance o r even pre- load. They can be
supp l i ed i n spec ia l o r u l t r a - p r e c i s i o n execut ion.
bore t o enable the r i n g t o be d r i v e n up a tapered
6.3.7 Needle R o l l e r Bear ings (Fig.7)
These a r e s i m i l a r t o c y l i n d r i c a l r o l l e r bear ings and have h i g h l oad -ca r ry ing
They a r e made i n va r ious designs and a re p a r t i c u l a r l y s u i t a b l e f o r capaci ty .
a p p l i c a t i o n s i n conf ined spaces. These bear ings a re used i n gudgeon p i n s and
un ive rsa l j o i n t s , b u t they a r e n o t recomnended where the re i s l i k e l y t o be mis-
al ignment o r s h a f t bending o r any c o n d i t i o n where the r o l l e r s can be subjected
t o t i l t i n g fo rces which can cause a c l u t c h ac t i on .
F ig .7 Needle R o l l e r Bearing. F ig .8 Taper R o l l e r Bear ing.
The diameters o f needle r o l l e r s a r e smal l , u s u a l l y 1 . 5 t o 5 mm and the r o l l e r
l eng th i s approximately 2 . 5 t imes the diameter.
137
6.3.8 Taper R o l l e r Bearings (F ig .8)
The r o l l i n g elements i n these bear ings a r e t runca ted cones and the t r a c k o f
both the i nne r r i n g (cone) and the o u t e r r i n g (cup) a r e tapered.
the r o l l e r s and the i nne r r i n g have a common apex on the bear ing a x i s and the
g rea te r t he i n c l i n a t i o n o f t h e r o l l e r s , t he g rea te r t he a x i a l c a r r y i n g capac i t y .
As w i t h s i n g l e row angular con tac t b a l l bear ings, taper r o l l e r bear ings must
always be mounted i n p a i r s o r ad jus ted towards another bear ing capable o f deal-
i ng w i t h a x i a l forces a c t i n g i n the oppos i te d i r e c t i o n . The taper bear ing can
a l s o be supp l i ed as a p a i r e d u n i t w i t h a common inne r o r o u t e r r i n g . This
bear ing i s used e x t e n s i v e l y i n the Automobile Indus t r y .
The tapers of
6.3.9 Double Row Spher ica l R o l l e r Bear ings (F ig .9)
Due t o t h e s i z e , shape, and number o f r o l l e r s , together w i t h the accuracy
w i t h which the r o l l e r s a re guided, t h i s bea r ing has e x c e l l e n t load-carry ing
capac i t y i n bo th a r a d i a l and a x i a l d i r e c t i o n . Since the bear ing i s s e l f -
a l i g n i n g , angular misal ignment between s h a f t and housing has no de t r imen ta l
e f f e c t and f u l l capac i t y i s always a v a i l a b l e . Accurate r o l l e r guidance i n the
normal ' C ' design i s by means of a loose r e l a t i v e l y narrow guide r i n g of t h i c k
r a d i a l sec t i on between the rows o f r o l l e r s and by the cage.
Fig.9 Double Row Spher ica l R o l l e r Fig.10 Spher ical R o l l e r Thrust Bear ing Bear ing
Spher ica l r o l l e r bear ings a re manufactured w i t h c y l i n d r i c a l o r tapered bores.
They can be mounted on tapered sleeves and are used i n Railway and Heavy Engin-
eer ing. These bear ings a re regarded as the heavy-weight o f the standard bear ing
138
types and a re used e x t e n s i v e l y i n plummer b locks.
6.3.10 Spher ica l R o l l e r Thrust Bearings (Fig.10)
Un l i ke most o the r types o f t h r u s t bear ing, t he l i n e o f a c t i o n o f the load
a t the contacts between the t racks and the r o l l e r s forms an angle w i t h the
bear ing a x i s which makes these bear ings s u i t a b l e f o r c a r r y i n g a r a d i a l load up
t o 55% o f the s imul taneously a c t i n g a x i a l load.
i ng washer prov ides s e l f - a l i g n i n g p r o p e r t i e s which pe rm i t a c e r t a i n angular
displacement o f the s h a f t r e l a t i v e t o the housing, due t o mounting e r r o r s o r
s h a f t d e f l e c t i o n .
The sphered t r a c k o f t he hous-
I n order t o prevent unacceptable s l i d i n g a t the r o l l e r - t o - t r a c k contacts
under the i n f l u e n c e o f c e n t r i f u g a l f o r c e and gy ra to ry moments, i t i s necessary
t o apply a c e r t a i n minimum a x i a l load t o the bear ing.
The s h a f t washer, r o l l e r s and cage form an assembly which i s separable from
the housing washer, a l l o w i n g the washers t o be mounted independent ly. These
bear ings a re s u i t a b l e f o r heavy loads and f o r r e l a t i v e l y h i g h speeds.
6.3.11 B a l l Thrust Bearings (Fig.11)
These cons is t o f a row o f b a l l s , r e t a i n e d i n p o s i t i o n by a cage and two
washers known as the s h a f t and housing washer, each w i t h a shal low b a l l
groove. The s h a f t washer has a smal ler bore than the o t h e r washer and
located by the s h a f t . The housing washer has a l a r g e r o u t s i d e diameter
the s h a f t washer f o r l o c a t i o n i n a housing.
These bear ings are o n l y capable o f c a r r y i n g a x i a l load i n one d i r e c t
t r a c k
5
than
on.
F i g . 1 1 B a l l Thrust Bear ing Fig.12 Bear ing w i t h Spher ica l Outside Sur- face and Extended Inner Ring Width.
139
6.3.12 Bearings w i t h Spher ica l Outside Surface and Extended Inner Ring Width (Fig.12)
These bear ings a r e e s s e n t i a l l y a v a r i a t i o n o f a deep groove b a l l bea r ing and
are normal ly used i n con junc t i on w i t h a range o f cas t i r o n o r pressed s t e e l
housings. They have a sphered o u t e r sur face and t h i s , when f i t t e d i n t o a
sphered sea t ing i n the housing, a l l ows the u n i t t o accommodate any i n i t i a l mis-
alignment o f t he s h a f t which may occur du r ing mounting.
have an extended inne r r i n g w i t h some f o r m o f l o c k i n g dev ice which i s used to
lock the bear ing and, hence, t he u n i t t o the s h a f t . Al though the bear ing i s
made t o the same degree o f accuracy as deep groove b a l l bear ings, the method
o f l ock ing the bear ing t o the s h a f t does n o t g i v e the same cen te r ing accuracy
as when bear ings are mounted w i t h an i n t e r f e r e n c e f i t on the s h a f t .
bear ings a r e used e x t e n s i v e l y i n machinery where the r o t a t i o n a l accuracy requi re-
ments a re n o t too s t r i n g e n t , such as a g r i c u l t u r a l machinery and conveyors, e t c .
where they o f f e r a s imple and economical s o l u t i o n .
These bear ings normal ly
These
6.4. FATIGUE LIFE AND LOAD-CARRYING CAPACITY
The concept o f a r o l l i n g bear ing i s t h a t load between the s t a t i o n a r y and
r o t a t i n g machine components i s t r a n s f e r r e d through the bear ing by means of
interposed r o l l i n g elements.
usua l l y a combinat ion o f r o l l i n g and s l i d i n g .
f unc t i on o f t he bear ing type, but 90-99% o f the l oad i s r e l a t e d t o r o l l i n g motion
and 1-10% to s l i d i n g .
Pure r o l l i n g seldom occurs and the ne t e f f e c t i s
The exact p ropor t i ons a re a
I f a bear ing i s p roper l y handled, c o r r e c t l y mounted, l u b r i c a t e d , and protec-
ted, a l l causes o f f a i l u r e a re e l i m i n a t e d except one - f a t i g u e o f t he m a t e r i a l .
The l i f e o f a r o l l i n g bear ing as def ined by IS0 ( I n t e r n a t i o n a l Standards Organ-
i s a t i o n ) i s the number o f r e v o l u t i o n s (o r number o f ope ra t i ng hours a t a g iven
constant speed) which the bear ing i s capable o f endur ing be fo re f a t i g u e occurs
on one o f i t s r i n g s o r r o l l i n g elements. Repeated t e s t s have v e r i f i e d t h a t
when a group o f apparent ly i d e n t i c a l bear ings a re run under the same cond i t i ons
o f load and speed they have d i f f e r e n t l i v e s . A c l e a r d e f i n i t i o n o f the term
" l i f e " i s , t he re fo re , essen t ia l f o r t he c a l c u l a t i o n o f bear ing s i z e . Dynamic
load r a t i n g s g i ven i n bear ing manufacturers catalogues a re based on the l i f e
t h a t 90% o f a s u f f i c i e n t l y l a r g e group o f apparent ly i d e n t i c a l bear ings can be
expected t o a t t a i n o r exceed.
nominal l i f e ) and agrees w i t h the IS0 d e f i n i t i o n .
This i s c a l l e d the bas i c r a t i n g l i f e ( o r the
A t y p i c a l l i f e d i spe rs ion curve is shown i n Fig.13, and i t can be seen t h a t
h a i f t he bear ings achieve an average l i f e f i v e t imes g rea te r than the nominal
l i f e on which the c a l c u l a t i o n s are based.
140
F %
The classic curve of hearing life shows the life distribution for identical bearings run under idcntical conditions of load and speed. S is the perccntagc of still serviceable brarings, F the percentagc showing signs of fatigue, and L, the life of an individual bearing, given in t e r n of I.,, as explained in the text
S,%
Fig.13 A Typ ica l Bear ing L i f e D ispe rs ion Curve.
6.4.1 The Her t z ian Contact
W h i l s t the Her t z theory r e l a t i n g t o the con tac t between s o l i d bodies i s
s t i l l of i n t e r e s t i n connect ion w i t h c a l c u l a t i n g con tac t pressures and deform-
a t i ons i n r o l l i n g bear ings, t he g rea tes t i n t e r e s t i s i n the shear s t resses
beneath the con tac t su r face as these a re considered t o be the cause o f bo th
p l a s t i c d e f o m a t i o n s and f a t i g u e cracks i n the con tac t zone.
F igure 14 shows the s t r e s s d i s t r i b u t i o n i n a Her t z ian con tac t and from a
f a t i g u e aspect the shear s t resses a t the edge o f the con tac t zone a r e the most
dangerous ones.
beneath the sur face. As these shear s t resses change d i r e c t i o n w i t h the pass-
age o f the r o l l i n g body, m a t e r i a l f a t i g u e w i l l occur i f any weak p o i n t such
as a s l a g i n c l u s i o n comes w i t h i n the subsurface zone where these shear s t resses
a re considerable. I t has been proved t h a t even t i n y s l a g i n c l u s i o n s can con-
s t i t u t e weak p o i n t s o f t h i s k i n d and t h a t under the i n f l u e n c e o f the a l t e r n a -
t i n g shear s t resses these i n c l u s i o n s r e s u l t i n m ic ro cracks which subsequently
increase i n s i ze , f i n d t h e i r way up t o the sur face, and lead t o sur face f a t i g u e
f a i l u r e (see Fig.15).
They a re p a r a l l e l t o the con tac t sur face and a r e s i t u a t e d
141
Fig .14 Shear Stresses i n the Contact Zone.
max T z g r e a t e s t shear s t r e s s i n the con tac t zone and occurs a t a depth z max
below the con tac t su r face fo rming an ang le o f 45' w i t h the p lane o f
t he con tac t su r face .
and zmax z 0.786 b where ' b ' i s h a l f t h e w i d t h o f t h e con tac t zone.
For l i n e con tac t o f i n f i n i t e l eng th T~~~~ 0.30Uo
Tzy max g r e a t e s t shear s t r e s s p a r a l l e l t o the p lane o f the con tac t sur face
and occurs at a depth"zo" beneath the con tac t su r face . For l i n e con-
t a c t o f i n f i n i t e l eng th T~~ max = 0.25Uo and zo = 0.50 b.
6.4.2 R e l a t i o n s h i p between Load and L i f e
The r e l a t i o n s h i p between bear ing load and l i f e shown i n F igu re 16 has been
ob ta ined by t e s t i n g bear ings under loads o f d i f f e r e n t magnitude. Three t e s t
s e r i e s were run under loads P1, P 2 and P3 . The l i v e s ob ta ined a r e i n d i c a t e d
by t h e do ts on the h o r i z o n t a l l i n e s . The l i v e s ob ta ined by 90% o f the bear-
ings i n each t e s t se r ies , i . e . t he nominal bea r ing l i f e , has been i n d i c a t e d
by the numerals 1, 2 and 3. I t w i l l be seen i n the graph t h a t these th ree
p o i n t s l i e on an approx imate ly s t r a i g h t l i n e . T h i s means t h a t i n v iew o f the
f a c t t h a t t he sca les o f bo th axes a r e l oga r i t hm ic , t he l i f e can be expressed
as a power o f t he load, hence:
where L = nominal bea r ing l i f e i n m i l l i o n s o f r e v o l u t i o n s ;
P = e q u i v a l e n t bea r ing load i n Newtons
C b a s i c dynamic load r a t i n g o f the bear ing i n Newtons p 3 f o r b a l l bear ings and 10/3 f o r r o l l e r bear ings .
142
Fig .15 Normal Fa t i gue F a i l u r e
The bas i c dynamic load r a t i n g o f t he bear ing i s de f i ned as the load t h a t
g i ves a nominal bea r ing l i f e o f one m i l l i o n r e v o l u t i o n s . On the graph, the
load a t t he p o i n t a t t he i n t e r s e c t i o n o f t he o r d i n a t e f o r t he l i f e one m i l l i o n
r e v o l u t i o n s w i t h the l i n e rep resen t ing the r e l a t i o n s h i p between load and l i f e
i s t he b a s i c dynamic l oad r a t i n g " C " . Th is i s the load r a t i n g g i ven i n SKF
catalogues and i s used f o r t he c a l c u l a t i o n o f the bear ing l i f e .
143
Fig.16 Re la t i onsh ip between Load and L i f e .
6.4.3 Fu r the r Development o f the L i f e Equat ion
With the advancement o f technology, g rea te r r e l i a b i l i t y i s requ i red from
r o l l i n g bear ings and o f the c a l c u l a t i o n methods used. I t i s necessary t o con-
s i d e r f a c t o r s n o t inc luded i n the bas i c l i f e equat ion even though the r e s u l t s
g iven by the bas i c l i f e equat ion a re s a t i s f a c t o r y i n the m a j o r i t y o f cases.
IS0 has suggested t h a t the l i f e equat ion should be as fo l l ows : -
al = r e l i a b i l i t y f a c t o r , which enables the bear ing l i f e t o be ca l cu la ted f o r
any g iven p r o b a b i l i t y o f f a t i g u e (al :: 1 f o r 90% p r o b a b i l i t y ) .
a = ma te r ia l f a c t o r (a 1 f o r a r o l l i n g bear ing o f good q u a l i t y s t e e l w i t h
normal hardness and s t ruc tu res .
a l u b r i c a t i o n f a c t o r (a, = 1 f o r normal l u b r i c a t i o n ) .
Al though the new l i f e equat ion i s a development o f t h e o l d equat ion, a m u l t i -
p l i c a t i v e combinat ion o f the th ree mod i f i e rs
r e s u l t i n an improvement. The negat ive e f f e c t o f inadequate l u b r i c a t i o n , f o r
instance, i s n o t compensated f o r by using say a vacuum remelted s t e e l bear ing
m a t e r i a l .
a 1, az, and a does n o t always
144
6.5 BOUNDARY D I HENS I ONS AN0 I NTERNAL CONTROLS
Every s tandard b e a r i n g w i t h m e t r i c boundary dimensions belongs t o a "dimen-
s i o n a l s e r i e s " which forms p a r t o f genera l p l a n s prepared by t h e I n t e r n a t i o n a l
O r g a n i s a t i o n f o r S t a n d a r d i s a t i o n . For any g i v e n b o r e t h e r e a r e a s e r i e s o f
d i f f e r e n t o u t s i d e d iameters and w i t h i n each d iameter s e r i e s t h e r e a r e b e a r i n g s
o f d i f f e r e n t w i d t h s , as shown i n F ig .17 . Each s tandard b e a r i n g b e l o n g i n g to
a dimension s e r i e s i s des ignated by a t w o - d i g i t number. The l a s t two d i g i t s i n
a f o u r o r f i v e d i g i t number a r e 1/5 o f t h e bore when t h i s i s i n m i l l i m e t r e s ;
i . e . b e a r i n g 6004 has a bore o f 20 mm.
Supplementary symbols f a l l i n t o v a r i o u s groups r e l a t i n g t o i n t e r n a l des ign
f e a t u r e s such as cage, m a t e r i a l , s e a l i n g , i n t e r n a l r a d i a l c l e a r a n c e and pre-
c i s i o n e t c .
The r a d i a l c learance f o r s i n g l e row deep groove b a l l b e a r i n g s has been
agreed b o t h i n t e r n a t i o n a l l y and by B r i t i s h Standards O r g a n i s a t i o n s . A general
summary i s as f o l l o w s : -
SKF I . S . O . & B . S . A l s o r e f e r r e d t o as
Des ignat ion
c2
Norma I
c 3 c 4
Group
2
Norma 1
3 4
0
00
000
OD00
SKF a l s o manufacture c e r t a i n bear ings w i t h s m a l l e r r a d i a l c learances (CI )
and l a r g e r r a d i a l c learances (C5).
6 .6 USAGE
C 1 - used on machine t o o l s p i n d l e s where minimum movement and maximum
r i g i d i t y a r e r e q u i r e d .
145
C2 - s u i t a b l e f o r f r a c t i o n a l horse power motors f o r domestic appl iances
p a r t i c u l a r l y where s i l e n t running i s requi red.
Normal c learance - used f o r normal a p p l i c a t i o n s where the re are no tempera-
ture, speed, o r i n t e r f e r e n c e f i t problems.
C 3 - o f t e n used on l a rge e l e c t r i c motors, p a r t i c u l a r l y where the inner r i n g
temperature exceeds t h a t o f the o u t e r r i n g , o r when the r i n g s a re
mounted w i t h heavy i n te r fe rence f i t s .
C4 - used f r e q u e n t l y on a p p l i c a t i o n s such as l a r g e t r a c t i o n motors f o r
d i e s e l e l e c t r i c and e l e c t r i c locomotives where the re are temperature
d i f f e r e n t i a l s between the inner and o u t e r r i n g and the i n te r fe rence f i t s
a re much g rea te r than those used f o r C3 bear ings.
C5 - t h i s c o n t r o l i s o f t e n used f o r bear ings i n furnace t rucks where there
a re l a rge temperature d i f f e r e n t i a l s between the i nne r and o u t e r r i n g s
coupled w i t h heavy i n te r fe rence f i t s .
6.7 SPEED LIMITS
Due t o the many f a c t o r s combining t o determine the maximum speed l i m i t s f o r
r o l l i n g bear ings such as bear ing type, s i ze , r a d i a l c learance, c o o l i n g condi-
t i ons , method o f l u b r i c a t i o n , load, degree of p r e c i s i o n and environment e t c . , i t
i s d i f f i c u l t t o g i v e p rec i se l i m i t s and a l l at tempts t o g i v e l i m i t i n g speeds
must be t o p rov ide an approximate general guide.
For comparison purposes between speed and bear ing s i ze , r o l l i n g bear ing
speeds a re u s u a l l y expressed i n terms o f "ndm" and n JbFi, where ndm = A x f l x f2 f o r r a d i a l bear ings
and n /% = A x f x f f o r t h r u s t bear ings 1 2 n = speed, rpm
dm = mean diameter o f bear ing, 0.5(d+D)mm
d = bear ing bore, rnm
D = bear ing ou ts ide diameter, mm
H = h e i g h t o f s i n g l e t h r u s t bear ing, mm
1
A = constant , which i s a f u n c t i o n o f the l u b r i c a n t and l u b r
f c o r r e c t i o n f a c t o r f o r bear ing s i z e (Fig.18)
f2 = c o r r e c t i o n f a c t o r f o r bear ing load (Fig.19)
method (Fig.20).
c a t i o n
Al though the speed l i m i t formula i s based on p r a c t i c a l exper ience extens ive
research i n t o more accurate determinat ion o f the f a c t o r s A, f l and f2 has
r e s u l t e d i n l i m i t i n g speeds i n c lose approximat ion t o the cond i t i ons found i n
p r a c t i c e .
Approximate speed l i m i t s f o r a load g i v i n g the bear ing a minimum l i f e
Lh = 100,000 hours ( f 2 = 1) a re shown on Figs.21 and 22. I f the load i s greater
146
1
0.5
50 100 2 0 0 300 500 1000
for bcaring size (d,,,=mean dm
Correction factor diameter o (1 bearing, mm)
Fig.18 Correction Factor f o r Bearing S i z e .
fi 1
0.5
I , , , , , f I I I J 50 100 Mo 300 500 lo00
Correction factor f* for bearing load, expressed as the resulting life L b in hours (d,=mcm diameter of bearing, mm)
d?n
Fig.19 Correction Factor for Bearing Load
147
Bearing type
Radial bearing^:
" dn = . f , f a A Deep groove ball bearings
Self-aligning ball bearings
Single row angular contact
Double row angular con-
ball bearings
tact hall bearings
Cylindrical roller bearings
Spherical roller radial bear- ings
Taper roller bearings
%wt bearitigJ:
" I / D H = f , f , A Ball thrust bearings
Spherical roller thrust bear- ings
Factor A
Normally Maximum
Vormally Maximum
Normally Maximum
Normally Maximum
Normally Maximum
Normally Maximum
Normally Maximum
Normally Maximum Normally
Maximum
500'000 1 000 000
500 000 800 000
500 OOO 600 000
200 000 400 000
400 000 600 000
200 000 400 OOO
200 OOO 400 000
1OOOOO 200 000 200 000
300 000
Remarks
Pressed-steel cages Solid brass cages
Pressed-steel cages Solid brass cages
Pressed-steel cages Solid brass cages
Presstd-steel cages Solid brass cages
In the case of a predominant thrust load 20-40% lowei limit values apply, depending on the working conditions
Good, natural cooling gene rally sufficient. Effective cooling necessary.
At high speeds the bearing slackness must usually be greater than normal
Thrust bearings operating at high speeds must carry a certain minimum load Fa nir, as shown in the graph in Fig.
F i g . 20
148
the l i m i t values a re reduced by modi fy ing the f a c t o r f2. Figures 21 and 22
show two speed l i m i t s (normal and maximum) f o r each bear ing type. Bear ings
operat ing up tomorma1 l i m i t s can be f i t t e d w i t h standard cages and, as a
general r u l e , grease l u b r i c a t i o n may be used. I t should be noted t h a t B a l l
Thrust and Spher ica l R o l l e r Thrust bear ings shown i n Fig.22 should be mounted
f r e e from slackness and c a r r y a c e r t a i n minimum a x i a l load, o therwise the t racks
may be damaged (due t o smearing) as a r e s u l t o f the gy ra to ry fo rces a c t i n g on
the r o l l i n g elements.
UBAlllNGS
M- d-trr (1 ) ---I far dumrr mrs: -_ u 2 _ _ I8 zu 24 25
26 21 31 34 M n
43 46 4 1 54 54 60
60 65 65 70 13 18
7B 85 83 93 9 0 %
1 5 1 1113 03 I 110 U8 I 118
I5 I25 2U 133 25 14U
13 148 411 155 54 168
65 180 75 195
In 225 I5 240 Jo 250
4u 265 55 uy) Ml 3IU
UI YO W 370 511 3 w
all 420 1x1 4w w 4 P
50 505 7u 51) UI 5uJ
m 5w 845 615 87U 645
tMJ 675 1111 718 $55 755
IM 795 135 M! i75 8%
us 94,
zn 22
n8 210
-
-I_
3 4
23
2 9 3 4
3 2 4 0 36 46 44 53
51 60 58 68 65 1 75
-_ 25 n
73 I 03 m : 90 0 %
95 I05 113 I13 10 I25
18 I33 25 140 33 141
40 1% 48 168 58 175
65 183 75 195 9O 215
05 235 20 1 25U 35 265
w m 65 r)5
ml 310
95 325 I0 3u 40 )80
70 410 OU 4w YI I 4 7 5
60 w5 h&d kariuEs. Approximate s p e d limits for a load (predorninurtly rcrdi.1 in the 95 535 case o f roller bcarinp) givin the bearings a minimum life L, of IOOOM) houn. Lower
limits apply to axially loadtd d i a l roller bearings 55 UI5 See adjoining table for thc values d. u u o 10 615
570
Fig.21 Approximate speed l i m i t s . Radial Bearings.
149
n .C.
m
BALL
- Ill
15
16
16 19 2a
u 25
P 31 35
a UI 41
U 45 46
51 s
62 11 15
79 16
P M 96
00 16
36 I3 59
b9
-
m
m
n
m
m
-
- 113 -
31
36 (0 45
(P 54 61
61 64 11
n n 86
P n
121 1% I39
141 1U 1%
I W IS3 191
im
- 114 -
Y
U 51 51
63 M 16
81
95
101 107 It4
111 IW IU
111
I(0
I14
M
m
im in
m
- - E
N.
I1 I3 14
15 I6 I1
11 XI 21
11 24 26
P 30 3a
34
- -
f 40 44 U
52 56
U 6( 12
16
M
M 92 w QI I24 id0
m 130
m
m
-
- L I '8.
- m -
I43
141 151 115
IN IU Ed
no !I4 I35
up !43 !59
6 2 L78 w 112 I35 -
> rn
Tkvrr bariag~. Approximate 8 Umiu for a Land giving the bearings a pinimum lice ef ll&MO houn. Scc adjoining uble for the values P H . It rhould be noted that the value 0.001 C, C p s u t i c carrying cayty of bm+d it often grater than tte numerial value o F. .I. obtam from the graph. It should .Is0 h noted chat the F . i lina have diffuent nlua for
ball mnd mUu baring$
The former value i8 then t a k a .
Fig.22 Approximate speed 1imits.Thrust bearings.
150
I n the top p a r t o f Fig.21 the speed curves w i t h A = 1500000 and A = 3000000
have been drawn i n and the broken l i n e shows the extreme values which have been
achieved w i t h bearing types i n a few known cases. Such speeds, however, do
requi re experience i n mounting and the gr
6.8 FRICTION
The extensive use o f r o l l i n g bearings
than s l i d i n g f r i c t i o n . F r i c t i o n a l losses
a t e s t care must be exercised.
s due t o r o l l i n g f r i c t i o n being less
i n r o l l ing bearings are usual ly very
low, hence the term a n t i f r i c t i o n bearings.
The f r i c t i o n a l resistance o f a r o l l i n g bearing i s dependent on several fac-
t o rs such as bearing load, speed o f r o t a t i o n , and the p roper t i es o f the l u b r i -
cant. Under c e r t a i n condi t ions (bearing load P 0.1C. adequate l u b r i c a t i o n
and normal operat ing condi t ions) i t i s poss ib le t o ca l cu la te the f r i c t i o n a l
resistance w i t h s u f f i c i e n t accuracy using the c o e f f i c i e n t s o f f r i c t i o n given
i n Table 6.1. Where rubbing seals are used t h e i r f r i c t i o n a l resistance, which
can be greater than t h a t i n the bearing, must a l so be taken i n t o account. The
f r i c t i o n torque "M" o f a bearing i s obtained from the equation:
M =
where IJ = c o e f f i c i e n t o f f r i c t i o n f o r the bearing (see Table 6.1)
d x F x - (Nmn) 2
F = bearing load, N
d = bearing bore, mm.
TABLE 6.1 Coe f f i c i en t o f f r i c t i o n
Bearing type IJ
Sel f a1 ign ing B a l l Bearings .001
Cy l i nd r i ca l Ro l l e r Bearings .0011
Thrust Ba l l Bearings .0013
Deep Groove B a l l Bearings .0015
Spherical Ro l l e r Bearings .0018
Taper Ro l l e r Bearings .0018
Spherical Ro l l e r Thrust Bearings .0018
Angular Contact Ba l l Bearings
Single Row .002
Double Row .0024
Needle Ro l l e r Bearings .0025
Cy l i nd r i ca l Ro l l e r Thrust Bearings .004
Needle Ro l l e r Thrust Bearings .004
151
Higher c o e f f i c i e n t o f f r i c t i o n values than those given i n Table 6.1 are
obtained w i t h new bearings and t h i s appl ies p a r t i c u l a r l y t o r o l l e r bearings
which have no t been run i n . Higher values w i l l a l so be achieved when s t a r t i n g
and using excessive q u a n t i t i e s o f l ub r i can t .
The f r i c t i o n loss i s :
wF = 9.8 x M x n (Watts) -1 03
where M = f r i c t i o n moment, Nmm
n speed, rpm.
6.9 LUBRICATION
R o l l i n g bearings must be l ub r i ca ted t o prevent m e t a l l i c contact between the
r o l l i n g elements, t racks and cage and a l so t o p ro tec t the bearing from corrosion
and wear. The most favourable running temperature f o r a r o l l i n g bearing i s
achieved when the minimum o f l ub r i can t necessary t o ensure r e l i a b l e l u b r i c a t i o n
i s used.
Lub r i ca t i ng p roper t i es de te r io ra te due t o ageing and mechanical working and
a l l lubr icants become contaminated i n serv ice and must, therefore be replenished
o r changed p e r i o d i c a l l y .
R o l l i n g bearings may be l ub r i ca ted w i t h grease or o i l , o r i n special cases
w i t h a s o l i d l ub r i can t . When considering l u b r i c a t i o n f o r bearings the choice
is between o i l and grease and various aspects need t o be considered. Grease
l u b r i c a t i o n has c e r t a i n advantages which are:-
1. Costs involved i n mounting are lower than w i t h o i l .
2 . Less maintenance i s required and i t i s not necessary t o
p ip ing o r pumping equipment.
3 . Constant o i l l eve l devices not required.
4. Easier t o conta in grease i n housing than o i l .
5. Cleaner t o use grease as there i s no splashing as w i t h o
6. Cheaper t o seal f o r grease than f o r o i l .
ncorpora t e
1 .
7. Grease ass i s t s i n seal ing against the en t r y o f moisture and o the r
i mpur i t i es .
Tests have shown t h a t on l y small amounts o f grease adhere t o the surfaces
o f the bearing.
faces o f the bearing.
i n the grease c a v i t i e s of the housing and, as a r e s u l t , t h i s i s usual ly i nac t i ve .
I t can be argued t h a t t h i s reserve o f grease helps t o mainta in an o i l bleed t o
the bearings, bu t experience suggests tha t although t h i s reserve may be i n
reasonable q u a n t i t i e s i n the cav i t i es , i t i s s t i l l poss ib le f o r the bearing to
Reservoirs o f grease form on the cage and against the side
The bu lk o f the grease c o l l e c t s ou ts ide the bearing and
152
f a i l due t o i n s u f f i c i e n t l u b r i c a n t . The bas i c r u l e f o r a normal bear ing arrange-
ment i s t h a t the bear ing should be w e l l packed w i t h grease w i t h the housing no
more than h a l f f u l l . I f the space round the bear ing i s excess i ve l y f i l l e d w i t h
grease then churn ing o f the grease i n the bear ing can occur which cou ld lead to
a r a p i d breakdown o f t he grease s t r u c t u r e due t o overheat ing.
the grease sof tens and the o i l i n the grease tends t o b leed from the soaps. The
s t i f f n e s s o r hardness o f a grease i s c a l l e d consis tency and i s u s u a l l y quoted i n
terms o f the Nat ional L u b r i c a t i n g Grease I n s t i t u t e (NLGI) sca le and Consistencies
2 o r 3 a re complete ly s a t i s f a c t o r y i n normal a p p l i c a t i o n s f o r b a l l and r o l l e r
bear i ng 1 ubr i c a t i on .
I n such cases
A ' 3 ' consistency grease would be used i n an a p p l i c a t i o n such as an axlebox
o r t r a c t i o n motor where the re i s cons iderable v i b r a t i o n and a r i s k o f the grease
slumping.
6.9.1 Greases
L u b r i c a t i n g greases a r e th ickened minera l o i l s o r s y n t h e t i c f l u i d s . Metal
soaps a re main ly used as the th i cken ing agent. The consis tency o f the grease
depends most ly on the type and q u a n t i t y o f the th i cken ing agent used.
s e l e c t i n g a grease, i t s consistency, temperature range and r u s t - i n h i b i t i n g pro-
p e r t i e s are the most impor tant f a c t o r s t o be considered.
When
6.9.1.1 Temperature Range
-30 t o +8OoC, a l though some spec ia l vers ions may be used up t o +120°C Sodium
base greases a re water-so lub le, i .e . they abso rbwa te r t o a c e r t a i n e x t e n t and
form a r u s t - i n h i b i t i n g emulsion wichout t h e i r l u b r i c a t i n g p r o p e r t i e s be ing
impaired. These greases w i l l p r o t e c t t he bear ings s u f f i c i e n t l y aga ins t r u s t
p r o v i d i n g t h a t water cannot e n t e r t he bear ing arrangement.
enter , such greases a re e a s i l y washed o u t o f t he bear ing housing.
( i i )
( i ) Sodium Base Greases. These greases may be used a t temperatures between
Where water c2n
Calcium Base Greases. Most ca lc ium based greases a re s t a b i l i s e d w i t h
1 t o 3% water. With increased temperature the water evaporates and separat ion
o f the grease i n t o minera l o i l and soap occurs. The upper temperature l i m i t
f o r these greases i s t h e r e f o r e approximately +6OoC.
base
Some hea t -s tab le ca lc ium
greases a re a v a i l a b l e which pe rm i t ope ra t i ng temperatures up t o +120°C.
( i i i ) L i t h i u m Base Greases. These greases a r e g e n e r a l l y s u i t a b l e f o r use a t
temperatures
fo r working temperatures up t o +150°C.
between -30 t o +llO°C, b u t a few greases o f t h i s type a r e s u i t a b l e
L i t h i u m and ca lc ium base greases a r e v i r t u a l l y i n s o l u b l e i n water and do n o t
therefore g i ve p r o t e c t i o n aga ins t co r ros ion . Such greases should t h e r e f o r e
never be used unless they con ta in a r u s t - i n h i b i t o r .
153
For heavi ly- loaded r o l l i n g bearings, e.g. r o l l i n g - m i l l bearings, greases
conta in ing EP add i t i ves are used since these increase the load-carrying a b i l i t y
o f the l ub r i can t f i l m . Such greases are a l s o genera l ly recommended f o r the
l u b r i c a t i o n of medium and large sized r o l l e r bearings. The rus t i n h i b i t i n g
p roper t i es o f calcium and I i thium base greases conta in ing EP add i t i ves (mainly
lead compounds) are good.
we l l as being i nso lub le i n water. They are, therefore, p a r t i c u l a r l y su i tab le
f o r app l i ca t i ons where water can penetrate the bearing arrangement, e.g. paper-
making machines o r r o l l i n g - m i l l s .
These greases adhere we l l t o the bearing surfaces as
Creases conta in ing inorganic thickeners instead o f metal soaps, e.g. c lay o r
s i l i c a , may be used f o r shor t periods a t higher temperatures than l i t h i u m base
greases. Synthetic greases, e.g. those made from d i e s t e r or s i l i c o n e f l u i d s ,
may be used a t both higher and lower temperatures than greases made from mineral
o i l s .
Grease r e - l u b r i c a t i o n i n t e r v a l s as recomnended by SKF are given i n graphs i n
Fig.23; these i n t e r v a l s are conservative and are known t o g i ve a wide safety
margin .
Relubrication interval hours 01 operation
t b
20000
300 200 160 100
a Radial wl bearings b Cylindrical roller bearings,needc roller bearings c Sphericd mler bearings. taper roller bearings,
tnrust ball bearings
- n r/min
Fig.23 Re- lubr icat ion I n t e r v a l s
164
To prevent the p o s s i b i l i t y o f m ix ing incompat ib le greases w i t h i t s inherent problems, i t i s adv i sab le t o ensure t h a t o n l y greases hav ing the same th i ckener
and w i t h a s i m i l a r base o i l a r e used when r e - l u b r i c a t i n g .
There i s o f t e n doubt as t o the q u a n t i t y o f grease t o be used i n a bea r ing
and w h i l s t i t i s d i f f i c u l t t o be exact , the f o l l o w i n g guide can be used:
G 0.005 x D x B
where G = grease q u a n t i t y , grams
D = bear ing o u t s i d e diameter, m
B bear ing width, mm.
For high-speed bear ings n e c e s s i t a t i n g f requent r e - l u b r i c a t i o n i n t e r v a l s i t i s
essen t ia l t o avo id o v e r - f i l l i n g the housings, s ince t o o much grease causes the
grease t o churn, r e s u l t i n g i n an excess ive r i s e i n temperature. Churning can
lead t o a breakdown i n the l u b r i c a t i n g p r o p e r t i e s o f the grease w i t h a f u r t h e r
r i s e i n temperature and the bear ings opera t i ng i n a pre- load cond i t i on . This
problem can be avoided by u t i l i s i n g a grease escape va l ve arrangement, as shown
i n Fig.24. The va l ve cons is t s o f a d i s c which r o t a t e s w i t h the s h a f t and, i n
con junc t i on w i t h a housing end cover, forms a narrow r a d i a l gap. Excess grease
Cnum d v m for mnd-fnmm typ. m l e r t c motor
Fig.24 Grease Escape Valve Arrangement
i s thrown o u t by the d i s c i n t o an angular c a v i t y and e jec ted through an opening
on the underside o f the end cover.
usefu l f rom a maintenance aspect as examinat ion o f the e jec ted grease can g i v e
va luable evidence regard ing the c o n d i t i o n o f the bear ing, i .e . i f the bear ing
i s f i t t e d w i t h a brass cage and t h i s i s beginn ing t o wear the grease w i l l become
Th is system o f escape va l ve i s extremely
165
50-
P-
discoloured, and t h i s can be e a s i l y seen and recognized. F igu re 25 shows the
temperature e f f e c t o f ope ra t i ng bear ings a t h i g h speeds w i t h and w i t h o u t grease
valves. Experiments i n the machine t o o l i n d u s t r y where bear ings a r e run w i t h
on l y a l i g h t smear o f grease on the t racks i n d i c a t e d t h a t i n i t i a l l u b r i c a t i o n
could be as l i t t l e as 1 cm per 50 mm mean bear ing diameter. The use o f such
small q u a n t i t i e s i n 9Omm bore c y l i n d r i c a l r o l l e r bear ings i n j i g bo re rs pre-
loaded 0.0025 mm and opera t i ng a t a speed o f 2500 rpm has reduced the temper-
a t u r e r i s e t o a slow as 8OC.
achieved us ing t h i s technique, t he method i s extremely d e l i c a t e and necess i ta tes
spec ia l t r a i n i n g f o r t he f i t t e r s .
3
Although long-running pe r iods have a l s o been
' d k t L -
I I I
- u - Y L
a " 0 L Y a E Y I-
60
10
2 0 1 0 24 48 72 96 120 144 188 l.54
h
a a a
C A h A I 1
0 ,& 46 n 96 h
Typical comparison between lubication with a grease valve and ordinary grease lubication for a high-speed bearing in a cloaed housing. The steady high tempera- ture in diagram (b) indicates that the grease has de- teriorated during the first high temperature running period. Grease has seeped out through the seals and less grease remains than in the housing with grease valve. The temperature peaks in diagram (a) nor- mally occur at each lubrication and a r e permissible with soda-base greases.
b
2 24 48 72 96 120 144 168 192 1 1 1 1 1 1 1 I
h
h
(a) Roller bearing 22328M running at 1100rev/min in conjunction with grease valve
(b) Same type of roller bearing without grease valve
(c) Ball bearing 6310 MA running at 8000rev/min in conjunction with grease valve
(d) Same type of bearing running at 12400rev/min in conjunction with grease valve
x = relubrication point
Fig.25 E f f e c t on Temperature o f I nco rpo ra t i ng Grease Valve.
6.9.2 O i l L u b r i c a t i o n
O i l has severa l advantages compared t o grease such as ease o f d r a i n i n g and
r e p l e n i s h i n g when necessary and p a r t i c u l a r l y when the r e l u b r i c a t i o n i n t e r v a l
f o r grease i s very sho r t .
o r ope ra t i ng
necessary t o d i s s i p a t e f r i c t i o n a l o r .appl ied heat f rom the bear ing.
t i o n o f a l u b r i c a t i n g o i l i s e a s i e r than the choice o f a grease.
un i fo rm i n t h e i r c h a r a c t e r i s t i c s and i f r e s i s t a n t t o o x i d a t i o n , gumming, and
evaporat ion can be se lected on the bas i s o f a s u i t a b l e v i s c o s i t y (Fig.26).
O i l l u b r i c a t i o n i s g e n e r a l l y used when h i g h speeds
temperatures p r o h i b i t the use o f grease and i s use fu l when i t i s
The selec-
O i l s a re more
156
Graph for selection of oil. d = bore of bearing. mlp., n P spccd, I. m. Example : spheric11 roller thxust b r i n g 29469 : d = &O mm, I - 500 r.p.m. UK an oil which has a minimwm viscosity of 13.5 cSt at working temperature. When the working I C I n p c n N r C is known the ap mximarc riscosit of the oil required at 50' C. can b;obuinef; with the aid of t ie thin. oblique lines. At a working tcmpcnturc of 70" C. the oil in the cnmplc should hare a viscosit of appmx. 27 cSr at 50" C. Select an oil from those avaiible who= viscosity is thc ~ r a ( i / this d u e . for cmmplc. Ckwylc
DTE Oil Medium (27.3 cSt at 50" C.)
Figure 26. Selection o f Oil.
Due t o the f a c t t h a t heat-generated i n r o l l i n g bear ings increases w i t h v i s c -
o s i t y , i t i s necessary to se lec t a t h i n o i l f o r high-speed opera t i on , o therwise
the bear ing temperature would be t o o h igh . For ve ry s low speeds, a p p l i c a t i o n s
such as spher i ca l r o l l e r t h r u s t bear ings i n cranes, an extremely t h i c k o i l
(minimum v i s c o s i t y 400 c S t a t 5D°C) i s used t o ensure a s u f f i c i e n t l y s t rong
o i l f i l m . For normal ambient temperatures and work ing cond i t i ons , an o i l w i t h
a v i s c o s i t y o f 12-22c S t a t 5OoC i s s u i t a b l e .
o i l i s requ i red for bear ings i n gear-boxes s ince they can be l u b r i c a t e d by the
gear o i l prov ided the bear ings a re adequately p ro tec ted aga ins t wear p a r t i c l e s
from the gears e n t e r i n g the bear ings.
bear ings must be separa te l y l u b r i c a t e d , u s u a l l y w i t h o i l . Grease can be used,
bu t the seals must have the a b i l i t y t o prevent t he gear o i l f l u s h i n g the grease
o u t o f the bear ing. I t must be mentioned tha t , as w i t h grease, an excess ive
q u a n t i t y o f o i l can cause churn ing and considerable heat; t he re fo re , for normal
r e s e r v o i r systems i t i s e s s e n t i a l f o r t he maximum o i l l e v e l t o be no h ighe r
than the cen t re o f the lowest r o l l i n g element. I f the o i l l e v e l exceeds t h i s
the re could be a temperature r i s e due t o churn ing.
on temperature r i s e and f r i c t i o n torque o f i nc reas ing the q u a n t i t y o f o i l . When
the q u a n t i t y o f o i l reaches a minimum l e v e l , i . e . t he d o t t e d zone, meta l - to-
metal con tac t occurs, r e s u l t i n g i n r a p i d temperature r i s e and p o s s i b l e bea r ing
se izure. Bear ing f r i c t i o n torque i s a l s o a f u n c t i o n o f o i l q u a n t i t y and i t can
be seen t h a t the torque increases w i t h the q u a n t i t y o f o i l . As t h i s cou ld
represent cons iderable power loss i t i s e s s e n t i a l t o ensure t h a t the o i l l e v e l
does nmt exceed a l e v e l compat ib le w i t h adequate l u b r i c a t i o n . O i l l u b r i c a t i o n
A t moderate speeds, no spec ia l
I f t h i s cannot be prevented, then the
F igure 27 shows the e f f e c t
167
can be by c i r c u l a t i o n , d r i p feed, wick feed, o r o i l m i s t . I n a c i r c u l a t o r y
system the o u t l e t must be g rea te r than the i n l e t t o prevent the p o s s i b i l i t y o f
excessive o i l i n the bear ing. For high-speed a p p l i c a t i o n s such as g r i n d i n g
sp ind les, o i l m i s t l u b r i c a t i o n i s o f t e n used. I n t h i s system a m i s t o f o i l and
a i r i s t ranspor ted through p ipes t o the bear ings. Condensing n i p p l e s immediately
t o be suppl ied t o the bear ing i n
can be accu ra te l y regulated, conse-
b l e . F igure 28 shows a t y p i c a l o i l
before each bear ing pos
d r o p l e t form. The smal
quent ly the l u b r i c a t i o n
m is t u n i t .
t i o n cause the o i l
q u a n t i t i e s o f o i l
f r i c t i o n i s n e g l i g
The dependence of burlng friction on quintlcy of 011 (in It+/hr) Lower curve: frlction torque, Ib. In.
Upper curve: outer rln; tempenwre. 'C
Fig.27 E f f e c t on Bear inq F r i c t i o n o f Varying a u a n t i t y o f O i l
6 .10 SEALS
Se lec t i ng the c o r r e c t bear ings f o r a p a r t i c u l a r a p p l i c a t i o n necess i ta tes
more than a s c e r t a i n i n g c o r r e c t type and s i ze .
s a t i s f a c t o r i l y , they must be p r o p e r l y l u b r i c a t e d and p ro tec ted f rom the oper-
a t i n g environment by means o f c o r r e c t l y designed seals.
I f bear ings a re t o f u n c t i o n
Seals a r e normal ly intended t o prevent f o r e i g n ma te r ia l e n t e r i n g the bear ing
and i n c e r t a i n cases t o prevent t he ingress o f mois ture and c o r r o s i v e media.
The f r i c t i o n developed by a rubbing seal must be considered r e l a t i v e t o the
power input . The seal o r s ide p l a t e must a l s o r e t a i n the l u b r i c a n t i n the
bear ing o r housing. Ro ta t i ona l speed o r rubbing seals must a l s o be considered
t o ensure excessive heat i s n o t developed.
158
Pipeline layout for condensed oil mist lubricating system
1 Compressed air cut-off valve 5 Mstribution box
I OU mint lubricator 6-8 Condenstng nipples
3 Pressure control valve 9 Branch pipelines
4 Mein pipeline 10 Spray nomle
Fig.28 O i l Mis t Lub r i ca t i on System
Simple gap seals are e f f i c i e n t using grease l u b r i c a t i o n , the purpose o f the
gap seals being t o keep ou t r e l a t i v e l y small amun ts o f f o re ign mater ia l and t o
r e t a i n the grease wi thout excessive temperature problems. A gap seal should be
long a x i a l l y and as small as poss ib le r a d i a l l y . For normal app l i ca t i ons the
gap should be 0.1 t o 0 . 3 mm. Gap seals can be used w i t h o i l l u b r i c a t i o n , but
i t i s advisable t o incorporate a groove i n the sha f t o r , a l t e r n a t i v e l y , f i t a
r i ng , both arrangements causing the o i l t o f l o w back i n t o the housing. A f u r -
ther a l t e r n a t i v e would be t o machine a small groove i n the shaf t adjacent t o
the gap seal (which has a d r a i n hole) thereby encouraging the o i l t o f l ow back
i n t o the housing.
Labyr in th seals are extens ive ly used w i t h grease l u b r i c a t i o n i n d i r t y and
wet condi t ions. The l a b y r i n t h consis ts o f a number o f r a d i a l l y separated
tongues w i t h a small r a d i a l clearance and can be considered as e laborate grooved
gap seals. The grooves are o f t e n f i l l e d w i t h grease t o prevent the ingress o f
169
d i r t and a t y p i c a l l a b y r i n t h system f o r a t r a c t i o n motor bearing i s shown i n
Fig.29.
lkaring srbcmc /or rrartion motor armatwe
Fig.29 Bearing Scheme f o r Tract ion Hotor Armature
6.11 N O I S E
Noise o r ig ina tes from an o s c i l l a t i n g body which produces a moving l ong i tud ina l
wave cons is t i ng o f v a r i a t i o n i n pressure. In order t o reduce noise i r r i t a t i o n t o
a minimum there i s a constant demand f o r s i l e n t running bearings p a r t i c u l a r l y i n
domestic and o f f i c e equipment such as vacuum cleaners, f l o o r po l ishers, c i r c u -
l a t o r y pump motors f o r cen t ra l heating and fans etc .
A clean h igh q u a l i t y r o l l i n g bearing runs w i t h an even pu r r i ng sound and
depending on the bearing arrangement, the noise may be so low as t o be inaudib le
o r i t can be unpleasant and d is turb ing. Bearing noise i s a func t i on o f the leve l
o f v i b r a t i o n i n the bearing and the qu ie te r the bearing the lower the l eve l o f
v ib ra t i on . V ibrat ions i n a bearing depend on many fac to rs such as surface
f i n i s h , speed, load and accuracy o f geometric form e tc .
Clearance i n a bearing i s a f u r t h e r con t r i bu to ry fac to r t o no ise and i t i s
necessary t o se lect bearings and f i t s t o achieve zero clearance i n the bearing
under operat ing condi t ions.
bearing clearances.
Fig.30 shows the e f f e c t on noise of d i f f e r e n t
Freedom from clearance i n mounted b a l l bearings can a l so be
160
achieved by a d j u s t i n g the bear ing a g a i n s t each o t h e r by means o f d i s c sp r ings
as shown i n Fig.31. Th is method ensures p r a c t i c a l l y zero c learance under a l l
working cond i t i ons . The t o t a l s p r i n g pressure should be approx imate ly 5
Newtons per mm o f s h a f t d iameter i .e . 100 Newton pressure f o r a 20 mm s h a f t .
Clearance in mounted bearing
The dependence of motor noise on bearing clearance. Measurement of vibration of electric motor over the fre- quency range 400--800 c/s with progressive variation of
A=clearance range obtained with C2 bearings and normal fits B=clearance range obtained with normal bearings and nor-
bearing clearance
mal fits
Fig.30 E f f e c t o f Noise on D i f f e r e n t Bear ing C 1 earances
For b a l l and c y l i n d r i c a l r o l l e r bear ings opera t i ng a t h i g h speeds (ndm 500000
and 400000 r e s p e c t i v e l y ) t he temperature d i f f e r e n t i a l between the i nne r and
ou te r r i n g s i s g r e a t e r than a t normal speeds and the e f f e c t o f t h i s must be
taken i n t o account. Any r e d u c t i o n i n c lea rance due t o temperature d i f f e r e n t i a l
must be compensated f o r by us ing bear ings w i t h g r e a t e r i n i t i a l r a d i a l c learance.
Wh i l s t v i b r a t i o n l e v e l s i n bear ings can be reduced, i t i s e q u a l l y impor tan t
t o ensure t h a t t he o t h e r components i n the machine a r e a l s o manufactured t o
s i m i l a r accuracy t o ensure they a r e n o t t h e cause o f v i b r a t i o n , o the rw ise the
q u i e t running p r o p e r t i e s o f t he bear ing w i l l n o t be u t i l i s e d . From a no ise
aspect t he o v a l i t y and tape r o f t h e s h a f t and housing sea t ings should be accur-
a t e and l i e w i t h i n h a l f t h e to le rance range f o r grades IT5 and IT6 f o r s h a f t
and housing respec t i ve l y . Bear ing a l ignment must a l s o be cons idered, as mis-
a l ignment can a l s o be a source o f no ise .
ponding d i a m e t r i c a l l y opposed low spots on a lobed type o f out-of-roundness,
and Fig.33 shows the e f f e c t o f a l ignment e r r o r s on n o i s e i n t e n s i t y . There a re
F igu re 32 shows t h e h i g h and co r res -
161
Fig.31 Spring preloading of deep groove ball bearings. 1 with spring washers, 2 with helical springs.
Fig.32 Lobing. For every high spot there is a corresponding There are always an low spot diametrically opposite.
odd number of lobes.
162
I
Noise intensity
dR
- Errors of alignment -
Fig.33 E f f e c t o f Alignment E r ro rs on Noise
many a p p l i c a t i o n s where no ise must be t o l e r a t e d i n o rde r t o achieve a s a t i s -
f a c t o r y bear ing l i f e . I n e l e c t r i c t r a c t i o n motors geared t o the d r i v i n g ax les
o f r a i l w a y veh ic les , the opera t i ng cond i t i ons d i c t a t e heavy f i t s which i n t u r n
necess i ta te bear ings w i t h l a r g e i n i t i a l r a d i a l c learance. The increased r a d i a l
c learance r e s u l t s i n combined r o l l e r and cage drop as they come o u t o f t he load
zone which can cause increased bear ing noise, a l t hough i t has no e f f e c t on the
l i f e o f t h e bear ing and cannot be heard when t h e motor is f i t t e d under the
locomotive.
w i t h c y l i n d r i c a l r o l l e r bear ings, d e s p i t e adequate l u b r i c a t i o n . Th is has been
a t t r i b u t e d t o resonance caused by c e r t a i n f requencies o f the r o l l i n g elements
c o i n c i d i n g w i t h t he n a t u r a l frequency of t h e bear ing end frame and, i n c e r t a i n
cases, whine has been s i g n i f i c a n t l y reduced by mod i f y ing t h e mass d i s t r i b u t i o n
o f the housing.
A h i g h p i t ched whine can o c c a s i o n a l l y occur a t c e r t a i n speed ranges
6.12 ANTICIPATING BEARING DAMAGE
The running performance o f r o l l i n g bear ings from a n o i s e aspect can be checked
w i t h reasonable accuracy us ing a wooden l i s t e n i n g s t i c k and l i s t e n i n g through
the handle t o the t ransmi t ted noise.
t h a t i t t ransmi t s no i se r e l a t i n g t o t h e c o n d i t i o n o f t he bear ing and c u t s o u t
most o f the extraneous no ise from o t h e r machine components which can cause pro-
blems f o r the more s e n s i t i v e stethoscope. With exper ience, an operator becomes
tuned t o c e r t a i n no ises and can a s c e r t a i n t h a t bear ing examinat ion should be
c a r r i e d ou t a t t he next shutdown pe r iod . N a t u r a l l y , if say a cons is ten t knocking
develops w i t h the normal no i se l e v e l , then an immediate i nspec t i on o f t he bear ing
A wooden s t i c k i s p a r t i c u l a r l y use fu l i n
163
should be made.
low p i t c h no ise and inadequate r a d i a l c learance produces m e t a l l i c no ise.
Damaged t racks caused by ca re less mounting produce pronounced
6.13 DETECTION OF BEARING DAMAGE BY SHOCK PULSE MEASUREMENT
When a r o l l i n g bear ing s u f f e r s f a t i g u e f a i l u r e , f l a k i n g occurs i n the r o l l i n g
element sur faces o r i n the t racks. A bear ing component coming i n t o con tac t
w i t h the damaged zone causes a mechanical shock, causing t r a n s i e n t v i b r a t i o n s
which a r e t ransmi t ted t o t h e bear ing housing. These v i b r a t i o n s generate e l e c -
t r i c vo l tages i n an accelerometer and t h e i r ampl i tude i s determined by a shock
pulse meter. The c o n d i t i o n o f t h e bear ing i s monitored, t a k i n g readings a t
s u i t a b l e i n t e r v a l s . I n c i p i e n t damage can, t h e r e f o r e be detected a t an e a r l y
stage and bear ing replacement planned acco rd ing l y .
Earphones can be used t o l i s t e n t o the rhythm o f t h e shocks and, prov ided
the damage i s no t t oo complex, t he rhythm w i l l suggest which component o f the
bear ing i s damaged.
Temperature i s a f u r t h e r method f o r gauging bear ing c o n d i t i o n , and bear ing
f a i l u r e s a r e sometimes preceded by a drop i n temperature fo l l owed by a r a p i d
increase i n temperature, u s u a l l y caused by metal- to-metal con tac t w i t h subse-
quent f a t i g u e f a i l u r e o r even se izure. The o l d method o f checkina the bear ing
temperature by f e e l i n g the housing i s n o t s a t i s f a c t o r y , and i n a p p l i c a t i o n s
where breakdowns cause maintenance problems i t i s usual t o use thermal-couples
pos i t i oned as c l o s e t o the bear ing o u t e r r i n g as poss ib le .
temperature method o f checking bear ing c o n d i t i o n i t must be noted t h a t t he re
w i l l always be a temperature r i s e w i t h new o r f reshly-greased bear ings.
o n l y a p p l i e s du r ing i n i t i a l running and once t h e new l u b r i c a n t has d i s t r i b u t e d
i t s e l f , t he temperature w i l l s t a r t t o f a l l .
When us ing t h e
This
6.14 FITS (SHAFT AND HOUSING)
Tolerances f o r the bore and ou ts ide diameter o f m e t r i c r o l l i n g bear ings a re
i n t e r n a t i o n a l l y s tandard ised and the requ i red f i t s a re achieved by s e l e c t i n g
s h a f t and housing to lerances us ing the IS0 t o le rance system ( incorporated i n
BS 4 5 0 0 : Part 1:1969). Only a smal l s e l e c t i o n o f t he IS0 to lerance zones need
t o be considered f o r r o l l i n g bear ing, and Fig.34 shows these r e l a t i v e t o the
bear ing bore (a) and bear ing ou ts ide diameter (b) .
The s h a f t t o le rance i s i nd i ca ted by a smal l l e t t e r and a number and the
housing bore by a c a p i t a l l e t t e r and a number. A t y p i c a l s h a f t and housing
bore to le rance combinat ion would be w r i t t e n j 6 - J7, the values f o r each being
obta ined from to le rance tab les .
164
b
Fig.34 Shaft and Housing F i t s .
The various symbol gradings a re as fo l lows:-
Shafts
Clearance Bearings always have clearance f i t i f the sha f t to lerance
T rans i t i on Bearings can be clearance or in ter ference f i t depending on
is llfll
the actual dimensions, i f the shaft to lerance is w i t h i n the
range 'lg - j". Inter ference Bearings w i l l always have an in ter ference f i t i f the shaf t
to lerance range i s w i t h i n "k - r'l.
Housings
C I earance Bearings w i l l always have clearance i f the housing tolerance
range i s I'G - H". Trans i t i on Bearings w i l l have a clearance or an in ter ference f i t depend-
ing on the actual dimensions i f the housing tolerance i s
w i t h i n the range "J - N".
In ter ference Bearings w i l l have i n te r fe rence f i t i f the housing tolerance is llpl l
The most important f ac to rs t o consider when se lec t i ng f i t s are;-
1 . Conditions o f r o t a t i o n
2 . Magnitude o f the load
3 . Temperature condi t ions
165
6.15 CONDIT IONS OF ROTATION
When a bearing r i n g i s t o r o t a t e r e l a t i v e t o the d i r e c t i o n of the r a d i a l
load, i t must be mounted w i t h s u f f i c i e n t in ter ference on the sha f t o r i n the
housing t o prevent " r o l l " o r "creept'.
dry surfaces, the surfaces undergo rap id wear. I t i s therefore essent ia l t ha t
the f i t used i s t i g h t enough t o
develop due t o the ac t i on o f the load. I f the load i s always d i rec ted towards
the same p o i n t on the r i n g , however, no creep o'ccurs and a clearance f i t i s
genera l ly permissible.
I f creep occurs between heavily-loaded
ensure t h a t no clearance e x i s t s and none can
Various loading condi t ions can be c l a s s i f i e d as fo l l ows ; -
( i ) Rotat ing inner r i n g load. The sha f t ro ta tes r e l a t i v e t o the d i r e c t i o n
o f the load so t h a t a l l po in t s on the inner r i n g t r a c k are subjected t o load
dur ing one revo lu t i on .
Example - Shaft w i t h b e l t d r i ve .
( i i ) Stat ionary inner r i n g load. The sha f t i s s ta t i ona ry r e l a t i v e t o the
d i r e c t i o n o f loading so t h a t the load i s always towards the same sector o f the
inner r i n g .
Example - Automobile f r o n t hub.
( i i i ) Stat ionary outer r i n g load. The bearing housing remains s ta t i ona ry
r e l a t i v e t o the d i r e c t i o n o f loading so t h a t t he load i s always d i rected
towards the same sector o f the outer r i n g .
Example - Shaft w i t h b e l t d r i ve .
( i v ) Rotating outer r i n g load. The bearing housing ro ta tes r e l a t i v e t o the
d i r e c t i o n o f loading so t ha t a l l po in ts on the outer r i n g t r a c k come under load
dur ing one revo lu t i on .
Example - Automobile f r o n t hub.
I n many app l i ca t i ons operat ing condi t ions cannot be re la ted t o any o f these
simple loading cases and va r iab le external loads o r out-of-balance forces
in f luence the d i r e c t i o n o f loading, an appropr ia te f i t being c l a s s i f i e d as
" d i r e c t i o n o f loading indeterminate". I n t h i s instance, bearing r i n g "creep"
can on ly be prevented by using an in ter ference f i t for both r i n g s and i n such
cases bearings w i t h increased r a d i a l clearance a re usua l l y necessary.
6.16 INFLUENCE OF LOAD AND TEMPERATURE
The load compresses the inner r i n g i n a r a d i a l d i r e c t i o n which s t re tches the
r i n g i n a c i r cumfe ren t ia l d i r e c t i o n and compresses the sha f t , thereby loosening
the f i t . S im i la r l y , as the bearing inner r i n g t rack is warmer dur ing operat ion
than the bearing bore, t h i s a l so has the ef fect of loosening the f i t o f the
166
bear ing inner r i n g on i t s j o u r n a l .
In t h e m a j o r i t y o f a p p l i c a t i o n s i t i s unnecessary to c a l c u l a t e t h e requ i red
i n te r fe rence s ince exper ience i s a good guide i n s e l e c t i n g s u i t a b l e f i t s f o r
d i f f e r e n t ope ra t i ng cond i t i ons and o n l y i n p a r t i c u l a r l y d i f f i c u l t or unusual
cond i t i ons i s i t necessary t o r e s o r t t o spec ia l c a l c u l a t i o n s .
6.17 BEARING APPLICATIONS
When design ing bear ing arrangements t h e r e a r e c e r t a i n bas ic r u l e s which
should be fo l l owed . Wherever p o s s i b l e o n l y one bear ing should be used f o r
l o c a t i o n purposes. This means t h a t t he bear ing ou te r r i n g should be he ld
a x i a l l y i n i t s housing w i t h t h e inner r i n g l oca ted on t h e s h a f t i n a s i m i l a r
manner. All o the r bear ings on t h e same s h a f t should be a x i a l l y f r e e , e i t h e r
on the s h a f t o r i n the housing, as shown on Fig.35. I f t h i s bas ic r u l e i s
ignored and two a x i a l l y located bear ings a r e used, any s h a f t expansion occu r r -
ing due t o generated o r ex te rna l heat could cause severed lock ing (pre loading)
across the bear ings, r e s u l t i n g i n premature bear ing f a i l u r e . I n c e r t a i n a p p l i -
ca t i ons us ing angular con tac t b a l l o r taper r o l l e r bear ings i t i s n o t p o s s i b l e
t o use only one bear ing f o r l o c a t i o n and the bear ings must be adjusted by end
covers.
FIG. 35 FTG. 36
With these bear ings g rea t ca re must be taken t o ensure t h a t t h ? bear ings can
accommodate a x i a l v a r i a t i o n s and i n sane a p p l i c a t i o n s a smal l gap i s l e f t
between the bear ing ou te r r i n g and the abutment cover, as shown i n Fig.36.
The a x i a l expansion problem can be resolved by i nco rpo ra t i ng a c y l i n d r i c a l
r o l l e r bear ing a t t he non- located p o s i t i o n , t h i s arrangement being used exten-
s i v e l y on e l e c t r i c motors where a b a l l bear ing i s used a t t he commutator end
and a r o l l e r bear ing a t t he d r i v e end. I n a d d i t i o n t o r e s o l v i n g the thermal
expansion problem such an arrangement has an added advantage i n t h a t a c y l i n -
d r i c a l r o l l e r bear ing has s i g n i f i c a n t l y h igher Ioad -ca r ry ing capac i t y (compared
t o a d imensional ly equ iva len t b a l l bear ing) which makes i t p a r t i c u l a r l y s u i t -
ab le f o r r e a c t i n g heavy d r i v e fo rces .
167
6.18 BEARING CARE
Cleanliness i s o f paramount importance when handling and mounting bearings,
but i n s p i t e o f the fac t t ha t bearings a re p rec i s ion made, one has on ly to look
around the average workshop t o see open bearings l e f t on benches o r i n dusty
environments. I t should be noted tha t i f fo re ign matter i s allowed t o enter a
bearing the r o l l i n g elements i n passing over i t dur ing serv ice can cause indent-
a t i ons leading t o f a t i g u e f a i l u r e i n the r o l l i n g elements o r t racks which can
shorten the l i f e o f the bearing considerably. As a general r u l e , i n order t o
ensure the bearings remain f r e e from impur i t i es , they should no t be removed
from t h e i r o r i g i n a l packing u n t i l they a r e required f o r i n s t a l l a t i o n . Before
packing bearings are u l t r a - s o n i c a l l y washed and coated w i t h a bearing preserva-
t i v e which mixes r e a d i l y w i t h most lubr icants , apar t from c e r t a i n c lay o r syn-
thetic-based greases, i n which case i t i s advisable t o contact the bearing manu-
facturers .
Although i n i t i a l washing o f bearings by the user i s no t recommended due t o
the p o s s i b i l i t y o f the washing f l u i d not completely evaporating, i f washing i s
necessary then the bearings should be washed w i t h c lean whi te s p i r i t a f t e r which
the bearings must be thoroughly d r i e d before adding l ub r i can t .
6.19 BEARING MOUNTING
I f the bearing inner r i n g i s t o be a t i g h t f i t on the sha f t , i t can be dr iven
onto the sha f t journal by means of a tubular d r i f t which should bear evenly
against the face o f the inner r i ng , as shown i n Fig.37. I f the bearing outer
r i n g i s t o be a t i g h t f i t i n i t s housing then the r i n g should be d r i ven i n t o the
housing by apply ing fo rce t o the outer r i n g .
Hollow drift for bearings with an Hollow drift for bearings with an interference fit both in the housing and
on the shaft interference fit on the shaft
I
F I G - 37 I FIG. 38
Pressure must n e v e r be appl ied t o the inner o r outer bearing r i n g i n order
t o mount the other r i n g which has a t i g h t f i t , otherwise there i s a danger o f
the t racks being indented and the bearing would probably f a i l i n serv ice a f t e r
168
a sho r t t ime. I f both the inner and o u t e r r i n g s a r e a t i g h t f i t , then a too l
o f t he type shown i n Fig.38 must be used which con tac ts both s i d e faces o f t h e
bear ing r i n g s .
Care must be taken t o ensure t h a t t h e bear ing r i n g being pressed on i s
c o r r e c t l y a l i gned , p a r t i c u l a r l y i n the case o f smal l r i n g s . Excessive m isa l i gn -
ment pu ts severe s t r e s s on the cage, which can r e s u l t i n premature bear ing
f a i l u r e .
C y l i n d r i c a l r o l l e r bear ings sometimes g i v e assembly problems due t o t h e
r o l l e r s being scored as they a r e fed over t h e t racks .
solved by us ing a spec ia l mounting s leeve as shown on Fig.39.
This problem can be r e -
Mounting sleeves simplify the mounting of cylindrical roller bearings and prevent damage to the tracks
FTG. 39
For l a r g e r o l l i n g bear ings w i t h c y l i n d r i c a l bores where the inner r i n g i s a
t i g h t f i t , mounting can be s i m p l i f i e d by hea t ing the complete bear ing i n an o i l
bath a t a temperature o f 70° - 8OoC above ambient.
t ranspor ted some d i s tance i t can be heated t o a h ighe r temperature t o compen-
sate f o r c o o l i n g down d u r i n g t r a n s i t .
t h i s way, b u t t h i s temperature should n o t be exceeded as, above 120°C, t h e r e i s
a danger o f reducing the bear ing m a t e r i a l hardness.
I f the bear ing needs t o be
Bearings can be heated up t o 12OoC i n
6.20 DISMOUNTING BEARINGS
I f an i n te r fe rence f i t has been used t o l o c a t e a bear ing on a s h a f t i t i s
essen t ia l t o use a s u i t a b l y designed withdrawal t o o l t o remove the bear ing.
The bas ic p r i n c i p l e s for mounting a bear ing app ly a l s o t o dismount ing.
mounting f o r c e must be app l i ed t o the bear ing r i n g having t h e i n t e r f e r e n c e f i t
and no t t o the loose r i n g and through t o the o the r r i n g by means o f the r o l l i n g
The d i s -
169
elements. Claw-type p u l l e t s a r e o f t e n used t o remove b a l l bear ings, but design-
e rs must take t h i s i n t o c o d s i d e r a t i o n du r ing the des ign s tage by i nco rpo ra t i ng
s u i t a b l e grooves i n the abutment shoulder ad jacen t t o t h e bear ing face t o enable
the bear ing inner r i n g t o be g r ipped by the p u l l e r c laws. N a t u r a l l y , the p u l l e r
f o r c e must be app l i ed t o the i nne r r i n g and no t t ransmi t ted through the r o l l i n g
elements. I f the inner r i n g i s i naccess ib le i t i s necessary t o p u l l on the
ou ter r i n g , bu t bear ing damage can be avoided by l o c k i n g the p u l l e r c e n t r e
screw w i t h a spanner and r o t a t i n g the c laws. The o u t e r r i n g w i l l then r o t a t e
du r ing the w i thdrawl process, d i s t r i b u t i n g t h e load over t h e t r a c k s and thereby
p reven t ing the p o s s i b i l i t y o f i nden ta t i ons . The bear ing i s then p u l l e d o f f
s u f f i c i e n t l y t o enable t h e inner r i n g t o be gr ipped w i t h t h e p u l l e r (F ig .40) .
Extractor
U FIG. 40
6.21 MOUNTING AND DISMOUNTING B Y O I L INJECTION
I n o rde r t o overcome many o f t h e mounting and dismount ing problems SKF, i n
1940, in t roduced an o i l - i n j e c t i o n techn ique which i s now w ide ly used i n many
branches o f eng ineer ing . O i l under h i g h pressure i s i n j e c t e d between the bear-
ing i nne r r i n g and s h a f t j o u r n a l du r ing mounting o r dismount ing. An o i l f i l m
i s formed which bo th separates and l u b r i c a t e s the con tac t sur faces . The o i l
separates the con tac t sur faces except f o r a narrow zone a t each end o f t he r i n g .
The su r face pressure i s g rea te r i n these zones due t o the i n f l u e n c e o f t he s h a f t
m a t e r i a l beyond t h e end o f t h e r i n g and t h e zones a c t as an o i l l ock which
r e t a i n s t h e o i l between the con tac t sur faces . When the o i l p ressure i s released
the o i l i s f o rced a u t o m a t i c a l l y back through t h e supp ly duc ts , thereby r e s t o r i n g
the o r i g i n a l f r i c t i o n .
The advantage o f u s i n g o i l - i n j e c t i o n i s t h a t t he f o r c e requ i red t o move the
component i s s i g n i f i c a n t l y reduced and t h a t by e l i m i n a t i n g d i r e c t con tac t and
the r e s u l t i n g f r i c t i o n between the con tac t sur faces , t he p o s s i b i l i t y o f damage
t o the sur faces o c c u r r i n g du r ing the mounting o r dismount ing process i s minimised.
170
A f u r t h e r advantage i s t h a t components can be dismounted o r ad jus ted w i t h o u t
the r i s k o f the f i t d e t e r i o r a t i n g .
I f the bear ing r i n g i s mounted on a tapered j o u r n a l w i t h a s e l f - r e l e a s i n g
taper , the r i n g w i l l be e j e c t e d w i t h some f o r c e when the o i l i s i n t roduced and
some f o r o f a x i a l r e s t r a i n t such as a n u t w i l l be necessary. I f , however, a
l ock ing taper i s used then an ex te rna l f o rce a d d i t i o n a l t o the o i l i n j e c t i o n
fo rce w i l l be requ i red . F igures 41, 42 and 43 show t y p i c a l arrangements.
Position of distribution groove in a tapered and a cylindrical seating for a rolling bearing
Cylindrical seating
FIG. 41
u
Tapered seating F I G . 42
The f i r s t two show a bear ing on a p a r a l l e l s h a f t and a bear ing on a tapered
s h a f t . The t h i r d shows o i l i n j e c t i o n be ing a p p l i e d t o a gear wheel mounted on
a p a r a l l e l s h a f t . I t should be no ted t h a t , w i t h a p a r a l l e l sha f t , once the
bear ing r i n g i s
and the r i n g cou ld l ock . I t has been found, however, t h a t i f the r i n g i s w i t h -
drawn r a p i d l y then i t w i l l t r a v e l over the remaining area w i t h o u t t o o much d i f f -
past t he o i l e n t r y h o l e t h e r e i s no longer any o i l pressure
i c u l t y
d rawa 1
by SKF
groove
When o
Large r o l l i n g bear ings w i t h a taper bore and mounted on adaptor o r w i th -
s leeves can be e a s i l y mounted o r dismounted us ing h y d r a u l i c nu ts designed
The h y d r a u l i c n u t cons i s t s o f an i n t e r n a l l y threaded s t e e l r i n g w i t h a
i n one face i n t o which i s f i t t e d an annu lar p i s t o n sealed w i t h O- r ings .
1 i s pumped i n t o the annu lar space behind the p i s t o n i t i s fo rced ou t -
wards, thereby f o r c i n g the bear ing on o r p u l l i n g the bear ing o f f the s leeve o r
s h a f t . F igu re 4 4 shows a h y d r a u l i c nu t be ing used t o mount a spher i ca l r o l l e r
bea r ing on an adaptor s leeve and Fig.45 shows an arrangement f o r d ismount ing a
bea r ing on an adaptor s leeve. An arrangement f o r d ismount ing a bea r ing f rom a
withdrawal s leeve i s shown on Fig.46.
171
Cylindrical mating surface having two distribution grooves
F I G . 43
HMV nut and stop rings in position to press loose an adapter sleeve
HMV nut for driving up a bearing on an adapter sleeve
HMV nut used to free a with- drawal sleeve
F I G . 46
6.22 CLEANING OF BEARINGS
A s p r e v i o u s l y mentioned, i t i s n o t necessary t o c lean bear ings taken d i r e c t
from t h e i r packages and any a t tempt t o do so cou ld be de t r imen ta l f rom bo th a
c lean l i ness and f u t u r e l u b r i c a t i o n aspect. There a r e occasions, however, when
bear ings need t o be dismounted, cleaned, and inspected i n s p i t e o f the f a c t
t h a t unnecessary dismount ing may cause d e t e r i o r a t i o n o f t he s h a f t and housing
f i t s as w e l l as p o s s i b l e damage t o the t racks .
172
A f t e r dismount ing f o r c lean ing purposes a l l used grease should f i r s t be care-
f u l l y removed. Small bear ings should be immersed i n w h i t e s p i r i t o r o t h e r c lean-
i ng f l u i d and s w i r l e d round, the res idua l grease and d i r t be ing removed us ing a
good q u a l i t y brush. Care must be taken t o ensure t h a t none o f the b r i s t l e s a re
t rapped between the cage and r o l l i n g elements.
A f t e r t h i s p r e l i m i n a r y washing the bear ings should be washed i n one o r two
a d d i t i o n a l b a t h s o f w h i t e s p i r i t .
should then be f lushed through the bear ing as i t i s s low ly ro ta ted . Under no
circumstances should the bear ing be r o t a t e d u n t i l the o i l passes through i t ,
otherwise any f o r e i g n p a r t i c l e s i n the bear ing w i l l be r o l l e d i n t o the t racks
causing i nden ta t i ons . I f the bear ings a re n o t intended f o r immediate mounting
they should be l u b r i c a t e d w i t h a good q u a l i t y o i l con ta in ing a wa te r - repe l l an t
a d d i t i v e .
A warm (lOO°C maximum) l i g h t minera l o i l
6.23 RECOGNITION OF BEARING FAILURES
Obviously premature bear ing f a i l u r e s occur , b u t i n the m a j o r i t y o f cases i f
the symptoms a r e recognised the cause o f the f a i l u r e s can be e l im ina ted .
r o l l i n g bear ings a r e p roper l y mounted, sealed, l u b r i c a t e d and maintained, they
w i l l run u n t i l f a t i g u e f a i l u r e ( f l a k i n g ) occurs on the bear ing i nne r o r ou te r
r i n g o r r o l l i n g elements. As a l ready mentioned, bear ing l i f e based on f a t i g u e
f a i l u r e can be p r e d i c t e d as a f u n c t i o n o f t he l i f e d i spe rs ion curve, and i f a
bear ing f a i l s w e l l sho r t o f i t s normal L,o l i f e i t i s impor tant to a s c e r t a i n
the cause o f the f a i l u r e be fo re f i t t i n g new bear ings.
I f
6.23.1 Wear
I f a bear ing housing i s inadequately sealed, abras ive p a r t i c l e s can e n t e r the
bear ing causing wear i n the t racks , r o l l i n g elements and cage, and i n c e r t a i n
bear ings wear i n the guide f langes. The worn sur faces a r e d u l l i n appearance
and an example o f worn t racks i s shown i n Fig.47. Wear can a l s o occur between
the inner r i n g bore and i t s j o u r n a l o r between the o u t e r r i n g o u t s i d e diameter
and housing bore due t o creep. Th is i s u s u a l l y the r e s u l t o f excessive c l e a r -
ance and i s p a r t i c u l a r l y ser ious because n o t o n l y the bear ing b u t a l s o the s h a f t
o r housing could be damaged and r e q u i r e rep lac ing . Creep between mat ing sur-
faces causes f r e t t i n g co r ros ion and wear, and p a r t i c l e s o f r u s t from the co r ro -
ded areas may a l s o e n t e r the bear ing causing wear i n the t racks and r o l l i n g e l e -
ments. Wear can a l s o be caused by inadequate l u b r i c a t i o n , the worn surfaces
hav ing a h i g h l y p lan ished f i n i s h . This type o f wear develops i n t o f a t i g u e
f a i l u r e .
173
F i g . 4 7 Worn Tracks due t o Abrasive P a r t i c l e s .
174
6.23.2 I n c o r r e c t Mount ing
F a u l t y mounting o f t e n r e s u l t s i n the bear ing be ing sub jec ted t o h i g h loads
which can cause m a t e r i a l f a t i g u e i n the bear ing r i n g s o r r o l l i n g elements w e l l
be fo re the nominal c a l c u l a t e d l i f e o f t he bear ing has been reached. Indenta-
t i o n s w i t h the same p i t c h as the r o l l i n g elements can be caused by the mount ing
f o r c e be ing a p p l i e d through t h e r o l l i n g elements. Dur ing s e r v i c e an ove r load
occurs each t ime a r o l l i n g element passes over an i nden ta t i on , and a f t e r a r e l a -
t i v e l y s h o r t t ime smal l fragments o f bea r ing m a t e r i a l break away, the c o n d i t i o n
be ing g e n e r a l l y r e f e r r e d t o as " f l a k i n g " . F igu re 48 shows the i nne r r i n g o f a
deep groove b a l l bea r ing w i t h advanced areas o f f l a k i n g on the r i gh t -hand s i d e
o f the t r a c k .
F ig .48 F l a k i n g caused by F a u l t y Mounting
F igu re 49 shows the i n n e r and o u t e r r i n g o f a b a l l bea r ing w i t h a f i l l i n g s l o t .
The f l a k e d wear on the r i gh t -hand s i d e o f the i nne r r i n g and on the l e f t -hand
s i d e o f t he o u t e r r i n g show t h a t f a i l u r e has been caused by a heavy t h r u s t l oad
due t o c r o s s - l o c a t i o n . Fa t igue f a i l u r e o f t h i s type begins a t a p o i n t below the
su r face o f t he t r a c k o r r o l l i n g elements. Small c racks develop, which g r a d u a l l y
work up t o the su r face where,under repeated ove r load a t such weak p o i n t s , even-
t u a l l y cause fragments t o break away, these fragments be ing r o l l e d i n t o o t h e r
p a r t s o f the t r a c k thereby i n i t i a t i n g f u r t h e r areas o f weakness.
175
Fig.49 F a i l u r e caused by a Heavy Thrus t Load due t o Cross Locat ion .
6.23.3 Cage F a i l u r e s
With an adequately l u b r i c a t e d bear ing opera t i ng i n normal c o n d i t i o n s , the
cage i s t h e most l i g h t l y - l o a d e d component. I f the l u b r i c a t i o n i s inadequate,
wear w i l l occur where the cage makes con tac t w i t h the r i n g s and r o l l i n g elements
and e v e n t u a l l y t he cage may f r a c t u r e .
assoc ia ted w i t h inadequate cage l u b r i c a t i o n . Excessive misal ignment o f t he
inner and o u t e r bear ing r i n g r e l a t i v e t o each o t h e r i s another cause o f cage
f a i l u r e and severe misal ignment can r e s u l t i n the cage, o r even the bear ing
r i n g s , c rack ing . Wear can a l s o be caused by f o r e i g n p a r t i c l e s e n t e r i n g the
bear ing, the p a r t i c l e s jamming o r wedging between the cage and r o l l i n g elements.
F igure 50 shows a spher i ca l r o l l e r bear ing case w i t h worn r o l l e r prongs and
wear i n t h e bottom o f the r o l l e r pockets. F igure 51 shows a more advanced case
where the prongs have a c t u a l l y sheared.
H i g h l y p lan i shed sur faces a r e u s u a l l y
176
F i g . 5 0 Spherical Rol ler Bearing Cage with Worn Rol ler Prongs and Wear in the Bottom of the Rol ler Pockets.
F i g . 5 1 More Advanced Case than F i q . 5 0 , with Prongs Sheared.
6.23.4 Smearing
Smearing i s a t rans fe rence o f m a t e r i a l from one body t o another when two
inadequately l u b r i c a t e d surfaces s l i d e aga ins t each o t h e r . Local s t r e s s con-
c e n t r a t i o n s a r e induced by smearing, and these produce cracks i n the sur face
l aye rs and subsequent f l a k i n g . F igu re 52 shows a smeared c y l i n d r i c a l r o l l e r
which has been caused by r a p i d a c c e l e r a t i o n o f t he r o l l e r i n the loaded zone
coupled w i t h inadequate o r i n c o r r e c t l u b r i c a t i o n . I f c y l i n d r i c a l r o l l e r s a r e
sub jec ted t o severe t h r u s t f o rces combined w i t h inadequate o r u n s u i t a b l e l u b r i -
ca t i on , smearing develops a t t he ends o f t he r o l l e r and on the gu ide f langes
Fig.52 Smeared C y l i n d r i c a l R o l l e r caused by Rapid Acce le ra t i on o f the R o l l e r i n t h e Loaded Zone coupled w i t h Inadequate o r I n c o r r e c t L u b r i c a t i o n .
o f bea r ing r i n g s , and Fig.53 shows smearing on the end o f a r o l l e r . Smearing
can a l s o occur on l i g h t l y - l o a d e d high-speed b a l l t h r u s t bear ings where the
g y r a t o r y moment may f o r c e the b a l l s t o s l i d e t a n g e n t i a l l y t o t h e d i r e c t i o n o f
178
Fig.53 Smearing Caused by Severe Thrust Forces combined with Inadequate or Unsuitable Lubrication.
rolling, causing spiral-shaped smearing. Lightly-loaded high-speed ball thrust
bearing must therefore always have a small pre-load or spring load when mounted
as given on Fig.22.
6.23.5 Vibrations
A phenomenon known as 'False Brinelling' can occur in rolling bearings fitted
to stationary machines, due to vibrations from adjacent machinery. Bearings in
ships ancillary equipment subject to vibrations from the ship's machinery are
particularly prone to this damage and this can apply to bearings in machines
being transported by sea.
179
Fi9.54 Vibration Damage.
Fig.55 Vibration Oamage.
180
Rotor clamp for preventing damage due to vibration during transport
FIG. 56
Fig.57 Corrosion a t the Contact Surfaces due t o Water.
181
A c h a r a c t e r i s t i c f e a t u r e o f v i b r a t i o n damage i s t h a t t he damaged areas a r e
spaced a t t he same p i t c h as the r o l l i n g elements.
movements a t the con tac t surfaces between the r o l l i n g elements and the r i ngs ,
and the r e s u l t i n g wear causes microscopic p a r t i c l e s o f m a t e r i a l t o break away.
These p a r t i c l e s o x i d i s e and, on m ix ing w i t h the l u b r i c a n t , cause a lapping
ac t i on , and hence an increase i n the o x i d a t i o n ra te . The h igher the f requency
o f t he v i b r a t i o n a l movement a t the con tac t sur faces, t he more r a p i d the damage,
each r o l l i n g element g r a d u a l l y forming a cavity’.
The v i b r a t i o n s induce small
C y l i n d r i c a l r o l l e r and needle r o l l e r bear ings a r e more prone t o t h i s type o f
damage than b a l l bear ings and spher i ca l r o l l e r bear ings, and one method o f
overcoming the problem i s t o use spr ing- loaded b a l l bear ings.
should be approx imate ly equal t o 20 Newtons per m i l l i m e t e r o f s h a f t diameter.
Before pre- loading, however, i t i s necessary t o check t h a t the combinat ion o f
ex te rna l f o rces and a d d i t i o n a l pre- load does n o t over load the bear ing.
The pre- load
Typica l examples o f v i b r a t i o n damage t o bear ings du r ing t r a n s p o r t a t i o n due
t o r e l a t i v e movement between the s h a f t and housing, whether by sea, r a i l , o r
road, can be resolved by d r i v i n g a wooden wedge between the s h a f t o r coup l i ng
and a robust p a r t o f the c r a t e .
l a t i v e t o the housing by means o f a clamp, as shown i n Fig.56.
A l t e r n a t i v e l y , the s h a f t can be locked re -
6.23.6 Rust and Other Types o f Corros ion
R o l l i n g bear ings w i l l r u s t if water o r mois ture i s present and the bear ings
a re n o t p ro tec ted by a l u b r i c a n t con ta in ing a r u s t - i n h i b i t o r . P i t t i n g develops
i n the corroded areas, r e l e a s i n g small p a r t i c l e s o f r u s t , and i f the re a re such
areas i n the t r a c k s the r u s t mixes w i t h grease and a c t s as a lapping agent.
Fig.57 shows c o r r o s i o n on a spher i ca l r o l l e r bea r ing and i n t h i s case the
grease was unable t o p r o t e c t the bear ing f rom water.
t o the water has r e s u l t e d i n c o r r o s i o n a t t he con tac t sur faces between the
r o l l e r s and r i n g s .
E l e c t r o l y t i c a c t i o n due
6.23.7 Passage o f E l e c t r i c Current Through Bear ings
E l e c t r i c c u r r e n t passing through a r o l l i n g bear ing causes damage t o the
t racks o r r o l l i n g elements, which can r e s u l t i n premature f a i l u r e o f the bearing.
A l t e r n a t i n g and d i r e c t c u r r e n t have a s i m i l a r e f f e c t and where the re i s a r i s k
o f e l e c t r i c a l leakage, r o l l i n g bear ings must be p ro tec ted aga ins t t he passage
o f c u r r e n t . I f a cont inuous c u r r e n t passes through a r o t a t i n g bear ing a dark
co loured f i l m i s produced on the t racks and r o l l i n g elements which g radua l l y
develop i n t o a washboard fo rma t ion as shown i n Fig.58.
The b a l l s i n b a l l bear ings subjected t o e l e c t r i c c u r r e n t do n o t u s u a l l y
develop the washboard surface, b u t become u n i f o r m l y dark-coloured over the whole
surface. Th is i s due t o the b a l l s sp inn ing when the bear ing ro ta tes . The
182
Fig.58 \,/ashboard Effect Caused by Continuous Passage of Electric Current.
rings have fluting similar to that on rollers, but the bands are much narrower.
On applications such a s traction motors, craters and burns occur instead of
the washboard formation and a typical burn is shown on Fig.59. Generally
pitting from electrical leakage does not mean rapid failure and on railway
applications, it is known that pitted bearings have run for very long periods
before requiring replacing. The main problem is removing the debris released
during the formation of the craters, and provided this is carried out and
fresh lubricant added, preferably by means of a grease escape valve, the
bearings should function successfully for a further period.
There are many problems relating to ball and roller bearings, but providing
the basic principles governing bearing selection and a knowledge of lubrication
is developed, then the calculated nominal bearing life can be achieved.
183
F i g . 5 9 Typical Burn Caused by E l e c t r i c Current Leakage i n A Traction Motor.
184
I- 4 PRACTICAL GEAR TRIBOLOGY
T. I . FOWLE, Consul tant, Tenterden
7.1 INTRODUCTION
The i d e n t i f i c a t i o n o f the causes o f t he va r ious forms o f d i s t r e s s appear ing
on gear t e e t h i s seldom an easy mat ter because o f t h e i r g rea t v a r i e t y and
because few engineers have the oppor tun i t y t o see even a m i n o r i t y o f them a t
f i r s t hand. The troubdeshoot ing c h a r t s g iven a t the end o f & i s chapter a re
intended t o s i m p l i f y t he i d e n t i f i c a t i o n of p o s s i b l e causes and the s e l e c t i o n o f
app rop r ia te remedies f r o m the observed symptoms. Systemat ic cons ide ra t i on o f
the var ious p o s s i b i l i t i e s should a t l e a s t narrow down the number and suggest
t e s t s which might be app l i ed t o con f i rm the f i n a l i d e n t i f i c a t i o n . The fo l l ow ing
notes a re g i ven t o a m p l i f y and e x p l a i n the reasons f o r the most impor tant
e f f e c t s .
7 .2 ALIGNMENT
There are two aspects o f gear a l ignment : ex te rna l and i n t e r n a l . E r ro rs i n
ex te rna l a l ignment, t h a t i s a l ignment w i t h the connected machines, p lace over-
loads on the bear ings and coupl ings, r i s k i n g f a i l u r e o r a t l e a s t no i se which
could be erroneously a t t r i b u t e d t o the gears themselves. E r ro rs i n i n t e r n a l
a l ignment cause uneven d i s t r i b u t i o n o f the load a long the gear t e e t w i t h conse-
quent r i s k o f damage and no isy running and a r e o f p a r t i c u l a r concern i n t h i s
chapter.
With p a r a l l e l - s h a f t gears the re a re b a s i c a l l y two e r r o r s o f i n t e r n a l a l i g n -
ment: the s h a f t s may n o t be p a r a l l e l , and they may n o t be i n the same p lane.
Both these e r r o r s may, o f course, be present toge the r . The va r ious combinat ions
produce p a t t e r n s of con tac t , o r o f damage such as p i t t i n g o r s c u f f i n g , as shown
i n F igure 1. Only i n a few cases a r e gears prov ided w i t h means for a d j u s t i n g
the p a r a l l e l i s m o f the two sha f t s , and indeed, w i t h modern machining i t i s
u n l i k e l y t h a t the sha f t s w i l l n o t be p a r a l l e l . However, i f t h e gearcase i s n o t
evenly supported on i t s f e e t by the foundat ions, t he s h a f t s w i l l n o t be i n the
same plane.
Checking t h a t the two s h a f t s a re i n the same p lane i s best c a r r i e d o u t by
removing the top o f the gear cas ing and, i f necessary, t he top halves o f bearings
o r bear ing keeps.
ence i n diameter suppor t ing a p r e c i s i o n s p i r i t l e v e l can then be placed across
A s t r a i g h t edge w i t h a spacer b lock t o a l l o w f o r any d i f f e r -
185
both ends o f t he s h a f t s i n t u r n t o check t h a t t h e ends a r e i n the same plane.
An accuracy o f 1 pe r 60 000 i s s a t i s f a c t o r y . The th ickness o f the shims requ i red
can then be e a s i l y determined.
€2 Gear
€3
iii
Q Gear
ozzl Gear
Single helical and spur
Double helical
axes parailei but not co-planar
Single helical and spur
axes co-pianar but not parailei
Single helical and spur
axes neither co-planar nor parallel
Fig .1 I n t e r n a l misal ignment p a t t e r n s on p a r a l l e l s h a f t gears
I n t e r n a l a l ignment can a l s o be checked by de termin ing the ex ten t o f t o o t h
con tac t w i t h marking b lue .
t h e i r bear ings and t h a t t he j o u r n a l s a r e l u b r i c a t e d , a t h i n coa t ing o f marking
b l u e i s a p p l i e d i n a t h i n a x i a l band on one o f the gears.
t u rned so t h a t t h e marking i s t r a n s f e r r e d on to the o t h e r gear.
then be taken by means of a S e l l o t a p e impression.
depends on the c o n d i t i o n s
i s based on B S 1807 f o r t u r b i n e gears and s i m i l a r d r i v e s .
A f t e r ensu r ing t h a t t h e j o u r n a l s cannot l i f t ou t o f
The gears a r e then
A record may
The degree o f con tac t required
o f s e r v i c e as i n d i c a t e d i n F igu re 2(a) t o (d) which
' S p l i t Marking' as
186
shown i n F igu re 2 (e ) , which i s due t o the hob n o t hav ing been concen t r i c w i t h
i t s mandrel, i s undes i rab le because i t prevents t h e most favourab le p a r t o f the
t o o t h p r o f i l e , i .e . t h a t w i t h the l e a s t s l i d i n g , f rom suppor t i ng the load and
t r a n s f e r s i t ins tead t o l e s s favourab le p a r t s .
l a )
(6)
25% 25% 50%
T I I
I I
I I
I I
F ig.2 Contact area
(a ) , (b) and (c ) represent t h e minimum con tac t areas requ i red by BS 1 8 0 7 f o r t u r b i n e gears and s i m i l a r d r i v e s , c lasses B, A 2 and A1 r e s p e c t i v e l y . The s p e c i f i c a t i o n does n o t s t i p u l a t e the d i s t r i b u t i o n o f t h e con tac t areas, o n l y t h e i r depth and t o t a l l eng th . Classes A 2 and A1 a r e f o r gears w i t h p i t c h l i n e speeds over 50 m / s , A 2 be ing suggested f o r 50-100 m/s and A1 f o r 50-150 m/s . (d ) represents the requirements o f some a u t h o r i t i e s f o r p r e c i s i o n gears. ( e l S p l i t marking.
187
Wi th n o n - p a r a l l e l s h a f t gears, such as bevel gears and worm gears, a l lowance
has t o be made f o r t he i n e v i t a b l e d i s t o r t i o n under load, as shown i n F igures 3 and 4. I n a l l cases con tac t shou ld n o t extend t o t h e ends o f t he t e e t h as loads
No load Full load
F ig .3 Contact marks on bevel gears
' .+
No load Full load
F ig .4 Contact marks on worm gears
t h e r e would be h i g h l y concent ra ted .
be c learance where the worm en te rs t h e con tac t , even a t t he h ighes t loads, so
t h a t t h e o i l on i t s su r face i s no t scraped o f f by t h e edge o f t he too th . I f
t h i s c learance i s n o t a v a i l a b l e the f r i c t i o n and wear a r e g r e a t l y increased and
the t r a n s m i t t e d power 1 im i ted .
I n t h e case o f worm gears the re must a l s o
7 . 3 TOOTH ACTION
An unders tand ing o f t o o t h a c t i o n he lps t o e x p l a i n many aspects o f gear t r i b o -
logy. I n spur, h e l i c a l and bevel gear t o o t h a c t i o n i s as represented i n F ig .5 .
The p o i n t o f con tac t moves con t inuous ly over bo th t e e t h and i n t h i s sense they
r o l l over one another . The t r a n s i e n t n a t u r e o f the con tac t enables very heavy
pressures t o be c a r r i e d . A t t he p i t c h p o i n t bo th t o o t h sur faces a r e moving i n
t h e same d i r e c t i o n a t t he same speed and so momentar i ly r o l l over one another
w i t h o u t s l i d i n g . A t a l l o t h e r p o i n t s the sur faces a r e moving a t d i f f e r e n t
speeds so t h a t t h e r e i s s l i d i n g as w e l l as r o l l i n g .
188
Dr
Opposing surface slides in these directions
Line of action
Fig.5 Contact c o n d i t i o n i n Spur, H e l i c a l and Bevel t ee th . The hatch ing i nd i ca tes the d i r e c t i o n o f d i s t o r t i o n o f t he su r face layers.
On the sur face o f t he d r i v i n g gear the s l i d i n g i s always away from the p i t c h
l i n e , w h i l e on the d r i v e n t o o t h i t i s always towards the p i t c h l i n e . When
l u b r i c a t i o n i s
away from the p i t c h l i n e o f the d r i v e r and t o p i l e i t up a t the p i t c h l i n e o f
the d r i v e n too th .
( r e f e r symptom 1 1 o f the Trouble-shoot ing c h a r t ) .
of s l i d i n g i s t h a t on t h e dedendum su r face o f bo th gears any cracks a r e p u l l e d
open i n advance o f the contact . The cracks being a l s o i n c l i n e d i n the most
favourable d i r e c t i o n , t he o i l r e a d i l y en te rs the crack. Fur ther movement o f
the contact then seals the mouth o f the c rack and compresses the o i l i n i t t o
extend the crack s t i l l f u r t h e r .
s lope away from the on-coming con tac t which, fur thermore, tends t o push the
s ides o f the crack together i n advance o f con tac t so t h a t o i l i s n o t encouraged
t o en te r the crack. The r e s u l t i s t h a t f a t i g u e p i t t i n g tends t o occur almost
e x c l u s i v e l y on the dedendum su r face o f gear teeth, bo th d r i v e r and d r i ven .
inadequate the s l i d i n g shear forces thus tend t o draw m a t e r i a l
Th i s produces the c o n d i t i o n known as groov ing and r i d g i n g
Another e f f e c t o f t h i s system
I n con t ras t , any cracks on the addendum surfaces
An impor tant f e a t u r e o f spur and h e l i c a l gears i s t h a t t he re i s no s l i d i n g
along the l i n e o f con tac t , which i s a l s o v i r t u a l l y t he case w i t h bevel gears.
I n hypoid gears and worm gears, however, t he re i s a considerable component o f
s l i d i n g a long the l i n e o f contact which makes f o r g rea te r d i f f i c u l t y i n
189
l u b r i c a t i o n , because any a s p e r i t y on one su r face i s i n con tac t w i t h the o t h e r
f o r a g rea te r t ime and d is tance. I n worm gears p a r t i c u l a r l y , t h i s s l i d i n g con-
s ide rab ly increases f r i c t i o n and temperature r i s e . Much o f the f r i c t i o n , more-
over, merely serves t o d i s t o r t t he wheel towards c r i t i c a l cond i t i ons o f contact
(see symptom 20).
7 .4 TOOTH SURFACE DISTRESS
O f t he th ree main forms o f sur face d i s t r e s s : p i t t i n g , ab ras i ve wear and
s c u f f i n g o r adhesive wear, t he f i r s t tends t o occur a t the lower end o f the
speed range w h i l e s c u f f i n g tends t o occur a t t he h ighe r end as i n d i c a t e d i n
F igure 6 . S c u f f i n g may f o low p i t t i n g , bu t p i t t i n g does no t occur where the re
i s s c u f f i n g o r where the re i s ab ras i ve wear. The idea l c o n d i t i o n o f f u l l - f i l m
o r elastohydrodynamic l u b r c a t i o n (EHL) occurs a t low loads and h igh speeds.
I 'tr U 0 4
Bending fatigue failure, impact f radure
Partial E.H.L.
Speed - Fig.6 Zones o f gear t o o t h d i s t r e s s .
The d i f f e r e n t slopes o f l i m i t s f o r p i t t i n g i n hard and i n s o f t gears i l l u s t r a t e separate ly the two e f f e c t s o f speed on f i l m th ickness and t o o t h e r r o r s .
7.4.1 P i t t i n g
About the most common form o f su r face d i s t r e s s i n s o f t o r through-hardened
gears i s p i t t i n g , which i s a type o f f a t i g u e f a i l u r e .
p i t t i n g i s r a t h e r unusual i n surface-hardened gear ing. Var ious sub-types and
As i nd i ca ted i n F igure 6 ,
190
causes a r e l i s t e d aga ins t sympton 2, and an example i s shown i n F igu re 7. I n
t h i s F igu re t h e t y p i c a l o y s t e r - s h e l l shaped c r a t e r s produced by t h e o i l p r i s i n g
t h e fragments loose can be seen.
F ig .7 Sur face f a t i g u e p i t t i n g .
Note c h a r a c t e r i s t i c o y s t e r - s h e l l shape on t h e l e f t . Note a l s o t h e e f f e c t s o f ' s p l i t mark ing ' . (Photo cour tesy o f She l l I n t e r n a t i o n a l Co.Ltd.)
A s well as t h e s t r e n g t h o f the m a t e r i a l , t he o i l f i l m th ickness , o r r a t h e r
the r a t i o o f t h e o r e t i c a l o i l f i l m th ickness t o the su r face roughness o f t he
harder t o o t h , i s a most impor tan t f a c t o r , and t h i c k f i l m s o r f i n e f i n i s h e s o r
bo th a r e d e s i r a b l e t o reduce t h e inc idence o f p i t t i n g . Shock- loading i s a l s o
t o be avoided as much as p o s s i b l e as i t cons ide rab ly increases the su r face
s t resses . T h i s i s i n d i c a t e d by t h e nega t i ve s lope o f t he p i t t i n g l i m i t f o r s o f t
gears i n F igu re 6.
P i t t i n g may be e i t h e r o f t h e i n i t i a l or t h e p rog ress i ve type . Wi th t h e
former t h e removal o f a s p e r i t i e s and prominent areas by p i t t i n g increases the
ac tua l area o f con tac t and reduces the s t resses so t h a t i t a r r e s t s i t s e l f .
Wi th over loaded sur faces , however, p i t t i n g c o n t i n u a l l y reduces the area o f
191
ac tua l contact and promotes f u r t h e r p i t t i n g . But i f one gear i s a b l e t o r e s i s t
p i t t i n g and can ma in ta in i t s shape, p i t t i n g w i l l progress less r a p i d l y on the
o the r (see sympton 2 ( f ) ) .
The s i z e o f the p i t s i s g e n e r a l l y r e l a t e d t o the s i z e o f the gears, bu t there
may be wide v a r i a t i o n s due, perhaps, t o su r face a s p e r i t i e s and o t h e r d e t a i l s .
I n p a r t i c u l a r , where the o i l f i l m th ickness i s r e l a t i v e l y very t h i n the sur face
t r a c t i o n can be very h i g h and produce very l a r g e shal low p i t s (see 2 ( j ) ) .
The e f f e c t o f extreme pressure (EP) a d d i t i v e o i l s i s va r ied . I n l abo ra to ry
t e s t s us ing very accurate gears o r d i scs , EP o i l s a r e g e n e r a l l y found t o accel -
e r a t e p i t t i n g , bu t where p r a c t i c a l gears hav ing su r face undulat ions o r s i m i l a r
inaccurac ies a r e concerned, EP o i l s a r e o f t e n a b l e t o de lay o r suppress the
problem. Apparent ly, a t l e a s t some EP agents can prevent the c r e s t s o f such
undulat ions from work-hardening so the con tac t zones can deform t o increase t h e i r
area and thus reduce s t resses and temperatures.
7 . 4 . 2 S c u f f i n g
Severe adhesive wear takes several d i f f e r e n t forms i n gear t e e t h according t o
cond i t i ons and i s g i ven even more d i f f e r e n t names. For example, s c u f f i n g ,
scor ing, g a l l i n g and p luck ing . The nomenclature i s even f u r t h e r confused by the
fac t t h a t what i s c a l l e d s c u f f i n g i n the UK i s known as sco r ing i n the USA. I n
l abo ra to ry t e s t r i g s the va r ious forms a r e no t t oo d i f f i c u l t t o recognise and
are reasonably w e l l def ined, as f o r example s c u f f i n g and sco r ing i n I P 166 f o r
the I A E Gear Rig, b u t i n non-standardised t e s t s d i s t i n c t i o n s a re sometimes made
between high-speed and low-speed s c u f f i n g ( l ess than about 4 m/s p i t c h l i n e
speed accord ing t o some a u t h o r i t i e s ) and between se l f -p ropaga t ing and s e l f -
hea l i ng forms i n both cases.
I n p r a c t i c a l gears appearances may vary t o an even g rea te r ex ten t , bu t a re
d i f f e r e n c e s i n degree, n o t o f k i n d . I n comparison w i t h l abo ra to ry t e s t gears i t
should be borne i n mind t h a t i n the l a t t e r a l a rge p a r t o f the power i s used t o
overcome t o o t h f r i c t i o n i n the t e s t gears so t h a t when l u b r i c a t i o n f a i l s and
s c u f f i n g occurs, t h e r e i s a considerable increase i n the power requi red. There
i s thus an immediate f a l l i n speed which f u r t h e r increases f r i c t i o n and noise,
as w e l l as producing smoke and sometimes sparks. Under these cond i t i ons
s c u f f i n g o f t e n does n o t l a s t f o r more than a f r a c t i o n o f a minute and, i n the
FZG r i g , f o r example, never more than 15 minutes. I n p r a c t i c e , however, the
e x t r a power absorbed when gear t e e t h s c u f f i s on l y a very smal l f r a c t i o n of the
power being t ransmi t ted so t h a t t he onset o f the damage i s u s u a l l y unnot iced and
may cont inue f o r many hours.
as ' g a l l i n g ' a r i s e s .
Under such cond i t i ons the very severe type known
Except p o s s i b l y a t low speeds, scu f f i ng appears t o be an e s s e n t i a l l y thermal
phenomenon due t o t o o t h s l i d i n g f r i c t i o n . The speed of s l i d i n g depends mainly on
192
Fig.8
the pe r iphe ra l speed o f the gears and the s i z e o f the tee th . And s ince l a r g e
t e e t h can be more h e a v i l y loaded than smal l t e e t h , t o o t h s i z e tends tohave a
preponderant i n f l u e n c e on the inc idence o f s c u f f i n g , as i n d i c a t e d i n Table 7.1
below, which summarises general exper ience, a l s o i n sec t i on 3 ( f ) .
Scuf fed p i n i o n t o o t h f rom FZG t e s t . (Photo cou r tesy o f She l l I n t e r n a t i o n a l Co. Ltd . )
Table 7.1 Danger o f s c u f f i n g i n spur, h e l i c a l and bevel gears.
Module p i t c h (mm) Danger o f s c u f f i n g
1.25 None
2.5
5 10
Only a t very h i g h speeds w i t h t h i n o i l .
A t moderate speeds even w i t h medium o i l .
A t low speeds even w i t h heavy o i l .
193
A s i n d i c a t e d i n t h e above Tab le , and a l s o i n symptom 3 ( d ) , t h e h i g h e r t h e
v i s c o s i t y grade o f t h e o i l t h e g r e a t e r t h e p r o t e c t i o n i t a f f o r d s a g a i n s t s c u f f -
ing . I f t h e grade i s t o o v iscous , however, t h e r e cou ld be d i f f i c u l t i e s i n
s t a r t i n g up f rom c o l d and t h e power loss and tempera ture r i s e i n high-speed
bear ings migh t be excess ive . Compromises have t o be made, t h e r e f o r e , and l i g h t e r
grades have t o be used f o r high-speed gears which, i n any case, a r e more accur -
a t e l y made and have s m a l l e r t e e t h and c o r r e s p o n d i n g l y s m a l l e r loads . V i s c o s i t y
grade recommendations f o r spur , h e l i c a l and bevel gears a r e , t h e r e f o r e , o f t e n
g iven i n terms o f speed o n l y , as i n T a b l e 7.2 below, which i s f o r gears opera-
t i n g a t ambient temperatures between about 10°C and 25OC.
T a b l e 7 . 2
O i l v i s c o s i t y grades f o r spur, h e l i c a l and bevel gears
P i t c h l i n e speed Range o f IS0 V i s c o s i t y Grades
m/s f t / m i n cSt a t 4OoC
0.5 100 460 - 1000
1 . 3 250 320 - 680
2.5 500 220 - 460
5 1000 150 - 320
12.5 2500 100 - 220
25 5000 68 - 150
50 10000 46 - 100
H igher v i s c o s i t y grades may be needed where t h e ambient tempera ture exceeds
2S0C, where t h e gears a r e s u b j e c t t o shock loads, o r where b o t h gears a r e made
o f through-hardened n icke l -chrome s t e e l s .
where t h e ambient tempera ture i s below 10°C, o r where t h e t e e t h have been g i v e n
a s c u f f - r e s i s t a n t c o a t i n g t o a s s i s t r u n n i n g - i n .
Lower v i s c o s i t y grades may be used
For d o u b l e - r e d u c t i o n g e a r i n g t h e low v i s c o s i t y end of t h e range f o r t h e low
speed t r a i n shou ld be taken, and f o r m u l t i p l e t r a i n s t h e mean f o r t h e two slowest
t r a i n s .
Worm gears need t o be t r e a t e d r a t h e r more generous ly because t o o t h f r i c t i o n
i s o f much g r e a t e r importance. To m i n i m i s e t o o t h f r i c t i o n H V I o i l s a r e p re-
f e r r e d t o o t h e r types and v i s c o s i t y grades a r e h i g h e r than f o r o t h e r types o f
gear ing , a s i n d i c a t e d i n Tab le 7 .3 below.
For d e s i g n e r s , t h e r i s k o f s c u f f i n g can be assessed by t h e use o f B l o k ' s
C r i t i c a l Contact Temperature t h e o r y o r t h e Niemann and S e i t z i n g e r b u l k t o o t h
temperature c r i t e r i o n [l] and a d e c i s i o n made on whether t o recommend t h e use
o f s t r a i g h t o r EP o i l s . For f i e l d use s i m p l e r guidance i s needed and cons ider -
a t i o n o f T a b l e 7.1 i s recommended. EP o i l s should, f u r t h e r m o r e , be cons idered
f o r p a r a l l e l - s h a f t gears where t h e t o o t h c o n t a c t markings do n o t reach t h e ex ten t
194
requ i red by t h e speed as i nd i ca ted i n F igure 2, where the gears are subject t o
dynamic over loads, and where t h e gears s tep up the speed.
Table 7.3 IS0 o i l v i s c o s i t y grades requ i red f o r enclosed worm gears
Output o r wormwheel r.p.m. Centre d i s tance ( inches) (mm) 50 and under 800 150 and over
2.5 64 HVI 1000 HVI 460 HVI 320 4 100 460 320 320
10 250 3 20 220 220
20 500 320 2 20 220
EP o i l s should g e n e r a l l y be used f o r hypoid gears because o f t h e i r h igh com-
ponent o f s l i d i n g a long the l i n e o f con tac t , t h e i r l a r g e t e e t h and t h e i r l i a b i l -
i t y t o considerable dynamic over loads itl automot ive se rv i ce . They should not ,
however, be used f o r worm gears un less the o i l temperature i s c o n s i s t e n t l y below
about 6OoC because o f the r i s k o f excessive c o r r o s i v e wear o f t he bronze. Since
s c u f f i n g i s a thermal phenomenon, overheat ing can be a cause, and i t s courses
and the approp r ia te counter-measures d e t a i l e d under symptom 23 should be consi -
dered.
7.4.3 Abras ive Wear
There a r e t w o k inds of ab ras i ve wear. One, as i n symptom 4, where a rough,
hard su r face rubs aga ins t a s o f t e r one, which i s known as 'two-body' abras ion;
t he o the r , as i n symptom 5, where ab ras i ve d i r t a c t s between two rubbing surfaces,
which i s known as ' three-body ' abrasion.
The f i r s t k i n d occurs where a rough surface-hardened p i n i o n runs aga ins t a
s o f t s t e e l o r p l a s t i c wheel, and a l s o where a rough surface-hardened worm runs
against a bronze wheel. I n the l a t t e r case the associated h i g h f r i c t i o n may so
d i s t o r t the gears t h a t con tac t i s brought on to the i n l e t edge o f the wheel t e e t h
and f r i c t i o n f u r t h e r increases t o the ex ten t t h a t power t ransmiss ion may be
l i m i t e d (see symptom 20).
smoother f i n i s h , e.g. 0.5 t o 0.2 microns Ra.
The remedy i s t o stone o r l a p the harder member t o a
Typ ica l contaminants causing three-body abras ion a r e sand and m i l l s c a l e .
F i l t r a t i o n down t o 20 microns (nominal) i s normal ly the best p r a c t i c a b l e so lu t i on ,
though smal ler p a r t i c l e s can s t i l l cause abras ion. The maximum amount o f conta-
minant t o l e r a b l e i n the o i l depends on i t s hardness r e l a t i v e t o t h a t o f the gears
concerned. For example, i n s t e e l m i l l p r a c t i c e a t y p i c a l l i m i t f o r m i l l s c a l e i s
0.3% w by D I N 51 592 , i .e. re ta ined on a 0.45 micron m i l l i p o r e f i l t e r ) . The
corresponding l i m i t f o r sand would be 0.1% w. Greases a r e p a r t i c u l a r l y l i a b l e
196
t o pe rm i t abras ion s i n c e they tend t o keep the ab ras i ve contaminants and t h e
wear p roduc ts i n t h e v i c i n i t y o f t h e mesh.
F i g . 9 Three-body ab ras ion o f a gear t o o t h by sand i n t h e o i l
Note t h e sho r t l e n g t h o f t h e scars.
(Photo cour tesy o f She l l I n t e r n a t i o n a l Co.Ltd.1
7 .4 .4 Other forms o f gear wear
The o t h e r forms o f gear wear l i s t e d under symptoms 6 t o 19 a r e r e l a t i v e l y
r a r e and space does no t pe rm i t spec ia l d i scuss ion here .
7 . 5 PROBLEMS I N LUBRICATION SYSTEMS
The main problems a r i s i n g i n supp ly ing and c o n t r o l l i n g t h e f l o w o f o i l t o
and from the t o o t h mesh a re : ove rhea t ing , windage ( i n ve ry h i g h speed gears) ,
196
excess ive foaming ( i n ba th l u b r i c a t e d systems) and excess ive a e r a t i o n ( i n l a r g e
c i r c u l a t i o n systems).
One f requent source o f ove rhea t ing i n ba th systems i s when one gear d i p s too
deeply i n t o t h e ba th (see symptom 23(a) , (b) and ( c ) ) . Th i s can sometimes be
avoided by sur round ing t h e lower p a r t o f t h e d i p p i n g gear by a spec ia l t rough.
When t h e gear i s a t r e s t t he t rough f i l l s w i t h o i l , bu t d u r i n g o p e r a t i o n t h e
excess i s thrown o u t and t h e meshing t e e t h rece ive s u f f i c i e n t o i l f o r l u b r i c a -
t i o n and c o o l i n g f rom ho les i n the bottom o f t h e t rough.
Above about 15 m/s p i t c h l i n e speed even t h i s spec ia l fo rm o f ba th l u b r i c a t i o n
tends t o cause excess ive power loss and temperature r i s e so t h a t spray l u b r i c a -
t i o n f rom c i r c u l a t i o n systems i s no rma l l y adopted. Pump pressures a r e u s u a l l y
i n t h e range 0.3 t o 1.5 bar . Pressures below 0.3 bar may be s a t i s f a c t o r y , b u t
whenever t h e gauges read below 1 ba r a v i s u a l check should be made t o ensure
t h a t t he o i l i s reach ing t h e p a r t s requ i red . (Upper l i m i t s t o o i l p ressure may
be d i c t a t e d by t h e s a f e t y l i m i t s f o r pumps, coo le rs , f i l t e r s , e t c . ) .
A t speeds above about 50 m/s windage e f f e c t s become n o t i c e a b l e and a t around
100 m/s and over they need to be s p e c i a l l y ca te red for i n t h e des ign by arrang-
i n g f o r p l e n t y o f space between t h e gears and t h e housing, by ar rang ing compl-
e t e l y separa te o r even dual d r a i n l i n e s w i t h s h i e l d s , o the rw ise t h e cas ing may
become choked w i t h o i l .
The l o c a t i o n o f t he o i l sprayers needs spec ia l c o n s i d e r a t i o n where p i t c h l i n e
speeds a r e above 50 m/s. Above t h a t speed t h e sprayers should no t be d i r e c t e d
s t r a i g h t into t h e mesh, b u t a l i t t l e i n advance. In t h i s way a l l t h e o i l
serves t o coo l t h e gears, t h e excess above t h a t requ i red f o r l u b r i c a t i o n i s
thrown o f f be fo re t h e gears mesh. T h i s arrangement i s e s p e c i a l l y requ i red f o r
'down-going' gears. Above approx imate ly 75 m/s, about 80-90% o f t he f l o w
should be d i r e c t e d on to t h e ou tgo ing s i d e f o r coo l i ng , w i t h t h e remainder be ing
sprayed s l i g h t l y i n advance o f mesh f o r l u b r i c a t i o n .
Excessive foaming and a e r a t i o n o f t h e o i l i s due e i t h e r t o a d e t e r i o r a t i o n
o f t h e p r o p e r t i e s o f t h e o i l f rom contaminat ion o r t o excess ive ingress o f a i r
i n t o t h e o i l . The former may be r e a d i l y checked by c a r r y i n g o u t t he a p p r o p r i a t e
t e s t s on the o i l , e.g. I P 146 and I P 313, t h e l a t t e r by re fe rence t o symptom
24(a ) , (b) and ( c ) .
REFERENCES
1 Fowle, T. I . , Lubn. Engg., 1976, 32 NO. 1, 17.
GEAR PROBLEMS : CAUSES AND REMEDIES
Symptom Possib le Causes Remedies
1 . Broken t e e t h Temporary c o r r e c t i o n s may be made by c u t t i n g o u t broken t e e t h and cracked sec t i ons u n t i l o n l y sound t e e t h a re l e f t , and running a t p r o p o r t i o n a l l y reduced
(a) Fat igue
( i ) Load unevenly d i s t r i b u t e d . ( i i ) Sharp corner i n t o o t h roo t . ( i i i ) Notches i n t o o t h r o o t caused by load; by i n s e r t i n g pegs and b u i l d i n g up
( i v ) Overload. by reshaping . improper f i l i n g o r g r i n d i n g . new t e e t h on them by weld ing, fo l lowed
(v) To rs iona l v i b r a t i o n s . ( v i ) Bruises on teeth, e.g. caused
( v i i ) Coarse m a r t e n s i t i c s t r u c t u r e
( v i i i ) B l o w - h o l e s i n roo ts o f cas t t ee th . ( i x ) Surface f a t i g u e p i t s ad jacent
Apart from e l i m i n a t i n g over loads the o n l y permanent c o r r e c t i o n i s , however, t o procure gears w i t h o u t t he f a u l t s l i s t e d , t o have them c o r r e c t l y l ined-up and run w i t h any shock loads l i m i t e d by shock absorbing coupl ings.
by dropping.
i n hardened s t e e l .
t o r o o t o f t oo th .
(b) F rac tu re
( i ) Repeated heavy loads. ( i i ) Hard f o r e i g n ob jec ts jammed i n
gear mesh.
2 . P i t t i n g
(a) Small w ide l y sca t te red (a) Fat igue a t sur face a s p e r i t i e s du r ing
(b) P i t s concentrated a t one (b) Concentrat ion o f load due t o s l i g h t
p i t s on working sur faces. i n i t i a l running.
o f t o o t h o r h e l i x . misal ignment.
( c ) Concentrat ion o f p i t s i n (c) Concentrat ion o f load on su r face regu la r bands a long undulat ions. t o o t h w id th .
(a) None: the p i t t i n g w i l l cease t o spread and w i l l be o f no consequence.
(b) Check al ignment o f sha f t s and cas ing f o r d i s t o r t i o n . The p i t t i n g may cease t o spread.
(c) The p i t t i n g may cease t o spread, bu t lapping o r s ton ing adv isable i n severe cases.
SvmDtom Possib le Causes Remed i es
2. P i t t i n g (contd. )
(d) P i t s concentrated along p i t c h l i n e .
(e) P i t s concentrated on dedendum surfaces o f one gear.
( f ) P i t s on dedendum sur- faces o f both gears, o f t e n w i t h d i s t i n c t s tep a t t he p i t c h l i n e .
(g) P i t t i n g i n case- hardened gears.
(h) M i c r o - p i t t i n g o r ' f r o s t - i n g ' i n su r face hardened gears ( c l o s e l y spaced p i t s smal ler than 0 .1mm across) .
(d) (i) Fau l t y p r o f i l e . ( i i ) Excessive wear o r s c u f f i n g has
produced a r i d g e a long the p i t c h l i n e which becomes overloaded.
(e) ( i ) I n s u f f i c i e n t hardness o f s o f t e r gear o r excessive f r e e - f e r r i t e i n rnicro- s t r u c t u r e r e l a t i v e t o loading.
m e t a l l i c i nc lus ions i n the meta l .
and o i l v i s c o s i t y too low.
s iona l o s c i l l a t i o n s and h i g h s t a r t i n g torque.
( i i ) Excessive amounts o f hard non-
( i i i ) I n i t i a l su r face f i n i s h too rough
( iv ) Overload, e s p e c i a l l y by shock, t o r -
( f ) I n s u f f i c i e n t hardness o f both gears r e l a t i v e t o the loading.
(g) Teeth too s o f t e i t h e r f rom acc iden ta l deca rbu r i sa t i on o r inadequate quenching.
(h) ( i ) N i t r i d e d gears: t h i n su r face l a y e r o f b r i t t l e super- r ich n i t r i d e s (whi te l a y e r ) .
! i i ) Inadequate o i l f i l m .
(d) ( i ) As above. ( i i ) Use o i l o f h ighe r v i s c o s i t y o r
o f h ighe r EP a c t i v i t y .
(e) The p i t t i n g w i l l cease t o spread i f opposing gear unaf fected. An increase i n o i l v i s c o s i t y could be b e n e f i c i a l
F i t shock absorbing coupl ing.
( f ) Reduce load ing i n s e r t shock absorbing type o f coupling,between source o f shock and gear. I f poss ib le , increase opera t i ng v i s c o s i t y o f o i l . Change t o EP o i l , main ly t o prevent subsequent s c u f f i n g o f remaining con tac t areas. I f poss ib le , l a p o r stone t e e t h t o improve su r face and rub i n d r y MoS2 powder.
(9) Reduce load.
(h) ( i ) A 25 micron l a y e r o r less i s gen- e r a l l y harmless bu t removal o f t h i c k e r l aye rs by lapping o r gr ind- i ng should be considered.
t he o i l . ( i i ) Increase opera t i ng v i s c o s i t y o f
Symptom Poss ib le Causes Remed i es
2 . P i t t i n g (contd.)
( i ) Very l a r g e oyster-shaped p i t s extending over p r a c t i c a l l y t he whole of t he a c t i v e f l a n k o f case- l ess than 0.5 micron. hardened tee th .
( i ) High t o o t h f r i c t i o n due t o d i r e c t contact between t h e tee th ; t h e o r e t i c a l o i l f i l m th ickness
3. Scu f f i ng (a) Tooth l oad ing too h igh.
(b) I nsuf f i c i en t 1 ub r i c a t i on ( i ) O i l -ba th l e v e l t oo low. ( i i ) O i l sprayer nozzles choked.
( i i i ) Pump suc t i on s t r a i n e r choked.
( iv ) Bearings rob o i l from sprayer nozz les.
( v ) Windage i n h i g h speed gears d e f l e c t s o i l spray.
(v i ) Lub r i can t channels a t low s t a r t i n g temperatures.
(v i i ) Clearance between gears and cas ing t o o sma l l : o i l cannot f l o w back t o bath.
( c ) Operat ing temperatures excessive.
(d) O i l v i s c o s i t y too low ( p a r t i c u l a r l y s o f t o r through-hardened gears).
( i ) Increase opera t i ng v i s c o s i t y o f the o i l .
NB. P i t t i n g o f s o f t and through-hardened s t e e l gears i s so common t h a t i t cannot be counted as a f a i l u r e . But, i f poss ib le , i t i s prudent t o c o r r e c t as d e t a i l e d above. In some cases, a f r e s h s t a r t can be made by tu rn ing round one o r both gears and lapping them together as approp r ia te . I n severe cases where noise i s excess ive o r reduced areas o f contact cannot be prevented from scu f f i ng , o r t he re i s a danger o f sur face p i t t i n g i n i t i a t i n g t o o t h breakage, i t may be necessary t o rep lace gears.
(a) Reduce loading
(b) ( i ) Raise o i l l e v e l so t h a t gear d ips 1-3 t o o t h he igh ts when running.
( i i ) Check o i l nozzles, c l e a r as necessary, f i l t e r o r change o i l .
( i i i ) C h e c k suc t i on s t r a i n e r , c lean as necessary, f i l t e r o r change o i l .
( i v ) R e s t r i c t o i l supply t o bear ings.
(v) Increase o i 1 pressure. Reposi t ion
( v i ) Use l u b r i c a n t w i t h lower channel p o i n t .
( v i i ) I f p o s s i b l e use lower v i s c o s i t y o i l .
sprayer nozzles.
Increase clearance.
( c ) Reduce temperatures (see Symptom 23)
(d) Use h igher v i s c o s i t y grade; improve c o o l i n g .
N 0 Symptom Poss ib le Cause Remed i es 0
3. S c u f f i n g (contd . ) (e) Lub r i can t has i n s u f f i c i e n t EP a c t i v i t y (e) Use more a c t i v e EP o i l . ( p a r t i c u l a r l y hardened gears) .
( f ) Teeth have excess ive addendum he igh t f o r t h e speed.
(9) Teeth do no t have s u f f i c i e n t con tac t because o f undu la t i ons , s p l i t markings o r misal ignment.
(h) Teeth a r e no t adequate ly r e l i e v e d .
( i ) F u l l - l o a d a p p l i e d be fo re gears have been adequate ly run - in .
( j ) Both gears have h i g h nickel-chromium conten t and a r e no t case-hardened.
4. Wear o f s o f t e r member Sur face f i n i s h o f harder member t o o o n l y o f gear p a i r . coarse.
5. Wear o f harder member Abras ive d i r t i n l u b r i c a n t becoming i s g rea te r . embedded i n s o f t e r inember.
6. Wear a t low speed. Lub r i can t f i l m t o o t h i n .
7. Wear a t h i g h speed. Excessive f r i c t i o n caused by over load, overspeed, loss o f backlash, o r f a u l t y l u b r i c a t i o n .
( f ) Check des ign . Use more a c t i v e EP o i l .
(g) Reduce undu la t i ons by s ton ing , l app ing o r shaving. Check a l ignment . Check t h a t cas ing has no t d i s t o r t e d f rom uneven s e t t l i n g o f foundat ions . Check t h a t bear ings a r e riot worn.
(h) Apply t i p - r e l i e f by shaving o r s ton ing .
( i ) Use a c t i v e EP o i l t o p revent f u r t h e r s c u f f i n g w h i l e cont inued runn ing makes sur faces smoother.
(j) Use a c t i v e EP o i l o r h ighe r v i s c o s i t y grade.
NB. Provided s c u f f i n g has no t roughened t h e sur faces unduly, e s p e c i a l l y w i t h s p i r a l bevel and h e l i c a l gears, once t h e bas i c cause has been e l i m i n a t e d f u r t h e r runn ing , p r e f e r a b l y w i t h an EP o i l , w i l l c o r r e c t t h e problem.
Stone o r l a p t e e t h t o a f i n e r f i n i s h .
Change o i l o r pass i t through a f i n e f i l t e r . F i t f i l t e r s on a i r ven ts .
Use h ighe r v i s c o s i t y l u b r i c a n t .
Reduce over loads o r overspeed, improve o i l f l o w and d i s t r i b u t i o n .
Symptom Poss ib le Causes Remed i es
8. Wear o f worm wheel teeth.
9 . Teeth t i p s rounded and dedendum surfaces gouged.
10. P l a s t i c f l o w o f t o o t h sur- faces w i t h pronounced f i n a t t i p s .
11. Groove a long p i t c h l i n e o f d r i v i n g t e e t h and r i d g e along p i t c h l i n e o f d r i v e n teeth.
12. Grooves a long p i t c h l i n e o f both d r i v i n g and d r i v e n tee th .
1 3 . Ripp l i ng .
14. Bulk p l a s t i c deformat ion o f t e e t h e s p e c i a l l y a t middle o f t o o t h w id th .
Ma te r ia l combinat ion may be unsu i tab le . (See a l s o No.20).
I n te r fe rence : gears no t p roper l y matched o r cen t re d i s tance t o o sma l l .
(a) Combination o f s o f t ma te r ia l and repeated shock loads leading t o separat ion o f t e e t h w i t h re-contact i n s u f f i c i e n t l y damped by o i l f i l m (peening) .
m a t e r i a l s ( r o l l i n g ) . (b) Very heavy steady loads and s o f t
When associated w i t h s c u f f i n g o f t he r e s t o f t h e t e e t h i t may be due t o complete f a i l u r e o f t he l u b r i c a n t supply. When not , o i l f i l m may be too t h i n .
Eros ion by spark discharge.
Excessive su r face f r i c t i o n a t low speeds.
Very severe overheat ing due t o f a i l u r e o f o i l supply.
I f p o s s i b l e use case-hardened s t e e l worm and c e n t r i f u g a l l y cast phosphor bronze.
Check design. d is tance.
(a) Reduce shock loads, use h igher v i s c o s i t y
I f p o s s i b l e extend cen t re
o i l , reduce backlash, change t o harder gear m a t e r i a l .
(b) Reduce loads, increase gear su r face hardness.
Check o i l supply, e.g. t h a t o i l bath l e v e l i s c o r r e c t when gears a re running, t h a t o i l supply p ipes, f i l t e r s and sprayer nozzles a r e n o t choked.
E s t a b l i s h source o f s t r a y e l e c t r i c cu r ren ts and lead t o e a r t h by (a) e a r t h i n g brushes o f generous s i ze , and (b) s topping o t h e r poss ib le paths through the gear mesh by i n s u l a t i n g pads under pedesta ls and insu la - t i n g bushes f o r holding-down b o l t s .
Use hypoid type f u l l EP o i l .
Repair t e e t h o r renew gears. E l im ina te cause o f o i l supply f a i l u r e . I n s t a l l alarms so t h a t u n i t can be stopped q u i c k l y i n event o f o i l supply f a i l u r e .
N 0 N
Remed i es System Poss ib le Causes
15. I ndenta t ions.
16. Cracks i n su r face of hardened gears, o f t e n i n n e t - l i k e p a t t e r n .
17. Long i tud ina l c rack ing and f l a k i n g i n case- hardened gears .
18. Red-brown spots on su r - faces o f case-hardened gears.
19. Red-brown con tac t marks on t e e t h and red- t -own powder i n nomina l l y s t a t i o n a r y gears.
20. Worm gear f a i l s t o t ransmi t f u l l to rque,
Hard p a r t i c l e s i n system, o f t e n swar f , o c c a s i o n a l l y f rom EP o i l carbonised on h i g h l y ra ted hea te rs i n system.
Overheat ing d u r i n g g r i n d i n g , i n c o r r e c t heat t rea tments , o r both.
Case i s t o o t h i n and co re too s o f t so t h a t su r face has co l l apsed under load.
A t tack by c o r r o s i v e substances such as s a l t s f rom hardening process.
F r e t t i n g due t o v i b r a t i o n w h i l e under s t a t i o n a r y load.
(a) I n s u f f i c i e n t a l lowance f o r d i s t o r - t i o n under load has brought con tac t o n t o e n t r y s i d e o f wheel t e e t h .
(b) Excessive t o o t h f r i c t i o n , due e i t h e r t o rough worm su r face o r t o u n s u i t - a b l e combinat ion o f gear m a t e r i a l s , causing excess ive d i s t o r t i o n .
Thoroughly c lean system, check hea te r surfaces, c lean and reduce su r face temperature as necessary .
Check w i t h manufac turer .
Check w i t h manufac turer .
Clean spots w i t h emery s t i c k . Clean, f l u s h and r e f i l l l u b r i c a t i o n system.
Arrange f o r t h e gears t o be f l ushed w i t h o i l and s l o w l y r o t a t e d .
(a) Ad jus t p o s i t i o n o f wheel so t h a t even under f u l l load con tac t i s n o t on e n t r y s i d e o f wheel t e e t h and o i l can be drawn i n t o con tac t .
o i l temperature o r use h i g h e r v i s c o s i t y grade. Best m a t e r i a l combinat ion i s case- hardened s t e e l worm and c e n t r i f u g a l l y cas t phosphor bronze wheel.
(b) Improve su r face f i n i s h o f worm, reduce
Symptom Poss ib le Causes Remed i es
21. V i b r a t i o n (a) De fec t i ve bear ings o r coup1 ings. (a) Check bear ings and coup l i ng and rep lace as necessary.
(b) Shaf ts misa l igned. (b) Check t h a t cas ing i s not d i s t o r t e d , r e a l i g n sha f t s .
( c ) Check and rebalance as necessary. ( c ) Check and rebalance as necessary.
Check these p a r t s and rep lace as necessary.
(d) Stone o r l a p t e e t h and rub i n MoS2 powder.
(e) Reduce excessive o i l supply t o mesh by
22. Unusual noise. (a) De fec t i ve r o l l i ng element bear ing.
(b) De fec t i ve o i l pump.
( c ) De fec t i ve coupl ing.
(d) Tooth sur faces excess ive ly p i t t e d o r roughened.
(e) Continuous t o o t h double h e l i c a l gears running 'down-going' w i t h apex t r a i l i n g , s q u i r t i n g o i l ou t o f mesh. o f mesh.
reducing immersion i n ba th o r reducing f l o w t o sprayers and d i r e c t i n g spray i n advance
23. Overheat ing. (a) O i l l e v e l t oo h i g h i n bath. (a) Adjust l e v e l when gear running t o d i p 1-3 t o o t h he ights .
(b) O i l v i s c o s i t y too h igh .
( c ) Speed t o o h i g h f o r ba th l u b r i c a t i o n .
(d) TOO much o i l sprayed too c lose t o ingoing mesh o f high-speed gears.
(e) Inadequate drainage from housing. (e) Improve drainage o r r e s t r i c t amount o f o i l t o
( f ) Clogged coo le r .
(b) Change t o lower v i s c o s i t y grade.
( c ) Change t o spray l u b r i c a t i o n system.
(d) D i r e c t o i l spray f u r t h e r i n advance o f mesh; r e s t r i c t amount o f o i l t o sprayers.
sprayers.
( f ) Check o i l and water s ides and c lean as necessary
(9) Cooler inadequate. (9) Change coo le r f o r l a r g e r s i z e o r swi tch t o coo le r water supply.
(h) Heat rad ia ted from surroundings. (h) In terpose r a d i a t i o n s h i e l d s .
PJ 0 w
tu 0 P Symptom Possib le Causes Remed i es
2 3 . Overheating (contd.) ( i ) Inadequate a i r f l o w over gearbox. ( i ) Increase v e n t i l a t i o n o f surrounding a i r space.
( j ) D i r t accumulat ions on casing. ( j ) Clean d i r t away.
(k) (Worm gears) , unsu i tab le l u b r i c a n t . (k) Change t o HVI minera l o i l o r , p r e f e r a b l y ,
(1) Excessive power loss i n p l a i n (1) Check bear ing design.
p o l y g l y c o l type s y n t h e t i c o i l .
bear ings.
24. Excessive foaming and (a) Gear d ips too deeply i n t o o i l bath. (a) Adjust l e v e l when gear running t o d i p 1 - 3
(b) A i r leaks on s u c t i o n s i d e o f (b) Remake suc t i on -s ide j o i n t s i nc lud ing pump
(c) O i l cascades down v e r t i c a l r e t u r n
(d) O i l contaminated by grease, j o i n t i n g (d) Renew o i l charge.
ae ra t i on . t o o t h he igh ts .
c i r c u l a t i n g system. gland.
p ipes i n t o r e s e r v o i r . i n t o tank below o i l l e v e l .
compound, another, and incompat ib le o i l e t c .
(c) Rearrange r e t u r n 1 ines t o a1 low smooth f l o w
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MATERIALS FOR TRIBOLOGICAL APPLICATIONS
D. SCOTT, Consul tant, E d i t o r o f Wear
8.1 I NTRODUCT I ON
Engineer ing design i s the c r e a t i o n o f i n s t r u c t i o n s f o r making an a r t i c l e t o
s a t i s f y a s p e c i f i c requirement.
e r i a l s o f c o n s t r u c t i o n and the
i ca t i ons . For t r i b o l o g i c a l app
are those p r o p e r t i e s which must
t o w i ths tand the mechanical and
the e f f e c t s o f t he environment
From a t r i b o l o g i c a l p o i n t o f v iew, the mat-
u b r i c a n t a r e impor tan t f a c t o r s i n such s p e c i f -
i c a t i o n s the impor tan t p r o p e r t i e s of m a t e r i a l s
be taken i n t o account i n des ign ing a component
thermal s t resses t o which i t w i l l be exposed and
n which i t has t o f u n c t i o n [1 ,2 ] . There i s a
cont inuous demand f o r m a t e r i a l s o f improved p r o p e r t i e s and w i t h b e t t e r s t r e n g t h
t o we igh t r a t i o s . Mechanisms operaLing under arduous c o n d i t i o n s o f h i g h speed,
heavy l oad o r extremes o f environment r e q u i r e m a t e r i a l s o f h igh s t reng th . I f
sub jec ted t o r e l a t i v e mot ion they may r e q u i r e m a t e r i a l s o f g rea t hardness, wear
and c o r r o s i o n r e s i s t a n c e and s t r u c t u r a l and dimensional s t a b i l i t y [31. Newer
m a t e r i a l s [ 4 ] may meet s t r i n g e n t design requirements beyond the c a p a b i l i t i e s o f
the more commonly used m a t e r i a l s bu t a v a i l a b i l i t y and c o s t make convent iona l
m a t e r i a l s more a t t r a c t i v e commercial ly and encourage innovat ion .
The des igner has a vas t range o f m a t e r i a l s f rom which t o s e l e c t . One r e f -
erence book [ 5 ] g i v e s 35000 p r o p r i e t a r y m a t e r i a l compos i t ions . The des igner
however, besides search ing f o r improved m a t e r i a l s must o f t e n seek the cheapest
ma te r ia l t o s a t i s f y h i s requirements and sometimes the more r e a d i l y a v a i l a b l e
i nd ig inous m a t e r i a l s . A determin ing f a c t o r i n the ex tens i ve use o f many mat-
e r i a l s i s t he a m e n a b i l i t y o f t he m a t e r i a l to man ipu la t i on and the e x t e n t t o
which the des igner can c o n t r o l and vary p r o p e r t i e s such as s t reng th , hardness
and d u c t i l i t y w i t h i n the range o f s p e c i f i c eng inee r ing requirements. The cho ice
o f m a t e r i a l s i s o f t e n r e s t r i c t e d by the manufac tur ing f a c i l i t i e s d t h i s disposal.
8.2 TYPES OF MATERIALS
M a t e r i a l s may be conven ien t l y d i v i d e d i n t o f o u r p r i n c i p a l t ypes : - f e r rous ,
non- fe r rous , non -meta l l i c and composi te m a t e r i a l s . The abundance o f i r o n and
i t s a l l o y s compr is ing the b u l k o f meta ls made, t h e i r f avou rab le economics and
d i ve rse p r o p e r t i e s make f e r r o u s m a t e r i a l s the d e s i r a b l e cho ice f o r t r i b o l o g i c a l
app l i ca t i ons . Modern c a s t i r o n s and s t e e l s f i n d ex tens i ve use i n t r i b o -
206
engineer ing as a l l o y i n g and heat- t reatment enables them t o be t a i l o r e d t o
s p e c i f i c a p p l i c a t i o n s . The p r i n c i p a l methods o f s t rengthening s t e e l inc lude
work-hardening, decreasing the g r a i n s i ze , s o l i d s o l u t i o n and d i spe rs ion
strengthening. I n c u r r e n t l y used s tee l s , t he mar tens i te t rans fo rma t ion pro-
duces the best combinat ion o f s t reng th and d u c t i l i t y b u t as hardness increases,
d u c t i l i t y decreases and a t the h ighes t s t r e n g t h l e v e l s produced by convent ional
heat t reatment procedures, d u c t i l i t y i s d imin ished t o l e v e l s considered a t
present unacceptable f o r most engineer ing a p p l i c a t i o n s [ 6 ] . I f a thermo-
mechanical t reatment i s used whereby a u s t e n i t e i s s t r a i n hardened be fo re t rans -
format ion t o mar tens i te unusual d u c t i l i t y , f a t i g u e and impact p r o p e r t i e s a r e
obtained. Ausforming may thus a l l o w increased s t reng ths above the present
usable l i m i t s w i thou t s a c r i f i c e of d u c t i l i t y [7]. With m a t e r i a l s genera l l y , h i g h hardness i s u s u a l l y assoc iated w i t h a h igh
m e l t i n g p o i n t . Powder me ta l l u rgy has widened the f i e l d o f a v a i l a b l e hard
m e t a l l i c m a t e r i a l s by making poss ib le metal combinat ions unobta inable by con-
ven t iona l m e l t i n g and c a s t i n g techniques. Hard s i n t e r e d carb ides may be used t o
advantage i n many a p p l i c a t i o n s r e q u i r i n g a h igh degree o f wear res is tance.
However, such m a t e r i a l s a r e u s u a l l y expensive t o manufacture and d i f f i c u l t t o
form and sur face t reatments and coa t ings on o r d i n a r y m a t e r i a l s may be used t o
increase s t reng th and improve wear res is tance.
For use a t e leva ted temperatures meta ls must form a dense, tough, impervious
ox ide l a y e r which r e s i s t s c rack ing under load and prevents a t t a c k o f the metal
by h o s t i l e environments. The es tab l i shed non- fer rous metal a l l o y systems i n
c u r r e n t use a r e based on n i c k e l r i c h and c o b a l t r i c h a l l o y s . I n both cases the
necessary res i s tance t o o x i d a t i o n and co r ros ion i s con fe r red by the i n t r o d u c t i o n
o f chromium. The n i c k e l based a l l o y s a r e s t i f f e n e d p r i n c i p a l l y by the a d d i t i o n
o f t i t a n i u m and aluminium. I n c o b a l t a l l o y s s t i f f e n i n g i s e f f e c t e d by complex
carb ides o f molybdenum, n iob ium and tantalum.
High speed t o o l s t e e l s and s i m i l a r spec ia l s t e e l s a r e a l s o used f o r e leva ted
temperature serv ice. Corros ion res i s tance requ i res c a r e f u l m a t e r i a l s e l e c t i o n
and the use o f s t a i n l e s s s tee l s , non- fer rous meta ls such as aluminium, n i c k e l ,
chromium, t i t a n i u m and t h e i r a l l o y s o r non -meta l l i c m a t e r i a l s such as p l a s t i c s
o r elastomers. To r e s i s t severe ab ras i ve wear, cemented carb ides, cermets or
even diamond may be requi red.
As convent ional m a t e r i a l s have been improved by or thodox methods almost t o
the l i m i t o f t h e i r p o t e n t i a l mechanical p r o p e r t i e s , new types o f m a t e r i a l a r e
being developed. Composites which combine m a t e r i a l s o f d i s s i m i l a r mechanical
and phys i ca l p roper t i es , can have p r o p e r t i e s super io r t o one o r bo th o f t h e i r
cons t i t uen ts . There a r e two p r i n c i p a l types. In one, a m a t r i x may be r e i n -
forced w i t h f i b r e s o r p a r t i c l e s t o improve i t s p r o p e r t i e s . I n the o t h e r the
207
the m a t r i x i s e s s e n t i a l l y a g l u e to h o l d toge the r f i b r e s o r p a r t i c l e s which
have des i rab le p r o p e r t i e s bu t which by themselves cannot be used as eng ineer ing
ma te r ia l s . Cer ta in d i f f i c u l t i e s r e q u i r e t o be surmounted be fo re composites
achieve t h e i r f u l l p o t e n t i a l .
lems w i t h s t r e s s concen t ra t i on a t t h e i r ends which can in f l uence c rack
i n i t i a t i o n . Convent ional methods a r e no t s u i t a b l e f o r t he manufacture o f com-
ponents f rom composites no r f o r t h e fo rma t ion and j o i n i n g o f f i b r e r e i n f o r c i n g
ma te r ia l s . By u s i n g re in fo rcements o f ox ide and non-meta l l i c wh iskers which
approach the t h e o r e t i c a l s t reng th , very h i g h u l t i m a t e s t reng ths i n composites
are poss ib le . Glass, carbon, s i l i c o n n i t r i d e and alumina a r e a t t r a c t i v e non-
metals. Besides r e p l a c i n g meta ls , ceramics may be used as coa t ings t o comple-
ment d e s i r a b l e metal c h a r a c t e r i s t i c s by add ing r e f r a c t o r y p r o p e r t i e s , i n s u l a t i o n ,
and eros ion , wear, o x i d a t i o n and c o r r o s i o n res i s tance .
Whiskers and f i b r e s a r e expensive and have prob-
The s t reng then ing o f meta ls f o r use a t h i g h temperatures can be achieved by
d ispers ing non-meta l l i c p a r t i c l e s i n them t o ma in ta in u s e f u l p r o p e r t i e s t o
w i t h i n 5O-10O0C o f t h e m e l t i n g p o i n t o f t he m a t r i x meta l .
the d i s p e r s o i d a r e r e q u i r e d and n i c k e l a l l o y s w i t h thor ium, TD n i c k e l , a r e
commercially a v a i l a b l e . Other newer m a t e r i a l s f o r arduous k o n d i t i o n s i nc lude
syn the t i c diamond and sapphire, new g r a p h i t e s and m a t e r i a l s such as the carbides,
bor ides and n i t r i d e s o f c e r t a i n meta ls which approach the hardness o f diamond.
Only smal l amounts o f
8.3 MATERIALS FOR SPECIFIC APPLICATIONS
Adequate m a t e r i a l p r o p e r t i e s f o r design a r e u s u a l l y ensured by i n d i r e c t means
mainly by the des igner s p e c i f y i n g chemical a n a l y s i s , heat - t rea tment and mech-
an ica l p r o p e r t i e s a l though such s p e c i f i e d p r o p e r t i e s may no t be d i r e c t l y repres-
e n t a t i v e i n se rv i ce . For instance, the most impor tan t m a t e r i a l p r o p e r t y may be
res is tance t o ab ras ion o r res i s tance t o s c u f f i n g and se i zu re , o r t o r o l l i n g
contac t f a t i g u e o r t o l u b r i c a n t a t t a c k o r co r ros ion .
dimensional s t a b i l i t y may comple te ly determine the s e r v i c e l i f e . As the u l t -
imate assessment o f a m a t e r i a l i s performance i n p r a c t i c e , f u l l sca le t e s t i n g
and se rv i ce s i m u l a t i o n t e s t i n g a r e u s u a l l y reso r ted t o as a means o f m a t e r i a l
se lec t i on .
A p r o p e r t y such as
Tr ibo-eng ineer ing depends i n many ins tances upon bear ings , components which
a l l o w r e l a t i v e mot ion between members o f a mechanism w h i l s t t r a n s f e r r i n g load.
Bearings may take many forms bu t t he most w ide ly used types a r e p l a i n bear ings ,
gears and r o l l i n g bear ings .
8.3.1 P l a i n Bear ings
In p l a i n bear ings the load i s t ransmi t ted between moving p a r t s by s l i d i n g
contac t and the c r i t e r i o n o f s a t i s f a c t o r y bear ing performance i s minimum wear o f
the components toge the r w i t h freedom from se izu re and freedom f rom mechanical
f a i l u r e by deformat ion o r f a t i g u e . To c a r r y a hard s t e e l s h a f t , u s u a l l y spec-
i f i e d f o r i t s mechanical p r o p e r t i e s , a bea r ing m a t e r i a l must be compara t ive ly
s o f t t o avo id wear o f t he harder m a t e r i a l y e t s t rong enough t o w i ths tand heavy
loads w i thou t d i s t o r t i o n and w i t h o u t s u f f e r i n g f a t i g u e . So f t bea r ing m a t e r i a l s
a l s o a l l o w ab ras i ve p a r t i c l e s t o become embedded and thus reduce ab ras i ve wear.
As a low hardness i s u s u a l l y assoc ia ted w i t h a low m e l t i n g p o i n t , h i g h spots o f
s o f t bear ings a re removed by s l i d i n g con tac t w i t h o u t damage t o the mat ing su r -
face and w i thou t t he r i s k o f se i zu re . However, low hardness i s u s u a l l y assoc-
i a t e d w i t h low f a t i g u e s t r e n g t h and, as s t r e s s l e v e l s a r e ra i sed , the demand i s
f o r harder bea r ing m a t e r i a l s t o improve the load c a r r y i n g capac i t y bu t w i t h the
minimum loss o f f r i c t i o n and wear p r o p e r t i e s . As a general r u l e i t i s adv i sab le
t o use the s o f t e s t bea r ing m a t e r i a l poss ib le .
White meta l , a w ide ly used p l a i n bea r ing m a t e r i a l i s based on t i n o r l ead o r
t h e i r in te rmed ia te a l l o y s . A t y p i c a l t i n based a l l o y con ta ins 7-10% Sb and
3-5% Cu, the p r i n c i p a l c o n s t i t u e n t s be ing SbSn, CueSns and a te rna ry p e r i t e c t i c
complex, Fig.1. Hardness and mechanical p r o p e r t i e s a r e l i t t l e a f fec ted by
composi t ion [ 8 ] . A t l O O " C , t he hardness ranges f rom 11-16 HV and the f a t i g u e
s t reng th f o r l o 7 cyc les f rom 1.6-1.9 MN/m2.
Lead based a l l o y s c o n t a i n i n g Sb and Sn and Cu i n the fo rm o f i n t e r m e t a l l i c
SbSn and CusSns may be cheaper than s i m i l a r t i n based a l l o y s b u t a r e s l i g h t l y
i n f e r i o r rega rd ing wear and f a t i g u e p r o p e r t i e s . In te rmed ia te a l l o y s o f h i g h
lead and t i n conten t a r e w ide ly used b u t appear t o have no advantages over the
o t h e r wh i te metals. The success o f w h i t e meta ls i s g e n e r a l l y regarded as be ing
due t o the c o r r e c t compromise between so f tness t o avo id wear and s t r e n g t h t o
r e s i s t f a t i g u e .
Copper-based a l l o y s , s t ronger bear ing m a t e r i a l s than the wh i te meta ls a t
ope ra t i ng temperatures, range from the phosphor bronzes (10% Sn, 0.5% P) through
the leaded bronzes t o the copper lead a l l o y s o f up t o 50% Pb, Fig.2. The wear
p r o p e r t i e s o f leaded bronze a r e b e t t e r than those o f wh i te meta l . The hardness
o f copper- lead v a r i e s accord ing t o composi t ion f rom 30-70 H.V. Journa l wear
increases w i t h increase i n hardness bu t f a t i g u e s t reng th increases rough ly i n
the same p r o p o r t i o n as j o u r n a l wear. Increased j o u r n a l hardness can h e l p t o
minimize wear. A d isa ivantage o f copper- lead a l l o y s i s t h e i r s u s c e p t i b i l i t y t o
l u b r i c a n t c o r r o s i o n of the lead phase.
A compromise between a l l o y s s o f t enough t o avo id wear, those hard enough t o
r e s i s t f a t i g u e and those a b l e t o r e s i s t c o r r o s i o n has evo lved by the use o f
ove r lay bear ings i n which a s t rong me ta l , such as copper-based meta l , has a
s o f t metal ove r lay . For economic reasons, the copper - lead may be used as an
i n t e r l a y between a s t e e l base and the ove r lay , Fig.3. To avo id f a t i g u e under
the a p p l i e d loads, t he ove r lay i s u s u a l l y t h i n n e r than 5 urn. The o v e r l a y p l a t e d
209
Fig.1 (x75) S t r u c t u r e o f g r a v i t y F i g . 2 (x75) Structure o f copper-lead c a s t t i n based whi te meta l . on s t e e l base.
copper-lead bear ing i s widely used f o r h igh duty engine bearings but the con-
tinuous search i s f o r super ior r e a d i l y a v a i l a b l e m a t e r i a l s .
Fig.3 (x75) S o f t o v e r l a y on copper F ig .4 (x120) S t r u c t u r e o f aluminium lead bearing. 20% t i n bear ing.
210
Aluminium, a comparat ive ly cheap m a t e r i a l i n abundant supply, has met w i t h
some success by the convent ional approach o f us ing hard metal compounds i n the
aluminium m a t r i x t o produce a s t r u c t u r e analogous t o t h a t of wh i te meta l . The
use o f another s o f t metal , t i n , w i t h aluminium has produced good r e s u l t s . By
s u i t a b l e c o l d working fo l l owed by heat- t reatment and r e c r y s t a l l i z a t i o n the
d i s t r i b u t i o n o f about 20% t i n produces a r e t i c u l a r s t r u c t u r e , Fig.4, w i t h
adequate bear ing p roper t i es . The a d d i t i o n o f a smal l amount o f hardener such as
copper i s b e n e f i c i a l and seems t o o f f e r t he bes t combinat ion of load c a r r y i n g
capac i t y , wear and co r ros ion r e s i s t i n g p r o p e r t i e s c u r r e n t l y a v a i l a b l e . Su i tab le
ove r lays f o r aluminium bear ings a i d running- in , minimise j o u r n a l wear and a l l o w
reduc t i on o f t i n content t o about 9%. S i m i l a r a luminium-lead bear ings e f f e c t
f u r t h e r economies.
For j o u r n a l s , the cheapest poss ib le m a t e r i a l i s u s u a l l y chosen. M i l d s t e e l
i s used f o r l ess arduous app l i ca t i ons , and p l a i n carbon s t e e l s can be heat-
t rea ted t o meet most convent ional a p p l i c a t i o n s , the p r o p e r t i e s improving w i t h
increase o f carbon content . Medium-carbon s t e e l s used f o r smal ler s izes o f
j o u r n a l s and engine c ranksha f t s have low h a r d e n a b i l i t y .
which a r e requ i red i n the hardened and tempered c o n d i t i o n , l ow-a l l oy s t e e l s a re
needed t o enable s a t i s f a c t o r y p r o p e r t i e s t o be obtained. Depending upon the
s p e c i f i c p r o p e r t i e s requi red, manganese, n i c k e l , chromium, molybdenum and
vanadium, used separate ly o r i n va r ious combinat ions, p rov ide a wide range o f
m a t e r i a l s f o r h i g h l y s t ressed t ransmiss ion components and the more s t r i n g e n t
app l i ca t i ons . Surface-hardening techniques, such as c a r b u r i z i n g and n i t r i d i n g ,
a re b e n e f i c i a l f o r p r o v i d i n g an ext remely hard, wear - res i s tan t sur face u s u a l l y
on s p e c i a l l y manufactured low-carbon manganese o r tough a l l o y s t e e l s con ta in ing
small amounts o f manganese, chromium, n i c k e l and molybdenum. N icke l s t e e l s a re
p a r t i c u l a r l y s u i t a b l e f o r case hardening as such s t e e l s p rov ide a strong, tough,
wear- res is tant case w i t h a d u c t i l e core w h i l s t the hardest n i t r i d e d cases a r e
obta ined w i t h aluminium-containing s tee l s .
. For more massive p a r t s ,
Compatabi l i ty o f meta ls i n s l i d i n g con tac t i s a dominant f a c t o r i n bear ing
performance and bo th the bear ing m a t e r i a l and the harder s h a f t r e q u i r e c a r e f u l
se lec t i on .
have been surveyed [9 ] . M a t e r i a l s f o r h i g h temperature bear ings and s l i d i n g a p p l i c a t i o n s
8.3.2 Gears
Gears i n se rv i ce a r e subjected t o r o l l i n g , s l i d i n g , abras ive, chemical ,
v i b r a t o r y and shock- loading a c t i o n .
scu f f i ng , p i t t i n g , f r e t t i n g , abras ion, co r ros ion and f r a c t u r e . Gear m a t e r i a l s
must be chosen t o r e s i s t these phenomena.
are the carbon-manganese s t e e l s ; manganese c o n t r i b u t e s markedly t o s t reng th
The i r use fu l l i f e may be terminated by
The most e x t e n s i v e l y used gear s t e e l s
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and hardness bu t i t s e f f e c t depends upon the carbon content . I t a l s o enhances
ha rdenab i l i t y , and f i ne -g ra ined manganese s t e e l s a t t a i n unusual toughness and
s t rength. For more s t r i n g e n t gear a p p l i c a t i o n s , a l l o y s t e e l s , hea t - t rea ted t o
prov ide the optimum p r o p e r t i e s , a r e used. N icke l p rov ides s o l i d - s o l u t i o n
s t rengthening and increases toughness and res i s tance t o impact, p a r t i c u l a r l y a t
low temperatures, lessens d i s t o r t i o n i n quenching, improves c o r r o s i o n r e s i s t -
ance and a l l ows more l a t i t u d e i n heat t reatment. Chromium increases harden-
a b i l i t y and has a s t rong tendency t o form s t a b l e carb ides which hamper g r a i n
growth and p rov ide f ine-gra ined, tough s t e e l s . Vanadium forms s t a b l e carb ides
t h a t do n o t r e a d i l y go into s o l u t i o n and which a r e n o t prone t o agglomerat ion
by tempering. It i n h i b i t s g r a i n growth, thus impar t i ng s t reng th and toughness
t o hea t - t rea ted s tee l s . Molybdenum and vanadium a r e genera l l y used i n combin-
a t i o n w i t h o t h e r a l l o y i n g elements.
f a s t e r machining ra tes , increased product ion and longer t o o l l i f e .
Lead may be added t o gear s t e e l s t o a t t a i n
Surface hardening t o reduce wear i s e x t e n s i v e l y a p p l i e d t o gear s t e e l s
w i thou t s a c r i f i c i n g d e s i r a b l e co re p r o p e r t i e s . Carbon and a l l o y s t e e l s can be
l i q u i d , gas o r pack carbur ized. N i t r i d i n g i s u s u a l l y a p p l i e d t o spec ia l alum-
in ium-conta in ing s tee l s . Flame and induc t i on hardening methods a r e a l s o used.
Other sur face t reatments such as Sulphinuz, phosphat ing and s o f t n i t r i d i n g
which reduce f r i c t i o n and a i d l u b r i c a t i o n , can be b e n e f i c i a l .
8.3.3 R o l l i n g Bear ings
Al though b a l l and r o l l e r bear ings a r e b a s i c a l l y r o l l i n g elements, i n oper-
a t i n g mechanisms they a r e a l s o subjected t o some wear by s l i d i n g and t o
chemical a t t a c k by l u b r i c a n t and environment. The i r use fu l l i f e i s u s u a l l y
l i m i t e d by sur face d i s i n t e g r a t i o n , p i t s be ing formed by a f a t i g u e process
dependent upon the p r o p e r t i e s o f the m a t e r i a l , the na tu re o f the l u b r i c a n t and
the environment [10,11,12]. The p r i n c i p a l q u a l i t i e s o f b a l l - b e a r i n g m a t e r i a l s
are dimensional s t a b i l i t y , h i g h hardness t o r e s i s t wear, h i g h e l a s t i c l i m i t
t o avo id p l a s t i c deformat ion under load, and good f a t i g u e res i s tance t o contend
w i t h h i g h a l t e r n a t i n g s t resses. A high-carbon s t e e l s a t i s f i e s these requ i re -
ments i f a carb ide- forming element i s incorporated t o increase hardness, g i v e
h a r d e n a b i l i t y and a l l o w o i l quenching t o minimize d i s t o r t i o n du r ing heat-
t reatment. EN 31, 535A99 o r SEA 52100 (1.0% C , 1.5% Cr) through hardening
s tee l i s used f o r convent ional bear ings, Fig.5. Vacuum degassed, vacuum re -
melted a i d e l e c t r o - s l a g r e f i n e d ma te r ia l o f improved mechanical p r o p e r t i e s may
be used f o r increased r o l l i n g con tac t f a t i g u e res is tance. For convenience,
i n the manufacture o f the l a r g e r s i zes o f r o l l e r bear ings case-hardening s t e e l s
con ta in ing chromium, n i c k e l and molybdenum accord ing t o the degree o f harden-
a b i l i t y , shock res i s tance and core hardness requ i red a r e used. For use i n a
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c o r r o s i v e environment m a r t e n s i t i c s t a i n l e s s s t e e l s a r e used w i t h some loss of
f a t i g u e res is tance.
For use a t e leva ted temperatures, convent ional r o l l i n g bear ing s t e e l s a r e no t
s a t i s f a c t o r y owing t o loss o f hardness and f a t i g u e res i s tance and h i g h speed
t o o l s t e e l s w i t h h igh tempering temperatures a r e used, Fig.6. High speed t o o l
s t e e l s con ta in ing p r i n c i p a l l y tungsten, molybdenum and vanadium a r e a l s o l ess
prone t o d e l e t e r i o u s l u b r i c a n t e f f e c t s than EN 31 s t e e l [ l l ] . Mate r ia l com-
b i n a t i o n and m a t e r i a l l u b r i c a n t combinat ion a re impor tant t o ensure adequate
r o l l i n g con tac t f a t i g u e l i f e [13,14,15].
Fig.5 ( ~ 4 0 0 0 ) S t ruc tu re o f EN 31 b a l l Fig.6 (~6,500) S t ruc tu re of 18%W bear ing s t e e l . h igh speed t o o l s t e e l .
Under c o n d i t i o n s o f u n l u b r i c a t e d r o l l i n g con tac t , f a i l u r e occurs n o t by the
usual f a t i g u e mechanism b u t by excessive wear l i m i t i n g use fu l l i f e , owing t o
v i b r a t i o n and rough, no i sy running. Var ious supera l loys w i t h base composi t ion
o f chromium, molybdenum and c o b a l t and con ta in ing s i g n i f i c a n t amounts o f
n i c k e l , tungsten, vanadium and o t h e r a l l o y i n g elements, a l s o cermets and
ceramics, a re p o t e n t i a l l y s u i t a b l e and have been used under arduous t e s t con-
d i t i o n s . Under c e r t a i n t e s t c o n d i t i o n s 1131, tungsten ca rb ide was the bes t o f
the m a t e r i a l s t r i e d , g i v i n g the lowest wear r a t e and being r e l a t i v e l y una f fec ted
by temperature. The best r e s u l t s were obta ined w i t h the smal lest ca rb ide s i z e
and the lowest percentage o f m a t r i x m a t e r i a l , Fig.7. Hot pressed s i l i c o n n i t -
r i d e was a l s o s u i t a b l e .
213
Fine grained Coarse grained
Fig.7 ( ~ 5 0 0 ) S t r u c t u r e o f 6% Co tungsten ca rb ide .
8.3.4 Wear Res is tan t M a t e r i a l s
Wear r e s i s t a n t m a t e r i a l s r e q u i r e the c o r r e c t combinat ion o f hardness t o
r e s i s t abras ion and d u c t i l i t y t o contend w i t h shock loads and c y c l i c s t ress ing .
AS these a r e c o n f l i c t i n g requirements, s u i t a b l e m a t e r i a l s e l e c t i o n i nvo l ves
compromise. Other f a c t o r s such as the types o f wear, t he na tu re o f any
abrasive, the o p e r a t i n g temperature and the environment a f f e c t t he cho ice o f
ma te r ia l .
For c o n d i t i o n s o f h i g h s t r e s s and impact, the toughness and work hardening
p roper t i es o f a u s t e n i t i c manganese s t e e l a r e u s u a l l y requ i red . For lower s t r e s s
s l i d i n g c o n d i t i o n s where toughness i s l e s s impor tan t , depending upon the c o s t
f ac to r , hardened s t e e l s , a l l o y c a s t i r ons , hard f a c i n g m a t e r i a l s , conc re te o r
ceramics may be used. Where maximum wear r e s i s t a n c e i s requ i red and c o s t i s
immater ia l , cemented ca rb ides may be used. Cor ros ion res i s tance r e q u i r e s the
use of s t a i n l e s s meta ls , s u i t a b l e rubbers o r p l a s t i c s . S u i t a b l y r e i n f o r c e d
polymer m a t e r i a l may be used where a low c o e f f i c i e n t o f f r i c t i o n as w e l l as wear
res is tance i s requ i red . High chromium a l l o y s o f i r o n and s t e e l o f f e r t he bes t
wear res i s tance t o e leva ted temperature problems o f c rack ing , s p a l l i n g and
thermal shock.
The wear res i s tance o f a metal v a r i e s w i t h d i f f e r e n t abras ives and the
e f f e c t i v e hardness o f an ab ras i ve has been de f i ned as the maximum va lue o f hard-
ness o f a metal t h a t can be abraded by i t . Th is i s o f importance i n m a t e r i a l
se lec t i on where a known ab ras i ve i s t o be encountered i n p r a c t i c e . I t i s a l s o
o f importance i n ab ras i ve s e l e c t i o n fo r m a t e r i a l removal. Next t o diamond, the
hardest ab ras i ve a v a i l a b l e , s i l i c o n ca rb ide i s t h e most impor tan t ab ras i ve f o r
lapping and g r i n d i n g and f o r g r i n d i n g wheels. Diamond can be syn thes ized from
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carbon and i n a s i m i l a r manner, c u b i c c r y s t a l l i n e boron n i t r i d e of s i m i l a r
hardness can be produced.
e r i a l o f economic s i g n i f i c a n c e even though the present i n i t i a l c o s t i s h igh.
I t s extreme hardness and d u r a b i l i t y make t h i s mat-
8.3.5 Tools
The e v o l u t i o n o f modern product ion methods has been l a r g e l y dependent on the
development o f tool s t e e l s capable of opera t i ng e f f i c i e n t l y under i nc reas ing
arduous cond i t i ons . High hardness i s a r e q u i s i t e o f almost every t o o l s t e e l
b u t t he optimum hardness depends upon t h e a p p l i c a t i o n and, as hardness and
toughness vary i nve rse l y , maximum hardness can o n l y be used i n the absence of
shock loading. Hardness a t e leva ted temperature i s sometimes e s s e n t i a l as w e l l
as res i s tance t o abras ion and d i s t o r t i o n . The choice o f s t e e l i s u s u a l l y based
on a cons ide ra t i on o f t he r e l a t i v e importance of the p r o p e r t i e s requi red. As
no s i n g l e s t e e l possesses a l l d e s i r a b l e p roper t i es , a compromise i s g e n e r a l l y
necessary. Economic aspects can r a r e l y be ignored.
The s implest t o o l s t e e l s a r e p l a i n carbon (0.6 t o 1.4% C ) types, which by
water quenching, develop a hard case.
V f o r ca rb ide s t a b i l i z a t i o n and t o r e t a r d g r a i n growth, and up t o 0.5% C r t o
increase h a r d e n a b i l i t y and case depth. Oi l -hardening manganese s t e e l s (1.0 t o
2.0% Mn) p rov ide h i g h hardness w i t h minimum d i s t o r t i o n . Chromium may be added
t o h e l p h a r d e n a b i l i t y w h i l s t a d d i t i o n s of tungsten and vanadium improve wear
res i s tance and c o n t r o l g r a i n growth. For the more arduous a p p l i c a t i o n s the
high-carbon (2.0% C ) , high-chromium (14% C r ) s t e e l s a r e used t o r e s i s t wear and
co r ros ion and t o ma in ta in h igh dimensional s t a b i l i t y .
c u t t i n g edge f o r c o l d b lank ing t o o l s .
Low a l l o y v a r i e t i e s con ta in up t o 0.25%
They ma in ta in a good
I f shock res i s tance i s requi red, s t e e l s o f lower carbon content (0.4 t o
0.5% C) are u s u a l l y requ i red and a l l o y i n g elements such as s i l i c o n , tungsten,
vanadium and chromium may be added t o improve abras ion res is tance. For ho t -
working, high-speed t o o l o r h o t d i e s t e e l s which r e t a i n h i g h hardness and a
good c u t t i n g edge a t e leva ted temperatures a r e needed.
type i s the 18-4-1 ( tungsten, chromium, vanadium) type and h o t hardness may be
increased by r a i s i n g the vanadium content o r by a d d i t i o n o f c o b a l t . It i s
poss ib le t o p a r t l y rep lace tungsten w i t h molybdenum as i s the case w i t h , f o r
example, M.2, 6.5.4.2 (W, Mo, C r , V) s t e e l . Tungsten, chromium and molybdenum
chromium ho t d i e s t e e l s may be regarded as l ean type high-speed t o o l s t e e l s i n
which improved toughness can be developed i f the carbon content i s reduced t o
below 0.4% C. N i cke l , chromium, molybdenum s t e e l s g e n e r a l l y used as s t r u c t u r a l
s t e e l s , may by s u i t a b l e heat t reatment be used as t o o l s t e e l s . They do n o t
possess the h i g h hardness o r abras ion res i s tance o f high-speed t o o l s t e e l s b u t
advantage can be taken o f t h e i r super io r toughness.
The most w ide ly used
21 5
8.3.6 C u t t i n g Tools
From an economic p o i n t o f view, one o f the most impor tan t f a c t o r s i n
machining i s t he r a t e a t which c u t t i n g t o o l s wear 1161. Genera l l y the hardes t ,
most wear - res i s tan t , t o o l m a t e r i a l which r e s i s t s b r i t t l e f r a c t u r e should be
used.
High-speed s t e e l s have good shock res i s tance and can be r e a d i l y shaped by
fo rg ing and machining, so a r e the p r e f e r r e d c u t t i n g t o o l m a t e r i a l s f o r a l a r g e
range o f a p p l i c a t i o n s . Surface t rea tments which increase hardness and minimize
adhesion t o the workpiece a r e b e n e f i c i a l . Cast cobal t -chromium-tungsten a l l o y s ,
the S t e l l i t e s a r e s u i t a b l e f o r a p p l i c a t i o n s i n v o l v i n g h i g h temperatures, where
coo l i ng i s imprac t i cab le bu t where impact i s absent. S in te red ca rb ide t o o l s
may be s u c c e s s f u l l y used f o r most m e t a l - c u t t i n g opera t i ons and t h e i r except -
ional performance r e s u l t s f rom h i g h hardness and compressive s t reng ths . The
s t r a i g h t tungs ten ca rb ides have the h ighes t hardness and a r e used f o r general
app l i ca t i ons , bu t grades c o n t a i n i n g t i t a n i u m and tan ta lum ca rb ide a r e more
r e s i s t a n t t o c r a t e r i n g and used g e n e r a l l y f o r machining s t e e l . The use o f t h i n
CVD coa t ings o f T i c improves t o o l l i f e cons ide rab ly .
The i n t r o d u c t i o n o f new m a t e r i a l s which a r e d i f f i c u l t t o machine and the
focus on h i g h p r o d u c t i v i t y , has accentuated the need f o r harder c u t t i n g t o o l s .
Ceramic and cermet t o o l s a r e now i n use. The h e a t - r e s i s t i n g p r o p e r t i e s o f
ceramic t o o l s enable them t o be used a t speeds u n a t t a i n a b l e by ca rb ide t o o l s
w i t h consequent ly improved s tock removal ra tes . T h e i r h i g h r i g i d i t y p revents
c h a t t e r i n g and a l l ows smoother c u t s and super io r work f i n i s h .
ness and wear - res i s tan t p r o p e r t i e s a l l o w c l o s e maintenance o f dimensions and
enable the machining o f h i g h hardness m a t e r i a l s . Being non-meta l l i c , ceramics
do no t weld t o t h e m a t e r i a l be ing c u t and t h e i r extreme r e f r a c t o r i n e s s ob-
v i a t e s t h e need f o r coo lan ts . Diamond t o o l s a r e used f o r spec ia l purpose
c u t t i n g where the h i g h cos t can be j u s t i f i e d ; t o o l l i f e , however, i s super io r
t o t h a t o f ca rb ide .
The i r h i g h hard-
8.3.7 P i s t o n Rings
Apart f rom speed, temperature and load, t he p r i n c i p a l f a c t o r s which i n f l uence
wear o f p i s t o n r i n g s a r e co r ros ion , f requency o f use and d i r t . W h i l s t h i g h
temperatures may cause wear by adverse ly a f f e c t i n g l u b r i c a t i o n , low temper-
a tu res cause c y l i n d e r and r i n g wear due t o c o r r o s i o n by the condensate o f
combustion produc ts , t h i s i s p a r t i c u l a r l y so w i t h i n f r e q u e n t l y used engines.
A i rborne dus t can be a se r ious problem.
Grey cas t i r o n i s p robab ly the most w ide ly used m a t e r i a l f o r compression and
o i l r i n g s b u t harder m a t e r i a l s , such as carbon s t e e l o r even En31 b a l l bear ing
s t e e l , f i n d use. For g r e a t e r hardness and wear res i s tance , chromium p l a t i n g
i n s u f f i c i e n t t h i ckness (0.005 inch) can be used on r i n g s and g i ves reduced
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wear ra tes f o r both c y l i n d e r and r i ngs . Chromium p l a t i n g does no t run s a t i s -
f a c t o r i l y against i t s e l f , so on l y one mating sur face i s p la ted . Al though occ-
a s i o n a l l y used f o r l i g h t e r se rv i ce on passenger ca r engines, chromium p l a t i n g i s
genera l l y used on compression r i n g s on heavy duty engines; i t i s w ide l y used i n
d iese l and n a t u r a l gas engines. As chromium p l a t i n g reduces f a t i g u e s t reng th
i t i s usual t o p l a t e m a t e r i a l s o f h igh f a t i g u e s t reng th such as h igh carbon
s tee l s . Molybdenum coa t ings o f hardness over 1000 HV and a p o r o s i t y o f 15-25%
may now be favoured i n preference t o chromium p l a t i n g . S in te red i r o n p i s t o n
r i n g s are f i n d i n g increas ing use. Carbon-graphite, used f o r many years as a
p i s t o n r i n g ma te r ia l f o r un lub r i ca ted compressors, i s a s a t i s f a c t o r y ma te r ia l
but i t has l i t t l e inherent f l e x i b i l i t y , needs support and depends f o r i t s low
wear p r o p e r t i e s on the presence o f condensed water [17 ] . Non-meta l l ic p i s t o n
r ings, p a r t i c u l a r l y f a b r i c r e i n f o r c e d PF res ins , have been used where there a re
doubts about l u b r i c a t i o n so t h a t , i n the event o f l u b r i c a t i o n f a i l u r e , t he s o f t
r i n g s w i l l no t damage the c y l i n d e r w a l l .
Dry running PTFE p i s t o n r i n g s a re successful i n medium pressure oxygen com-
pressors as they possess low f r i c t i o n a l c h a r a c t e r i s t i c s , dimensional s t a b i l i t y
a t ope ra t i ng temperatures, good wear res is tance, h i g h s t reng th and load-
c a r r y i n g a b i l i t y and chemical iner tness. However, f o r non - lub r i ca ted h igh -
pressure app l i ca t i ons , dimensional s t a b i l i t y can o n l y be re ta ined i f the PTFE
i s re in fo rced .
ab le than s t e e l re in forcement . Other f i l l e r s commonly used a re carbon, powd-
ered metals, MoS2, ceramics and carbon f i b r e s . The concept o f avoidance o f
damage t o the c y l i n d e r w a l l and inc reas ing exper ience w i t h PTFE r i n g s on unlub-
r i c a t e d compressors have l e d t o the idea o f r e p l a c i n g m e t a l l i c r i n g s on l u b r i c -
a ted compressors w i t h PTFE r i n g s t o a l l o w reduced l u b r i c a n t feed ra tes . A s t a t e
o f the a r t rev iew o f t h e i r use has been presented [ I S ] .
ma te r ia l and m e t a l l u r g i c a l aspects o f p i s t o n r i n g s c u f f i n g has been c a r r i e d
o u t [ l g ] .
G lass - f i b re re in forcement appears t o be p o t e n t i a l l y more s u i t -
A l i t e r a t u r e survey o f
8.3.8 Cams and Tappets
In modern high-speed, h igh-output , small automtoive u n i t s d i s t r e s s o f cams
and tappets can be a major problem due t o acce le ra ted normal wear, p i t t i n g ,
s c u f f i n g and burn ish ing. Hardenable grey cas t i r o n i s t he most w ide ly used
camshaft ma te r ia l .
h i g h carbon content a re used i n the through-hardened c o n d i t i o n .
s t e e l s may a l s o be used w i t h s e l e c t i v e f lame o r i nduc t i on hardening o f sur face
areas.
genera l l y used w i t h i n d i v i d u a l cams sur face hardened. Tappet m a t e r i a l s a re
u s u a l l y through-hardened h i g h carbon, chromium o r molybdenum types o f ca rbu r i zed
l ow-a l l oy s tee l s . The most commn tappet m a t e r i a l s i n automot ive a p p l i c a t i o n s
Water-quenched high-carbon o r oi l -quenched a l l o y s t e e l s o f
Carbu r i z ing
For automot ive engine cams, chromium and molybdenum con ta in ing i r o n s a r e
217
are grey hardenable cas t i r o n con ta in ing chromium, molybdenum and n i c k e l o r
c h i l l e d cas t i ron. Oxide coa t ings on hardened s t e e l tappet faces improve
f r i c t i o n a l q u a l i t i e s and acce le ra te wear ing- in . Surface f i n i s h e s a re important
t o ensure adequate l i f e ; s t r e s s - r e l i e v i n g can be b e n e f i c i a l . Sal t -bath
n i t r i d i n g t reatment can be b e n e f i c i a l t o cam fo l l owers .
8.3.9 F r i c t i o n M a t e r i a l s
Technological progress i n the aeronaut i c a ? and automobi le i n d u s t r i e s makes
demands on f r i c t i o n m a t e r i a l s i nc reas ing l y more severe. High q u a l i t y brake
ma te r ia l s must have h igh c o e f f i c i e n t s o f f r i c t i o n , s t a b i l i t y a t a l l ope ra t i ng
temperatures, good wear res i s tance and s t reng th , h i g h thermal c o n d u c t i v i t y and
co r ros ion s t a b i l i t y . For arduous a p p l i c a t i o n s where sur face temperatures a re
high, o rgan ic f r i c t i o n ma te r ia l i s no longer s u i t a b l e . A t ope ra t i ng temper-
a tu res above 350"C, wear o f organic m a t e r i a l becomes ext remely severe.
base fac ings can operate up t o lO0O'C b u t new m a t e r i a l s a re under development
f o r ae ronau t i ca l a p p l i c a t i o n s where rubbing sur faces may w e l l reach temperatures
we l l above t h i s . S in te red m a t e r i a l s o f f e r a wide range o f f r i c t i o n a l cha rac te r -
i s t i c s by the d isposal o f c a r e f u l l y graded non-meta l l i c p a r t i c l e s i n a m e t a l l i c
ma t r i x ;
l eve l .
many a p p l i c a t i o n s .
Copper-
the composi t ion can be ad jus ted t o minimise wear, se izure and noise
Cermet and carbon f r i c t i o n m a t e r i a l s a r e be ing success fu l l y used i n
8.3.10 P l a s t i c Bear ings
Since the f i r s t appearance on the market i n the 1 9 3 0 ' s o f bear ings based on
thermoset t ing res ins , t he re has been a cont inuous increase i n the u t i l i z a t i o n
o f polymers and polymer based composites i n the bear ing f i e l d , [ 2 0 , 2 1 ] . Nylon
was the f i r s t o f t he thermoplast ic m a t e r i a l s used fo l l owed by PTFE and more
r e c e n t l y t he po lyace ta l s . P l a s t i c s o f f e r a number o f advantages over metals.
Thei r phys i ca l and mechanical p r o p e r t i e s can be v a r i e d over a wide range by
s u i t a b l e choice o f polymer type, f i l l e r and re in forcement ; some a r e cheap and
easy t o shape. Many polymers a re r e s i s t a n t t o chemical a t t a c k and e x h i b i t low
c o e f f i c i e n t s o f f r i c t i o n du r ing u n l u b r i c a t e d s l i d i n g . Thei r wear r a t e s s l i d i n g
against smooth metal counter faces a r e low and they do n o t normal ly e x h i b i t
s c u f f i n g o r se izure.
when l u b r i c a t e d , polymers a l l o w elastohydrodynamic l u b r i c a t i o n more r e a d i l y than
metals.
u l t i m a t e s t rengths, e l a s t i c modul i , creep res i s tance and c o e f f i c i e n t s o f ex-
pansion. Polymers may a l s o r e a d i l y absorb f l u i d s r e s u l t i n g i n dimensional
i n s t a b i l i t y . The i r low thermal c o n d u c t i v i t i e s can cause problems w i t h d i s s -
i p a t i o n o f f r i c t i o n a l heat. They a l s o have temperature l i m i t a t i o n s regard ing
L u b r i c a t i o n by f l u i d s can o f t e n be dispensed w i t h , bu t
They have disadvantages compared w i t h metals, e s p e c i a l l y regard ing
218
sof ten ing, m e l t i n g and thermal degradat ion.
The pr ime v i r t u e s o f thermo-set t ing r e s i n based bear ings, c l o t h o r f i b r e
re in fo rced , i s t h e i r h igh s t reng th and .exce l l en t performance under c o n d i t i o n s
o f water l u b r i c a t i o n . A t y p i c a l use i s r o l l neck bear ings o f s t e e l r o l l i n g
m i l l s where heat removal by water i s e s s e n t i a l . On a smal ler scale, re in forced
r e s i n bear ings a re used i n pumps f o r water c i r c u l a t i o n and w i t h o i l and grease
l u b r i c a t i o n i n automot ive app l i ca t i ons .
Nylon and po lyace ta l bushes p rov ide good d ry bear ing l i v e s i f the PV f a c t o r
i s kept low. These m a t e r i a l s s i n t e r e d t o p rov ide c o n t r o l l e d p o r o s i t y are used
f o r o i l impregnated bear ings a t much g rea te r PV values w i t h low wear. Thermo-
p l a s t i c bear ings f i n d ex tens i ve use i n f r a c t i o n a l HP e l e c t r i c motors, automot ive
a p p l i c a t i o n s and washing machines.
a re used i n p ros thes i s human j o i n t s .
Polyethy lene and h igh dens i t y po lyethy lene
PTFE based bear ings a re now used almost anywhere where c lean l i ness , heat
res is tance, freedom from se izu re and l u b r i c a n t e l i m i n a t i o n i s requi red. Thin
f i l m s o f PTFE f i n d e f f e c t i v e use i n many a p p l i c a t i o n s , p a r t i c u l a r l y t o avo id
f r e t t i n g . Depending upon the a p p l i c a t i o n and the p r o p e r t i e s requi red, PTFE
may be f i l l e d w i t h g lass, asbestos, carbon f i b r e s , MoS2, g raph i te , bronze,
n i c k e l and i r o n ox ide. A p r i n c i p a l a p p l i c a t i o n o f PTFE i s f o r gas l u b r i c a t e d
bear ings t o contend w i t h s topping and s t a r t i n g .
8.4 SURFACE TREATMENTS AND COATINGS
Mate r ia l s f o r t r i b o l o g i c a l a p p l i c a t i o n s must f u l f i l two impor tant f unc t i ons .
They must have s t r u c t u r a l p r o p e r t i e s f o r load c a r r y i n g and sur face character-
i s t i c s t o a l l o w r e l a t i v e mot ion w i t h low f r i c t i o n and the minimum amount o f wear
damage. Surface treatments and coa t ings a l l o w design scope t o meet these
demands. Also, w i t h wear problems i t i s o f t e n d i f f i c u l t t o make wor thwhi le
innovat ions i n design, l u b r i c a t i o n o r m a t e r i a l s se lec t i on , bu t many sur face
t reatments and coa t ings a re a v a i l a b l e which can e f f e c t wear res i s tance and
improved f r i c t i o n a l p r o p e r t i e s r Z Z , Z 3 ] . Hard wear r e s i s t a n t m a t e r i a l s a re
u s u a l l y expensive t o manufacture and d i f f i c u l t t o shape, and f o r economic
reasons, wear- res is tant sur face t reatments may be app l i ed t o more common,
cheaper t o produce and more e a s i l y shaped m a t e r i a l s .
Many sur face t reatments a re a v a i l a b l e t o combat wear, and the choice
depends l a r g e l y upon the type and s e v e r i t y o f wear invo lved. Some treatments
a re sho r t l i v e d bu t the more common sur face t reatments a r e those which are
expected t o l a s t t he l i f e o f the component. Both types o f t reatment may be com-
b ined t o e f f e c t p r o t e c t i o n du r ing arduous c o n d i t i o n s o f running i n and long
se rv i ce e f fec t i veness . Treatments expected t o l a s t the l i f e o f a component
i nvo l ve sur face hardening w i thou t changing the composi t ion, sur face hardening by
d i f f u s i o n treatments and by the a p p l i c a t i o n o f sur face coat ings. Carbon and
219
alloy steels of a hardenable composition and cast iron may be surface hardened
by flame or induction methods.
An extensively used method of surface hardening to improve wear resistance
depends upon diffusing specific elements into the surfaces of metal by such
treatments as carburizing, carbo-nitriding, nitriding, chromosing, boronising
and siliconising. Care must be taken to ensure a sufficient depth of case of
the correct structure and hardness and a satisfactory transition zone, Fig.8.
Shot peening work hardens the surface of metals.
Electro-deposition of hard metal such as chromiumprovides hard surface
coatings and alloy coatings may be used to improve wear resistance. Composite
coatings may be produced by the co-deposition of hard particles and electro-
deposited metal, Fig.9. Coatings of silicon carbide in nickel, tungsten carbide
in nickel and cobalt and silicon nitride in nickel have been used effectively.
Diamond containing coatings have been developed for specific applications 1241.
Hard surfacing or facing finds wide use in many applications to provide
specific wear resistant alloy or ceramic coatings. Almost any metal or alloy
which can be cast may be used as a welding rod to apply a coating.
spraying even the most refractory materials can be deposited with good surface
bonding .
With plasma
Fig.8 (x75) Micro hardness survey of a case hardened gear tooth with superimposed HV.
Fig.9 ( ~ 1 5 0 ) Co-deposited Fe-b%Ni and chromium carbide.
220
Some sur face t rea tments , besides e f f e c t i n g wear res i s tance , may a l s o a i d
l u b r i c a t i o n . Thus Sulphinuz [25] t rea tment can n i t r o g e n harden a su r face w i t h
the assoc ia ted compressive s t resses b e n e f i c i a l t o f a t i g u e res i s tance , as w e l l
as produce a s u l p h u r - r i c h su r face l a y e r w i t h good l u b r i c a n t p r o p e r t i e s under
boundary c o n d i t i o n s .
an oxygen- ion r i c h , low f r i c t i o n su r face b e n e f i c i a l i n reduc ing wear and a i d i n g
l u b r i c a t i o n under arduous cond i t i ons . Sulphinuz and s o f t n i t r i d i n g t rea tments
may s o f t e n hard m a t e r i a l s .
which incorpora tes a quenching t rea tment o f t he Su f t BT process [25] which pro-
duces a su lphur r i c h l a y e r , by an e l e c t r o l y t i c t rea tment i n a low temperature
s a l t ba th . Var ious types o f phosphat ing t rea tments p rov ide a t h i n , porous
c r y s t a l l i n e l u b r i c a t i n g sur face f i l m o f i n s o l u b l e phosphate, Fig.10, which can
a l s o r e t a i n l u b r i c a n t o r p rov ide an e f f e c t i v e base f o r a s o l i d l u b r i c a n t .
S o l i d l u b r i c a n t f i l m s such as PTFE and MoS2 can a l s o c o n t r o l wear, reduce
f r i c t i o n and a i d l u b r i c a t i o n .
S o f t n i t r i d i n g c a r r i e d o u t i n an o x i d i s i n g ba th produces
Th is can be avoided w i t h the Noskuff process [25]
Fig.10 Phosphate coa t ing .
A very hard, s t r o n g l y adherent, homogeneous t h i n f i l m u s u a l l y o f hard
ca rb ide such as TIC may be a p p l i e d t o metal by chemical vapour d e p o s i t i o n (CVD)
[26,27]. Re f rac to ry coa t ings such as alumina and s i l i c o n n i t r i d e may a l s o be
chemical vapour deposi ted.
Phys ica l vapour d e p o s i t i o n techniques a r e a v i a b l e approach t o the p roduc t i on
o f su r face f i l m s o f extreme v e r s a t i l i t y i n depos i t compos i t ion . V i r t u a l l y any
meta l , a l l o y , r e f r a c t o r y o r i n t e r m e t a l l i c compound, some po lymer ic type
m a t e r i a l s and t h e i r m ix tu res can be depos i ted [ 2 8 ] . Metal f i l m s depos i ted by
221
i on -p la t i ng a re s t r o n g l y bonded t o the sur face as the f i l m i s depos i ted on a
surface cleaned by s p l u t t e r e t c h i n g [29,30]. S o f t metal l u b r i c a n t f i l m s may
a lso be bonded t o a metal su r face by i o n - p l a t i n g [31]. I o n - n i t r i d i n g speeds up
the n i t r i d i n g opera t i on .
8.5 CONCLUSIONS
Mechanisms can o n l y per fo rm s a t i s f a c t o r i l y i f the des ign and the m a t e r i a l s
o f c o n s t r u c t i o n a r e c o r r e c t l y chosen t o contend w i t h the o p e r a t i n g c o n d i t i o n s
and the environment i n which they a r e requ i red . I t i s o n l y by e f f i c i e n t s e l -
ec t i on and c a r e f u l s p e c i f i c a t i o n t h a t the most e f f e c t i v e use o f m a t e r i a l s can be
accomplished t o e f f e c t m a t e r i a l cpnse rva t i on and energy sav ing i n t h e i r p ro-
duct ion.
Economics i n m a t e r i a l s and manufac tur ing cos ts can o f t e n be made by j u d i c i o u s
use o f sur face coa t ings and su r face t rea tments .
REFERENCES
1 2
3
4
5 6
7
8 9
10
1 1 12
13 14 15
16 17 18
19 20
21
22
Scott,O., T r i bo logy , 1968, 1, 14. Scott,D, I n T r ibo logy - Proc. J. Res iden t ia l Course, 1968, Paper 1 , I n s t . M e t a l l u r g i s t s , London. Scott,D., Proc. I n s t . Mech. Engrs., London I n t . Conf. on L u b r i c a t i o n and Wear, 1967, 182, ( 3 A ) , 325. New Eng ineer ing M a t e r i a l s , Proc. I n s t . Mech. Engrs, London, 1965/66, 180, ( 3 D ) . mdman,N.E., Eng ineer ing A l l o y s , 1962, Chapman & H a l l , London. Zakay,V.F. and Justusson,W.M., I n High St rength S tee ls - I . S . I . Special Report 76, 1962, 14. I r o n & S tee l l ns t . , London. Ouckworth,W.E., Leak,D.A. and P h i l l i p s , R . , I n High S t reng th S tee ls , I . S . I . Special Report 76, 1962, 22, I r on & S tee l l n s t . , London. Forrester,P.G., Metal1 Rev., 1960, 5, (ZO), 507. Amateau,M.F., Nicholson,D.W. and Glaeser,W.A., 1961, O.T.S. PB 171625, O f f i c e o f Techn ica l Services, Washington D.C. Scott,O., I n Fa t i gue i n R o l l i n g Contact , 1963, 103, I n s t . Mech. Engrs. London. Scott,D., I n Low A l l o y S tee ls , 1968, 203, I r o n and Stee l I n s t . London. Scott,D., I n R o l l i n g Contact Fa t igue, (Ed. Tourret ,R., and Wright,E.P.), 1977, 3, Heyden, London. Scott,D., Wear, 1977, 9, 71. Scott,D., Proc. I n s t . Mech. Engrs., 1969, 183, (3L), 9. Scott,D., R o l l i n g Contact Fa t igue, In , Wear, (Scott,D., Ed.), T r e a t i s e on M a t e r i a l s Science and Technology, 1978, 13, 321, Academic Press, NY. Trent,E.M. Metal Cu t t i ng , 1977, B u t t e r w o x h s , London. Summers-Smith,D., Wear, 1966, 9, 425. Summers-Smith,D., Proc. T r ibo logy Conf., 1971, Paper C93/71, I n s t . Mech. Engrs. Scott,D., Smith,A.I., Ta i t , J . and Tremain,G.R., Wear, 1975, 2, 293. Pratt,G.C., P l a s t i c Based Bear ings i n "Lub r i ca t i on and Lubr ican ts" , (E.R. B r a i t h w a i t e - Ed.), 1967, 377, E l s e v i e r , Amsterdam. Evan5,F.C. and Lancaster,J.K., The Wear o f Polymers, i n Wear, (Scott ,D.) , T r e a t i s e on M a t e r i a l s Science and Technology, 1978, - 13, 85, Academic Press, NY. Scott,D., Wear, 1978, 48, 283.
222
23
24 25 26 27
28 29
30 31
Wilson,R.W., Proc. 1st Euro, Tribology Congr., 1973, 165, Inst. Mech. Engrs., London. Sharp,W.F., Wear, 1975, 32, 315. Gregory,J.C., Tribology, 1970, 3, 73. Gass,H. and Hintermann,H.E., SwTss Patent 452.205, 1968. Hintermann,H.E. and Aubert,F., Proc. 1 s t Euro Tribology Congr., 1973, 207, Inst. Mech. Engrs., London. Bunshah,R.F. and Juntz,R.S., J. Vac. Sci. Technol., 1972, 9, 1389. Spalvins,T., Przbyszewski,J.S. and Buckley,D.H., NASA Tech. Note TN D - 3707, 1966. Teer,D.G., Tribology, 1975, 1, 245. Sherbiney,M.A. and Halling,J., Wear, 1977, 3, 211.
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!) SELECTION OF LUBRICANTS
A.R. LANSDOWN, D i r e c t o r , Swansea T r ibo logy Centre, U.K.
9.1 I NTRODUCT I ON
The v a r i e t y o f a v a i l a b l e l u b r i c a n t s i s enormous. I f we simply consider
broad, bas i c types the re a r e probably many hundreds: i f we take i n t o account
minor d i f f e r e n c e s i n composi t ion and the va r ious commercial brands, t he re a re
probably tens o f thousands. To the non-spec ia l i s t t he problem o f proper l u b r i -
cant s e l e c t i o n can t h e r e f o r e seem very confus ing.
For many a p p l i c a t i o n s t h e s e l e c t i o n o f a l u b r i c a n t i s i n f a c t no t c r i t i c a l ,
and a wide v a r i e t y o f l u b r i c a n t s could work q u i t e s a t i s f a c t o r i l y .
app l i ca t i ons the o b j e c t o f l u b r i c a n t s e l e c t i o n should probably be t o ensure the
lowest o v e r a l l l i f e cos t f o r t he system as a whole.
For such
For some a p p l i c a t i o n s , however, t he s e l e c t i o n o f l u b r i c a n t may be very c r i t i -
cal indeed, and the re may be very few o r even no l u b r i c a n t s capable o f ensur ing
s a t i s f a c t o r y ope ra t i on .
Because the re a r e thousands o f d i f f e r e n t l u b r i c a n t s a v a i l a b l e , and because
many a p p l i c a t i o n s a re n o t c r i t i c a l , t he re i s o f t e n a tendency t o leave l u b r i c a n t
se lec t i on t o a very l a t e stage i n the design process. The r e s u l t may be t h a t a
design i s completed and a machine const ructed f o r which no s u i t a b l e l u b r i c a n t
ex i s t s , and t h e r e have been cases where very expensive m o d i f i c a t i o n has been
necessary t o reso lve the problem o f l u b r i c a t i o n .
One important p r i n c i p l e i s t h e r e f o r e t h a t l u b r i c a n t requirements should
always be considered a t an e a r l y stage i n design.
In orde r t o approach l u b r i c a n t s e l e c t i o n r e a I i s t i c a l l y , we should be c l e a r
as t o what a r e the o b j e c t s o f l u b r i c a t i o n .
The pr imary o b j e c t o f l u b r i c a t i o n i s t o reduce f r i c t i o n o r wear, o r u s u a l l y
both f r i c t i o n and wear. Th is i s the f a c t o r which de f i nes a ma te r ia l as a l u b r i -
cant . There a r e i n a d d i t i o n th ree secondary func t i ons o f a l u b r i c a n t : -
( i ) To a c t as a coo lan t . I n some systems t h i s w i l l be a v i t a l f u n c t i o n o f
t he l u b r i c a n t because f r i c t i o n a l o r process heat must be removed and no
a l t e r n a t i v e c o o l i n g f l u i d can be used.
To remove wear d e b r i s o r o t h e r contaminants, o r t o prevent o t h e r contami-
nants from e n t e r i n g the system.
( i i i ) To p r o t e c t meta ls aga ins t co r ros ion .
( i i )
There i s no good reason why a
224
l u b r i c a n t should be expected t o p rov ide such p r o t e c t i o n , bu t because min-
e r a l o i l s a re very e f f e c t i v e co r ros ion prevent ives, many designers have
come t o expect the same o f a l l l u b r i c a n t s .
The l u b r i c a n t w i l l o f t e n s a t i s f a c t o r i l y f u l f i l a l l these tasks, bu t where the
a v a i l a b l e choice o f l u b r i c a n t s i s l i m i t e d , i t may be necessary t o choose the
l u b r i c a n t o n l y t o meet the f r i c t i o n and wear requirements, and t o use o the r
techniques t o so lve the coo l i ng , contaminat ion and co r ros ion problems.
9.2 SELECTING THE LUBRICANT TYPE
Lubr i ca t i on systems as such a re ou ts ide the scope of t h i s chapter, bu t the
problem o f l u b r i c a n t s e l e c t i o n cannot e n t i r e l y be separated from t h a t o f selec-
t i n g the l u b r i c a t i o n system.
The most s t r a i g h t f o r w a r d way t o s e l e c t both l u b r i c a n t and l u b r i c a t i o n system
i s probably t o s t a r t w i t h the s implest technique and t o progress from t h a t on l y
as f a r as i s necessary t o ensure s a t i s f a c t o r y ope ra t i on . The s implest technique
w i l l u s u a l l y have the lowest i n i t i a l cost , and w i l l o f t e n also be the most
re1 i ab le .
The s implest system c o n s i s t s o f a smal l q u a n t i t y o f p l a i n minera l o i l i n
p lace i n the l u b r i c a t e d component, w i t h o u t any f a c i l i t y f o r r e - l u b r i c a t i o n .
Such a system w i l l cope w i t h a s u r p r i s i n g l y wide v a r i e t y o f mechanisms,
i nc lud ing watches and c locks and many o t h e r p r e c i s i o n instruments, door- locks
and hinges, sewing-machines, t y p e w r i t e r s , b i c y c l e s , r o l l e r skates, skate-boards,
and so on.
I t ceases t o cope when the re i s t oo much load o r speed o r heat o r deb r i s , o r
when the l i f e requ i red i s so long t h a t t he o i l ox id i ses . evaporates, o r creeps
away from the bear ing surfaces. I t i s then necessary t o use a more sophis t icated
o i l , a grease, a s o l i d l u b r i c a n t , o r sometimes even a gas l u b r i c a n t , o r t o use
a more complex l u b r i c a t i o n system.
Table 9.1 shows some of the poss ib le choices o f a l t e r n a t i v e systems when the
s implest system i s no longer adequate. Some o f these choices a re concerned w i t h
the l u b r i c a t i o n system r a t h e r than the choice o f l u b r i c a n t , b u t whatever l u b r i -
c a t i o n system i s used, a choice o f l u b r i c a n t i s needed.
As the demands on a bear ing increase, a p o i n t i s even tua l l y reached where a
p l a i n minera l o i l i s no longer adequate, and i t i s a t t h i s p o i n t t h a t the pro-
blem o f l u b r i c a n t s e l e c t i o n begins.
A convenient approach t o l u b r i c a n t s e l e c t i o n w i l l be t o consider f i r s t the
p r o p e r t i e s which i n f l uence the s e l e c t i o n o f d i f f e r e n t minera l o i l s , then the
var ious o the r o i l s which can be used, and f i n a l l y the a l t e r n a t i v e s t o l u b r i c a -
t i n g o i l s , namely greases, s o l i d l u b r i c a n t s and gases.
I t may be use fu l t o r e f e r t o F igure 1, which i nd i ca tes broad l i m i t s o f speed
and load w i t h i n which d i f f e r e n t c lasses o f l u b r i c a n t can be used.
SPEED ( f t /m in ] - 2 10 100 boo0 10.000 100.000
1,ooo. 000 100.000
2 . For 1 iqiiid Iiihricnnts t h c rmlnmcntnl pronerty is v i s c o s i t y , :ind t h i s f i m i r c LISSIU~CS t h a t t he corrert v i s c o s i t y h;is
1:xtcrnalIy nressiiriscd hcnrinps can he uscd ovcr a wide rnnp,c of snecd :ind lo:1d. t hecn chosen. 10.000
3.
- N
c \ e
1,000 - . _ E > 10.000
t looeooo - N
Y a -
a
1,000 100
100 14.5 10 100 1.000 10.000 rw,ooo r,ooo,ooo
SPEED AT BEARING CONTACT lrnm/s) --C
F i g . 1 Speed/load limitations for different types o f lubricant.
N tu cn
226
Table 9.1
Poss ib le choices when a smal l q u a n t i t y o f p l a i n minera l o i l i n p lace i s no lonqer adequate.
Too much load
Too much speed (which may lead t o too much heat)
Too much heat
Too much d e b r i s
Contamination
More viscous o i l Grease Extreme pressure o i l Extreme pressure grease E x t e r n a l l y p ressu r i sed l u b r i c a t i o n Sol i d l u b r i c a n t
Greater q u a n t i t y o f o i l Less viscous o i l O i l c i r c u l a t i o n system Gas l u b r i c a t i o n
O i l w i t h a n t i - o x i d a n t More viscous o i l Greater q u a n t i t y o f o i l O i 1 c i r c u l a t i o n system Syn the t i c o i l Sol i d l u b r i c a n t
Greater q u a n t i t y o f o i l C i r c u l a t i o n system w i t h f i l t r a t i o n
O i l c i r c u l a t i o n system Grease S o l i d l u b r i c a n t
Long 1 i f e requ i red O i l w i t h a n t i - o x i d a n t More viscous o i l Great q u a n t i t y o f o i l R e l u b r i c a t i o n system Syn the t i c o i l Grease Sol i d l u b r i c a n t
9.3 PROPERTIES OF MINERAL OILS
9.3.1 V i s c o s i t y
V i s c o s i t y i s the most impor tant s i n g l e p roper t y o f a l u b r i c a t i n g o i l , s i nce
i t i s the so le p roper t y which determines whether the o i l i n a bear ing a t a
s p e c i f i c load and speed w i l l g i v e f u l l f l u i d f i l m separat ion o f the bear ing
surfaces.
V i s c o s i t y may be expressed i n any one o f a number o f d i f f e r e n t u n i t s . Pro-
bably the most commonly used i n Engl ish-speaking coun t r i es i s the cen t i s toke
(cSt) which p roper l y descr ibes Kinematic V i s c o s i t y , and i s d i r e c t l y measured by
the va r ious standard I n s t i t u t e o f Petroleum, ASTM, B r i t i s h Standard and IS0
t e s t methods. The Absolute V i s c o s i t y i s commonly expressed i n terms o f the
cen t ipo i se (cP) and i s the v i s c o s i t y used i n the va r ious engineer ing ca l cu la - t i o n s such as equat ions ( 1 ) t o (3) .
227
Each o f these u n i t s has i t s S t equ iva len t , b u t the i n t r o d u c t i o n o f the S I
Uni ts has been the sub jec t of considerable d ispute. The SI equ iva len t o f t he
cent is toke i s the m i l l i m e t r e z per second (mm2 s - ' ) and the S I equ iva len t o f the
cent ipo ise i s t he m i l l inewton second per metre2 o r m i l 1 ipascal second ( m N s / m Z ) .
There a re a l s o th ree o t h e r u n i t s which a r e s t i l l w ide l y used, a l though
current p o l i c y i s t o d i scon t inue them i n favour o f k inemat ic v i s c o s i t y i n c e n t i -
stokes. They a r e Redwood Seconds, used i n B r i t a i n , Saybol t Seconds (SUS) i n the
United States, and degrees Engler, used i n Germany and o t h e r p a r t s o f con t i nen ta l
Europe.
There a re several equat ions which r e l a t e the o i l f i T m th ickness t o the o i l
v i scos i t y . For general hydrodynamic l u b r i c a t i o n the Reynolds Equation [ l ] appl ies,
( 1 ) ah h3 * + h3 3 = 6pU + 6ph + 12pV
ax [ ax1 aY I a Y 1
f o r elastohydrodynamic l u b r i c a t i o n w i t h c y l i n d r i c a l elements, as i n r o l l e r
bearings and many gear c o n f i g u r a t i o n s , an equat ion o f the Dowson-Higginson type,
121 app l i es ,
"0.7 G0.54 = 2.65
hmin/R w0.03
whi le f o r e lastohydrodynamic l u b r i c a t i o n w i t h spher i ca l elements, as i n b a l l -
bearings, t he Archard-Cowking Equation [3 ] a p p l i e s ,
where U i s p r o p o r t i o n a l t o v i s c o s i t y .
10-1
z I-
LL u. lL 0
I- z W
U U W
P u
- 2
8
10-2 1 0 . ~ 10-8
PV/p DIMENSIONLESS
F i g . 2 Typica l S t r i beck curve
228
Figure 2 , sometimes c a l l e d a S t r i b e c k curve, shows how the v i s c o s i t y o f a
l u b r i c a n t a f f e c t s the l u b r i c a t i o n mode and the c o e f f i c i e n t o f f r i c t i o n i n a
bear ing . By the use of t he approp r ia te equat ion the o i l v i s c o s i t y requ i red f o r any
bear ing can be ca l cu la ted , b u t i f t h i s i s done f o r every bear ing i n every mach-
ine i n a f a c t o r y the r e s u l t w i l l p robably be a l i s t of several hundred v i s c o s i -
t i e s . The number o f a v a i l a b l e minera l o i l s i s so grea t t h a t i t might w e l l be
poss ib le t o o b t a i n o i l s w i t h every v i s c o s i t y requi red, b u t t he re a r e some very
good reasons f o r n o t do ing so.
( i ) I f a l l the bear ings i n a complex machine can use the same
a s i n g l e o i l r e s e r v o i r and c i r c u l a t i o n system can be used,
g e n e r a l l y be cheaper and more r e l i a b l e .
( i i ) A l a r g e number o f d i f f e r e n t l u b r i c a n t s causes s torage prob
u b r i c a n t , then
and t h i s w i l l
ems.
( i i i ) The g rea te r the number o f l u b r i c a n t s i n use i n one p l a n t , t he g rea te r i s
the p r o b a b i l i t y o f t he wrong one being used.
Most bear ings a r e requ i red t o operate over a range o f temperature, so the
v i s c o s i t y o f the o i l w i l l o n l y be a t t h e optimum value f o r a p a r t o f the
opera t i ng time.
( i v )
Fo r tuna te l y the v i s c o s i t y requirement i s o f t e n n o t c r i t i c a l . As long as the
v i s c o s i t y i s h igh enough t o ensure the requ i red o i l f i l m th ickness, t he re i s no
harm i n going t o a s l i g h t l y h ighe r v i s c o s i t y . I n any case an increase i n v isco-
s i t y i s p a r t l y sel f -compensat ing. The e f f e c t of i nc reas ing v i s c o s i t y i s t o
increase viscous f r i c t i o n , which increases t h e power d i s s i p a t e d i n the bear ing.
The excess power i s converted t o heat, the heat r a i s e s the o i l temperature, and
the h igher temperature reduces the o i l v i s c o s i t y . So the end r e s u l t of us ing a
more viscous o i l i s t h a t t he power consumption and the temperature s t a b i l i s e a t
a s l i g h t l y h ighe r l e v e l .
I t i s t he re fo re p o s s i b l e t o use a smal l number o f v i s c o s i t y grades to f i l l a
wide range o f o i l requirements, and i n f a c t many n a t i o n a l and i n t e r n a t i o n a l
standards ( i n c l u d i n g B r i t i s h Standard 4231 "V iscos i t y C l a s s i f i c a t i o n f o r Indus-
t r i a l L i q u i d Lubr icants") r e s t r i c t t h e number o f l u b r i c a n t s i n such a way tha t
the v i s c o s i t y o f each o i l i n cen t i s tokes a t 4OoC i s 50% h igher than t h a t o f the
preceding grade. T h i s g i ves o n l y e ighteen grades t o cover the whole range from
2 cSt t o 1500 cSt .
So the f i r s t t h i n g to do i n m ine ra l o i l s e l e c t i o n i s t o c a l c u l a t e the lowest
v i s c o s i t y which w i l l ensure f u l l f l u i d f i l m l u b r i c a t i o n o f a l l t he bear ings i n
the system, and then to i d e n t i f y t he next h ighe r standard v i s c o s i t y grade.
O°F, 7OoF, 100°F
The v i s c o s i t y o f an o i l i s always quoted a t a s t a t e d temperature, u s u a l l y
o r 130°F, bu t i nc reas ing l y 4OoC. These temperatures w i l l
229
r a r e l y be those a t which your bear ings a r e intended t o operate, so the second
important p roper t y t o consider i s the v a r i a t i o n o f v i s c o s i t y w i t h temperature.
9.3.2 V i scos i ty/Tempe r a t u r e Re1 a t i onsh i p
For a l l l i q u i d s the v i s c o s i t y decreases as the temperature increases, bu t
the r a t e o f decrease v a r i e s considerably f rom one l i q u i d t o another . I t fo l l ows
t h a t i f we p l o t graphs o f o i l v i s c o s i t y aga ins t temperature, which g ives some-
t h i n g approaching s t r a i g h t l i n e s , t he s lope o f the l i n e s i s d i f f e r e n t f o r d i f f -
e ren t o i l s . F igure 3 shows some t y p i c a l v i scos i t y / tempera tu re r e l a t i o n s h i p s
f o r d i f f e r e n t o i l s .
Standard curves a r e r e a d i l y a v a i l a b l e showing the v iscos i ty / tempera
r e l a t i o n s h i p f o r d i f f e r e n t o i l s , so t h a t we can conver t our requ i red v
t o the v i s c o s i t y a t a standard reference temperature, prov ided t h a t we
which l i n e ( i . e . which s lope) r e l a t e s t o the o i l under cons ide ra t i on .
u r e
scos i t y
know
The p roper t y which i s most w ide l y used t o descr ibe the v i scos i t y / tempera tu re
behaviour o f an o i l i s t he V i s c o s i t y Index (V.1). Th is index r e l a t e s the
change o f v i s c o s i t y w i t h temperature t o two a r b i t r a r y o i l s , one based on a
Pennsylvania Grude o i l (V. I . - 0) and one on a Gu l f Coast o i l (V.1. - 100).
v i s c o s i t i e s a t 4OoC and 100°C by means of equat ion
The v i s c o s i t y index o f an unknown o i l can be ca l cu la ted from the measured
V.I. 100 (L-u) / (L-H)
where U i s the v i s c o s i t y a t 4OoC o f t he o i l sample i n cen t i s tokes , L i s t he
v i s c o s i t y a t 4OoC o f an o i l o f 0 v i s c o s i t y index having the same v i s c o s i t y a t
100°C as the o i l sample, and H i s the v i s c o s i t y a t 4OoC o f an o i l of 100 v i s -
c o s i t y index hav ing the same v i s c o s i t y a t 100°C as the o i l sample. Tables o f
values f o r L and H a re conta ined i n the standard I n s t i t u t e o f Petroleum and
ASTM t e s t methods.
B r i e f l y , the V i s c o s i t y Index w i l l be low i f the v i s c o s i t y changes r a p i d l y
w i t h temperature, and h igh i f v i s c o s i t y i s l ess a f f e c t e d by temperature. B r i t i s h
Standard D.S.4231 r e f e r s t o minera l o i l s having V i s c o s i t y Ind i ces of 0, 50 o r 95.
The h ighes t V . I . f o r a s imple minera l o i l i s s l i g h t l y over 100, w h i l e c e r t a i n
s y n t h e t i c o i l s have V . I . g rea te r than 200.
General ly, a V . I . h ighe r than 100 i s o n l y obta ined w i t h a minera l o i l by the
use o f a po lymer ic a d d i t i v e c a l l e d a V i s c o s i t y Index Improver. V. I . Improvers
a r e used i n mu l t i - g rade engine o i l s and i n h y d r a u l i c f l u i d s which a re requi red
t o operate over a wide temperature range.
Most bear ings have t o operate over a range of temperatures, and i t i s the
w i d t h o f t h i s range which determines what V . I . i s requ i red . A V . I . o f 0 may be
acceptable i f the temperature range i s very narrow, o r i f a l a r g e change i n
v i s c o s i t y i s acceptable. On the o t h e r hand, a V . I . o f 160 o r more i s s p e c i f i e d
230
Fig.3 Viscosity/temperature characteristics of various oils
231
f o r an a i r c r a f t h y d r a u l i c f l u i d which i s requ i red t o opera te f rom -4OOC t o +15OoC.
The e f f e c t i v e n e s s o f a V.I . Improver i s a f f e c t e d by the shear r a t e , t he r a t e
a t which the o i l has t o pass through con f ined spaces, and a t h igh shear r a t e i n
a high-speed bear ing the e f f e c t i v e v i s c o s i t y o f a V . I . improved o i l may be
l i t t l e o r no d i f f e r e n t f rom t h a t o f t he base o
Having chosen t h e requ i red V i s c o s i t y Index,
a t the re fe rence temperature can be worked o u t
Figure 3 o r a c h a r t such as t h e one i n B.S.423
9.3.3 V i scos i t y /P ressu re Re la t i onsh ip
Before l eav ing t h e sub jec t o f v i s c o s i t y , i t
1 .
t h e v i s c o s i t y o f t he se lec ted o i l
by means o f a graph such as
i s perhaps d e s i r a b l e f o r complete-
ness t o ment ion the v a r i a t i o n o f v i s c o s i t y w i t h p ressure .
l u b r i c a t i n g o i l s increases as the pressure i s increased. I n p r a c t i c e the r a t e
o f increase i s very low, and i n p l a i n hydrodynamic o r e x t e r n a l l y p ressu r i sed
bearings i t can s a f e l y be neg lec ted .
The v i s c o s i t y o f a l l
I n elastohydrodynamic l u b r i c a t i o n , however, t he pressure generated i n the
l u b r i c a n t can be s u b s t a n t i a l . The increase i n l u b r i c a n t v i s c o s i t y i s co r res -
pondingly h igh , and i s a major f a c t o r i n ensu r ing success fu l l u b r i c a t i o n o f
gears and r o l l i n g bear ings .
I t i s r a r e t o s p e c i f y o r quote a p ressure c o e f f i c i e n t o f v i s c o s i t y f o r a
l u b r i c a t i n g o i l , and t h i s i s no rma l l y o n l y done f o r very c r i t i c a l s i t u a t i o n s
such as i n c e r t a i n spacec ra f t a p p l i c a t i o n s .
9.3.4 A n t i -Wear, Extreme Pressure and A n t i -Fr i c t i o n P r o p e r t i e s
I n theory , f rom t h e p o i n t o f v iew o f pure phys i ca l l u b r i c a t i o n , we have
completely s p e c i f i e d the o i l when we have s p e c i f i e d the v i s c o s i t y , t he V . I . and
perhaps the pressure c o e f f i c i e n t o f v i s c o s i t y . I n p r a c t i c e t h i s i s n o t always
so, and i t i s o f t e n necessary t o s p e c i f y the an t i -wear , extreme pressure o r
a n t i - f r i c t i o n p r o p e r t i e s o f t he o i l .
Re fe r r i ng aga in t o F igu re 2, t h e boundary and mixed l u b r i c a t i o n reg ions
represent s i t u a t i o n s i n which the bear ing sur faces a r e n o t comple te ly separated
by a f i l m o f l u b r i c a n t , bu t exper ience some degree o f s o l i d - t o - s o l i d con tac t .
Even i n l i g h t l y - l o a d e d bear ings s o l i d - t o - s o l i d con tac t can occur a t low
speed when a bear ing i s s t a r t i n g o r s topp ing , and t h i s i s p a r t i c u l a r l y impor tan t
i f a mechanism i s in tended t o ope ra te i n t e r m i t t e n t l y . The r e s i d u a l o i l f i l m
w i l l o f t e n g i v e some p r o t e c t i o n under these c o n d i t i o n s , b u t g r e a t e r p r o t e c t i o n
can be ob ta ined by t h e use o f an an t i -wear a d d i t i v e . T h i s i s u s u a l l y an organic
a c i d such as s t e a r i c a c i d o r a n a t u r a l o i l such as rapeseed o i l , b u t s y n t h e t i c
o rgan ic phosphorus compounds such as t r i - x y l y l phosphate a r e a l s o e f f e c t i v e .
I n heav i l y - l oaded bear ings o r i n t e r m i t t e n t r o l l i n g bear ings o r c e r t a i n types
o f gear, a more a c t i v e type o f a d d i t i v e known as an Extreme Pressure (EP)
232
a d d i t i v e may be needed.
compounds con ta in ing phosphorus, su lphur o r c h l o r i n e , a l though lead naphthenate
i s s t i l l used i n c e r t a i n gear o i l s , and molybdenum d i s u l p h i d e i s occas iona l l y
used.
EP a d d i t i v e s a r e u s u a l l y r e a c t i v e s y n t h e t i c o rgan ic
EP p r o p e r t i e s a re assessed i n severa l d i f f e r e n t ways, b u t the most w ide ly
used t e s t methods a re probably the Timken and Four-Bal l machines. One c r i t e r i o n ,
measured by means o f the Four-Bal l Machine, i s the Mean Her t z load, and t h i s can
vary from 30 f o r a p l a i n minera l o i l t o 85 f o r a powerful EP gear o i l .
One extreme requirement f o r EP p r o p e r t i e s i s i n c e r t a i n types o f metalworking
operat ion, and the most powerful c h l o r i n e and su lphur-conta in ing a d d i t i v e s a re
used i n the l u b r i c a n t s f o r such a p p l i c a t i o n s . Powerful EP a d d i t i v e s can be
co r ros i ve , and should t h e r e f o r e n o t be used where they are n o t necessary.
A n a t u r a l o i l may be added t o an o i l t o decrease the f r i c t i o n i n boundary
l u b r i c a t i o n , and thus reduce the power consumption and the heat generated. This
may be p a r t i c u l a r l y impor tant i n a mechanism which i s s e n s i t i v e t o f r i c t i o n ,
such as a worm gear. The n a t u r a l o i l i s then sometimes c a l l e d a l u b r i c i t y addi-
t i v e , bu t the term i s i l l - d e f i n e d and genera l l y n o t recommended.
We have now considered a l l the p r o p e r t i e s requ i red t o g i v e adequate i n i t i a l
1 ubr i c a t ion,
f a c t o r y l u b r
9.3.5 Stab
An o i l w i
bu t t he re a r e two o t h e r cons ide ra t i ons i n ensur ing cont inued s a t i s -
c a t i on.
l i t y
1 decompose chemical ly i n se rv i ce because o f e i t h e r heat ( thermal
decomposition) o r a combinat ion o f heat and oxygen ( o x i d a t i o n ) . Thermal s t a b i -
l i t y can be improved i n manufactur ing i f the more uns tab le components o f t he o i l
can be removed, b u t o therwise the o n l y s o l u t i o n i s t o keep the temperature down.
I n general , however, thermal decomposition takes p lace a t much h ighe r tempera-
tu res than ox ida t i on , and i t i s the o x i d a t i v e s t a b i l i t y which determines the
maximum temperature a t which t h e o i l can be used.
i v e l y reduced by the use o f an t i -ox idan ts . I f the temperature i s h i g h i n the
presence o f oxygen, i t may t h e r e f o r e be d e s i r a b l e t o use an o i l con ta in ing an
a n t i - o x i d a n t .
Ox ida t i on can be very e f f e c t -
F igu re 4 shows the r e l a t i o n s h i p between o i l l i f e and temperature f o r t y p i c a l
minera l o i l s .
Many o f the a d d i t i v e s used i n o i l s w i l l a l s o decompose, and the use fu l l i f e
o f the o i l may depend on the d e p l e t i o n o f such a d d i t i v e s . An t i -ox idan ts a r e
used up i n prevent ing ox ida t i on , so t h a t where h i g h l y o x i d a t i v e cond i t i ons occur
the an t i -ox idan ts may be r a p i d l y exhausted and leave the o i l unprotected.
600
500 1 LOO
I
300 - W [L
200
2
5 100
W a
k
L L O W E R TEMPERATLIRE LIMIT IMPOSED BY 'POUR POINT WHICH VARIES WITH OIL,,+ SOURCE, VISCOSITY, TREATMENT AND ADDITIVES]
- 100
I I l l I I l l 1 2 3 4 5 10 20 30 LO 50 100 200 ,300 LOO 500 1,000 2,wO 3,000 L,OOo5.000 10,000
LIFE (HOURS) - Fig.4 Temperature/l i f e 1 i m i t s f o r minera l o i l s
N w W
234
9.3.6 Contaminants
The q u a l i t y o f a l u b r i c a n t w i l l a l s o d e t e r i o r a t e i n use because o f contamina-
t i o n .
s i t i o n produc ts from a f u e l o r the l u b r i c a n t i t s e l f , or they may be d i sso l ved
ac ids produced by o x i d a t i o n , o r water from condensat ion o r f u e l combustion. They
may a l s o e n t e r the system from ou ts ide , such as when atmospher ic dus t o r r a i n
e n t e r an o i l f i l l e r opening.
The contaminants may be s o l i d p a r t i c l e s o f wear d e b r i s o r s o l i d decompo-
Some o f these contaminants can be removed by f i l t r a t i o n and o the rs w i l l
s e t t l e o u t i n t h e r e s e r v o i r , bu t i t i s sometimes necessary t o use a de tergent o r
d i spe rsan t a d d i t i v e t o keep the contaminants i n a r e l a t i v e l y harmless dispersed
c o n d i t i o n . U l t i m a t e l y t h e o i l change p e r i o d i n some systems may be determined
by the ex ten t o f con taminat ion .
9 .4 CHOICE OF BASE OIL
9.4.1 L i m i t a t i o n s o f Minera l O i l s
Minera l o i l s a r e the most w ide ly used l u b r i c a t i n g o i l s because they a r e the
cheapest, and f o r many a p p l i c a t i o n s they a r e a l s o the best a v a i l a b l e . The most
impor tan t l i m i t a t i o n s on t h e i r use a r i s e f rom h igh temperatures, f l a m m a b i l i t y
and c o m p a t i b i l i t y problems, and the re a r e a number o f o t h e r base o i l s which can
then be used as a l t e r n a t i v e s .
Table 9.2 shows some o f the impor tan t f a c t o r s i n the s e l e c t i o n o f a l t e r n a t i v e
base o i l s .
9.4.2 High Temperatures
More and more i n d u s t r i e s a r e r e q u i r i n g l u b r i c a n t s t o opera te a t temperatures
too h igh f o r minera l o i l s , and a l t e r n a t i v e f l u i d s a r e t h e r e f o r e becoming more
and more w ide ly used. Es te rs were f i r s t developed f o r a i r c r a f t j e t engines, and
a re now probab ly the most common l u b r i c a n t i n a p p l i c a t i o n s which a r e too ho t f o r
minera l o i l s . At h igher temperatures va r ious types o f s i l i c o n e can be used, and
f o r even h ighe r temperatures polyphenyl e the rs a r e a v a i l a b l e , b u t bo th s i l i c o n e s
and polyphenyl e the rs have some se r ious disadvantages i n comparison w i t h minera l
o i l s and e s t e r s .
F igu re 5 shows the t e m p e r a t u r e / l i f e l i m i t s f o r severa l s y n t h e t i c l u b r i c a t i n g
o i l s . These l i m i t s depend on the acceptab le v i s c o s i t y , t he degree o f oxygen
access and the ex ten t t o which d e t e r i o r a t i o n i s acceptab le .
9 . 4 . 3 F lammabi l i t y
Some i n d u s t r i e s , such as a v i a t i o n and coa l mining, have long been concerned
w i t h the f l a m m a b i l i t y o f l u b r i c a n t s and h y d r a u l i c f l u i d s . Wi th i nc reas ing
p l a n t i n t e g r a t i o n and l u b r i c a n t system c a p a c i t i e s , t h i s concern i s now ex tend ing
Tab le 9.2 Impor tan t p r o p e r t i e s i n s e l e c t i n g d i f f e r e n t base o i l s .
F l u i d Di -ester Neopentyl Typica l Typica l Typica l Ch lo r i - Polyg lyco l Polyphenyl Ninera l Remarks po lyo l Phosphate Methyl Phenyl nated ( i n h i b i t e d ) Ether O i l
Esters (Conplex) Ester S i l i cone Hethy l Phenyl comparison ( f o r 1
S i l i cone Hethy l proper ty
.__ S i 1 icone
For es te rs t h i s temperature w i l l be Maximum temperature 250 300 I20 220 320 305 260 450 200 higher i n the absence o f ne ta l s
pC;bsence o f oxygen
This l i m i t i s a rb i t ravy . It w i l l be Maximum temperature 210 240 120 ICO 250 230 200 320 I5O higher i f oxygen concentrat ion i s low i n presence of and l i f e i s sho r t oxyygjen ("C)
Minimum temperature -35 -65 -55 -50 -30 -65 -20 This l i n i t depends on the power ava i l ab le t o o v e r c m the e f f e c t o f increased 0 to -50
due t o increase j n v i scos i t y v i s c o s i t v P C l
"..,~_.. -, This l i n i t depends on the power ava i l ab le t o o v e r c m the e f f e c t o f increased 0 to -50 Minimum temperature -35 -65 -55 -50 -30 -65 -20
due t o increase j n v i scos i t y v i s c o s i t v P C l
-____ Density (g lm l ) 0.91 1.01 1.12 0.97 1.06 1.04 1.02 1.19 0.88
0 V iscos i t y index 145 140 200 175 195 160 -60
F lash p o i n t (OC) 230 255 200 31 0 290 270 180 275
Spontaneous i g n i t i o n Low Medium Very h igh High High temperature
This re fe rs p r i m a r i l y t o anti-wear Boundary l u b r i c a t i o n Good Good Very good F a i r bu t F a i r but Good Very good F a i r Good p roper t i es when some metal contact
to 140 A h igh v i scos i t y index i s des i rab le
Above t h i s temperature the vapour o f the f l u idmaybe i g n i t e d by an open f l a w
Above t h i s temperature the f l u i d may i g - Very h igh Medium High LOW n i t e wi thout any f l a r e be ing present
- I 5 O to
_. ~
poor for poor f o r i s occurr ing s tee l on s tee l on s tee l s tee l
Tox i c i t y S l i g h t S l i g h t Sane Non-toxic Non-tox?c Non-toxic Believed Believed S l i c h t ~ ~ ~ ~ ~ ~ s ~ a ~ ~ ~ ~ ~ be t o x i c i t y t o be low t o be low
Sui tab le rubbers t i i t r i l e . S i l i cone Buty l , Neoprene, tieoprene, Viton, N i t r i l e (None f o r N i t r i l e s? 1 icone EPR v i t on v i t o n f l uo ro - very h igh
s i l i c o n e temvera-
~~ ~
Non- Non- E f fec t on metals S l i g h t l y Corrosive Enhances lion- tion- Corrosive Non- cowos ive corros ive corros ive corros ive t o some c o v o s i o n corros ive corros ive i n
fer rous fer rous presecce of water metals meta ls o f water t o fer rous
to non- nan- n presence when pure
when ho t m t a 1 s
Cost ( r e l a t i v e t o 5 10 10 25 5 C 60 5 250 d i t h q u a l i t y and supp1:r pos i t i on . N lhese are rough apprOXIPlatlOnS. and vary
w u1
m n e r a l 011)
N w 0)
-100
I I I 1 1 I 1 I I I l l -
' L-POUR POINT LIMIT FOR SILICONES AND ESTERS-'
F i g . 5 Temperature/life limits for some synthetic oils.
237
t o most i n d u s t r i e s .
The b e s t f i r e - r e s i s t a n t l i q u i d i s water , b u t i t i s a poor l u b r i c a n t and i n
i t s r e l a t i v e l y unmodi f ied form i s o n l y used as a h y d r a u l i c f l u i d . To mprove
i t s l u b r i c a t i n g performance for m e t a l - c u t t i n g , d e t e r g e n t s and EP a d d i t ves a r e
used. For more genera l l u b r i c a n t use, v a r i o u s techn iques a r e employed t o i n -
crease i t s v i s c o s i t y , i n c l u d i n g t h i c k e n i n g w i t h c l a y s and n a t u r a l o r g a n i c sub-
stances, m i x i n g w i t h p o l y g l y c o l s , and e m u l s i f i c a t i o n w i t h m i n e r a l o i l .
Other f i r e - r e s i s t a n t l i q u i d s w i t h good l u b r i c a t i n g p r o p e r t i e s i n c l u d e phos-
phate e s t e r s , c h l o r i n a t e d hydrocarbons and c h l o r i n a t e d s i l i c o n e s , b u t these a l l
have cor respond ing disadvantages.
9.4.4 C o m p a t i b i l i t y
Problems o f i n c o m p a t i b i l i t y can a r i s e w i t h any l i q u i d . M i n e r a l o i l s a r e
incompat ib le w i t h n a t u r a l rubber s e a l s and hoses, e s t e r s a r e i n c o m p a t i b l e w i t h
n i t r i l e s , and phosphate e s t e r s a r e i n c o m p a t i b l e w i t h many d i f f e r e n t rubbers .
Minera l o i l s and most o t h e r combust ib le l i q u i d s a r e i n c o m p a t i b l e w i t h h i g h -
p ressure oxygen, and c e r t a i n s p e c i a l f l u o r i n e - c o n t a i n i n g l u b r i c a n t s a r e p r e -
f e r r e d f o r b r e a t h i n g oxygen systems. M i n e r a l o i l s a r e u n s a t i s f a c t o r y i n con-
t a c t w i t h red-ho t s t e e l s because they produce c a r b u r i s a t i o n , and rapeseed o i l
may be used t o a v o i d t h i s problem.
Most c o m p a t i b i l i t y problems can be overcome by c a r e f u l s e l e c t i o n o f s u i t a b l e
base o i l s and a d d i t i v e s , b u t i n some i n d u s t r i e s such as f o o d s t u f f s , pharmaceu-
t i c a l s and chemicals, even t h e s m a l l e s t l e a k o f any convent iona l l u b r i c a n t may
be unacceptable. I n such a case i t may be p o s s i b l e t o use a process f l u i d as a
l u b r i c a n t . For example, i n sugar r e f i n i n g t h e h i g h v i s c o s i t y o f syrups and
molasses enables them t o p r o v i d e e f f e c t i v e l u b r i c a t i o n o f bear ings . Where t h e
a v a i l a b l e process f l u i d s have i n s u f f i c i e n t v i s c o s i t y f o r e f f e c t i v e hydrodynamic
l u b r i c a t i o n , e x t e r n a l p r e s s u r i s a t i o n can be used. Such use o f process f l u i d s
may sometimes be p r e f e r r e d t o c o n v e n t i o n a l l u b r i c a t i o n , because i t may e l i m i n a t e
t h e need for s e a l s and g lands .
9.5 GREASES
A l u b r i c a t i n g grease i s a l i q u i d l u b r i c a n t which has been t h
s e m i - s o l i d c o n s i s t e n c y . The base o i l may be m i n e r a l o i l , e s t e r
one o f t h e o t h e r s y n t h e t i c o i l s , and many o f t h e a d d i t i v e s used
o i l s a r e e q u a l l y e f f e c t i v e i n greases. I n a d d i t i o n , s o l i d l u b r
ckened t o a
s i l i c o n e , o r
i n l u b r i c a t i n g
cants such as
g r a p h i t e or molybdenum d i s u l p h i d e can c o n f e r impor tan t advantages i n greases.
I t has been s a i d t h a t more i n d u s t r i a l bear ings a r e l u b r i c a t e d by grease than
by o i l . To understand t h e reason f o r t h i s we shou ld c o n s i d e r a g a i n t h e s imp les t
fo rm o f l u b r i c a t i o n descr ibed i n S e c t i o n 9.2, namely t h e use o f a smal l quan-
t i t y o f p l a i n m i n e r a l o i l i n p l a c e i n t h e b e a r i n g . There a r e two disadvantages
600
500
1 00
300
2 00
100
0
- 100
WITH UNLIMITED OXYGEN I
~~
1 2 3 1 5 10 20 30 10 50 100 200 300 100500 1.000 2.000 3.0001.wO 10,000
LIFE (HOURS1 - Fig.6 Tempera tu re / l i f e l i m i t s f o r greases.
239
o f t h a t system which a r e due o n l y to t h e f l u i d n a t u r e o f t h e o i l . The f i r s t i s
t h a t i t i s d i f f i c u l t t o r e t a i n t h e o i l i n t h e b e a r i n g f o r any l e n g t h o f t ime.
The second i s t h a t i t i s d i f f i c u l t t o s t o p d i r t o r o t h e r contaminants g e t t i n g
i n t o t h e b e a r i n g .
The use o f a grease may overcome b o t h o f these problems w h i l e r e t a i n i n g t h e
s i m p l i c i t y and economy o f t h e system. The grease i n a b e a r i n g o f t e n d i s t r i b u t e s
i t s e l f q u i t e q u i c k l y i n such a way t h a t a v e r y smal l p r o p o r t i o n i s a c t i v e l y
l u b r i c a t i n g t h e b e a r i n g , w h i l e t h e b u l k i s o u t s i d e t h e c r i t i c a l b e a r i n g sur -
faces, fo rming a sea l a g a i n s t loss o f l u b r i c a n t o r i n g r e s s o f contaminants.
The c h i e f l i m i t a t i o n on t h e use o f greases i s t h e l a c k o f c o o l a n t f l o w t o
remove s u r p l u s heat when t h e b e a r i n g i s l a r g e , f a s t o r h e a v i l y loaded, b u t t h e r e
a r e greases wh ich w i l l o p e r a t e
t e m p e r a t u r e / l i f e l i m i t s f o r some t y p i c a l greases.
a t temperatures as h i g h as 250°C. F i g u r e 6 shows
9.6 S O L I D LUBRICANTS
S t r i c t l y speaking, s o l i d l u b r i c a n t s a r e s o l i d m a t e r i a l s which can be i n t r o -
duced between b e a r i n g sur faces t o reduce f r i c t i o n or wear, b u t t h e t i t l e i s
normal ly used i n a more r e s t r i c t e d sense t o cover o n l y r e d u c t i o n i n f r i c t i o n o r
p r e v e n t i o n o f adhesion o r s e i z u r e . I t t h e r e f o r e excludes hard w e a r - r e s i s t a n t
coa t ings and f r i c t i o n sur faces such as non-sk id c o a t i n g s and brake m a t e r i a l s .
Table 9 . 3 l i s t s a l a r g e number o f d i f f e r e n t s o l i d l u b r i c a n t s , b u t t h e m a j o r i t y
o f appl i c a t i o n s use e i t h e r molybdenum d i s u l p h i d e , g r a p h i t e or PTFE ( p o l y t e t r a -
f l u o r o e t h y l e n e ) . They can a l l be used i n a v a r i e t y o f ways, i n c l u d i n g f r e e
powder, rubbed f i l m , bonded f i l m o r s p u t t e r e d f i l m . G r a p h i t e and PTFE may a l s o
be used i n t h e fo rm o f s o l i d b l o c k s , as may many o t h e r polymers, and i n t h i s
form they a r e commonly known as d r y b e a r i n g m a t e r i a l s .
The i m p o r t a n t p r o p e r t i e s o f molybdenum d i s u l p h i d e , g r a p h i t e and PTFE a r e
l i s t e d i n T a b l e 9.3.
The advantages o f t h e common s o l i d l u b r i c a n t s a r e t h e i r wide temperature
ranges, chemical i n e r t n e s s , low v o l a t i l i t y , and t h e f a c t s t h a t they do n o t need
s e a l i n g , do n o t need feed systems and g e n e r a l l y do n o t cause contaminat ion .
T h e i r d isadvantages compared w i t h o i l or grease a r e r e l a t i v e l y h i g h f r i c t i o n ,
l a c k o f heat removal, f a i l u r e t o p revent c o r r o s i o n , and steady wear i n s e r v i c e .
They a r e t h e r e f o r e o n l y used where t h e i r advantages over o i l s and greases o u t -
weigh t h e i r d isadvantages, such as a t h i g h o r v e r y low temperatures, i n vacuum,
where very long l i f e i s r e q u i r e d w i t h o u t r e l u b r i c a t i o n , o r where contaminat ion
i s c r i t i c a l .
9.7 GAS LUBRICATION
Gases can be used as l u b r i c a n t s i n t h e same way as o i l s , and t h e p h y s i c a l
laws govern inq hydrodynamic o i l l u b r i c a t i o n aoo lv a l s n tn naqe4 The v p r v I n w
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v i s c o s i t y o f gases means t h a t the f i l m th ickness i s a l s o very low, so t h a t
hydrodynamic gas bear ings ("gas-dynamic bear ings") a r e 1 im i ted t o c o n d i t i o n s
o f h i g h speed, low load, smal l c learances and very c l o s e l y c o n t r o l l e d to le rances .
For t h i s reason e x t e r n a l l y p ressu r i sed gas bear ings a r e more commonly used
which can c a r r y h ighe r loads, a r e l ess c r i t i c a l on c learances and to le rances ,
and can be used a t lower o r even zero speeds.
Gas l u b r i c a t i o n can be used a t h ighe r o r lower temperature than o i l o r grease,
but t h e i r main advantages a r e i n g i v i n g h igh s t i f f n e s s i n h i g h speed p r e c i s i o n
bear ings, such as i n den ta l d r i l l s and p r e c i s i o n g r i n d i n g sp ind les , and i n
avo id ing s e a l i n g and contaminat ion problems where a s u i t a b l e process gas i s
a v a i l a b l e t o a c t as t h e gas l u b r i c a n t .
242
I() LUBRICANT ADDITIVES Their Application , Performance and Limitations
D . M . SOUL, Lub r i zo l I n t e r n a t i o n a l Labora to r ies , Hazelwood, Derby, U.K.
10.1 I NTRODUCT I ON
The l u b r i c a t i o n o f i n d u s t r i a l p l a n t and equipment covers the whole range o f
requirements f o r machinery i n c l u d i n g the i n t e r n a l combustion engine and the gas
tu rb ine . Discussion o f such a wide sub jec t i n one chapter would be impossible
bu t a very wide f i e l d o f a d d i t i v e a p p l i c a t i o n s e x i s t s apa r t from those f o r
engine and t ransmiss ion systems. These inc lude h y d r a u l i c and gear l ub r i can ts ,
metal c u t t i n g and forming f l u i d s , t u r b i n e o i l s , compressor and r e f r i g e r a t o r
o i l s , f i r e - r e s i s t a n t f l u i d s , greases, and a m isce l l any o f spec ia l i sed products.
Lub r i can t a d d i t i v e s a r e chemicals incorporated i n t o e i t h e r a l i q u i d base
(mineral o i l , s y n t h e t i c f l u i d or water) , a semi - f l u id , o r a grease.
the a d d i t i v e s a r e so lub le o r u n i f o r m l y dispersed throughout the c a r r i e r medium.
I n t h i s respect t h i s d iscuss ion does n o t cover s o l i d l u b r i c a n t a d d i t i v e s such
as graphi te , molybdenum d l s u l p h i d e and polytetrafluoroethylene.
I n general ,
Add i t i ves are used t o improve the performance o f the base c a r r i e r ma te r ia l t o
prov ide a des i red phys i ca l , chemical, o r mechanical p roper t y . They a re manufac-
tu red t o s t r i c t q u a l i t y c o n t r o l standards t o assure cons is ten t performance i n
s i m i l a r a p p l i c a t i o n s , i n many environments throughout the wor ld , f o r both the
base ma te r ia l and the a d d i t i v e system. Many l u b r i c a n t s p e c i f i c a t i o n s cover ing
a wide range of i n d u s t r i a l processes can o n l y be s a t i s f i e d by the use o f these
a d d i t i o n a l chemicals and c e r t a i n t e s t s a r e designed s p e c i f i c a l l y t o assess such
fac to rs .
10.2 B A S I C PROPERTIES OF LUBRICANTS
All types o f l u b r i c a t i n g f l u i d s d i s p l a y c e r t a i n bas i c p r o p e r t i e s . These may
be used t o advantage i n s e l e c t i n g the most app rop r ia te l u b r i c a n t f o r any
243
a p p l i c a t i o n .
an unt reated base o i l o f t he c o r r e c t v i scomet r i c p r o p e r t i e s can prov ide a
p e r f e c t l y adequate l u b r i c a t i n g medium.
references f o r standard eva lua t i on t e s t s a r e g iven i n Table 10.1.
I n a p p l i c a t i o n s where hydrodynamic l u b r i c a t i o n f i l m s a re formed,
The p r o p e r t i e s w i t h app rop r ia te t e s t
Mineral o i l p r o p e r t i e s a re dependent on the crude o i l source and the r e f i n e r y
processes o f separat ion and treatment. L u b r i c a t i n g o i l f r a c t i o n s o f va ry ing
v i s c o s i t y a r e produced. By s e l e c t i o n o f the processes used i n the r e f i n e r y ,
proper t ies such as pour p o i n t , co lou r and v i s c o s i t y index may be c o n t r o l l e d o r
modif ied so as t o produce b e t t e r f i n a l products .
10.3 LUBRICANT ADDITIVES
I n Table 10.2 i s a l i s t o f the a d d i t i v e s normal ly employed i n l u b r i c a n t s .
This t a b l e g i ves the general name o f each type together w i t h a summary o f i t s
purpose and a suggested mechanism o f a c t i o n .
10.3.1 Detergent and Dispersant A d d i t i v e s
10.3.1.1 Detergent Types
A v a r i e t y o f metal con ta in ing compounds c a l l e d detergents' have been described
i n the l i t e r a t u r e .
o f p a r t i c u l a t e ma t te r and n e u t r a l i s i n g
d i r t and debr i s . Some o f these m a t e r i a l s a l s o f u n c t i o n as r u s t i n h i b i t o r s and
emu 1 s i f i e r s .
The i r f unc t i on appears t o be one o f e f f e c t i n g a d i spe rs ion
ac ids r a t h e r than one o f c lean ing up
The compounds a re g e n e r a l l y molecules hav ing a l a r g e hydrocarbon t a i l and a
"polar group" head. The t a i l sec t i on serves as a s o l u b i l i z e r i n the base o i l ,
whi le the p o l a r group serves as the f u n c t i o n a l p a r t o f the molecule, which
a t t r a c t s p a r t i c u l a t e contaminants i n the l u b r i c a n t .
The most w ide ly used members o f t he o rganometa l l i c c lass a r e the sulphonates
and phenates. These inc lude bo th the n e u t r a l barium, ca lc ium and magnesium
sa l t s , and the a l k a l i n e o r h i g h l y bas i c products prepared from these o rgan ic
substrates by i nco rpo ra t i ng metal i n excess o f the s t o i c h i o m e t r i c q u a n t i t i e s
i n t o those compounds.
as subst rates f o r metal con ta in ing a d d i t i v e s .
Phosphonates and s a l i c y l a t e s a r e a l s o used commercial ly
Commercial i n t e r e s t i n barium compounds i s d e c l i n i n g due t o cos t o f produc-
t i o n and environmental cons iderat ions.
The meaning o f the terms "neu t ra l " and "basic" metal s a l t used above:
Neu t ra l s a l t - i s a s a l t o f an a c i d which conta ins the s t o i c h i o -
m e t r i c amount o f metal requ i red f o r n e u t r a l i z a t i o n
o f the a c i d i c group present.
Bas ic s a l t - i s a s a l t o f an a c i d which con ta ins more metal than
i s requ i red f o r the n e u t r a l i z a t i o n o f the a c i d i c
groups present . Such excess metal may be present
Table 10.1
Standard L u b r i c a t i n g O i l Tests
Test Scope o f Test P r a c t i c a l use o f Data I .Pe t . Method Number and Related Methods
S p e c i f i c Gravi t y
V i scos i t y
F lash P o i n t
Colour
Pour/Cloud Po in t
A c i d i t y
Carbon Residue
Q u a l i t y o f o i l . Naphthenic h i g h , P a r a f f i n i c medium. Aromat ic low. Batch c o n t r o l .
Re la t i on o f 'body' o i l t o
Normal ly evaluated a t two
A lso var ious methods f o r low temper-
temperature.
temperatures,
a tu res and va ry ing shear ra tes .
V o l a t i l i t y , c l a s s i f i c a t i o n o f f l u i d s , i n f l a m m a b i l i t y .
Depth o f co lour r e l a t e d t o r e f i n e r y processes.
Re f in ing processes - dewaxing. E f f e c t o f low temperatures.
Trace residues from r e f i n e r y
Carbon res idue a f t e r burn ing o f f
processes.
o i l .
Volume t o weight conversions
V i s c o s i t y Index. R e f i n i n g processes. 'F low' o f o i l i n machinery.
Storage and opera t i ng cond i t i ons o f o i l . Taxat ion and sh ipp ing o f Petroleum products.
Contamination.
Low temperature v i s c o s i t y and opera t i on .
Normal ly low f o r new o i l . D e t e r i o r a t i o n o f used o i l .
Performance o f h i g h q u a l i t y base P a r a f f i n i c - h i g h . Naphthenic - low.
160/68, 59/72, 189179
71/79
34/75 (Pensky-Martin) 170/75 (Abel) 36/67 (Cleveland) 15/72
15/67 (Pour) 219/67 (Cloud)
1 /74
13/78 (Conradson) 14/65 (Ramsbottom)
Lu P P
245
due t o a t r u e bas i c s a l t s t r u c t u r e , b u t a more l i k e l y
exp lana t ion i s t h a t the excess metal i s present i n the
form o f d ispersed metal compounds.
10.3.1.1.1 Sul phonates
Normal metal sulphonates d e r i v e d from "mahogany" ac ids ( t h e mahogany-coloured
petroleum sulphonic ac ids obta ined as a by-product du r ing w h i t e o i l manufacture)
were f i r s t employed as detergent a d d i t i v e s i n commercial crankcase o i l s du r ing
World War 1 1 . Almost w i t h o u t except ion, the metal present i n such sulphonates
was ca lc ium o r barium. Petroleum sulphonates were n o t o n l y super io r t o e a r l i e r
add i t i ves w i t h respect t o detergency, b u t were much less c o r r o s i v e t o s e n s i t i v e
bear ing metal a l l o y s and responded w e l l t o co r ros ion i n h i b i t o r s . They can be
represented by the general formula:
RxArS03M
where R A r represents complex a l k y l a r o m a t i c r a d i c a l s de r i ved from petroleum and
M i s one equ iva len t o f a po l yva len t metal .
Supplementing the supply o f n a t u r a l petroleum sulphonates a re the s y n t h e t i c
sulphonates de r i ved from long-chain a l k y l s u b s t i t u t e d benzenes (e.g. polydodecyl
benzene bottoms) obta ined as by-products i n the manufacture o f household de te r -
gents o r which a r e manufactured s p e c i f i c a l l y f o r t h i s use.
H igh l y bas i c sulphonates con ta in from 3 t o 10 o r 15 t imes as much metal as
the corresponding normal sulphonates. Ca l l ed "overbased", "superbasic" o r
"hyperbasic" sulphonates, these products a r e manufactured by heat ing a m ix tu re
o f c e r t a i n promoters o r so l ven ts w i t h a n e u t r a l sulphonate and a l a r g e excess o f
metal ox ide o r hydroxide and carbonat ing w i t h carbon d i o x i d e t o conver t the metal
base to c o l l o i d a l l y - d i s p e r s e d metal carbonate. Overbased sulphonates possess
the a b i l i t y t o n e u t r a l i z e a c i d i c contaminants formed i n l u b r i c a t i n g o i l s and
thus reduce c o r r o s i v e wear o f engine components.
10.3.1.1.2 Phosphonates and/or Thiophosphonates
These detergent a d d i t i v e s can be represented by the general formula:
8 R - P (XM12
where R i s a l a r g e a l i p h a t i c r a d i c a l o f a t l e a s t 500 molecular weight . X i s
oxygen and/or su lphur , and M i s one equ iva len t o f a monovalent o r po l yva len t
meta l .
500 - 2,000 molecular weight w i t h a phosphorus reagent, g e n e r a l l y phosphorus
pentasulph ide, t o form a complex o rgan ic phosphorus-sulphur compound which i s
then n e u t r a l i z e d w i t h a metal base.
p r i o r t o o r d u r i n g n e u t r a l i z a t i o n , a p o r t i o n o r s u b s t a n t i a l l y a l l o f the sulphur
They a re prepared by f i r s t hea t ing a p o l y o l e f i n such as polybutenes of
By steam treatment o r prolonged hyd ro l ys i s
TABLE 10.2 Lubr icant Add i t i ve
Add i t i ve Chemicals Purpose o f Add i t i ve How Addit ive Works
Ant ioxidant
Corrosion I n h i b i t o r
Rust I n h i b i t o r
Detergent
Dispersant
F r i c t i o n Mod i f i e r
Antiwear
Hindered phenols hi nes Organic sulphides and polymers Zinc dithiophosphate
Zinc d i thiophosphates Sulphurized terpenes surfaces from corrosion Phosphosulphurized terpenes Sulphurized o l e f i n s
h i n e phosphates Sodi um.ca1 c i urn and Magnesi um sulphonates A l k y l succinic acids Fa t t y acids
Normal or basic, calcium, Reduces or prevents deposits i n b a r i um, magnesium phosphonates, phenates temperatures and sul phonates
Polymers such as n i t rogen containing polymethacrylates, format ion and deposi t ion under a l k y l succinimides, and succinate esters high condi t ions molecular weight amines and amides
F a t t y acids Fa t t y amines Fats
Zinc d i a l k y l - d i thiophosphate s tee l app l i ca t i ons T r i c resy l phosphate
Minimizes the formation o f resins, varnish, acids, sludge
Protects bearing and other metal
Protects ferrous metal surfaces against r u s t
engines operated a t h igh
Prevents and re ta rds sludge
low temperature operat ing
To increase o i l f i l m strength t o prevent o i l f i l m rupture
Reduce r a p i d wear i n steel-on-
Reduces organic peroxides terminat ing the oxidat ion chain. oxygen taken up i n the o i l . react ions.
Acts as an t i ca ta l ys t . Coats metal surfaces which p ro tec t against ac id and peroxide at tack.
Reduces format ion o f acids by decreasing Prevents c a t a l y t i c
Polar molecules are absorbed p r e f e r e n t i a l l y on the metal surface and serves as a b a r r i e r against water. Neutral izes acids.
Controls bui ldup of varnish and sludge by reac t i ng w i th ox ida t i on products t o form o i l soluble mater ia l which remains suspended i n the o i l .
Dispersants have a strong a f f i n i t y for d i r t p a r t i c l e s and surround each w i t h o i l soluble molecules which keep the sludge frm agglomerating and deposi t ing i n the engine.
Highly po la r molecules are absorbed on the metal surface and remain i n place t o cushion and keep metal surfaces apart .
Add i t i ve reacts w i th the metal t o form a compound which i s deformed by p l a s t i c f l ow t o al low a new d i s t r i b u t i o n of load.
Extreme Pressure
V iscos i t y Index Improver
Pour Depressant
Ant i foam
Emu1 s i f i er
Tackiness
An t i sep t i c
Metal Deactivator
Sulphurized fa t s , o le f i ns Chlor inated hydrocarbons Lead s a l t s o f organic acids. Amine phosphates
Polyisobutylenes Polymethacrylates Polyacryl ates Ethylene propylene
copolymers Styrene maleic es te r
copolymers Hydrcgenated styrene butadiene copolymers
Wax a l k y l a t e naphthalene Wax ak l y la ted phenols Polymethacrylates
S i l i cone polymers Polymthacrylates
Sodium s a l t s of sulphonic acids, sodium s a l t s o f organic acids, f a t t y m i n e s a l t s
Soaps, poly isobutylene and po lysc ry la te polymers
Prevents seizure and welding between metal surfaces under cond i t i on o f extreme pressure and temperature
Reduces the r a t e o f change o f v i s c o s i t y w i th temperature
Lowers the pour p o i n t of t he o i l
Prevents the fonnat ion o f s tab le foam
To nake mineral o i l m isc ib le w i t h water
To provide the o i l w i th greater cohension
Phenols, ch lo r i ne compounds, Increases emulsion l i f e and formaldehyde bases prevents odour
T r i a r y l phosphi tes Stop the c a t a l y t i c e f f e c t o f metals Sulphur compounds on ox ida t i on and corrosion Diamines. Dimercapto th iad iazo le de r i va t i ves .
EP agent reacts w i t h metal surfaces t o form new compounds having lower shear strength than the base metal and i s sheared p r e f e r e n t i a l l y t o the base metal.
Polymer molecule assumes a compact cur led form i n a poor solvent ( co ld o i l ) and an uncurled high surface area i n a b e t t e r solvent (ho t o i l ) . uncurled form thickens the o i l .
The
Retards the formation o f f u l l - s i z e wax c rys ta l s by coat ing o r c o - c r y s t a l l i z a t i o n w i th the wax.
Appears t o at tack the o i l f i l m surrounding each bubble reducing i n t e r f a c i a l tension. The small bubbles l i be ra ted combine t o form large ones which f l o a t t o the surface.
Emulsi f ier i s absorbed a t the o i l -wa te r i n te r face t o reduce i n t e r f a c i a l tension r e s u l t i n g i n an i n t ima te dispersion o f one l i q u i d i n the other.
Increases v i scos i t y . and s t r i ngy .
Prevents and reduces microorganism growth.
Mater ia ls themselves are tacky
A p ro tec t i ve f i l m i s absorbed on metal surfaces which prevents contact between corrosive agents and base metal.
N P 4
248
present i n the thiophosphonate group, -P(S) (SMl2, can be s u b s t i t u t e d w i t h oxygen
t o y i e l d a phosphonate.
The h i s t o r y o f these detergent a d d i t i v e s c l o s e l y p a r a l l e l s t h a t o f sulphonate
a d d i t i v e s i n t h a t bas i c and overbased s a l t s have rep laced normal s a l t s i n a l l
bu t a few commercial a p p l i c a t i o n s . The manufacture o f such b a s i c and overbased
s a l t s i s c a r r i e d ou t by the use o f methods l i k e those descr ibed i n the SULPHONATES
sect ion.
10.3.1.1.3 Phenates
Phenates and phenate-sulphides have p layed an impor tant r o l e as detergent
a d d i t i v e s ever s ince t h e i r i n t r o d u c t i o n du r ing World War 1 1 . Among the e a r l i e s t
a d d i t i v e s o f t h i s type t o ga in commercial acceptance were:
Calcium and bar ium phenates o f t e r t i a r y - o c t y l p h e n o l
su lph ide and ter t iary-amylphenol su lph ide hav ing
the general formula:
011
R R
Calcium phenates o f t e r t i a ry -amy lpheno l - formaldehyde
condensation products.
OH OH
R R
Calcium and bar ium phenates o f p a r a f f i n wax s u b s t i t u t e d
phenol hav ing the general formula:
OM
I n a d d i t i o n t o per forming a detergent r o l e , phenates - and e s p e c i a l l y phenate
su lph ides - e x h i b i t s u b s t a n t i a l a n t i - o x i d a n t p r o p e r t i e s and a r e p a r t i c u l a r l y
use fu l f o r h igh temperature f l u i d s .
L i k e the o t h e r f a m i l i e s o f detergent a d d i t i v e s discussed, bas i c and overbased
phenates have replaced normal phenates i n most a p p l i c a t i o n s . Manufactur ing
249
techniques f o r such products a r e s i m i l a r t o those employed f o r bas i c sulphonates
o r phosphonates.
10.3.1.1.4 A l k y l Subs t i t u ted S a l i c y l a t e s
These a d d i t i v e s can be represented by the general formula:
0
C - OM II
where R i s an o i l - s o l u b i l i z i n g o rgan ic r a d i c a l and M i s one equ iva len t o f a
po l yva len t meta l .
One o f the f i r s t a d d i t i v e s o f t h i s type t o see commercial use was the z inc
ca rboxy la te o f d i - i sop ropy l s a l i c y l i c ac id . More r e c e n t l y , ca lc ium carboxy l -
ates of long-chain a l k y l s a l i c y l i c ac ids have been used. The manufacture of
such a d d i t i v e s invo lves ca rboxy la t i on o f a metal phenate w i t h carbon d iox ide.
As w i t h the o t h e r detergent a d d i t i v e s discussed, overbasing techniques have
been employed t o prepare improved products .
10.3.1.2 Mode of A c t i o n of Detergent A d d i t i v e s
Al though the mechanism o f detergency i n non-aqueous media such as minera l
o i l s i s n o t f u l l y understood, researchers have found evidence f o r the ex i s tence
o f "soap m ice l l es " i n non-aqueous so lvents . There i s reason, then, t o be l i eve
t h a t detergent a d d i t i v e s i n minera l o i l s o l u t i o n can a c t i n a manner s i m i l a r t o
aqueous soap s o l u t i o n s .
t o n e u t r a l i z e harmful i no rgan ic and o rgan ic ac ids which accumulate i n crankcase
l u b r i c a n t s d u r i n g se rv i ce . They can a l s o a c t as h i g h temperature s t a b i l i s e r s as
means o f reducing thermal decomposi t ion o f o t h e r a d d i t i v e s by n e u t r a l i s i n g small
amounts o f a c i d i c products which cou ld cause c a t a l y t i c decomposition reac t i ons
t o occur.
Basic and overbased detergents a l s o possess the a b i l i t y
10.3.1.3 Dispersant A d d i t i v e s
The term "dispersant" i s p resen t l y used t o designate a d d i t i v e s which a r e
capable o f d i spe rs ing the "co ld s ludge' ' formed i n engines operated f o r t he most
p a r t a t r e l a t i v e l y low b u l k crankcase o i l temperatures. Unless maintained i n
f i n e suspension i n the l u b r i c a t i n g o i l , t h i s sludge deposi ts on o i l f i l t e r s ,
valve t r a i n components, and o i l c o n t r o l r i n g s where i t i n t e r f e r e s w i t h good
engine performance.
Since known meta l -conta in ing detergents d i d not appear t o o f f e r a s o l u t i o n
t o the c o l d s ludge problem, researchers turned t h e i r a t t e n t i o n t o me ta l - f ree
250
organ ic compounds i n the hope t h a t an "ashless detergent" would p rov ide the
answer.
Such products can a l s o be used t o e f f e c t d i spe rs ion o f i n s o l u b l e m a t e r i a l
i n o i l , i . e . c o l l o i d a l d ispers ions, and a l s o t o d isperse water i n o i l t o produce
s t a b l e i n v e r t emulsions con ta in ing up t o f o r t y percent water.
The compounds which are use fu l f o r t h i s purpose a r e again cha rac te r i zed by a
p o l a r group at tached t o a r e l a t i v e l y h i g h molecular weight hydrocarbon chain.
The p o l a r group genera l l y con ta ins one or more o f t he elements: n i t rogen , oxygen
and phosphorus. The s o l u b i l i z i n g chains a re based on po ly i sobu ty lene .
Dispersants may be d i v i d e d i n t o va r ious chemical f a m i l i e s .
10.3.1.3.1 Copolymers
Copolymers which con ta in a c a r b o x y l i c e s t e r f u n c t i o n and one o r more addi -
t i o n a l p o l a r f unc t i ons such as amine, imide, hyd roxy l , e the r , epoxide, phosph-
orus e s t e r , carboxy l , anhydr ide, o r n i t r i l e genera l l y have d i spe rsan t p roper t i es .
Some o f these polymers e x h i b i t v i s c o s i t y mod i f y ing p r o p e r t i e s and thus f i n d
a p p l i c a t i o n as m u l t i - f u n c t i o n a l a d d i t i v e s . Three types o f d ispersant V I improver
a r e i n used today: po lymethacry la tes, s tyrene-male ic e s t e r copolymers and
ethylene-propylene copolymers.
10.3.1.3.2 Subs t i t u ted Succinimides
Hydrocarbon polymers may be in t roduced i n t o molecules by chemical r e a c t i o n .
Typ ica l products o f t h i s type a r e obta ined by t r e a t i n g a p o l y o l e f i n , w i t h a mole-
c u l a r weight i n the range 500-2000, w i t h phosphorus c h l o r i d e and phosphorus sul-
phide and then w i t h reagents such as urea, e thy lene ox ide and b o r i c ac id .
The N-subs t i t u ted long cha in a l keny l succinimides con ta in the c h a r a c t e r i s t i c
succi n imide grouping
where R conta ins upwards o f 50 carbon atoms.
ex tens i ve l y i n many types o f crankcase l u b r i c a n t .
Th i s group o f products i s used
10.3.1.3.3 Amides
High molecular weight amides and polyamides a re u s u a l l y prepared by the
r e a c t i o n o f h ighe r f a t t y ac ids o r es te rs w i t h po l ya l ky lene amines. Products o f
t h i s type are g e n e r a l l y used i n the l u b r i c a t i o n o f two-cycle engines and l i t t l e
i n automot ive crankcase l u b r i c a n t s .
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10.3.1.3.4 Other Chemicals
Extensive research i n t o o t h e r c lasses o f detergent chemist ry cont inues. Pro-
ducts may be produced from the fo l l ow ing : -
Poly and benzyl amines
High molecular weight e s t e r s which may be used f o r
e m u l s i f i c a t i o n purposes
Amine s a l t s o f h igh molecular weight ac ids
10.3.1.4 Mode o f Ac t i on o f Dispersant A d d i t i v e s
Based on microscopy and chromatography s tud ies , several groups o f researchers
have concluded t h a t ashless d ispersants f u n c t i o n by adso rp t i on on contaminant
p a r t i c l e s present i n o i l s , thus keeping them i n suspension. Al though these re-
searchers have c o n t r i b u t e d much t o an understanding o f the mechanism o f d i spe r -
sancy, there i s wide agreement t h a t i t i s necessary t o evaluate the products i n
p rac t i ca l t r i a l s s ince the re i s l i t t l e i n the way o f l abo ra to ry t e s t i n g which
can compare products w i t h t h i s f u n c t i o n .
10.3.2 An t iox idan ts and Bear ing Corros ion I n h i b i t o r s
Ant iox idants a r e probably employed in a wider v a r i e t y o f l u b r i c a n t s than any
other k i n d o f a d d i t i v e . I n a d d i t i o n t o t h e i r use i n crankcase o i l s and steam
tu rb ine o i l s , they w i l l be found i n gas t u r b i n e l u b r i c a n t s , automat ic t rans-
mission f l u i d s , gear o i l s , c u t t i n g o i l s , greases and h y d r a u l i c f l u i d s .
The f u n c t i o n o f an o x i d a t i o n i n h i b i t o r i s the prevent ion of d e t e r i o r a t i o n
associated w i t h oxygen a t t a c k on the l u b r i c a n t base f l u i d . I n h i b i t o r s f u n c t i o n
e i t h e r t o dest roy peroxides o r t he f r e e r a d i c a l s de r i ved from perox ides. The
most w ide ly used a n t i o x i d a n t s i n the l u b r i c a n t f i e l d a r e the pheno l i c types,
su lphur ised p o l y o l e f i n s and the z i n c d i th iophosphates. The phenols a re consi -
dered t o be the chain-breaking type whereas the l a t t e r two a re be l i eved t o be
perox i de dest royers . The co r ros ion o f bea r ing meta ls i n i n t e r n a l combustion engines i s genera l l y
considered t o be due l a r g e l y t o a c i d o r a c t i v e su lphur a t t a c k on the bear ing
metals. The ac ids invo lved i n the a t t a c k o r i g i n a t e e i t h e r f rom products o f
incomplete combustion o f the f u e l which f i n d t h e i r way i n t o the l u b r i c a n t as
blow-by gases o r from the o x i d a t i o n o f t he l u b r i c a n t . Ox ida t i on i n h i b i t o r s can
reduce o r e l i m i n a t e the l a t t e r m a t e r i a l s and, hence, reduce bear ing co r ros ion .
General ly, i n most i n d u s t r i a l app l i ca t i ons , o x i d a t i o n i s the major cause o f a c i d
re 1 ease.
I n most environments i n which a l u b r i c a t i n g o i l i s employed, i t comes i n con-
tac t w i t h a i r o f t e n a t h igh temperatures and i n the presence of meta ls o r chemi-
ca l compounds which promote o x i d a t i o n o f the o i l . The o i l undergoes a complex
se r ies o f o x i d a t i o n reac t i ons and the harmful r e s u l t s i nc lude , p r i n c i p a l l y , an
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increase i n v i s c o s i t y o f t he l u b r i c a n t , the fo rma t ion o f a c i d i c contaminants
such as "petroleum oxyacids" and the development o f carbonaceous m a t e r i a l .
Among the more e f f e c t i v e chemicals employed as commercial a n t i o x i d a n t s today
are: -
10.3.2.1 Di th iophosphates
Z inc diorganodithiophosphates ( a l s o e f f e c t i v e i n the r o l e o f co r ros ion i n h i -
b i t o r s ) . Other metals may a l s o be used.
Ashless d i th iophosphates, rep lac ing metal by amine can be e q u a l l y e f f e c t i v e .
10.3.2.2 Hindered Phenol
Hindered phenols ( i . e . phenols i n which the hydroxy l group i s s t e r i c a l l y -
b locked or "hindered") :
2, 6-di-tertiary-butyl-4-methylphenol
4, 4' -methy 1 ene b i s (2,6-d i - t e r t i ary-buty 1 phenol )
4, 4' - t h i ob i s (2-methy 1-6- t e r t i ary-buty 1 phenol
10.3.2.3 N i t rogen Bases
Amines such as:
N-phenyl-alpha-naphthylamine
N-phenyl-beta-naphthylamine
Tetramethyldiaminodiphenylmethane
A n t h r a n i l i c a c i d
Phenothiazine and a l k y l a t e d d e r i v a t i v e s
10.3.2.4 Sulphur ised p o l y o l e f i n s , where the sulphur a c t s i n a s i m i l a r manner
t o n a t u r a l l y o c c u r r i n g su lphur chemicals found i n crude o i l b u t removed du r ing
r e f i n i n g .
O f a l l these an t i ox idan ts , the most w ide ly used a re the z i n c d io rganod i th io -
phosphates. These have a dual f u n c t i o n o f behaving a l s o as ant iwear agents t o
p r o t e c t cam and tappet s c u f f i n g when used i n crankcase o i l s . These compounds
a re a l s o used e x t e n s i v e l y as m i l d l oad -ca r ry ing a d d i t i v e s i n gear oils and i n
h y d r a u l i c o i l s . Performance requirements today genera l l y d i c t a t e the use o f
more than one a n t i o x i d a n t .
Hindered phenols a r e favoured f o r h y d r a u l i c and t u r b i n e o i l s because they
g i v e e x c e l l e n t o x i d a t i o n l i v e s f o r long f i e l d use. The phenyl naphthylamine
an t i ox idan ts , e s p e c i a l l y t h e beta form, or those con ta in ing t races o f i t , a r e
now regarded as carc inogenic , They a r e t h e r e f o r e n o t used to a g r e a t e x t e n t
and a re more s p e c i f i c t o some types o f grease.
10.3.2.5 Mode o f Ac t i on o f An t iox idan ts
There i s general agreement among independent i n v e s t i g a t o r s t h a t t he ox ida t i on
of a l u b r i c a t i n g o i l i nvo l ves a chain o x i d a t i o n r e a c t i o n i n which i n i t i a l l y -
253
formed o rgan ic peroxides a t t a c k unoxid ized o i l and a r e subsequently regenerated
by oxygen i n the a i r t o cont inue such a t t a c k . Accord ing t o t h i s w ide ly accepted
"peroxide theory", an e f f e c t i v e a n t i o x i d a n t i s a chemical compound which reduces
organic peroxides and consequent ly causes the chain r e a c t i o n t o cease.
10.3.3 Corrosion I n h i b i t o r s
I n the a d d i t i v e i ndus t r y , the term "co r ros ion i n h i b i t o r ' ' i s app1ied;to a
mater ia l which p r o t e c t s co r ros ion -suscep t ib le non-ferrous metal components,
p r i n c i p a l l y bear ings, aga ins t a t t a c k by a c i d i c contaminants i n the l u b r i c a t i n g
o i l . A d i f f e r e n t term - r u s t i n h i b i t o r - i s used t o des ignate m a t e r i a l s which
p ro tec t f e r rous metal sur faces aga ins t r u s t .
Among the e a r l i e s t types o f co r ros ion i n h i b i t o r s t o see commercial app l i ca -
t i o n were o rgan ic phosphi tes. Most o f these were n o t pure chemicals b u t r a t h e r
mixtures o f mono-, d i - , and t r i - o r g a n o phosphi tes obta ined from the r e a c t i o n of
alcohols o r hydroxyesters (e.g. methyl l a c t a t e , t r i m e t h y l c i t r a t e ) w i t h phos-
phous t r i c h l o r i de.
The major c lasses o f co r ros ion i n h i b i t o r s i n commercial use a t the present
time are:
10.3.3.1 Di th iophosphates
Metal diorganodithiophosphates, e s p e c i a l l y z i n c diorganodithiophosphates.
They possess t h e s t r u c t u r e :
RO \ QS
P
RO ' 'SM where R i s an a l i p h a t i c o r aromat ic r a d i c a l and M i s a po l yva len t metal such as
zinc o r n i c k e l . T h e i r manufacture i nvo l ves f i r s t heat ing an a lcohol o r phenol
w i th phosphorus pentasulph ide t o form diorganodithiophosphoric ac id , then neu-
t r a l i z i n g such a c i d w i t h a metal base.
10.3.3.2 Dithiocarbamates
Metal diorganodithiocarbamates, e s p e c i a l l y z i n c diorganodithiocarbamates
They are descr ibed by the general formula:
R \ HS N - C - SM
I R
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where R and M a r e as de f i ned i n 10.3.3.1. The i r manufacture i nvo l ves t h e reac-
t i o n o f an o rgan ic amine, carbon d i su lph ide , and a metal base.
10.3.3.3 Sulphur Products
Sulphur ized terpenes, f o r example, su lphur ized dipentene. These products a re
manufactured by hea t ing elemental su lphur w i t h a terpene hydrocarbon and then
o p t i o n a l l y washing the crude product w i t h aqueous a l k a l i o r a l k a l i metal su lph ide
t o remove d i sso l ved o r "corros ive" su lphu r .
10.3.3.4 Phosphorus-Sulphur Products
Phosphosulphurized terpenes, f o r example, phosphorus pentasulph ide- t reated
These a d d i t i v e s a r e manufactured by hea t ing phosphorus pentasul - t u rpen t ine .
phide w i t h a terpene hydrocarbon.
O f the fou r major c lasses o f co r ros ion i n h i b i t o r s l i s t e d above, metal d i t h i o -
phosphates - p a r t i c u l a r l y z i n c d ia l ky ld i t h iophospha tes - have achieved the widest
commercial acceptance. Many o f t he compounds l i s t e d above may n o t be t r u e corro-
s ion i n h i b i t o r s bu t f u n c t i o n by i n h i b i t i n g o x i d a t i o n and so reducing the forma-
t i o n o f c o r r o s i v e c a r b o x y l i c ac ids.
10.3.3.5 T r i a z o l e s and Che la t i ng Agents
Benzotr iazo le and i t s d e r i v a t i v e s a r e used t o form a su r face l aye r on copper
and s i l v e r based a l l o y s by c h e l a t i o n . T h i s process pass ivates the su r face and
reduces i t s a b i l i t y t o a c t as a c a t a l y s t towards f l u i d degradat ion by p reven t ing
s o l u b i l i s a t i o n o f smal l q u a n t i t i e s o f the meta l .
There a r e vers ions o f t he chemical which a r e n o t s o l u b l e and o t h e r which a r e
so lub le i n bo th s y n t h e t i c base f l u i d s and aqueous systems.
10.3.3.6 Dirnercapto Th iad iozo le D e r i v a t i v e s These a r e prepared by r e a c t i n g hydraz ine and carbon d i s u l p h i d e fo l l owed by
var ious reac t i ons t o make the m a t e r i a l o i l so lub le . They a r e e f f e c t i v e i n red-
uc ing co r ros ion .
10.3.3.7 Mode o f A c t i o n o f Corros ion I n h i b i t o r s
I t can be sa id t h a t compounds l i k e 10.3.3.5 and 10.3.3.6 a r e t r u e co r ros ion
i n h i b i t o r s . They f u n c t i o n by r e a c t i n g chemical ly w i t h the non-ferrous su r face
of t h e metal component: (e.g. i n an engine, copper- lead o r lead-bronze bear ings)
t o form thereon a c o r r o s i o n - r e s i s t a n t , p r o t e c t i v e f i l m . Th i s f i l m must adhere
t i g h t l y t o the bear ing su r face l e s t i t be removed by d ispersants o r detergents
and expose the under l y ing metal su r face t o a t t a c k by a c i d i c components
l u b r i c a t i n g o i l .
10.3.4 Rust I n h i b i t o r s
The presence o f water i n l u b r i c a n t s v a r i e s between very smal l l e v e l s as h igh as f o r t y percent (40) i n i n v e r t emulsion h y d r a u l i c f l u i d s and n
n the
t o values n e t y - f i ve
(95) percent i n emulsion f l u i d s . I n h i b i t i n g fe r rous surfaces aga ins t r u s t i n g i s
therefore a requirement i n a l l types o f o i l .
A range o f r u s t i n h i b i t i n g chemicals i s necessary t o cope w i t h d i f f e r e n t en-
vironments, and t h e i r chemical o r phys i ca l a c t i o n i s impor tant . The s t rong sur-
face adsorpt ion e x h i b i t e d by these chemicals t o r e s t r i c t t he contact w i t h water
means t h a t c a r e f u l choice i s necessary t o ensure t h a t o t h e r su r face a c t i v e chem-
i c a l s , no tab ly the ant iwear and extreme pressure members, a re ab le t o perform
t h e i r f unc t i on .
Typica l chemicals used f o r o i l so lub le systems inc lude the fo l l ow ing : -
Turbine, hydrau A l k y l t h i o a c e t i c ac ids and d e r i v a t i v e s
Alkenyl s u c c i n i c ac ids and d e r i v a t i v e s i c and c i r c u -
S
i v e o i l s .
) Subs t i t u ted imidazol ines Gear o i l s
Amine phosphates P rese rva t i ve o i
Sulphonates, n e u t r a l o r low base Engine preserva Storage, e t c .
Addi t ional f i l m s t reng th i s achieved by the a d d i t i o n o f f a t t y m a t e r i a l s such as
l a n o l i n i n the case o f t h i c k f i l m s torage composi t ion.
10.3.5 V i s c o s i t y Improvers
V iscos i t y improvers a re m a t e r i a l s which improve the v i s c o s i t y temperature
r e l a t i o n s h i p o f a l u b r i c a n t . They a r e g e n e r a l l y o i l s o l u b l e polymers w i t h mole-
cu lar weight ranging from approximately 50,000 t o 1,000,000. The polymer mole-
cule i n t e r a c t s w i t h the o i l t o e f f e c t t he f i n a l o i l v i s c o s i t y . The h ighe r the
tempature o f the system, the l a r g e r the polymer volume, the g rea te r the th icken-
ing e f f e c t , and hence the l ess the " th inn ing" tendendy o f the o i l due t o increa-
sed temperature.
I n a d d i t i o n t o v i s c o s i t y c h a r a c t e r i s t i c s , the performance o f these polymers
i s dependent on the shear s t a b i l i t y o r res i s tance t o shear and on t h e i r chemical
and thermal s t a b i l i t y . Wi th a g iven polymer, the shear s t a b i l i t y decreases w i t h
an increase i n molecular weight . The loss due t o shear i s r e f l e c t e d i n a de-
crease i n v i s c o s i t y o f the l u b r i c a n t i n the mechanical system.
V i s c o s i t y Index o r " V . I . " i s an a r b i t r a r y number - c a l c u l a t e d from the obser-
ved v i s c o s i t i e s o f a l u b r i c a n t a t two w ide ly separated temperatures - which
ind icates the res i s tance o f the l u b r i c a n t t o v i s c o s i t y change w i t h temperature.
The h igher the V . I . value, t he g rea te r the res i s tance o f t he l u b r i c a n t t o th icken
a t low temperatures and t h i n o u t a t h igh temperatures.
The s i g n i f i c a n c e o f V . I . i s much less w i t h cu r ren t o i l s p e c i f i c a t i o n s .
Measured low temperature v i s c o s i t y i s now most impor tant . For motor o i l s such
values a re repor ted us ing the Cold Cranking Simulator and f o r automat ic t rans -
mission f l u i d s and gear o i l s us ing the B r o o k f i e l d Viscometer.
The use o f hydrocarbon polymers o f low molecular weight makes i t poss ib le t o
formulate mul t igraded a x l e o i l s . Reduction i n o i l drag and consequent f u e l
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savings may be achieved w i t h lower v i s c o s i t y index values b u t a h i g h degree o f
shear s t a b i l i t y .
A l l important v i s c o s i t y improvers a re manufactured by processes o f po lymer i -
sa t i on . The groups o f polymers, s o l u b l e i n o i l , are : -
( i ) Poly isobutenes
( i i ) A l k y l methacry la te and a c r y l a t e copolymers
( i i i ) Rubber type chemicals such as o l e f i n e co-polymers (OCP) and butadiene-styrene copolymers
10.3.5.1 Mode o f Ac t i on o f V . I . Improvers
V . I . improvers e x e r t a g rea te r t h i cken ing e f f e c t on o i l a t h igh temperatures
than they do a t r e l a t i v e l y lower temperatures. The r e s u l t o f such s e l e c t i v e
th i cken ing i s t h a t t he o i l s u f f e r s less v i s c o s i t y change w i t h changing tempera-
tu re . I t i s be l i eved t h a t the s e l e c t i v e th i cken ing occurs because the polymer
assumes a compact, c u r l e d form i n a poor so l ven t such as c o l d o i l , and an un-
cu r led h igh surFace area form i n a b e t t e r so l ven t such as ho t o i l .
Polymers whose s o l u b i l i t y i n o i l changes very l i t t l e w i t h temperature a c t as
th ickeners, bu t a r e n o t as e f f e c t i v e V . I . improvers as a re those polymers whose
s o l u b i l i t y i s poor a t low temperatures b u t good a t h ighe r temperatures. V . I .
improvers a re more e f f e c t i v e i n i nc reas ing the v i s c o s i t y o rde r o f low v i s c o s i t y
o i l s and become p rog ress i ve l y less e f f e c t i v e as the v i s c o s i t y o f the base o i l
increases.
V . I . improvers undergo temporary v i s c o s i t y reduc t i on under shear because o f
the al ignment o f the polymer molecules i n the d i r e c t i o n o f f l ow . This temporary
v i s c o s i t y reduc t i on has the e f f e c t o f reducing f r i c t i o n i n h i g h shear zones and
g i ves a s i g n i f i c a n t advantage f o r t h e V . I . improver- t reated o i l over a base o i l
o f the same v i s c o s i t y . Another advantage i s t h a t i n low shear zones the v isco-
s i t y o f the t r e a t e d o i l remains h igh, thus m in im iz ing o i l consumption. Polymer-
t rea ted o i l s a l s o e x h i b i t lower bear ing wear than t h e i r comparable minera l o i l
counterpar ts . These f a c t o r s have been impor tant i n the acceptance o f mu l t i g rade
engine o i l s .
Polymers used as v i s c o s i t y - i n d e x improvers must be r e l a t i v e l y s t a b l e t o chain
s c i s s i o n under h igh shear ra tes . Molecular weight i s d r a s t i c a l l y reduced by
shearing, thus causing a d e t e r i o r a t i o n of t he p r o p e r t i e s which improve v i s c o s i t y
index.
and molecular weight d i s t r i b u t i o n and increases w i t h decreasing molecular weight.
I t can be seen t h a t shear s t a b i l i t y and v i s c o s i t y - i n d e x improvement r e q u i r e
d ivergent molecular weights . The molecular weights o f commercial polymers a r e
based on a compromise between these two p r o p e r t i e s . I n many cases, t h i s has
r e s u l t e d i n the use o f l a r g e r amounts of lower-molecular-weight polymers f o r a
g iven a p p l i c a t i o n .
The shear s t a b i l i t y o f a polymer type i s dependent on molecular weight
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Chemical and thermal s t a b i l i t y a re impor tant p r o p e r t i e s o f a good v i s c o s i t y -
index improver. For good s t a b i l i t y , i t i s important t h a t c a t a l y s t residues be
completely removed du r ing the manufactur ing process. Some commercial polymers
are formulated w i t h i n h i b i t o r s t o enhance t h e i r s t a b i l i t y . Several o f the
commercial v i s c o s i t y - i n d e x improvers a re m u l t i - f u n c t i o n a l i n t h a t they may be
e f f e c t i v e pour-point depressants and possess dispersency performance.
10.3.6 Pour P o i n t Depressants
Ever s ince l u b r i c a t i n g o i l s were prepared from crude o i l s , r e f i n e r s have
experienced d i f f i c u l t y w i t h congelat ion o f these products a t low temperatures.
Par t o f t he d i f f i c u l t y a r i s e s f rom a n a t u r a l t h i cken ing o f t he hydrocarbons
comprising the b u l k o f t he o i l ;
use o f a so l ven t such as kerosene t o reduce v i s c o s i t y . The r e s t o f t he d i f f i -
c u l t y - the more ser ious p a r t - a r i s e s from c r y s t a l l i z a t i o n a t low temperatures
o f the p a r a f f i n wax present i n almost a l l l u b r i c a t i n g o i l f r a c t i o n s . Upon c ry -
s t a l l i z a t i o n , t h i s wax tends t o form i n t e r l o c k i n g networks which adsorb o i l and
form a voluminous g e l - l i k e mass which r e s t r i c t s the f l o w o r "pour" o f the o i l .
something which can u s u a l l y be co r rec ted by the
Pour p o i n t depressants a r e chemicals which modify the wax c r y s t a l l i z a t i o n
process i n such a manner t h a t t h e o i l w i l l pour a t low temperatures. Al though
some monomeric compounds such as t e t r a ( long chain a l k y l ) s i l i c a t e s , phenyl t r i -
s tea ry loxys i l ane , and p e n t a e r y t h r i t o l t e t r a s t e a r a t e have been shown t o be e f f e c -
t i v e , a l l commercial ly impor tant pour p o i n t depressants a r e polymers:
( i )
(i i ) V i n y l c a r b o x y l a t e - d i a l k y l fumarate copolymers
( i i i ) A l p h a - o l e f i n polymers and copolymers
( i v )
A l k y l methacry la te polymers and copolymers
F r i e d e l - C r a f t s condensat ion products o f c h l o r i n a t e d wax and
aromat ic compounds such as naphthalene o r phenol (some i n v e s t i g a t o r s
do n o t c lass type ( i v ) products as polymers; o the rs ma in ta in t h a t
they a r e r e l a t i v e l y low molecular weight polymers having a p l u r a l i t y
o f aromat ic r i n g s and p a r a f f i n wax r a d i c a l s ) .
The molecular weight range o f polymers e f f e c t i v e as pour p o i n
i s genera l l y below t h a t o f polymers used as V . I . improvers, and
the area o f 500 t o 100,000.
10.3.6.1 Mode o f A c t i o n o f Pour P o i n t Depressants
Pour p o i n t depressants f u n c t i o n by adsorbing on o r co -c rys ta l
depressants
s u s u a l l y i n
i z i n g w i t h the
p r e c i p i t a t i n g wax, thus i n h i b i t i n g l a t e r a l c r y s t a l growth. This promotes growth
o f smal ler c r y s t a l s than the p l a t e l e t s formed i n the absence o f pour p o i n t
depressants. Th is change d imin ishes the a b i l i t y o f t he wax c r y s t a l s t o ove r lap
and i n t e r l o c k t o form l a r g e conglomerates o f wax which would impede the f low o f
the o i 1 .
258
There i s evidence t h a t a l i p h a t i c polymers f u n c t i o n by a c o - c r y s t a l l i z a t i o n
mechanism, and t h a t a l k y l a r o m a t i c t y p e s such as c h l o r i n a t e d p a r a f f i n wax-aromatic
compound condensat ion produc ts f u n c t i o n by adso rp t i on on the nascent wax c r y s t a l .
10.3.7 Extreme Pressure A d d i t i v e s
These a d d i t i v e s , commonly c a l l e d "E.P." agents, a re chemicals which a re addec
t o l u b r i c a n t s t o p revent d e s t r u c t i v e meta l - to -meta l con tac t du r ing l u b r i c a t i o n .
P l a i n minera l o i l s p rov ide good l u b r i c a t i o n when a f i l m o f o i l i s ma in ta ined be.
tween the moving sur faces (hydrodynamic l u b r i c a t i o n ) bu t f a i l t o p rov ide ade-
quate l u b r i c a t i o n when pressure and rubb ing speeds a r e such t h a t t he f i l m o f o i l
i s squeezed o r wiped o u t . The l a t t e r k i n d o f l u b r i c a t i o n , c a l l e d "boundary lub-
r i c a t i o n " , i s governed l a r g e l y by parameters o f t he c o n t a c t i n g sur faces such as
sur face f i n i s h , metal shear s t r e n g t h and t h e c o e f f i c i e n t o f f r i c t i o n between the
con tac t i ng metal sur faces . Unless such parameters can be chosen t o meet expected
pressures and rubb ing needs, d e s t r u c t i v e meta l - to -meta l con tac t w i l l take p lace .
10.3.7.1 Commercial E.P. A d d i t i v e s and t h e i r A p p l i c a t i o n
V i r t u a l l y a l l commercial E.P. a d d i t i v e s a r e o rgan ic compounds t h a t c o n t a i n onc
o r more elements o r f u n c t i o n s such as su lphur , halogen, phosphorus, carboxy l , o r
ca rboxy la te s a l t wh ich can r e a c t chemica l l y w i t h t h e metal su r face under condi-
t i o n s o f boundary l u b r i c a t i o n . The ease w i t h which an E.P.add i t i ve reac ts w i t h
t h e metal su r face , i . e . i t s " a c t i v i t y " , determines t o a l a r g e e x t e n t whether i t
would be used i n a l u b r i c a n t such as a c u t t i n g o i l , a hypoid gear o i l , h y d r a u l i c
o i l , o r a steam t u r b i n e o i l . An assignment o f l i k e l y f i e l d s o f a p p l i c a t i o n o f
commercial E .P.add i t i ves based on t h e i r r e l a t i v e a c t i v i t y i s shown i n Tab le 10.3
Tab le 10.3 F i e l d s o f A p p l i c a t i o n o f E.P. A d d i t i v e s
High A c t i v i t y A d d i t i v e s "Moderate" o r I n te rmed ia te " M i Id" o r Low A c t i v i t v A d d i t i v e s A c t i v i t v A d d i t i v e s
S t r a i g h t c u t t i n g o i l s Hypoid gear o i l s Worm gear o i l s Drawing compounds (e.g. mu l t i - pu rpose gear S p i r a l bevel gear o i l s
Meta l - fo rming l u b r i c a n t s I n d u s t r i a l gear o i l s Manual gear box o i l s
Some hypoid gear o i Is (e.g. "open gear" o i Is) I n d u s t r i a l gear o i l s f o r Steam t u r b i n e o i l s
genera l a p p l i c a t i o n
1 ub r i cants )
Motor o i l s
J e t a i r c r a f t t u r b i n e o i l s
Gas t u r b i n e o i l s
Automat ic t ransmiss ion
H y d r a u l i c o i l s ( f i r e
f l u i d
r e s i s t a n t emulsion
I n d u s t r i a l gear o i l s f o r c losed o r c i r c u l a t i n g systems
type)
259
10.3.7.2 Automotive E.P. Gear Oils
These o i l s a r e used t o l u b r i c a t e the worm, s p i r a l bevel , or hypoid gear
d r i ves o f automot ive veh ic les . Since hypoid gears r e q u i r e the g rea tes t measure
o f E.P. p r o t e c t i o n o f a l l commercial gear d r i v e s , most a d d i t i v e t reatments a re
designed t o g i v e s a t i s f a c t o r y performance i n t h i s environment.
H i s t o r i c a l l y , E . P . a d d i t i v e s f o r use i n gear s e r v i c e f a l l i n t he f o l l o w i n g
general ca tegor ies . Th is i n fo rma t ion should n o t be i n t e r p r e t e d as i n d i c a t i n g
t h a t e a r l y t reatments a r e obsolete; i n f a c t , a l l o f t he l i s t e d t reatments a re
i n commercial use today, many on i n d u s t r i a l a p p l i c a t i o n s . The more r e c e n t l y
developed a d d i t i v e s , however, enjoy most o f t he market.
E a r l i e s t successful t reatment - Lead soap (e.g. lead naphthenate p lus an
a c t i v e o r "corros ive" o rgan ic su lphur
compound.
La te 1930's - Ch lo r ine and "Moderately a c t i v e ' ' su lphur
present i n t h e same o r d i f f e r e n t o rgan ic
molecules
World War I I p e r i o d - Sulphur, c h l o r i n e and phosphorus i n s u i t a b l e
o rgan ic c a r r i e r s
Most recent types (1960- ) - Sulphur and phosphorus i n s u i t a b l e o rgan ic
c a r r i e r s
Typ ica l E . P . a d d i t i v e s used commercial ly i n fo rmu la t i ng gear l u b r i c a n t s have
included:-
Ch lo r ina ted p a r a f f i n wax (40% - 60% c h l o r i n e )
Chlornaphtha xanthate ( r e a c t i o n product o f c h l o r i n a t e d naphtha and a l k a l i
metal xanthate)
Ch lo r ina ted p a r a f f i n wax su lph ides ( r e a c t i o n product o f c h l o r i n a t e d para-
f f i n wax and a l k a l i metal su lph ides)
Sulphur ised f a t t y o i l s (e.g. su lphur ised l a r d o i l , su lphur ised f i s h o i l ,
su lphu r i sed sperm o i I ) Sulphur ised hydrocarbons such as polybutenes
Sulphur ised s y n t h e t i c e s t e r s (e.g. su lphu r i sed methyl o l e a t e o f f a t t y ac ids )
Sulphur c h l o r i d e - t r e a t e d f a t t y o i l s (e.g. S 2 C I 2 - t rea ted f i s h o i l )
A 1 i p h a t i c and aromat ic po lysu lph ides (e.g. benzyl d i su lph ide , ch lorobenzyl
d i su lph ide, b u t y l d i su l phide)
Phosphosulphurized f a t t y o i l s (e.g. l a r d o i l heated w i t h su lphur and phos-
phorus pen tasu 1 ph ide)
Organic phosphi tes (obta ined by t r e a t i n g a l coho ls w i t h P C I )
A l karylphosphates (octy lphenol t rea ted w i t h P205) 3
260
A l k y l phosphates (a l coho ls tea ted w i t h P 0 )
Lead naphthenate
Z inc and lead d i -organo d i th iophosphates
Z inc and lead d i - a l k y l d i th iocarbamates
2 5
An eva lua t i on o f t he p r a c t i c a l e f fec t i veness o f a hypoid gear l u b r i c a n t must
be c a r r i e d ou t i n f u l l - s c a l e equipment i n the l a b o r a t o r y and i n the f i e l d .
Bench t e s t r i g s have no t been a b l e t o per form t h i s task, a l though they a r e use-
f u l i n screening l i k e l y candidates f o r f u l l - s c a l e eva lua t i on . A l i s t o f these
t y p i c a l bench t e s t s i s g i ven i n Table 10.4.
10.3.7.3 Ant iwear A d d i t i v e s
Al though d i scuss ing p r i m a r i l y gear systems, the advent o f h i g h performance
engines f o r passenger cars posed new l u b r i c a t i o n problems. Engine i nspec t i on
began to reveal unmistakable evidence o f excessive wear and s c u f f i n g o f v a l v e
t r a i n components; h igh r o t a t i o n a l speeds and inc reas ing pressures between cams
and l i f t e r f o o t surfaces had apparent ly combined t o s h i f t l u b r i c a t i o n requ i re -
ments i n t o the boundary reg ion. The f i r s t and perhaps most e f f e c t i v e E.P. addi -
t i v e f o r c o n t r o l l i n g o r e l i m i n a t i n g wear and s c u f f i n g i n the va l ve t r a i n area
was found t o be t h a t v e r s a t i l e a d d i t i v e - z i n c d ia l ky ld i t h iophospha te .
Other a d d i t i v e s found use fu l f o r t he c o n t r o l o f va lve t r a i n wear inc lude:
T r i c r e s y l phosphate
D i l a u r y l phosphate
Didodecyl phosphi te
Sulphur ized terpenes
Sulphur ized sperm o i l
Ch lo r ina ted compounds
Z inc d i a l k y l d i th iocarbamate
I t has been mentioned e a r l i e r t h a t t he z i n c d iorganodi th iophosphate p lays a
very impor tant r o l e i n ant iwear h y d r a u l i c o i l s and i n m i l d EP gear o i l s . The
choice o f t he c o r r e c t type o f compound depends upon the des i red performance
c h a r a c t e r i s t i c s . A z i n c d i th iophosphate may be prepared us ing a v a r i e t y of
a l coho ls . Pr imary and secondary a l k y l groups a r e used f o r a p p l i c a t i o n s such a s
gaso l i ne engine and h y d r a u l i c systems and aromat ics ( s u b s t i t u t e d phenols) f o r
d i e s e l engines where the major r o l e i s as a h i g h temperature a n t i o x i d a n t .
I t i s essen t ia l t h e r e f o r e t h a t the performance o f these m a t e r i a l s i s r e l a t e d
t o the a p p l i c a t i o n . Genera l ly , good ant iwear performance means low thermal
s t a b i l i t y , bu t a good a n t i o x i d a n t . For h y d r a u l i c system use the a d d i t i v e must
be r e s i s t a n t t o h y d r o l y s i s a t temperatures t y p i c a l o f h y d r a u l i c c i r c u i t s .
An i n d i c a t i o n o f t he major performance p r o p e r t i e s o f a range o f z i n c diorgano-
d i th iophosphates i s g i ven i n Table 10.5.
Tab le 10.4
Lubr i can t , F r i c t i o n , Wear and Gear Tes t Machines
Other c h a r a c t e r i s t i c s Type o f Type o f Range o f Speed
Load i ng (kg)
m/s Load i ng Tes t Machine Type o f Contact Measurement
Almen-Weiland Conforming area, F r i c t i o n , wear, Mechanical 0-2000 0 - 0.2 A lso used f o r c o r r o s i o n t e s t p in /bea r ing s h e l l s l oad -ca r ry ing s teps on specimens a f t e r runn ing
Fa lex I P 241/77T M u l t i p l e l i ne /a rea F r i c t i o n , wear, Mechanical 0-2000 0.1-0.25 A lso f o r c o r r o s i o n t e s t l oad -ca r ry ing cont inuous
She l l 4 -Ba l l M u l t i p l e p o i n t F r i c t i o n , wear, Dead we igh t 0-1800 1500 A lso w i t h d i f f e r e n t loads speed I P 239179 T hardened s t e e l 1 oad- ca r r y i ng s teps rprn and specimens f o r wear t e s t s .
b a l Is "Roll ing" u n i t f o r bear ing s t u d i e s
T i mken B lock and r i n g . F r i c t i o n , wear, Dead we igh t 0-50 0-400 A lso f o r grease s tud ies I P 240/76 T L i n e l oad -ca r ry ing cont inuous (0-800rpm)
o r s tep
Niemann-FZG Gear. Two t o o t h Load-car ry ing Dead we igh t 1600 7.3 A lso runs a t 2175 r.p.m. IP 33h/77 T forms. P i n i o n tee th wear r a t e s tep , s t a r t 'Normal ' O i 1 temperature 9OoC
16. Wheel t e e t h 24. under load load Spur/Case hardened
I .A.E. Gear-one t o o t h Load-car ry ing Dead we igh t Up t o a t 7.9 A lso runs a t 2000 and 6000 rpm I P 166/77 form p i n i o n t e e t h s tep , s t a r t l e a s t 70 4000 O i l temperature 200O-6O0C,
16. Wheel 16. Spur/ under load Lever rpm 400O-7O0C Case hardened
Ryder FTMS 791a-6508 Gear-one t o o t h Load-car ry ing Hydrau l i c 10000 o i l temperature 7 4 ' ~
fo rm p in ion /whee l a p p l i e d when rpm S t a r t up a t no l oad t e e t h 28 runn ing N
Q, CL
262
TABLE 10.5 R e l a t i v e Performance o f Z inc Di th iophosphates
Substrate Performance
A1 coho1 Thermal A n t i - H y d r o l y t i c A n t i - Bear ing STabi 1 i t y Oxidancy Stabi 1 i t y Wear P r o t e c t i o n
Secondary-1 4 2 2 2 2
Secondary-2 5 1 1 1 1
Pr imary- 1 2 4 4 4 4
P r i ma ry -2 3 3 3 3 3
Aromatic-1 1 5 5 5 5
Aromatic-2 1 5 5 5 5
These a d d i t i v e s a r e t y p i c a l o f those used f o r l u b r i c a n t s
Rat ing 5 T: worst c o n d i t i o n
1 = best c o n d i t i o n
10.3.7.4 EP a d d i t i v e s f o r Turb ine O i l s
Advances i n the design o f steam tu rb ine , gas tu rb ine , and j e t a i r c r a f t t u rb -
i ne engines and t h e i r assoc iated gear ing have in t roduced problems o f boundary
l u b r i c a t i o n . To so l ve these problems, the a d d i t i v e i ndus t r y has developed E.P.
a d d i t i v e s o f the "low a c t i v i t y " v a r i e t y which p rov ide p r o t e c t i o n aga ins t excess-
i v e wear a n d s c u f f i n g o f t u r b i n e engine components. Such a d d i t i v e s must w i t h -
stand h igh temperatures -and i n the case o f steam tu rb ines , water contaminat ion - w i thou t promot ing co r ros ion o f t u r b i n e engine components. For use i n gas t u r -
b ine and j e t a i r c r a f t t u r b i n e engines formulated w i t h s y n t h e t i c ester-base
f l u i d s d i f f e r e n t chemicals may be requ i red .
For steam t u r b i n e o i l s a d d i t i v e s based on phosphorus and h y d r o l y t i c a l l y s tab le
c h l o r i n e con ta in ing chemicals have been used. C e r t a i n sulphur-phosphorus addi -
t i v e s a r e a l s o known t o have been used. In t h i s case, t he su lphur i s i n a c t i v e
towards copper a t normal temperatures and becomes re leased o n l y a t h i g h opera-
t i ng temperatures . For gas t u r b i n e a p p l i c a t i o n s phosphorus compounds, bo th phosphi tes and phos-
phates, a re used. I n c e r t a i n cases p o l y g l y c o l f l u i d s may a c t i n the same way - presumably by s t rong adso rp t i on of t h e hydroxy groups t o metal sur faces.
10.3.7.5 EP A d d i t i v e s f o r C u t t i n g O i l s
Very h igh pressures and temperatures a r e developed l o c a l l y between the work
and the c u t t i n g t o o l i n machining operat ions. The so-ca l led " s t r a i g h t c u t t i n g
o i l s " w ide l y employed t o cool and l u b r i c a t e t h e work and c u t t i n g t o o l a r e m i n -
e r a l 0 1 1 s which have been blended w i t h s u i t a b l e E.P. a d d i t i v e s . Since the
l u b r i c a t i o n of a m e t a l - c u t t i n g opera t i on i s almost completely w i t h i n the boundary
263
region, E . P . a d d i t i v e s o f h igh a c t i v i t y a r e requ i red f o r bes t r e s u l t s . I n the
case o f an o rgan ic su lphu r compound, i t s a c t i v i t y shou ld be such t h a t a d i l u t e
s o l u t i o n (e.g. 1 o r 2 pe rcen t ) o f t he compound i n a minera l o i l w i l l completely
blacken a copper s t r i p w i t h i n one hour a t 100°C. Sulphur compounds having t h i s
degree o f a c t i v i t y a re known i n the i n d u s t r y as " co r ros i ve su lphur " a d d i t i v e s
and a re used e x t e n s i v e l y i n the compounding o f commercial c u t t i n g o i l s . Examples
o f E .P . a d d i t i v e s developed f o r use i n c u t t i n g o i l s i nc lude su lphu r i zed minera l
o i l , su lphu r i zed f a t t y o i l s , su lphur c h l o r i d e - t r e a t e d f a t t y o i l s , su lphu r i zed
o l e f i n s , su lphur c h l o r i d e - t r e a t e d o l e f i n s , benzyl po l ysu lph ides , c h l o r i n a t e d
p a r a f f i n wax, and c h l o r i n a t e d m ine ra l o i l s .
However, metal work ing and fo rming opera t i ons r e q u i r e many types o f chemical
a d d i t i v e a p a r t f rom t h e a c t i v e o r c o r r o s i v e su lphur type. Genera l l y , t he a c t i v e
sulphur a d d i t i v e s a r e used i n metal removal ope ra t i ons . The l i s t below ind ica tes
the inc reas ing degree o f s e v e r i t y :
( i ) Turn ing and M i l l i n g
( i i ) D r i l l i n g and Reaming
( i i i ) Tapping and Threading
( i v ) Broaching
As ve ry h i g h emperatures a r e reached, the cho ice o f s u i
ends a l s o on the m a t e r i a l be ing machined, so t h a t t he bes t
f i n i s h and t o o l l i f e a r e re ta ined .
The chemical a c t i v i t y o f su lphu r compounds may be chosen
a b l e a d d i t i v e dep-
coo l i ng , su r face
t o g i v e p a r t i c u l a r
performance i n c u t t i n g opera t i ons . When such m a t e r i a l s a r e incorpora ted i n t o
f l u i d s hav ing a dual-purpose na ture , such as machine l u b r i c a t i o n , then the choice
has t o be made w i t h the requirements o f t h e h y d r a u l i c o r c i r c u l a t i n g o i l c i r c u i t
me ta l l u rgy matched aga ins t t h e metal work ing opera t i on .
Add i t i ves a r e a l s o used i n metal fo rming opera t i ons such as r o l l i n g , drawing,
stamping and f o r g i n g . For r o l l i n g o i l s t h e g r e a t e s t requirement i s h i g h sur -
face f i n i s h , which means t h a t t he a d d i t i v e s must n o t be chemica l l y a c t i v e . Gen-
e r a l l y , f a t t y a l c o h o l s and e s t e r s a r e p r e f e r r e d .
Drawing and f o r g i n g opera t i ons r e q u i r e h i g h l y s p e c i a l i s e d produc ts . F a t t y -
based systems a r e used c o n t a i n i n g a v a r i e t y o f a d d i t i v e s i n c l u d i n g s o l i d l u b r i -
cants such as g r a p h i t e and molybdenum d isu lph ide .
10.3.7.6 Mode o f A c t i o n o f E.P. A d d i t i v e s
E.P.add i t i ves f u n c t i o n by r e a c t i n g w i t h r e l a t i v e l y moving sur faces under
boundary l u b r i c a t i o n cond i t i ons t o fo rm an adherent f i l m which has lower shear
s t reng th than t h a t o f t h e metal sur faces themselves. T h i s f i l m a c t s as a s o r t
o f s o l i d l u b r i c a n t , and takes over t h e task o f l u b r i c a t i o n when the o i l i s no
longer ab le t o p rov ide a separa t i ng and p r o t e c t i v e l u b r i c a t i n g f i l m . W i th an
264
approp r ia te E.P. a d d i t i v e , t he re i s l i t t l e o r no fo rma t ion o f such " s o l i d l u b r i -
cant" under cond i t i ons o f hydrodynamic l u b r i c a t i o n . I t forms o n l y a t the eleva-
ted temperatures which develop l o c a l l y between metal surfaces under cond i t i ons
o f boundary l u b r i c a t i o n . Because temperature has been shown t o be t h e most
i n f l u e n t i a l parameter i n the f u n c t i o n o f E.P. l u b r i c a n t s , F.P.Bowden and co-
workers have suggested t h a t t he term "Extreme Temperature" l u b r i c a n t s and addi -
t i v e s might be more approp r ia te .
10.3.8 E m u l s i f i e r s
Genera l ly , an e m u l s i f i e r i s a chemical f o r d i spe rs ing e i t h e r water i n o i l o r
o i l i n water. The former, the more d i f f i c u l t system, produces f i r e - r e s i s t a n t
h y d r a u l i c f l u i d s , rock d r i l l l u b r i c a n t s , and many types of wire-drawing media.
The e m u l s i f i e r may be o f the a l keny l succ in imide type, f a t t y es te rs , o r o the rs
con ta in ing f a t t y residues. For metal working and h y d r a u l i c media o f t he so lub le
o i l ( o i l i n water) type, a l a r g e range o f e m u l s i f i e r s i s a v a i l a b l e . The choice
depends upon whether i t i s d e s i r a b l e t o have a chemical w i t h i o n i c cha rac te r i s -
t i c s , a s a l t , o r one w i t h non - ion i c p r o p e r t i e s such as es te rs , phenol e the rs
and o the r oxygenated chemicals. A shor t l i s t i s g iven below o f some o f t he
chemicals:
Sodium sulphonates
T a l l o i l amides
Ethanol amines
Quaternary Ammonium s a l t s
Po lya l ky lene phenol e the rs and associated oxygenated products
Ethoxy lated f a t t y ac ids
S a l t s o f f a t t y ac ids
E m u l s i f i e r s a r e c l a s s i f i e d accord ing t o a numerical value, t he HLB number,
obta ined by e s t i m a t i n g the emulsion s t a b i l i t y when prepared i n a standard way.
For o i l - i n - w a t e r s o l u b l e o i l emulsions, t he HLB range f o r e m u l s i f i e r s i s
12-15, f o r w a t e r - i n - o i l i n v e r t emulsions i t i s 4-6 when checked i n a naphthenic
base s tock .
10.3.9 F r i c t i o n M o d i f i e r s
Cer ta in d e f i c i e n c i e s a r e sometimes observed w i t h l u b r i c a n t s w i t h respect t o
t h e i r f r i c t i o n a l c h a r a c t e r i s t i c s . Where metal sur faces a r e designed t o s l i d e ,
two opposing requirements may occur:
( i )
( i i)
Smooth s l i d i n g w i t h no v i b r a t i o n and minimum c o e f f i c i e n t o f f r i c t i o n .
No s l i d i n g maximum c o e f f i c i e n t o f f r i c t i o n f o r engagement o f c l u t c h
surfaces o r f r i c t i o n l o c k i n g devices.
265
Addi t ives f o r ( i ) a r e genera l l y fa t ty-based, n a t u r a l l y - o c c u r r i n g products
such as f a t t y e s t e r s and amides. They f i n d use i n s l i d i n g mot ion systems such
as machine t o o l s l ideways.
I n the case o f type ( i i ) , t h e c l u t c h o r f r i c t i o n mechanism should engage o r
disengage smoothly w i t h o u t v i b r a t i o n .
t o con t ro l .
Two types o f a p p l i c a t i o n a r e d i f f i c u l t
A choice o f d i f f e r e n t products may be made:-
( i ) A n t i - c h a t t e r i n l i m i t e d s l i p ax les, o r o t h e r metal /metal f r i c t i o n
l ock ing u n i t s . I t i s necessary t o reduce the s t i c k - s l i p a c t i o n o f
l u b r i c a t e d s t e e l / s t e e l contacts .
Amide - metal d i th iophosphate combinations
Amine d i th iophosphates
( i i ) Anti-squawk a d d i t i v e s reduce v i b r a t i o n which g i ves r i s e t o aud ib le
no i se i n c lu t ches o f d i s s i m i l a r ma te r ia l (bronze on s t e e l , asbestos
on s t e e l , e t c . ) . Chemicals used f o r t h i s purpose inc lude:
N-acylsarcosines and d e r i v a t i v e s
Sulphur ised f a t s and e s t e r s
Organophosphorus a c i d and f a t t y a c i d m ix tu res
Esters o f d imer ised f a t t y ac ids
Formulat ions which meet the complex performance des i red i n automat ic t rans-
mission f l u i d s , un i ve rsa l t r a c t o r engine- t ransmiss ion o i l s o r machine t o o l sys-
tems a re t h e r e s u l t o f c a r e f u l matching o f f r i c t i o n m o d i f i e r s i n the whole
a d d i t i v e system.
10.4 CONCLUSIONS
The chapter w i l l serve t o i n d i c a t e the complex na tu re o f t he chemicals used
i n l u b r i c a n t f o rmu la t i ons . Main ly , t he d iscuss ion has been about a d d i t i v e s f o r
minera l l u b r i c a t i n g o i l s a l though t h e r e has been reference made t o s y n t h e t i c
f l u i d s and water con ta in ing h y d r a u l i c and metal working f l u i d s .
I n fo rmu la t i ng l u b r i c a n t s o f any type, t he presence o f these a d d i t i v e s re-
q u i r e s t h a t n o t o n l y should each i n d i v i d u a l product c a r r y o u t i t s des i red func-
t i o n b u t t h a t i n t e r a c t i o n s between more than one chemical must no t cause de te r -
i o r a t i o n o f performance. N e i t h e r must m ix tu res reac t together to form o i l in-
s o l u b l e by-products. The f i n a l o i l f o rmu la t i on must be s t a b l e over t h e range
o f ope ra t i ng and s torage temperatures.
M ix tu res o f a d d i t i v e s may be formulated and used as a package a t a g i ven
treatment l e v e l , r e q u i r i n g o n l y the c a r r i e r f l u i d . Again, t he re must be no
d e t e r i o r a t i o n a f t e r manufacture o r s torage and the combinat ion must be se lected
w i t h g rea t ca re so t h a t maximum performance i s achieved a t an economic cos t .
The au tho r wishes t o acknowledge the ass is tance o f h i s col leagues i n the
L u b r i z o l Corporat ion f o r rev iewing t h i s chapter and suggest ing a d d i t i o n a l data;
266
also to the Lubrizol Corporation for permission to compile the information
REFERENCES
1 Smallheer and Kennedy-Smith, Lubricant Additives, The Lezius-Hiles Co.,
2 C.V.Smallheer, Lectures on Lubricant Additives, Imperial College, London, Cleveland, Ohio.
March 1970.
267
11 CONSUMPTION AND CONSERVATION OF LUBRICANTS
A.R. LANSDOWN
D i rec to r , Swansea T r i b o l o g y Centre, U.K.
11.1 CONSUMPTION
Apart f rom the r e l a t i v e l y smal l q u a n t i t i e s o f vegetab le and animal o i l s ,
almost a l l modern l u b r i c a n t s a r e de r i ved f rom petroleum, e i t h e r by f r a c t i o n a l
d i s t i l l a t i o n o r chemical conversion. It i s now g e n e r a l l y recognised t h a t t he
wor ld ' s remaining supp l i es o f pe t ro leum have o n l y a l i m i t e d l i f e .
The va r ious es t imates o f the remaining l i f e o f pe t ro leum reserves depend on
many assumptions. The r a t e o f f u t u r e consumption i s i t s e l f dependent on
several assumptions, such as the a v a i l a b i l i t y and compet i t i veness o f a l t e r n a t i v e
fue ls , improvements i n e f f i c i e n c y o f u t i l i s a t i o n , and f i s c a l i ncen t i ves . The
est imates o f remain ing reserves a l s o depend on severa l assumptions, such as the
ra te o f d i scove r ing new reserves and the p o t e n t i a l f o r us ing low-grade sources.
For energy purposes t h i s l a s t f a c t o r reaches i t s l i m i t when the energy requ i red
t o e x p l o i t a source i s equal t o the energy u l t i m a t e l y ob ta ined from the source.
There i s a general tendency t o d iscuss the f u t u r e o f pe t ro leum p u r e l y f rom
the energy s tandpo in t , and t o r e l a t e i t t o the phasing i n o f a l t e r n a t i v e energy
sources. There a r e however many produc ts f rom pet ro leum which w i l l be l e s s
read i l y rep laced f rom non-petroleum sources, and l u b r i c a n t s a r e i n t h i s
category . A f u r t h e r l i m i t a t i o n on f u t u r e l u b r i c a n t supp l i es i s t h a t n o t a l l crude o i l s
can be used t o produce l u b r i c a n t s w i t h o u t expensive, and energy-expensive,
chemical p rocess ing . The Un i ted Kingdom's Nor th Sea o i l i s i n f a c t genera l l y
unsu i tab le f o r l u b r i c a n t manufacture.
We a r e thus faced w i t h the s i t u a t i o n t h a t w i t h i n the work ing l i f e o f some o f
our younger l u b r i c a n t t e c h n o l o g i s t s petroleum-based l u b r i c a n t s w i l l cease t o be
p l e n t i f u l and may become ex t remely expensive. Conservat ion o f l u b r i c a t i n g o i l s
should t h e r e f o r e a l ready be a ma t te r o f se r ious concern, and w i l l i n e v i t a b l y be
recognised as such w i t h i n one, o r a t most two, decades.
268
Table 11.1 shows a breakdown o f the t o t a l Uni ted Kingdom consumption o f
petroleum f o r the calendar year 1977, and i t can be seen t h a t l u b r i c a t i n g o i l s
and greases represent j u s t over one m i l l i o n m e t r i c tons, o r 1.17% o f t he t o t a l .
Accurate f i g u r e s f o r the breakdown o f l u b r i c a n t types a r e more d i f f i c u l t t o
ob ta in , bu t Table 11.2 g i ves est imates o f U.K. l u b r i c a n t consumption in some o f
the more important ca tegor ies . The b igges t s i n g l e category cons is t s o f auto-
mot ive engine o i l s , compr is ing perhaps one t h i r d o f t he t o t a l l u b r i c a n t con-
sumpt ion.
Table 11.1 Uni ted Kingdom Consumption o f Petroleum Products 1977 (F igures from I n s t i t u t e o f Petroleum "Petroleum S t a t i s t i c s " )
Product category Consumption (thousand tonnes)
Motor s p i r i t
Gas, d iese l and fue
A v i a t i o n f u e l s
Other f u e l s
Naphtha/Light d i s t i
17,336
o i l s 47,920
4,218
9,639
l a t e 5,179
L u b r i c a t i n g o i l s and greases 1,029
Other products 2,835
Tota 1 88,156
Table 11.2 Est imated Lubr i can t Consumption by Types
Consumption (%) End Use TY Pe
Av i a t ion 0.4
Marine 8.5 T rac to r 3 . 0 Motor O i l s 35.0
Motor Greases 0.1
I n d u s t r i a l Hydraul i c o i 1 s 16.0
I n d u s t r i a l Bear ing o i l s 22.5
lndust r i a l Metalworking 5.5
I n d u s t r i a l Process o i l s 5.5
I n d u s t r i a l I.C.E. o i l s 1.5
I n d u s t r i a l Greases 1 .o I n d u s t r i a l Fuel as l u b r i c a n t 1 .o
269
Overa l l l u b r i c a n t s account f o r o n l y a l i t t l e over 1 % o f t he t o t a l pe t ro leum
consumption, bu t t he importance o f t h i s 1 % i s increased by the two f a c t o r s
p rev ious l y mentioned, namely the g rea te r d i f f i c u l t y o f f i n d i n g s u b s t i t u t e
sources f o r l u b r i c a n t s and the l i m i t e d range o f crude o i l s which can be used t o
produce l u b r i c a n t s .
The u l t i m a t e f a t e o f the va r ious l u b r i c a n t s i s a l s o d i f f i c u l t t o assess
accura te ly . Table 11.3 shows some es t imates made i n 1973 o f t he f a t e o f au to-
mot ive engine o i l s . From the conserva t ion p o i n t o f v iew the re a re two i n t e r -
e s t i n g aspects o f these es t imates . The f i r s t i s t h a t some 185
apparent ly p o t e n t i a l p o l l u t e r s o f l and and water. The second
258,000 tons c o u l d t h e o r e t i c a l l y be reclaimed.
An a r t i c l e pub l i shed i n the AA "Dr ive" magazine i n January,
t h a t the 58,000 tons o f engine o i l s changed each year by motor
poss ib l y disposed o f as shown i n Table 11.4. The l a r g e r quant
garages was, however, g e n e r a l l y disposed o f more l e g a l l y .
000 tons were
s t h a t some
1974 suggested
s t s a t home was
t y changed i n
Since 1974 the re have been more d e t a i l e d surveys, bu t the i nc reas ing aware-
ness o f t he need f o r conserva t ion , has a l s o l e d t o a tendency t o longer o i l -
change p e r i o d s and t o a g r e a t e r c o n t r i b u t i o n o f r e - r e f i n i n g t o the d isposa l
prob 1 em.
Both o f these f a c t o r s , reduc t i on i n consumption and r e - r e f i n i n g , w i l l i n -
e v i t a b l y assume g r e a t e r importance i n coming years.
Table 11.3 Est imates o f t he Fate o f Motor O i l s
Burned (exhaust) 25% 95,000 tons
Changed (garages) 50% 190,000 tons
Changed (home) 15% 58,000 tons
Leaked 5% 19,000 tons
Scrapped w i t h v e h i c l e 1 % 4,000 tons
Spi 1 l e d 1% 4,000 tons
Railway ( ? ) 1% 5,000 tons
Table 11.4 Est imates o f O i l Disposal by "Do- i t - you rse l f " Car Owners
Bur ied i n garden
Poured down d r a i n s
Burned
20%
8%
18%
Taken t o garage e t c . f o r p roper d isposa l 17%
Otherwise disposed o f 37%
11 .2 REDUCING CONSUMPTION
There are o f course p u r e l y mechanical ways t o reduce consumption, such as
e l i m i n a t i n g leakage and improving s e r v i c e a b i l i t y o f engines. Improvement may
sometimes a l s o be poss ib le by the use o f a more v iscous o i l , b u t t h i s must
obv ious ly be done w i t h cau t i on . Not o n l y must care be used t o ensure t h a t a
s u i t a b l e v i s c o s i t y i s used, b u t as a general r u l e the use o f a h ighe r v i s c o s i t y
o i l w i l l mean h ighe r power consumption, and the r e s u l t i n g energy wastage w i l l
probably more than o f f s e t any l u b r i c a n t saving.
The best prospect f o r reducing consumption l i e s i n ensu r ing t h a t o i l changes
are no t c a r r i e d ou t any more f r e q u e n t l y than they need t o be.
Even i n i d e n t i c a l systems the r a t e o f o i l degradat ion can vary considerably .
The f o l l o w i n g a re some o f the f a c t o r s which l ead t o r a p i d degradat ion.
Dusty o r d i r t y environments
High temperatures
Very low temperatures
Temperature f l u c t u a t i o n s , leading t o condensat ion
High a l t i t u d e
Poor f i l t e r maintenance
Low o i l l e v e l s
Frequent s t o p - s t a r t ope ra t i on
Short journey lengths i n a v e h i c l e
Contamination by chemicals
Contamination by unburned f u e l
Contamination by combustion products
Contamination by wear d e b r i s
Wi th in the o i l f o rmu la t i on the f o l l o w i n g f a c t o r s can a l s o lead t o r a p i d
degradat ion.
Poor q u a l i t y base o i l , con ta in ing unstable molecules
Inadequate a n t i - o x i d a n t con ten t
I n s u f f i c i e n t d ispersant o r detergent a d d i t i v e s
I n s u f f i c i e n t ant i -wear o r EP a d d i t i v e s
I n s u f f i c i e n t bas i c a d d i t i v e s w i t h su lphu r -con ta in ing f u e l
Excess ive ly r e a c t i v e o r unstable a d d i t i v e s
Because o f a l l these va r iab les , the requ i red o i l change p e r i o d i n two
i d e n t i c a l engines may vary by a f a c t o r o f ten i n d i f f e r e n t ope ra t i ng cond i t i ons .
I n an e ra o f r e l a t i v e l y cheap l u b r i c a n t s , recommended o i l change pe r iods
w i l l tend t o be sho r t enough t o ensure s a t i s f a c t o r y q u a l i t y i n the worst set o f
cond i t i ons , because the economic pena l t y f rom us ing an o i l f i l l t o o long w i l l be
f a r g rea te r than the va lue o f the o i l saved.
271
The s o l u t i o n i s o i l q u a l i t y mon i to r i ng , which enables the o i l remain ing i n an
i nd i v idua l system t o be assessed p e r i o d i c a l l y , and o n l y changed when i t s con-
d i t i o n i s approaching an u n s a t i s f a c t o r y l e v e l . As a bonus, o i l mon i to r i ng w i l l
a l so g i ve va luab le i n fo rma t ion about the c o n d i t i o n o f the engine o r o t h e r lub-
r i c a t e d system.
O i l mon i to r i ng i s an impor tan t p a r t o f machinery h e a l t h mon i to r i ng , which i s
descr ibed i n d e t a i l i n Chapter 18. It i s t h e r e f o r e unnecessary t o descr ibe the
var ious techniques a t l eng th here, bu t t he re a r e two aspec ts which should be
mentioned.
The f i r s t o f these i s the need t o t a i l o r the mon i to r i ng techniques t o the
s ize and importance o f the o i l system be ing monitored. A l a r g e c r i t i c a l system
w i l l j u s t i f y the e f f o r t and c o s t invo lved i n f requent spec t rograph ic o i l a n a l y s i s
o r fe r rography . Even a smal l system, such as a c a r engine, may w e l l j u s t i f y the
use of a magnetic p l u g t o mon i to r wear deb r i s , examinat ion o f a drop o f o i l f rom
the d i p s t i c k by t h e f i l t e r paper technique, o r v i s c o s i t y by the F l o s t i c k method.
The second f a c t o r i s t he need t o e s t a b l i s h f o r any system a c r i t e r i o n f o r
dec id ing when t o change the o i l . Th i s may be a l e v e l o f a c i d i t y i n the To ta l
Acid Number, a degree o f v i s c o s i t y change, o r the v i s i b l e appearance o f contam-
inants on a f i l t e r - p a p e r , bu t an o b j e c t i v e c r i t e r i o n o r c r i t e r i a w i l l be
essent ia l i f o i l change i s t o be determined by o i l q u a l i t y . I t i s a s a l u t a r y
thought t h a t the inc idence o f i n f l i g h t engine f a i l u r e s i n c e r t a i n Un i ted States
A i r Force a i r c r a f t decreased when no t o n l y r o u t i n e o i l change, bu t r o u t i n e
engine overhaul pe r iods were d i scon t inued i n favour o f schedu l ing i n accordance
w i t h spec t roscop ic o i l ana lys i s .
11.3 RECLAMATION AND RE-REFINING
Even when a l u b r i c a n t has d e t e r i o r a t e d so t h a t i t i s no longer f i t f o r
service, the g r e a t e r p a r t o f i t w i l l s t i l l be unchanged. Much o f t h e degradat ion
i s by contaminat ion , w h i l e a f u r t h e r f a c t o r i s d e p l e t i o n o f a d d i t i v e s . Only a
very small p r o p o r t i o n o f the base o i l w i l l u s u a l l y have been degraded, and t h i s
w i l l c o n s i s t o f t he most uns tab le molecules, u s u a l l y o x i d i s e d t o aldehydes,
ketones o r c a r b o x y l i c ac ids .
I t i s t h e o r e t i c a l l y p o s s i b l e t o remove a l l t h e contaminants and the degraded
add i t i ves and base o i l molecules, t o add f r e s h a d d i t i v e s , and thus t o produce
a l u b r i c a n t which d i f f e r s l i t t l e i f a t a l l f rom the o r i g i n a l . The processes
used a re descr ibed as rec lamat ion o r r e - r e f i n i n g , depending on the e x t e n t o f
t reatment invo lved.
I n the s imp les t case the use o f an i n - l i n e f i l t e r t o remove s o l i d contam-
inants i s a rec lamat ion technique. Another example i s t h a t o f a t rans former
o i l , which degrades i n s e r v i c e t o generate a smal l bu t unacceptable l e v e l o f
272
o f e l e c t r i c a l c o n d u c t i v i t y and can be c leaned up i n s i t u t o recover t h e re -
qu i red i n s u l a t i o n l e v e l . Such techniques a r e commonly known as " launder ing"
and have been w ide ly used f o r many years .
A we l l -es tab l i shed system f o r more i n t e n s i v e r e - r e f i n i n g i s the ac id -c lay
system, which b a s i c a l l y c o n s i s t s o f t he f i v e components shown i n F ig .1 . The
s t r a i n e r a t the i n l e t t o the waste o i l s to rage tank removes major s o l i d con-
taminants, w h i l e water and sludge a r e d ra ined o f f f rom the bottom.
a t i n g f u e l o r o the r v o l a t i l e m a t e r i a l s a r e removed i n the steam s t r i p p e r .
c l a r i f i e d o i l f rom the i n te rmed ia te s to rage i s t r e a t e d w i t h ho t s u l p h u r i c a c i d ,
which reac ts w i t h most o f the r e a c t i v e compounds present and removes them as
a c i d sludge.
remaining p o l a r compounds, and the s o l i d s a r e f i n a l l y removed i n a f i l t e r - p r e s s .
The cleaned o i l ob ta ined by t h i s process can be made s u i t a b l e f o r use as l ub -
r i c a n t base o i l , bu t i s o f t e n blended i n t o heavy f u e l o i l s .
Contamin-
The
Treatment w i t h heated a c t i v e "ear th" or "c lay" removes the
Strainer -
sludge
treatment
Clay and clean oil
F igure 1 Ac id -c lay r e - r e f i n i n g process
The main disadvantage o f t h i s process i s t he problem of d i spos ing o f l a r g e
An a l t e r n a t i v e process developed by the amounts o f a c i d and a c i d c l a y .
I n s t i t u t e Franca is du P e t r o l e uses l i q u i d propane p r e c i p i t a t i o n t o remove the
degraded m a t e r i a l s and r e s i d u a l a d d i t i v e s . Th is cons ide rab ly reduces t h e
273
generat ion of waste by-products, but a t present the process i s probably no t
economic . A Matthys process c o n s i s t s e s s e n t i a l l y o f two d i s t i l l a t i o n stages. The
f i r s t i s a t atmospheric pressure and 180°C and removes water and l i g h t hydro-
carbons. The second takes p lace a t 3 4 O o - 3 6 O 0 C and i s designed so t h a t the
undesirable m a t e r i a l s form a coke which i s c a r r i e d o f f i n the l i q u i d products
and removed by c e n t r i f u g i n g .
Most r e - r e f i n i n g processes requ i re a f i n i s h i n g stage t o produce f r a c t i o n s
f o r re-use as l u b r i c a n t s and t h i s may be c l a y t reatment o r a convent ional hydro-
f i n i s h i n g .
The recovery o f o i l f rom d i l u t e emulsions o r o i l y p l a n t waste-water i s
d i f f i c u l t , bu t i s important no t o n l y f o r o i l conservat ion bu t f o r e l i m i n a t i o n
o f p o l l u t i o n . The standard procedure i s t o crack emulsions by chemical t r e a t -
ment and a l l o w the product t o s e t t l e i n ho ld ing tanks u n t i l the o i l can be
skimmed o f f . The o i l f r a c t i o n then tends t o con ta in a h i g h p ropor t i on o f d i r t ,
water and the treatment chemical. I t i s l i k e l y t o be uneconomical f o r re -
r e f i n i n g and i s o f t e n blended i n t o burner f u e l .
In recent years, techniques have been developed which use polymer ic membranes
t o f i l t e r o u t p a r t i c l e s and d r o p l e t s f rom the water, e i t h e r by d i r e c t u l t r a -
f i l t r a t i o n o r by reverse osmosis. These techniques a r e repor ted t o g i ve a much
cleaner o i l f r a c t i o n as w e l l as a water f r a c t i o n s u i t a b l e f o r d ischarge t o
sewers.
One problem associated w i t h r e - r e f i n i n g o f engine o i l s i s t h a t t he re may be
a s l i g h t bu i l d -up o f po lynuclear aromat ics, which a re carc inogenic i n higher
concentrat ions. I t may the re fo re be des i rab le t o t r e a t r e - r e f i n e d o i l s by a
f i n i s h i n g process such as so lvent e x t r a c t i o n which w i l l remove polynuclear
a roma t i c s .
11.4 ECONOMICS
The economics o f o p t i m i s i n g o i l -change pe r iods depends main ly on t a i l o r i n g
the mon i to r i ng techniques t o the s i z e and importance o f the system. Systems
which a re e i t h e r very l a rge o r o f c r i t i c a l importance a re a l ready being mon-
i t o r e d by soph is t i ca ted techniques. Smaller and less c r i t i c a l systems may
already j u s t i f y s imple mon i to r i ng techniques, and as l u b r i c a n t a v a i l a b i l i t y
decreases, the balance w i l l i n e v i t a b l y s h i f t i n favour o f increased l e v e l s o f
monitor ing.
The economics of r e - r e f i n i n g a l s o depends on a v a i l a b i l i t y o f l u b r i c a t i n g o i l .
During the second World War l a rge q u a n t i t i e s o f l u b r i c a t i n g o i l s were re - re f i ned
i n most coun t r i es , b u t the i ndus t r y dec l i ned i n the f o l l o w i n g t h i r t y years.
The ex ten t o f the d e c l i n e v a r i e d i n d i f f e r e n t coun t r i es , bu t nowhere d i d i t
274
comple te ly cease. The reason f o r t he d e c l i n e may have been p a r t l y psycho log ica l ,
i n t h a t r e - r e f i n e d o i l s were cons idered t o be i n f e r i o r i n some way, bu t t he main
reason was probab ly f i n a n c i a l . The economics o f r e - r e f i n i n g d i d n o t pe rm i t more
than a token reduc t i on i n r e t a i l p r i c e s o f r e - r e f i n e d o i l s .
I t has been p o s s i b l e f o r many years t o produce r e - r e f i n e d o i l s o f s i m i l a r
q u a l i t y t o new o i l s , b u t i n a p e r i o d o f r e l a t i v e a f f l u e n c e and p l e n t y most
people have been happ ier t o pay the m a r g i n a l l y h igher p r i c e f o r "new" o i l .
The main economic problem has been the c o s t o f c o l l e c t i n g and t r a n s p o r t i n g
waste o i l , bu t a second f a c t o r has been the d i f f i c u l t y o f persuading opera to rs
t o keep wastes o f d i f f e r e n t q u a l i t i e s c lean arld separate.
I n the Federal Republ ic o f Germany a d e l i b e r a t e move was made t o reduce
l u b r i c a n t consumption and encourage r e - r e f i n i n g by the i n t r o d u c t i o n in 1968 o f
the "Law on Measures t o Ensure the Disposal o f Waste O i l " . Th is law enabled
o rgan isa t i ons d i spos ing o f waste o i l s by approved methods t o c l a i m an al lowance
t o cover cos ts i nvo l ved i n d i sposa l . The approved methods inc luded r e - r e f i n i n g .
The funds were r a i s e d by a spec ia l du ty on imported l u b r i c a n t grade o i l , and
t h i s had the e f f e c t o f i n t r o d u c i n g a p r i c e margin i n favour o f r e - r e f i n e d o i l s .
As a r e s u l t , i t was es t imated i n 1973 t h a t up t o 30% o f t o t a l l u b r i c a t i n g
o i l consumption was be ing r e - r e f i n e d i n Germany, compared w i t h o n l y 5% i n
B r i t a i n .
With d e c l i n i n g a v a i l a b i l i t y o f pe t ro leum l u b r i c a n t s , t he p r o p o r t i o n o f re -
r e f i n e d l u b r i c a n t s w i l l p robab ly approach more and more the t h e o r e t i c a l l i m i t
o f 60-70%.
275
12 HEALTH AND SAFETY ASPECTS OF LUBRICANTS
A.R. EYRES, M.A., M.Sc.
Environmental Heal th & Product Safety Advisor , Mobil Europe Inc.
12.1 INTRODUCTION
The m a j o r i t y o f minera l o i l based l u b r i c a n t s and greases a re r e l a t i v e l y harm-
less t o man. The i r use normal ly invo lves no unusual hazards prov ided t h a t
reasonable care i s taken t o avo id excessive s k i n con tac t o r i n h a l a t i o n o f m is t s
and vapours. A small number o f products may, because o f composi t ional requ i re -
ments t o meet p a r t i c u l a r t echn ica l performance needs, present a h igher degree of
hazard. Because l u b r i c a n t s a r e main ly composed o f organic chemicals which have
some solvency f o r n a t u r a l components o f the sk in , i t i s u n l i k e l y t h a t p r o v i s i o n
o f completely safe products cou ld be poss ib le . Syn the t i c l u b r i c a n t s genera l l y
are s i m i l a r i n hazard p o t e n t i a l t o minera l o i l products. I n order t o rev iew the
hazards o f l u b r i c a n t s , i t i s f i r s t o f a l l necessary t o look b r i e f l y a t t h e i r
composition.
12.2 COMPOSITION OF LUBRICANTS
Mineral o i l based l u b r i c a n t s a re prepared from base o i l s manufactured from
n a t u r a l l y o c c u r r i n g crude petroleum o i l s .
wor ld and t h e i r composi t ion v a r i e s accord ing t o source. In a d d i t i o n t o complex
mixtures o f p a r a f f i n i c , i s o p a r a f f i n i c , naphthenic ( c y c l o p a r a f f i n i c ) and aromat ic
hydrocarbons, some compounds o f su lphur , oxygen and n i t r o g e n w i l l be present
p lus t races o f a number o f metals. D i s t i l l a t i o n o f crude o i l , f o l l owed by
var ious o t h e r r e f i n i n g processes such as so l ven t e x t r a c t i o n , hydrogenat ion o r
ac id t reatment y i e l d s va r ious f r a c t i o n s i n the broad ca tegor ies shown i n Table
12.1.
Crude o i l s occur i n many p a r t s o f the
A l u b r i c a t i n g o i l f r a c t i o n t y p i c a l l y con ta ins several thousand i n d i v i d u a l
hydrocarbon compounds.
naphthenic depending on the predominant type o f hydrocarbon compound present .
This i s a f u n c t i o n o f the crude source. Crude o i l a l s o no rma l l y con ta ins po l y -
c y c l i c aromat ic hydrocarbons, some o f which (4 t o 6 r i n g compounds) a re known
t o be carc inogenic , eg. benz(a) pyrene.
solvent o r severe a c i d t reatment , remove most o f these p o l y c y c l i c aromat ic com-
pounds so t h a t very few a re present i n the f i n a l l u b r i c a n t base stock. However,
A base s tock i s u s u a l l y descr ibed as p a r a f f i n i c o r
Some r e f i n i n g processes, such as
276
Table 12.1
Mol ecu 1 a r Boi 1 i ng Range, Size " C
s o 0 - 100
20 - 200
c1 - c4
'4 - c12
Gases
L i g h t Naphtha
Gasol ine
Heavy Naphtha/Kerosine
Gas O i l s
'3 - '8
'9 - '16
'10- '26
100 - 270
170 - 400
L u b r i c a t i n g O i l s C,7 upwards > 300
Residuum CZ6 upwards
i t should be no ted t h a t the p o l y c y c l i c compounds a r e present i n the e x t r a c t e d
m a t e r i a l . Because o f t h e i r b o i l i n g p o i n t s , t he 4 t o 6 r i n g p o l y c y c l i c compounds
a r e n o t no rma l l y p resent i n d i s t i l l a t e f r a c t i o n s b o i l i n g below about 370°C.
To o b t a i n l u b r i c a n t performance c h a r a c t e r i s t i c s which cou ld n o t be prov ided
by the base o i l s themselves, va r ious a d d i t i v e s such as a n t i o x i d a n t s , de tergents /
d ispersants , e m u l s i f i e r s , b ioc ides , a n t i - c o r r o s i v e s and ant i -wear/extreme
pressure compounds a r e incorpora ted . P o t e n t i a l h e a l t h hazards o f such a d d i t i v e s
t h e r e f o r e a l s o need t o be considered.
For spec ia l a p p l i c a t i o n s , a v a r i e t y o f s y n t h e t i c l u b r i c a n t s have been
developed. These a r e based on s y n t h e t i c hydrocarbons and e s t e r s , p o l y g l y c o l s ,
s i l i c o n e s and phosphate e s t e r s .
m inera l o i l based l u b r i c a n t s a r e g e n e r a l l y incorpora ted .
A d d i t i v e s o f s i m i l a r types t o those used i n
12.3 MINERAL BASE OIL FACTORS
12.3.1 Acute T o x i c i t y
A l l types o f m ine ra l o i l base s tocks have a low o rde r o f acu te ( sho r t - t e rm)
t o x i c i t y .
m o r t a l i t y ) f o r t h e r a t a r e w e l l above 109 pe r kg of body we igh t . Ex t rapo la ted
t o man t h i s equates t o i nges t i on o f m r e than one l i t r e o f o i l t o cause death.
Low v i s c o s i t y p roduc ts such as ke ros ine present a somewhat g r e a t e r hazard
because o f the danger o f a s p i r a t i o n i n t o the lungs, f o l l o w e d by pneumonit is, i f
vomi t i ng occurs.
Acute o r a l LD5O'S ( t h e dose t o t e s t an ima ls r e s u l t i n g i n 50 pe r c e n t
Minera l o i l s a r e a l s o e s s e n t i a l l y non - tox i c by abso rp t i on th rough t h e sk in .
Dermal L O 5 0 ' s f o r t he r a b b i t a r e w e l l above log per kg body we igh t , a l e v e l
genera l l y cons idered as harmless.
277
12.3.2 Dermat i t i s
Dermat i t i s i s undoubtedly the major p o t e n t i a l h e a l t h problem w i t h minera l
o i l products, r e s u l t i n g f rom repeated o r prolonged s k i n con tac t and inadequate
sk in care. Primary i r r i t a t i o n and d e f a t t i n g o f the s k i n can occur t o va ry ing
extents , depending on the type o f product and the degree o f exposure. The
l i g h t e r petroleum o i l s w i t h f i n a l b o i l i n g p o i n t s below about 350°C, f o r example,
kerosine, tend t o be d i r e c t s k i n i r r i t a n t s . Since the chemical na tu re o f
mineral o i l ensures t h a t t he re w i l l be some solvency e f f e c t on the n a t u r a l f a t s
o f the s k i n i t i s u n l i k e l y t h a t p o t e n t i a l problems can be t o t a l l y e l im ina ted by
product development. However, adopt ion o f s imple measures t o prevent repeated
and prolonged con tac t , together w i t h good personal hygiene p r a c t i c e s and care o f
the sk in , can v i r t u a l l y e l i m i n a t e d e r m a t i t i s problems. Dermat i t i s f rom lub-
r i c a n t s i s an avoidable occupat ional d isease i n the vas t m a j o r i t y o f cases.
12.3.3 O i l M is t
Al though i t has o f t e n been suggested t h a t i n h a l a t i o n o f o i l m i s t over
extended pe r iods may lead t o an increased r i s k o f lung cancer, evidence from
animal t e s t i n g and exposed human popu la t i ons does n o t support t h i s . For example,
animal exposures t o w h i t e o i l m i s t a t 5 and 100 mg/m3 showed no i n d i c a t i o n s o f
increased lung cancer r i s k . I n f u r t h e r work, no i n j u r y o r i n d i s p o s i t i o n from
inha la t i on o f o i l m i s t was observed amongst animals exposed f o r 18 months t o
m is t f rom a su lphur ised so l ven t -ex t rac ted naphthenic base o i l a t 50 mg per
cubic meter [1,2,3]. Decouf le o f the U.S. Nat ional Cancer I n s t i t u t e [4,5] has
publ ished two ep idemio log i ca l s tud ies o f the cancer m o r t a l i t y o f workers exposed
t o c u t t i n g o i l m is t s . The r e s u l t s i nd i ca ted t h a t exposure t o o i l m i s t s does
not pose a hazard i n terms o f r e s p i r a t o r y cancer and f a t a l non-malignant res-
p i r a t o r y disease, b u t may be associated w i t h a s l i g h t increase i n cancer o f the
g a s t r o i n t e s t i n a l system. A study o f workers i n metal machining p l a n t s i n
Germany by Oraschs [6 ] showed no adverse r e s p i r a t o r y e f f e c t s among 443 employees
i n 17 f a c t o r i e s .
been exposed t o h i g h o i l m i s t l e v e l s i n the range 40 t o 80 mg/m3. Comparison
of smokers and non-smokers i n exposed and non-exposed popu la t i ons suggested i n
fac t t h a t i n h a l a t i o n o f o i l m i s t may p rov ide some p r o t e c t i v e e f f e c t aga ins t the
harmful e f f e c t s o f smoking on the b ronch ia l system.
About 63 per cen t o f these employees were repo r ted t o have
The c u r r e n t Threshold L i m i t Value ( the atmospheric concen t ra t i on t o which i t
i s be l i eved most workers can be exposed f o r 8 hours d a i l y w i t h o u t adverse
e f f e c t s on hea l th ) publ ished by the American Conference o f Gernmental Ind-
u s t r i a l Hyg ien i s t s i s 5 mg/m3. This has been se t on the bas i s o f prevent ing
nuisance and unpleasantness t o workers r a t h e r than on h e a l t h e f f e c t aspects and
i t i s e s s e n t i a l l y an index o f good i n d u s t r i a l hygiene p r a c t i c e s . I t i s be l i eved
t h a t the 5 mg/m3 l i m i t p rov ides a s a f e t y f a c t o r o f a t l e a s t ten aga ins t even
r e l a t i v e l y minor changes i n t h e lungs.
As w i t h many chemicals, i n h a l a t i o n o f very h i g h concen t ra t i ons o f o i l m i s t s
( o r vapours o f more v o l a t i l e p roduc ts such as ke ros ine ) may cause i r r i t a t i o n o f
the lungs and may lead t o a chemical pneumonia.
12.3.4 O i l Vapours
A t normal temperatures, t y p i c a l l u b r i c a t i n g o i l s do n o t produce any s i g n i f -
i c a n t l e v e l s o f vapour i n the working atmosphere. Satura ted vapour concent-
r a t i o n s f o r a t y p i c a l l u b r i c a t i n g o i l have been c a l c u l a t e d by Sanderson [7]
t o be 0.016 ppm a t 20°C and 1.4 ppm a t 100°C - these a r e so low t h a t any
adverse h e a l t h e f f e c t s w i l l n o t occur. L i g h t e r p roduc ts such as d i e s e l o i l and
ke ros ine do have the p o t e n t i a l f o r evapora t ion o f l i g h t ends t o produce s i g n i f -
i c a n t vapour concen t ra t i ons . For example, the sa tu ra ted vapour concen t ra t i on
f o r keros ine a t 20°C i s about 2000 ppm. These l i g h t e r p roduc ts may produce
i r r i t a t i o n o f mucous membranes. I n metal machining opera t i ons , s i g n i f i c a n t
v a p o r i s a t i o n o f l u b r i c a t i n g o i l s may occur a t t he h i g h too l /workp iece temper-
a tu res , b u t on c o o l i n g i n the sur round ing atmosphere, t h i s vapour w i l l be con-
densed t o d r o p l e t s o f o i l m i s t .
12.3.5 Skin Cancer
I t has been known f o r many years t h a t some types o f minera l o i l can cause
s k i n cancer w i t h repeated and pro longed exposure over l ong pe r iods o f years. I t
should be no ted however t h a t s k i n cancer i s no rma l l y l e s s se r ious than o t h e r
forms o f cancer and i s u s u a l l y cu rab le w i t h e a r l y t rea tment .
I n 1922, L e i t c h [ 8 ] repo r ted t h a t S c o t t i s h sha le o i l d i s t i l l a t e s caused
cancer o f t he s k i n when p a i n t e d on t o animals. I t i s n o t p o s s i b l e t o rev iew a l l
o f the ex tens i ve s tud ies c a r r i e d o u t s ince t h a t t ime, b u t among the most
impor tan t were those repo r ted i n 1966 by Bingham and Horton 191, sponsored by
the American Petroleum I n s t i t u t e . I t was shown t h a t base s tocks prepared by
so l ven t r e f i n i n g , which removes p o l y c y c l i c a romat ic hydrocarbons (PCAH), d i d n o t
cause tumours i n mice whereas t y p i c a l a c i d r e f i n e d base s tocks d i d . I n 1968,
the UK Medical Research Counci l pub l i shed a r e p o r t "The Carcinogenic Ac t i on o f
Minera l O i l s : A Chemical and B i o l o g i c a l Study" [lo]. The s a l i e n t conc lus ions
from t h i s r e p o r t and o t h e r s tud ies can be summarised as f o l l o w s :
- Some f r a c t i o n s o f c e r t a i n crudes f rom which l u b r i c a t i n g o i l s a r e r e f i n e d
have been shown t o produce tumours on s k i n s o f r a b b i t s o r mice.
* So lvent e x t r a c t i o n methods o f r e f i n i n g which remove aromat ic compounds
markedly reduce the ca rc inogen ic a c t i v i t y o f the r e f i n e d l u b r i c a n t s .
279
. Carc inogen ic i t y o f m inera l o i l s appears t o be r e l a t e d t o the presence o f
PCAH's, some o f which a r e known t o be carcinogens.
I t has no t been p o s s i b l e t o d e f i n e any simple a n a l y t i c a l parameters
which c o r r e l a t e w i t h ca rc inogen ic a c t i v i t y .
*
An Ad-hoc Committee o f the UK I n s t i t u t e o f Petroleum s tud ied a l l a v a i l a b l e
evidence and adv ised member companies i n 1968 t h a t o i l s which have been so l ven t
re f i ned o r t r e a t e d i n o t h e r ways t o app rec iab l y reduce the con ten t o f p o l y c y c l i c
aromat ic compounds, were l e s s l i k e l y t o promote s k i n cancer
no t been thus r e f i n e d .
The d i f f i c u l t y o f d e f i n i n g a n a l y t i c a l parameters t o c o r r e
genic a c t i v i t y i s i l l u s t r a t e d by r e s u l t s repor ted by Scala [
12.2.
han o i l s which had
a t e w i t h ca rc ino -
11, shown i n Table
O I L
Table 12.2
A B C
Pyrene, ppm 3.5 18.3 4.4
Benz (a ) anthracene, ppm 6.6 7.9 2.7
Benz (a) pyrene, ppm 4.5 1.2 0.2
% C A (carbon i n a romat ic r i n g s ) 15.4 15.2 12.4
Cancer a c t i v i t y + +
The reason f o r t h i s i s t he unpred ic tab le e f f e c t s o f cocarcinogens, i n h i b i t o r s
and acce le ra to rs . For example, i t has been shown t h a t su lphur and some organo-
sulphur compounds can increase c a r c i n o g e n i c i t y , as a l s o can c e r t a i n types o f
hydrocarbons.
So f a r , desp i te ex tens i ve research, no e n t i r e l y s a t i s f a c t o r y method f o r
eva lua t i ng the p o t e n t i a l c a r c i n o g e n i c i t y o f an o i l has been devised. The bes t
method to da te i s the l ong term p a i n t i n g o f mice sk ins . Such t e s t s take two
years t o complete and r e q u i r e h i g h standards o f exper imental techniques; they
are t h e r e f o r e very c o s t l y . There i s a l s o the d i f f i c u l t y o f t r a n s l a t i n g r e s u l t s
o f animal t e s t s t o the human exposure s i t u a t i o n . I t i s t h e r e f o r e impossible t o
t e s t each i n d i v i d u a l f o rmu la t i on . However, a l a rge number o f t e s t s have been
done and these fo rm the b a s i s f o r c u r r e n t recommendations t o min imise hazards.
Cases o f occupat iona l sc ro ta1 cancer have been repo r ted f rom many c o u n t r i e s
i nc lud ing France, Sweden, the UK and the USA. The inc idence i n the UK i s
equ iva len t t o about 5 cases per m i l l i o n males per year whereas i n Sweden i t i s
lower, a t about 1 case pe r m i l l i o n males per year. Wahlberg [ 1 2 ] has repor ted
t h a t o n l y 21 pe r cen t o f Swedish cases had had d e f i n i t e exposure t o minera l o i l .
280
This con t ras ts markedly w i t h a f i g u r e o f 86 per cen t repo r ted f o r UK cases.
No obvious reason f o r t h i s d i f f e r e n c e has been i d e n t i f i e d , b u t f a c t o r s such as
p l a n t and personal hygiene may w e l l be s i g n i f i c a n t . As these a r e improved,
incidence o f scrota1 cancer can be expected t o decrease even tua l l y . The long
l a t e n t pe r iod f rom f i r s t exposure t o d iagnosis o f the cancer (10 t o 43 years)
means t h a t any changes i n inc idence cannot be observed q u i c k l y .
12.3.6 Eye I r r i t a t i o n
I n common w i t h a m u l t i t u d e o f commonly used m a t e r i a l s , eg. soapy water, many
l u b r i c a n t s may cause some i r r i t a t i o n i f splashes e n t e r the eye. Wi th the
m a j o r i t y o f products t h i s w i l l n o t be more than very s l i g h t , b u t some such as
neat so lub le o i l s , may, because they con ta in app rec iab le amounts o f sur face
a c t i v e m a t e r i a l s such as soaps, be somewhat more i r r i t a t i n g .
12.4 ADDITIVE FACTORS
Before use i n l u b r i c a n t s , a d d i t i v e s a re screened f o r t o x i c i t y and s k i n o r
eye i r r i t a n c y . This i s e s s e n t i a l t o assess p o t e n t i a l hazards and determine any
requ i red hand l i ng precaut ions du r ing b lend ing o f the f i n i s h e d l u b r i c a n t . If i t
appears l i k e l y t h a t an a d d i t i v e may lead t o any increased hazard i n the blended
l u b r i c a n t , f u r t h e r t e s t i n g may be done t o d e f i n e t h i s . The e x t e n t o f poss ib le
increased hazard i n a f i n i s h e d f o r m u l a t i o n must be assessed t o decide whether o r
no t the a d d i t i v e should be re jec ted .
I n the m a j o r i t y o f products, a d d i t i v e s a r e minor i ng red ien ts and the p o t e n t i a l
hazards a r e e s s e n t i a l l y those associated w i t h the base minera l o i l .
a d d i t i v e con ten ts a r e h ighe r , eg. i n neat so lub le o i l s and in some engine l ub -
r i c a n t s , the f i n a l product may be more i r r i t a t i n g than the base minera l o i l .
I f use o f these a d d i t i v e s i s e s s e n t i a l f o r t echn ica l performance reasons, t he
hazards must be c o n t r o l l e d by the implementat ion o f adequate hand l i ng and use
precaut ions.
Where
A wide range of a d d i t i v e s i s used to achieve improvements i n l u b r i c a n t per-
formance. For each a d d i t i v e type, va r ious chemical compounds have been found
t o be e f f e c t i v e .
p o t e n t i a l hazards. Among those which have been the cause o f some concern a r e
lead compounds, o r t h o isomers o f phosphate es te rs , c h l o r i n a t e d naphthalenes,
sodium n i t r i t e i n combinat ion w i t h amines, sodium mercaptobenzothiazole and
t r i c h l o r o e t h y l e n e .
Very few o f these have been found t o present any s i g n i f i c a n t
12.4.1 Lead Compounds
One o f the e a r l y e f f e c t i v e methods of improving ant iwear and extreme
pressure p r o p e r t i e s o f a l u b r i c a n t was t o i nco rpo ra te lead soap. Al though the re
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are no repor ted cases o f s u f f i c i e n t lead abso rp t i on t o cause adverse h e a l t h
e f f e c t s , Van Peteghem and Vos [13] repo r ted s l i g h t increases i n b lood lead
l e v e l s i n s t e e l m i l l employees w i t h f requent o r prolonged s k i n con tac t w i t h t h i s
type o f l u b r i c a n t . New a d d i t i v e technology has enabled t h i s type o f f o rmu la t i on
t o be l a r g e l y rep laced over the l a s t few years by unleaded l u b r i c a n t s .
12.4.2 Ortho Phosphates
Absorpt ion o f o r t h o t r i c r e s y l phosphate has been shown t o cause c e n t r a l
nervous system damage lead ing t o neurornuscular problems and va r ious stages o f
pa ra l ys i s . The para isomer does n o t have t h i s e f f e c t and i s e s s e n t i a l l y i n e r t
p rov id ing the content o f o r t h o isomer i s a t a ve ry low l e v e l . !Suppl iers o f
t r i c r e s y l phosphates have f o r many years r e s t r i c t e d the o r t h o content t o l ess
than 1 per cent i n o rde r t o avo id the p o s s i b i l i t y o f c e n t r a l nervous system
e f f e c t s .
12.4.3 Ch lo r ina ted Naphthalenes
Ch lo r ina ted naphthalenes were used f o r a sho r t p e r i o d many years ago as
e f f e c t i v e extreme pressure a d d i t i v e s i n c u t t i n g o i l s .
t i nued when an a s s o c i a t i o n w i t h ch loracne o f the s k i n was found.
c h l o r i n a t e d a d d i t i v e now used, eg. c h l o r i n a t e d p a r a f f i n s , do n o t cause t h i s
e f f e c t .
The i r use was discon-
The types o f
12.4.4 Sodium N i t r i t e and Amines
Sodium n i t r i t e i n combinat ion w i t h t r i and diethanolamines has been used f o r
many years t o p rov ide s a t i s f a c t o r y a n t i c o r r o s i o n p r o p e r t i e s i n aqueous g r i n d i n g
f l u i d s , and, a t lower concentrat ions, i n some so lub le c u t t i n g o i l s . Recent ly,
small amounts o f n i t rosamines, a type o f chemicalof which some a re known t o be
carc inogenic , have been found i n bo th concentrates and d i l u t e d ve rs ions o f
such products [14,15]. Ni t rosamines a r e a l s o found i n many foods, d r i n k s and
cosmetics, a r e present i n the atmosphere o f c i t y s t r e e t s and a r e a l s o formed
w i t h i n the body i t s e l f . Assessment o f any increased hazard f rom g r i n d i n g f l u i d s
i s d i f f i c u l t because o f the problems o f es t ima t ing exposure and e x t e n t o f
absorpt ion i n t o the body. Taking the worst poss ib le case, i t appears t h a t
absorpt ion cou ld approach t h a t f rom foods, bu t t y p i c a l l y i t i s l i k e l y t o be very
much less than t h i s . As a p recau t ion however, supp l i e rs have e l im ina ted the
combinat ion o f n i t r i t e and amine except i n some c r i t i c a l a p p l i c a t i o n s where i t
has no t y e t been p o s s i b l e t o m e t the techn ica l requirements w i t h a l t e r n a t i v e
formulat ions.
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12.4.5 Sodium Mercaptobenzothiazole
Th is compound was used a t one t ime as a very e f f e c t i v e c o r r o s i o n i n h i b i t o r
i n aqueous l u b r i c a n t s . I t was l a t e r i d e n t i f i e d as a s t rong s k i n s e n s i t i z e r and
i s no longer used i n a p p l i c a t i o n s where s k i n con tac t i s l i k e l y .
12.4.6 T r i c h l o r o e t h y l e n e
T r i ch lo roe thy lene was used w ide ly as a non-flammable d i l u e n t i n open gear
l u b r i c a n t s t o enab le a h i g h l y v iscous l u b r i c a n t f i l m t o be e a s i l y a p p l i e d t o
gear t e e t h sur faces .
weighted average TLV o f 100 ppm may be hazardous, caus ing depression o f t he
c e n t r a l nervous system w i t h v i s u a l d i s tu rbances and l a c k o f co -o rd ina t i on , p l u s
the p o s s i b i l i t y o f damage t o the l i v e r and kidneys. A d d i c t i o n t o s n i f f i n g
t r i c h l o r o e t h y l e n e vapours has a l s o occur red . Such exposures on a c o n t i n u i n g
b a s i s a r e u n l i k e l y t o occur except perhaps i n con f ined and p o o r l y v e n t i l a t e d
spaces. However, t o p rov ide a g r e a t e r margin o f sa fe ty , t r i c h l o r o e t h y l e n e can
be s u b s t i t u t e d w i t h a s l i g h t l y more expensive sa fe r a l t e r n a t i v e , 1.1.1 t r i -
ch lo roe thane, which has a TLV o f 350 ppm.
Exposure t o t r i c h l o r o e t h y l e n e vapour above the t ime
I t has a l s o been i n d i c a t e d t h a t t r i c h l o r o e t h y l e n e i s a carc inogen i n animal
t e s t s . However, t he doses g i ven by i n g e s t i o n i n t o the stomach were so massive
compared t o p o s s i b l e human exposure t h a t t he s i g n i f i c a n c e o f these t e s t r e s u l t s
can be s e r i o u s l y quest ioned.
12.5 BACTERIA AND BlOClDES
Water based l u b r i c a n t s and minera l o i l l u b r i c a n t s contaminated w i t h water , eg.
marine eng ine o i l s , can suppor t the growth o f b a c t e r i a , yeas ts and f u n g i . Growth
does n o t no rma l l y occur i n p roduc ts which do n o t c o n t a i n water. As supp l i ed t o
users, p roduc ts a r e no rma l l y f r e e o f b a c t e r i a , bu t con taminat ion occurs f rom a
number o f p o s s i b l e sources such as water f rom engine c o o l i n g systems, the water
used as d i l u e n t , r e s i d u a l b a c t e r i a i n p l a n t c i r c u l a t i o n systems, re fuse such as
c i g a r e t t e s thrown i n t o the coo lan t , employees s p i t t i n g i n t o the produc t o r even
f rom b a c t e r i a l con taminat ion o f t he a i r i n the p l a n t .
The b a c t e r i a , yeas ts o r f ung i which grow i n aqueous coo lan ts o r l u b r i c a n t s
Al though concern contaminated w i t h water a r e no t no rma l l y harmful t o humans.
has been expressed t h a t b a c t e r i a l con taminat ion may lead t o increased r e s p i r a t o r y
o r s k i n i n f e c t i o n s , i n d u s t r i a l medical adv i se rs respons ib le f o r l a r g e metal
machining p l a n t s have repo r ted t h a t they can f i n d no evidence f o r t h i s . The
types o f b a c t e r i a found a r e almost i n v a r i a b l y the non-pathogenic type which a r e
harmless t o humans a l though ve ry o c c a s i o n a l l y a p o t e n t i a l l y harmfu l pathogenic
type may be i d e n t i f i e d .
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For techn ica l reasons, i t i s des i rab le t o c o n t r o l b a c t e r i a l growth. With
aqueous coolants , t h i s i s achieved e i t h e r by i n c l u s i o n o f small amounts o f b i o -
c ides i n the o r i g i n a l product o r by a d d i t i o n o f b ioc ides du r ing use. By t h e i r
nature, b ioc ides a re moderately t o h i g h l y t o x i c by i nges t i on and may be s k i n o r
eye i r r i t a n t s i n the concentrated form. Therefore, t h e i r use must be c a r e f u l l y
c o n t r o l l e d t o avo id increased h e a l t h hazards. I n the concen t ra t i ons no ima l l y
used, and prov ided approp r ia te hand l i ng precaut ions a r e observed, b ioc ides
should present no hazard t o hea l th . However, use o f excessive concentrat ions i n
"topping-up" may cause s k i n i r r i t a t i o n . With marine engine o i l s , b a c t e r i a l
problems can be c o n t r o l l e d by c o r r e c t o i l s e l e c t i o n and approp r ia te ope ra t i ng
procedures [ 1 6 ] .
12.6 SYNTHETIC LUBRICANTS
A v a r i e t y o f chemical types a re used as s y n t h e t i c l u b r i c a n t s t o meet oper-
a t i o n a l requirements which cannot be s a t i s f i e d adequately w i t h minera l o i l
products. Types i nc lude va r ious e s t e r s o f organic f a t t y ac ids , s i l i c o n e s , syn-
t h e t i c hydrocarbons such as p o l y o l e f i n e s , p o l y g l y c o l s and phosphate es te rs .
There a re no unusual h e a l t h hazards associated w i t h these. D e f a t t i n g o f the
sk in , s i m i l a r t o t h a t w i t h minera l o i l , i s poss ib le i n most cases i f repeated o r
prolonged con tac t occurs. I n the case o f phosphate es te rs , the use o f the
o r t h o isomer should be avoided as i nd i ca ted i n 1 2 . 4 . 2 .
12 .7 USED AND RECLAIMED OR RE-REFINED OILS
There i s s t rong evidence t h a t PCAH con ten t o f minera l o i l based l u b r i c a n t s
increases d u r i n g use [ 1 7 ] . The ex ten t o f t he increase appears t o depend on the
type o f a p p l i c a t i o n , be ing up t o about t e n - f o l d f o r c u t t i n g o i l s and d i e s e l
engine o i l s , bu t perhaps one hundred- fo ld or more f o r gasol ine engine o i l s and
quenching o i l s . Non-engine i n d u s t r i a l l u b r i c a n t s such as hyd rau l i c , gear and
bear ing o i l s would n o t be expected t o show any s i g n i f i c a n t PCAH increase du r ing
use because o f t he l i m i t e d temperature increases t o which they a re subjected.
Much o f t he increase i n engine o i l s appears t o a r i s e f rom gaso l i ne combustion
products . The s i g n i f i c a n c e o f these increases i n PCAH content i n r e l a t i o n t o any
increased s k i n cancer r i s k i s n o t c l e a r a t present f o r the reasons discussed i n
1 2 . 3 . 5 . I n the case o f c u t t i n g o i l s , c a l c u l a t i o n s based on o i l m i s t concent-
r a t i o n s a t the TLV o f 5 mg/m3 i n d i c a t e t h a t PCAH l e v e l s w i l l be o f the same order
o f magnitude as background atmospheric l e v e l s . Provided o i l m i s t l e v e l s a r e
mainta ined below the TLV, i t appears the re should be no s i g n i f i c a n t increase i n
r i s k .
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For environmental conservat ion reasons, t he re a r e a t t r a c t i o n s i n rec la im ing
o r r e - r e f i n i n g used l u b r i c a n t s f o r f u r t h e r use. I n s p e c i f i c s i t u a t i o n s the re
may a l s o be economic j u s t i f i c a t i o n f o r reclamat ion. A v a r i e t y o f processes may
be employed from simple c e n t r i f u g i n g and e a r t h f i l t r a t i o n t o a c i d t reatment ,
r e d i s t i l l a t i o n and so l ven t e x t r a c t i o n . U n t i l f u r t h e r i n fo rma t ion i s a v a i l a b l e
on the p o t e n t i a l hazards, i t i s considered i t would be prudent t o l i m i t use o f
such o i l s t o a p p l i c a t i o n s i n which the re i s l i t t l e s k i n con tac t un less i t i s
c e r t a i n t h a t the o i l has o n l y been used where PCAH increase i s u n l i k e l y , o r has
been t r e a t e d by a process which w i l l remove PCAH, eg. so l ven t e x t r a c t i o n o f
aroma t i cs . Used gaso l i ne engine o i l s can c o n t a i n up to about 1 per cen t of l ead [ 1 8 ] .
This o r i g i n a t e s main ly f rom lead a d d i t i v e s i n gaso l i ne w i t h perhaps a minor
c o n t r i b u t i o n f rom wear o f engine p a r t s .
these o i l s may r e s u l t i n some increased abso rp t i on o f lead i n t o the body.
Repeated o r prolonged s k i n con tac t w i t h
Used c u t t i n g o i l s u s u a l l y con ta in smal l metal ch ips o r swarf which present an
a d d i t i o n a l hazard t o the sk in . Many o f these metal p a r t i c l e s a re need le - l i ke i n
shape and can cause m ic ro - les ions of t h e sk in , l ead ing t o a general i r r i t a t i o n .
I t i s a l s o be l i eved t h a t e n t r y o f swarf i n t o the s k i n may dest roy an e l e c t r o -
negat ive b a r r i e r beneath the sur face over about one square inch around the s i t e ,
thus a l l o w i n g o t h e r m a t e r i a l s such as the c u t t i n g f l u i d t o pene t ra te i n t o the
sk in .
12.8 HEALTH AND SAFETY PRECAUTIONS
12.8.1 S u p p l i e r ' s R e s p o n s i b i l i t i e s
Sect ion 6 o f the Heal th and Safety a t Mork Act e t c . 1974 places r e s p o n s i b i l -
i t i e s on supp l i e rs " t o ensure so f a r as i s reasonably p r a c t i c a b l e , t h a t the sub-
stance i s safe and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used" and t o make
a v a i l a b l e i n fo rma t ion on any re levan t t e s t s and "about any c o n d i t i o n s necessary
t o ensure t h a t i t w i l l be sa fe and w i t h o u t r i s k s t o h e a l t h when p r o p e r l y used."
Suppl iers a re a l s o requ i red t o e l i m i n a t e o r minimise r i s k s t o h e a l t h and sa fe ty ,
as f a r as i s reasonably p r a c t i c a b l e . Suppl iers cannot be expected t o produce
complete ly safe products ; indeed, i t can be s a i d t h a t t he re i s no such t h i n g - the re a r e o n l y safe ways o f us ing a product. Even pure water can be harmful i f
one d r i n k s too much o f i t and many people w i l l s u f f e r s k i n problems i f t h e i r
hands a r e immersed i n water f o r several hours d a i l y .
For many years, reputable l u b r i c a n t s u p p l i e r s have been assessing the
p o t e n t i a l hazards o f products. O f p a r t i c u l a r importance i s the assessment o f
t o x i c i t y o f poss ib le a d d i t i v e s and r e j e c t i o n o f those which may lead t o s i g n i f -
i c a n t l y increased r i s k . As i nd i ca ted i n Sect ion 12.4, a number o f a d d i t i v e s
which may present hazards under some c o n d i t i o n s o f use have been rep laced i n
recent years by a l t e r n a t i v e sa fe r m a t e r i a l s . Care must always be exerc ised
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however, t o ensure t h a t a ma te r ia l w i t h a known r i s k i s n o t rep laced by a new
mater ia l w i t h unknown r i s k which may i n f a c t be much g rea te r .
The Heal th and Safety a t Work Act does n o t r e q u i r e supp l i e rs t o d i s c l o s e
d e t a i l s o f product composi t ions t o users.
broad composi t ional i n fo rma t ion g e n e r a l l y a v a i l a b l e , o r even d e t a i l e d inform-
a t i o n on a c o n f i d e n t i a l bas i s t o a user h e a l t h p ro fess iona l where necessary,
formulat ions a r e p r o p r i e t a r y i n fo rma t ion t o the i n d i v i d u a l manufacturer i n a
compet i t ive business wor ld. Considerable research expendi ture may have been
incurred i n developing new a d d i t i v e s and i n s e l e c t i n g the best combinat ion o f
a d d i t i v e s t o p rov ide a performance b e n e f i t i n a p a r t i c u l a r a p p l i c a t i o n . Sup-
p l i e r s a r e n a t u r a l l y r e l u c t a n t t o increase the p o s s i b i l i t y t h a t t h i s inform-
a t i o n may pass t o t h e i r compet i tors . In any case, such in fo rma t ion i s u s u a l l y
o f l i t t l e value t o the user i n assessing p o t e n t i a l hazards. O f much g rea te r
value i s the t o x i c o l o g i c a l i n fo rma t ion and the recommended hand l i ng precaut ions
based on t h i s i n fo rma t ion .
Wh i l s t most s u p p l i e r s w i l l make
Most s u p p l i e r s a l s o p rov ide users w i t h book le ts o r l e a f l e t s recommending safe
handl ing precaut ions and rev iewing p o t e n t i a l h e a l t h hazards. Typ ica l recom-
mendations, together w i t h p r a c t i c e s o f l a rge user companies, have been reviewed
by the I n s t i t u t e o f Petroleum and incorporated i n a Code o f P rac t i ce f o r Metal-
working F lu ids , publ ished i n J u l y 1978 1191.
12.8.2 Skin P r o t e c t i o n
The bas i c requirement f o r avo id ing s k i n problems i s t o minimise con tac t .
With the m a j o r i t y of l u b r i c a n t s , occasional s k i n con tac t f o r sho r t pe r iods w i l l
cause no problems.
o r repeated s k i n con tac t w i l l be s u f f i c i e n t t o prevent d e r m a t i t i s and s k i n
i r r i t a t i o n problems. The precaut ions needed t o avo id d e r m a t i t i s and s k i n i r r i -
t a t i o n w i l l a l s o prevent s k i n cancer. Modern so l ven t r e f i n e d types o f minera l
o i l s ( o r equ iva len t ) a l s o minimise the r i s k o f s k i n cancer.
i r r i t a n t types o f product , such as keros ine and neat so lub le o i l s , occasional
very sho r t con tac t i s u n l i k e l y t o cause problems, b u t i f any apprec iab le con tac t
i s l i k e l y , s u i t a b l e p r o t e c t i v e measures should be employed.
Compliance w i t h the normal recommendation t o avo id prolonged
With the more
Contact can be minimised by us ing s u i t a b l e p r o t e c t i v e gloves and c l o t h i n g ,
b a r r i e r creams, and the proper i n s t a l l a t i o n and use o f splash guards on c u t t i n g
machines. P r o t e c t i v e c l o t h i n g which becomes contaminated w i t h o i l should be
changed f r e q u e n t l y and cleaned by any launder ing process (d ry o r wet, o r a
combinat ion o f both) which produces v i s u a l l y c lean garments.
comes g r o s s l y contaminated, eg. by spray o r s p i l l a g e , i t should be changed
immediately.
I f c l o t h i n g be-
Special aprons a r e a v a i l a b l e , c o n s i s t i n g o f an impervious back w i t h a
detachable absorbent f r o n t which can be e a s i l y removed f o r c lean ing . The use o f
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t h i s type o f p r o t e c t i o n by t o o l s e t t e r s , who a re p a r t i c u l a r l y l i k e l y t o be
exposed to heavy contaminat ion i n l ean ing over o i l y machines, i s s t r o n g l y
recommended. Use o f t h i s type o f apron removes the temptat ion t o stow o i l y rags
o r t o o l s i n t rouse r pockets, a p r a c t i c e which c o u l d r e s u l t i n the s k i n i n the
g r o i n area being i n prolonged con tac t w i t h o i l - soaked c l o t h i n g .
c l o t h i n g should be sho r t o r r o l l e d up t o avo id c o n t i n u a l f r i c t i o n between o i l -
soaked c u f f s and the s k i n o f forearms and w r i s t s . The golden r u l e i s - "Do n o t
wear o i l -soaked c lo th ing " . I t should be remembered t h a t t h i s a p p l i e s a l s o * t o
underc lo th ing which may become contaminated from o i l - soaked o v e r a l l s . Under-
c l o t h e s should a l s o be changed f requen t l y . To h e l p minimise c l o t h i n g contam-
i na t i on , separate l ocke r f a c i l i t i e s f o r work and s t r e e t c l o t h e s a r e d e s i r a b l e
i n changing rooms.
Sleeves o f
Employees who come i n t o con tac t w i t h o i l should wash exposed s k i n a t the end
o f any work per iod, us ing warm water and soap, m i l d detergent o r p r o p r i e t a r y
s k i n c leanser . Strong soaps and detergents and ab ras i ve type soaps o r c leansers
should be avoided. Kerosine, p e t r o l and o t h e r degreasing so l ven ts should n o t be
used f o r c lean ing the sk in . Hands should a l s o be washed be fo re e a t i n g , d r i n k i n g
or smoking and be fo re and a f t e r us ing the l ava to ry .
and t o i l e t f a c i l i t i e s should be a v a i l a b l e and should be w e l l maintained.
I d e a l l y , employees should shower a t the end of the work s h i f t t o remove a l l
t r aces o f o i l f rom the sk in . I f rags a re used t o wipe o i l f rom the s k i n o r
machinery, a p l e n t i f u l supply should be a v a i l a b l e so t h a t they can be changed
f requent ly .
i b i l i t y o f accumulat ion o f metal ch ips and swarf which may c u t o r sc ra tch the
sk in .
E a s i l y access ib le washing
P re fe rab ly they should be o f a d isposable type t o avo id the poss-
B a r r i e r creams a re o f t e n used on the assumption t h a t they p r o t e c t the s k i n
f rom d i r e c t con tac t w i t h o i l o r coo lan t . Al though t h e i r e f f e c t i v e n e s s i s some-
what unce r ta in , they do have a p a r t t o p l a y i n m in im is ing d e r m a t i t i s problems
by increas ing awareness o f the need f o r care o f the sk in . They can a l s o make
eventual washing o f the s k i n more e f f e c t i v e , p a r t i c u l a r l y where " d i r t y " work i s
involved. Use o f creams f rom reputable supp l i e rs i s recommended s ince long
exper ience has enabled them t o avo id use o f components which may harm the sk in .
I t i s important t o use the c o r r e c t type o f cream f o r the type o f o i l invo lved
s ince a water r e s i s t a n t type intended fo r use w i t h aqueous coo lan ts w i l l no t
g i v e e f f e c t i v e p r o t e c t i o n aga ins t minera l o i l based l u b r i c a n t s .
Use o f a s k i n r e c o n d i t i o n i n g cream a f t e r work i s a l s o impor tant t o he lp
rep lace the n a t u r a l f a t s and o i l s removed from the s k i n by exposure t o l ub -
r i c a n t s and by washing. This i s a very important p a r t o f a s k i n care programme
t o avo id d e r m a t i t i s problems. I t i s p a r t i c u l a r l y important w i t h o l d e r em-
ployees who tend t o have d r i e r sk ins , and i n the w i n t e r when low temperatures
281
and humid i t y also tend t o cause dryness and c rack ing o f t he sk in . Male
employees a r e o f t e n r e l u c t a n t t o use a s k i n cream because o f t h e i r a s s o c i a t i o n
w i t h the " s o f t femin ine touch" o f consumer a d v e r t i s i n g . There i s no doubt t h a t
the common a s s o c i a t i o n o f m a s c u l i n i t y w i t h tough and rough hands which do n o t
need s k i n creams has c o n t r i b u t e d t o numerous cases o f occupat iona l s k i n disease.
E f f o r t s to persuade more men t o use r e c o n d i t i o n i n g creams r e g u l a r l y would un-
doubtedly be w e l l rewarded i n reduced absence from work, o r need t o t r a n s f e r t o
o the r j obs , because o f d e r m a t i t i s .
b a r r i e r cream suppl i e r s .
S u i t a b l e creams a r e a v a i l a b l e f rom repu tab le
F i n a l l y , an impor tan t p a r t o f any s k i n p r o t e c t i o n programme i s t o ensure t h a t
a l l employees who use o r a re exposed t o any type o f l u b r i c a n t o r metalworking
f l u i d keep a c a r e f u l watch on a l l areas o f t h e i r s k i n and o b t a i n medical adv ice
a t the f i r s t s ign o f any abnorma l i t y . Medical a t t e n t i o n should be ob ta ined f o r
any c u t s and scra tches as w e l l as d i s c o l o r a t i o n , soreness, i t c h i n g , s w e l l i n g o r
warty growths. Awareness o f s k i n d i so rde rs and s k i n ca re can be promoted by
d i s p l a y i n g and d i s t r i b u t i n g pos te rs and l e a f l e t s pub l i shed by t h e Hea l th and
Safety Execut ive and by the o i l supp l i e rs . Employees should be aware o f and
observe any spec ia l i n s t r u c t i o n s on produc t package l a b e l s o r i n the s u p p l i e r ' s
p roduc t l i t e r a t u r e . Under the Hea l th and Safe ty a t Work Ac t , i t i s t he respon-
s i b i l i t y o f t h e employer t o i n fo rm h i s employees o f any known or p o t e n t i a l
hazards t o h e a l t h and t o i n s t r u c t them on the a p p r o p r i a t e p recau t ions t o be
fo l l owed . Constant reminders and proper superv i s ion a r e necessary t o ensure
t h a t t he contempt b red o f f a m i l i a r i t y does n o t o v e r r i d e prudence, o r t he
"problems o n l y happen t o o t h e r people" syndrome does n o t become predominant.
12.8.3 O i l M i s t and Vapour
Al though exposure i s u n l i k e l y t o c r e a t e a hea
o i l m i s t and vapour i n the p l a n t atmosphere shou
unpleasant environment. The o i l m i s t concen t ra t
t he Threshold L i m i t Value o f 5mg/m3 and p re fe rab
t h hazard, concen t ra t i ons o f
d be minimised t o avo id an
on should be main ta ined below
y below 2.5mg/m3. O i l m i s t i s
determined by use o f sampl ing pumps t o c o l l e c t m i s t on f i l t e r papers f o r ana ly -
s i s by we igh ing o r o t h e r methods. The Occupat ional Hygiene Sub-committee o f the
I n s t i t u t e o f Petroleum Adv isory Committee on Hea l th has pub l i shed d e t a i l s o f
s u i t a b l e techniques [20 ] . As a general guide, i f m i s t can be seen i n the p l a n t
atmosphere, i t i s l i k e l y t o be above 5mg/m3.
I f the o i l m i s t l e v e l i s excess ive , t he f i r s t s tep i s t o t r y t o reduce the
amount generated. Th is w i l l r e q u i r e an assessment o f t h e way i n which the m i s t
i s produced. There a r e two bas i c mechanisms by which m i s t i s formed:
( 1 ) I n some opera t i ons o i l may be atomised i n small d r o p l e t form. Th is may occur
i n some h i g h speed c u t t i n g opera t i ons o r f rom m i s t l u b r i c a t i o n systems through
288
over a p p l i c a t i o n o f m i s t o r poor r e c l a s s i f i c a t i o n o f t he l u b r i c a n t . In these
cases, m i s t concentrat ions may be reduced by adjustments t o the method o f app-
l i c a t i o n o r by m o d i f i c a t i o n t o the composi t ion o f the l u b r i c a n t . I n metal
machining the r a t e , volume f l o w and p o i n t o f a p p l i c a t i o n can a f f e c t the degree
o f m is t i ng . Proper p o s i t i o n i n g o f splash guards can c o n t r o l the escape o f o i l
m i s t i n t o the general p l a n t atmosphere. S p e c i a l l y formulated a n t i - m i s t c u t t i n g
o i l s have success fu l l y reduced p l a n t m i s t l e v e l s i n some a p p l i c a t i o n s . M i s t
l u b r i c a n t f o rmu la t i ons r e q u i r e a c a r e f u l balance between adequate m i s t i n g and
r e c l a s s i f i c a t i o n p r o p e r t i e s . The r a t e o f a p p l i c a t i o n o f the m i s t l u b r i c a n t and
the design o f the a p p l i c a t i o n system a re a l s o important.
( i i ) Vapor isat ion o f l u b r i c a n t may be fo l lowed by condensation t o form small
d r o p l e t s o f o i l m is t . I n t h i s case i t may be p o s s i b l e t o p rov ide a d d i t i o n a l
c o o l i n g by i nc reas ing the volume f l o w r a t e o f o i l app l i ed . I t i s o f t e n thought
t h a t h i g h v e l o c i t y j e t o f o i l i s t he most e f f e c t i v e means o f c o o l i n g whereas i n
f a c t a low v e l o c i t y h igh volume f l o w w i l l r e s u l t i n less o i l vapor i sa t i on . With
a h igh v e l o c i t y j e t t he re may a l s o be a g rea te r tendency t o format ion o f o i l
m i s t .
I f format ion o f o i l m i s t and vapour cannot be e f f e c t i v e l y c o n t r o l l e d by app-
l i c a t i o n o r f o rmu la t i on changes, l o c a l exhaust v e n t i l a t i o n should be used.
Systems inco rpo ra t i ng f i l t e r s t o remove o i l so t h a t c lean a i r can be re tu rned t o
the p l a n t atmosphere a re commercial ly a v a i l a b l e .
the exhaust hood should be located as c l o s e t o the p o i n t o f m i s t generat ion as
poss i b l e .
For maximum e f fec t i veness ,
O i l m i s t i s sometimes generated by the use o f a i r j e t s t o remove swarf from
machined pa r t s . If the re i s no o t h e r way o f per forming t h i s ope ra t i on , con-
s t r u c t i o n o f an enclosure w i t h l o c a l exhaust v e n t i l a t i o n may be needed.
12.8.4 Sk in Cancer
I f good q u a l i t y so l ven t r e f i n e d o i l s (o r those t r e a t e d adequately i n o t h e r
ways t o reduce the aromat ic contents) a re used i n a l l a p p l i c a t i o n s where any
s i g n i f i c a n t s k i n con tac t i s l i k e l y , s k i n cancer should n o t be a problem i n
f u t u r e , prov ided t h a t precaut ions discussed under Skin P r o t e c t i o n i n sec.12.8.2
a r e fo l lowed. Because o f the long l a t e n t p e r i o d f r o m i n i t i a l exposure t o
occurrence ( o f t e n more than 20 years) some cases associated w i t h prolonged o r
repeated exposure t o p o o r l y r e f i n e d minera l o i l s many years ago, o r w i t h unsat-
i s f a c t o r y hygiene p r a c t i c e s i n the past , can s t i l l be expected t o a r i s e .
However, f o l l o w i n g the i n t r o d u c t i o n o f so l ven t r e f i n e d o i l s i n c u t t i n g f l u i d
fo rmu la t i ons and the implementat ion o f b e t t e r hygiene p rac t i ces , t he re should be
fewer cases as t ime passes.
289
Because changes occur i n composi t ion o f used o i l s , p a r t i c u l a r l y w i t h gaso-
l i n e engine l u b r i c a n t s and quenching o i l s , i t would be prudent t o exe rc i se care
w i t h these products and avo id s k i n con tac t as much as poss ib le .
12.8.5 Bac te r ia and Bioc ides
Bacter ia i n aqueous coo lan ts and minera l o i l l u b r i c a n t s contaminated w i t h
water need t o be c o n t r o l l e d f o r t echn ica l performance reasons. As i n d i c a t e d i n
Section 12.5, t he re i s no evidence o f h e a l t h hazards associated w i t h the occur-
rence o f these bac te r ia . However, a d d i t i o n o f b ioc ides t o coo lan t systems may
present hazards i n hand1 ing these ma te r ia l s . B ioc ides a r e normal ly i r r i t a n t
and t o x i c . Therefore, the s u p p l i e r ' s handl ing recommendations should be care-
f u l l y fo l lowed.
12.9 CONCLUSIONS
Provided t h a t users a re aware o f p o t e n t i a l hazards and f o l l o w recommended
handl ing p r a c t i c e s i n combinat ion w i t h good personal and p l a n t hygiene standards,
l u b r i c a n t s should present no undue h e a l t h r i s k s . The major p o i n t s which need t o
be repeatedly s t ressed can be summarised as f o l l o w s :
Use good q u a l i t y so l ven t r e f i n e d ( o r equ iva len t ) minera l o i l s i f
the re i s t o be s i g n i f i c a n t s k i n contact .
Ensure t h a t adequate i n fo rma t ion i s a v a i l a b l e t o enable products t o
be used sa fe l y .
Develop proper awareness o f hazards through t r a i n i n g , caut ionary
no t i ces , s u p p l i e r p u b l i c a t i o n s e t c .
Avoid repeated or prolonged s k i n con tac t .
Encourage good personal hygiene w i t h proper s k i n c lean ing p rac t i ces .
Prov ide s u i t a b l e p r o t e c t i v e c l o t h i n g .
Encourage proper use o f b a r r i e r and recond i t i on ing creams.
Keep o i l m i s t concentrat ions w e l l below 5mg/rn3.
E s t a b l i s h good o i l and machine maintenance p rac t i ces .
Obtain e a r l y medical adv ice f o r any s k i n problems.
Ensure good p r a c t i c e s a r e maintained w i t h d i l i g e n t superv is ion.
REFERENCES
1 Wagner,W.D., e t . a l . , Am. Ind. Hyg. Assn. J. 1964, 5, 158. 2 Lushbaugh,C.C., e t . a l . , Arch. Ind. Hyg. 1950. 1, 237. 3 Wagner,W.D., e t . a l . , Unpublished r e s u l t s , USPHS, 1014 Broadway,
C i n c i n a t t i , U.S.A. 4 Decoufle,P., Ann. New York Acad. Sci . 1976, 271, 94. 5 Decoufle,P., J. Nat. Cancer Ins t . 1978, 5, 1025. 6 Drasche,H., e t . a l . , Zentbl . Arb. Med. Arbschutz 1974, lo. 7 Sanderson,J., O i l M i s t - Recent I n t e r e s t s i n Europe. Presented a t
Esso Symp. on O i l M is t and Nitrosamines, Stockholm, March 1977.
290
8 9
10
11 12 13 14 15
16
17
18 19
20
Lei tch,A., B r i t . Med. J., 1922, 2, 1004. Bingham,E., and Horton,A.W., 'Advances i n B io logy o f S k i n ' , Vol . V I I - Carcinogenesis, 1966, Pergamon Press, New York. Medical Research Counci l , 'The Carcinogenic A c t i o n o f Minera l O i l s : A Chemical and B i o l o g i c a l S tudy ' . Special Report Ser ies No. 306, 1968, H.M.S.O. London. Scala,R.A., J. Occ. Med. 1975, 1_z, 784. Wahlberg,J.E., Acta.Derrn. (Stockholm), 1974, 54, 471. Th. Van Peteghem and H. De Vos, B r i t . J. Zingmark,P.A. and Rapp,C. Ambio, 1977, 6, 237.
.NIDSH Techn ica l Report 'Con t ro l o f Exposure t o Meta lwork ing F l u i d s ' , February 1978, (Pub1 i c a t i o n No. 78-165). Technical B u l l e t i n , ' M i c r o b i o l o g i c a l Degradat ion o f L u b r i c a t i n g O i l s ' . Mobi l O i l Co. L td . , 1977. Thony,C., e t a l . , Arch. Mal. Prof de Med Trav e t Sec. SOC. ( P a r i s ) , 1975, 36, 37. Clausen,J., and Rastogi,S.C., B r i t . J. Ind. Med. 1977, 2, 208. I n s t i t u t e of Petroleum "Code o f P r a c t i c e f o r Meta lwork ing F lu ids" , 1978, Heyden & Son L td . , London. I n s t i t u t e o f Petroleum Occupat ional Hygiene Sub-committee, Ann. Occup. Hyg. 1975, 18, 293-297.
I n d . T e d . 1974, 2, 233.
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13 EFFECTIVE CONTAMINATION CONTROL IN FLUID POWER SYSTEMS
J.B.SPENCER, Manager, User Support D i v i s i o n , Sperry Vickers.
1 3 . 1 INTRODUCTION
The s e l e c t i o n o f a f i l t e r and i t s proper l o c a t i o n i n a h y d r a u l i c system needs
as much care and the same l e v e l o f e x p e r t i s e as the s e l e c t i o n o f o t h e r compo-
nents such as pumps, va lves and c y l i n d e r s . Many system designers look no f u r -
ther than the h y d r a u l i c equipment manufacturer 's catalogue f o r guidance, bu t
un fo r tuna te l y i t i s s t i l l common f o r h y d r a u l i c equipment manufactuers t o spec i f y
one general l e v e l o f f i l t r a t i o n such as 25 micrometre, w i t h o u t regard t o the
working pressure, environment, o r duty cyc le . With c e r t a i n types o f equipment
a lower standard may be acceptable; an example o f t h i s being the e a r l i e r designs
of pumps, many o f which g i v e long t r o u b l e - f r e e s e r v i c e p ro tec ted o n l y by a 0.13
mm s t r a i n e r . On the o t h e r hand, more modern equipment such as the m i n i a t u r i z e d
c o n t r o l s w i t h sma l le r c learances than many servo va lves, w i l l need much h igher
standards o f p r o t e c t i o n .
Usual ly t he nex t s tep i s t o decide on the l o c a t i o n o f t he f i l t e r and, again,
the genera l ized recommendations o f t he f i l t e r manufacturer a r e o f t e n accepted
w i thou t regard t o the p a r t i c u l a r system requirements. F i n a l l y , the s i z e o f the
f i l t e r i s f i x e d and sometimes the o v e r r i d i n g cons ide ra t i on i s s imply a d e s i r e t o
match the f i l t e r p o r t s i z e t o the diameter o f the a d j o i n i n g pipework.
w e l l ensure t h a t recommended v e l o c i t i e s a r e n o t exceeded bu t much more important
and o f t e n over looked i s the f i l t e r e f f i c i e n c y and d i r t ho ld ing capac i t y .
Th is may
I t must be admi t ted t h a t t he h i t -o r -m iss approach descr ibed above o f t e n
appears t o achieve an acceptable r e s u l t , b u t w i t h over 70% o f h y d r a u l i c system
f a i l u r e s known t o be due t o poor f l u i d c o n d i t i o n , t he re i s a c l e a r need f o r a
more systemat ic approach t o contaminat ion c o n t r o l .
The need has been accentuated by the i nc reas ing l y arduous cond i t i ons under
which systems operate. For example, a pressure o f about 70 bar was common i n
i n d u s t r i a l h y d r a u l i c systems f o r many years; today, 140-210 bar systems a r e
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commonplace and much o f t he contaminant fo rmer l y washed away i s now fo rced i n t o
the c learances where i t does considerable damage. Smal ler o i l r e s e r v o i r s mean
more r a p i d c i r c u l a t i o n and l e s s o p p o r t u n i t y f o r p a r t i c l e s t o s e t t l e ou t . Higher
ope ra t i ng temperatures r e s u l t i n t h i n n e r o i l , which i n some s i t u a t i o n s may g i ve
less p r o t e c t i o n aga ins t wear g i v i n g r i s e t o increased contaminat ion.
I n the face o f these trends, t he h y d r a u l i c equipment user wants improved
r e l i a b i l i t y and i t must be eas ie r and less c o s t l y f o r h im t o achieve t h i s i f he
works w i t h c lean o i l . I t i s n o t d i f f i c u l t t o keep the o i l i n good c o n d i t i o n
prov ided the machine design i s r i g h t , and the rewards f o r doing so a r e b e t t e r
r e l i a b i l i t y and longer l i f e f rom both the equipment and o i l .
1 3 . 1 . 1 A Systemat ic Approach t o F i l t r a t i o n
To work towards the most e f f e c t i v e p r o t e c t i o n cons is ten t w i t h economy, we
must f i r s t d e f i n e our aim. I t i s n o t , as i s w ide l y assumed, s imply t o separate
o u t p a r t i c l e s l a r g e r than a c e r t a i n s i z e chosen a r b i t r a r i l y . Instead, we must
achieve s t a b l e l e v e l s o f contaminat ion acceptable and approp r ia te t o var ious
p a r t s o f t h e system.
For a s t a b i l i s e d contaminat ion l e v e l , ' d i r t i n ' must equal ' d i r t o u t ' ( c o l l -
ected by the system f i l t e r s ) . ' D i r t i n ' i s made up o f i n - b u i l t contaminant,
contaminat ion i n the i n i t i a l charge o f o i l , and contaminat ion drawn i n from the
atmosphere through t h e a i r breather and c y l i n d e r sea ls - a l l o f which c o n t r i b u t e
t o the generat ion o f p a r t i c l e s by the process o f wear.
The e f f e c t o f t h e i n - b u i l t contaminat ion must be c a r e f u l l y considered. Inev-
i t a b l y i t w i l l be h igh, even when care i s taken i n the p repara t i on o f pipework
and man i fo ld blocks. F lushing w i l l remove some i n i t i a l contaminant, b u t t he re
a r e many systems where t h i s i s no t done, and a t t he f i r s t s t a r t - u p h i g h pressures
a re generated w i t h h igh contaminat ion l e v e l s present . The r e s u l t i s u s u a l l y
r a p i d pump wear and va l ve ma l func t i on , t he f i r s t o f which w i l l a lmost c e r t a i n l y
go undetected a t t h i s time. A l l h y d r a u l i c systems should be run i n an o f f - l o a d
c o n d i t i o n u n t i l the des i red contaminat ion l e v e l i s a t t a i n e d .
F igu re 1 shows a t y p i c a l r e l a t i o n s h i p between the design contaminat ion l e v e l
and the ac tua l l e v e l p r i o r t o s t a r t - u p . This convenient method o f p resen ta t i on
fo l lows n a t u r a l l y f rom the l o g a r i t h m i c d i s t r i b u t i o n o f p a r t i c l e s i z e t h a t occurs
i n p r a c t i c e . The r e l a t i v e s lopes o f t he i n i t i a l and acceptable Contamination
l i n e s a re a c l e a r p o i n t e r t o the type o f f i l t r a t i o n needed.
The methods used t o determine and c o n t r o l contaminat ion l e v e l s w i l l be d i s -
cussed i n more d e t a i l l a t e r , bu t a t t h i s stage we can summarize the p r a c t i c a l
and performance requirements o f t h e f i l t r a t i o n system as fo l l ows : -
293
Fig. 1: Typical relationship between the desired design contamination level and the actual level prior to start up. It is essential that the system is flushed and run at no-load until the acceptable contamination level is achieved.
Initial contamination level
level
5 10 15 25 50 100 Parliclasire - micrometres (lopa)
( i ) I t must be capable o f reducing the i n i t i a l contaminat ion t o the des i red
l e v e l w i t h i n an acceptable p e r i o d o f t ime, w i t h o u t causing premature
wear o r damage t o the h y d r a u l i c components.
I t must be capable o f ach iev ing and ma in ta in ing the des i red l e v e l ,
i n c l u d i n g a s u i t a b l e f a c t o r o f s a f e t y t o c a t e r f o r a concentrated ingress
which could occur; f o r example, when a system i s ' topped-up' .
( i i i ) The q u a l i t y o f maintenance a v a i l a b l e the end user l o c a t i o n must be
( i i )
acknowledged.
F i l t e r s must be e a s i l y access ib le f o r maintenance purposes.
I n d i c a t i o n o f f i l t e r c o n d i t i o n t o s u i t t he end u s e r ' s requirements must
be provided.
I n cont inuous process p l a n t , f a c i l i t i e s must be prov ided t o a l l o w
changing o f elements w i thou t i n t e r f e r i n g w i t h p l a n t ope ra t i on .
( i v )
(v)
( v i )
( v i i ) The f i l t e r s must p rov ide s u f f i c i e n t d i r t h o l d i n g capac i t y f o r an
acceptable i n t e r v a l between element changes.
( v i i i ) T h e i n c l u s i o n o f a f i l t e r i n the system must n o t produce undesi rab le
e f f e c t s on the opera t i on o f components, e.g. h i g h back pressures on
seal d ra ins .
Sampling p o i n t s must be prov ided t o mon i to r i n i t i a l and subsequent l e v e l s
of contaminat ion.
( i x )
294
13.2 DIRT INPUT - TYPES AND SOURCES OF CONTAMINATION
13.2.1 New O i l
A l though o i l i s r e f i n e d and blended under r e l a t i v e l y c lean c o n d i t i o n s , i t i s
u s u a l l y s to red i n drums o r i n a b u l k tank a t t he u s e r ' s f a c t o r y . A t t h i s p o i n t
i t i s no longer c lean, because the f i l l i n g l i n e s c o n t r i b u t e metal and rubber
p a r t i c l e s and the drum always adds f l a k e s o f metal o r scale. Storage tanks can
be a rea l problem because water condenses i n them t o cause r u s t i n g and contami-
n a t i o n f rom the atmosphere f i n d s i t s way i n unless s a t i s f a c t o r y . a i r b rea the r
f i l t e r s a re f i t t e d .
I f the o i l i s be ing s to red under reasonable cond i t i ons , the p r i n c i p a l contam-
i nan ts on d e l i v e r y t o a machine w i l l be metal , s i l i c a , and f i b r e s . With o i l s
from reputable supp l i e rs , sampling has shown average counts o f 30,000 t o 50,000
p a r t i c l e s above 5 micrometre pe r 100 m l , w i t h a r e l a t i v e l y low s i l t l e v e l . Using
a p o r t a b l e t r a n s f e r u n i t o r some o t h e r f i l t r a t i o n arrangement, i t i s p o s s i b l e t o
remove much o f t he contaminat ion present i n new o i l be fo re i t en te rs t h e system
and i s ground down i n t o f i n e r p a r t i c l e s .
I t must be s a i d i n passing t h a t contaminat ion a r i s i n g f rom d e l i v e r y and s t o r -
age v a r i e s w i t h the i ndus t r y . For example, a i r c r a f t ope ra t i on genera l l y needs
h i g h standards o f c l e a n l i n e s s and f a i r l y q u i c k turnover o f s to res , whereas much
longer s torage pe r iods a r e the r u l e f o r marine systems and the environment may
be more d i f f i c u l t t o c o n t r o l .
13.2.2 B u i l t - i n Contaminat ion
New machinery always has a c e r t a i n amount o f b u i l t - i n contaminat ion.
assembly and i n f l u s h i n g the system reduces t h i s , b u t never e l im ina tes i t .
Typ ica l b u i l t - i n contaminants a r e burrs , ch ips, f l a s h , d i r t , dust , f i b r e , sand,
moisture, p i p e dope, weld s p l a t t e r , p a i n t s , and f l u s h i n g s o l u t i o n s .
Care i n
The amount o f contaminant removed d u r i n g f l u s h i n g depends n o t o n l y on t h e
ef fect iveness o f t he f i l t e r used bu t a l s o the v e l o c i t y o f t he f l u s h i n g f l u i d .
Unless h i g h v e l o c i t i e s a re a t t a i n e d , much o f the contaminant w i l l no t be d i s -
lodged u n t i l t he system i s i n ope ra t i on , w i t h component f a i l u r e the almost
c e r t a i n r e s u l t . I r r e s p e c t i v e o f t he standard o f f l u s h i n g , an o f f - l o a d running-
i n pe r iod should be regarded as e s s e n t i a l . Some b u i l t - i n contaminant, such as
weld scale, o f t e n remains i n t a c t u n t i l h i g h pressure o i l i s forced between i t
and the parent meta l , thus loosening i t .
13.2.3 Environmental Contamination
Contaminants f rom t h e immediate surroundinas can be in t roduced i n t o a f l u i d
power system. On l a r g e i n s t a l l a t i o n s such as those w i t h i n steelworks, i t i s
r e l a t i v e l y easy t o a s c e r t a i n t h e environmental cond i t i ons , though they vary con-
s ide rab ly . For example, a coke oven system operates i n cond i t i ons very d i f f e r e n t
296
from a c o l d m i l l . Sometimes the best s o l u t i o n i s t o p r o t e c t t h e h y d r a u l i c
equipment by p rov id ing a c lean room where maintenance can be c a r r i e d o u t under
c o n t r o l l e d cond i t i ons . Un fo r tuna te l y , i t i s no t uncommon t o see h y d r a u l i c power
sources exposed t o the worst p o s s i b l e environment, w h i l e a longside the e l e c t r i c s
are protected by p ressu r i sed and temperature c o n t r o l l e d cabins.
I n most machine shops the r e l a t i v e l y l a r g e contaminant p a r t i c l e s o f 10-15
micrometres do no t demand a h igh standard o f a i r f i l t r a t i o n , a l though g r i n d i n g
machines w i thou t e f f e c t i v e e x t r a c t i o n equipment can r e s u l t i n l o c a l i s e d pro-
blems. On the o t h e r hand, foundr ies and stone quar r i es demand a very h igh
standard o f f i l t r a t i o n because o f t he a i r b o r n e ab ras i ve p a r t i c l e s .
The mobi le equipment f i e l d presents spec ia l problems because the o r i g i n a l
manufacturer u s u a l l y s e l l s a standard machine t o operate i n a wide v a r i e t y of
environmental cond i t i ons .
13.2.4 Entry Po in ts f o r Environmental Contaminat ion
13.2.4.1 A i r breathers. Very l i t t l e i n fo rma t ion appears to be a v a i l a b l e on
what the f i l t e r w i l l a c t u a l l y achieve and p u r e l y nominal r a t i n g s a r e u s u a l l y
spec i f i ed . There have been instances where the element has shrunk l eav ing a
f r e e passage f o r t h e a i r , which h i g h l i g h t s the need f o r more r i g i d engineer ing
standards on t h i s type o f product . The amount o f a i r passing through the f i l -
t e r w i l l depend on draw-of f , which means f o r example t h a t s i n g l e a c t i n g c y l i n -
ders i n bad environments must r e s u l t i n a g rea te r ingress o f contaminant.
I t i s encouraging t o see some manufacturers now o f f e r i n g b e t t e r grades of
f i l t e r paper and t h a t i n c e r t a i n areas the combined f i l l e r / b r e a t h e r has become
unacceptable. A separate breather i s more e f f i c i e n t and helps t o re lease the
a i r w h i l e f i l l i n g takes p lace through a s u i t a b l e gauze s t r a i n e r .
13.2.4.2 Power u n i t access p la tes . I n some p l a n t s i t cannot be assumed t h a t
access p l a t e s w i l l always be replaced, though happ i l y t h i s s t a t e o f a f f a i r s i s
not as common as i t once was. I n power u n i t design, good s e a l i n g i s v i t a l , and
i n bad environments such items as s t r a i n e r s should no t be pos i t i oned i n s i d e the
rese rvo i r i f access requ i res t h e r e f i t t i n g o f removable p l a t e s . Other removable
items w i l l a l l o w ingress du r ing maintenance and good design p r a c t i c e should
minimize t h i s .
13.2.4.3 Cy l i nde r seals . Wiper seals cannot be 100% e f f e c t i v e i n removing
very f i n e contaminant f rom the c y l i n d e r rod.
I f they were, they would remove the o i l f i l m f rom t h e p i s t o n rod, producing
a r e s u l t t h a t i s u s u a l l y diagnosed as a leaky seal . I n any case, a completely
dry rod would q u i c k l y wear o u t t he seals. Where c y l i n d e r s remain extended i n a
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h e a v i l y contaminated atmosphere considerable q u a n t i t i e s o f f i n e p a r t i c l e s can
g e t i n t o the system unless p r o t e c t i o n such as a be l lows i s prov ided.
I t has been shown t h a t c y l i n d e r p i s t o n rod seals n a t u r a l l y ingress about
one p a r t i c l e over 10 micrometre f o r each square cen t ime t re of swept rod area.
Wear o f seals o r wipers can increase the i ng ress ion r a t e considerably . Thus
i n bad ambient cond i t i ons a 50mm diameter rod i n a 100mm bore c y l i n d e r ,
c y c l i n g a t a speed o f 12 metres per minute, could ingress about 20,000 p a r t i c l e s
over 1Omicrometres every minute, and t h i s q u a n t i t y cou ld increase by a f a c t o r o f
100 f o r every 100 hours o f running.
13.2.5 Generated Contaminat ion
Contamination i s created i n t e r n a l l y by t h e opera t i on o f a h y d r a u l i c system.
The generated contaminants a r e products o f wear, co r ros ion , c a v i t a t i o n , and
f l u i d breakdown, i . e . decomposition, ox ida t i on , e t c . Exper ience shows t h a t i n
a system which has been c a r e f u l l y f l ushed and has f i l t e r e d o i l added t o the
sealed r e s e r v o i r ( i n c o r p o r a t i n g an e f f e c t i v e b rea the r ) , t h e contaminat ion w i l l
be main ly system generated.
I f the i n i t i a l l e v e l i s n o t s a t i s f a c t o r y , t h i s induces wear which g r e a t l y
accelerates the bu i l d -up o f generated contaminant.
13.3 EFFECTS OF TYPES AND S I Z E S OF PARTICLES
We know t h a t contaminant p a r t i c l e s a r e o f a l l shapes and s i zes and t h a t t he
f i n e r they are, t h e more d i f f i c u l t i t i s t o count them and to determine t h e
m a t e r i a l of which they a r e composed. However, we can say t h a t t h e m a j o r i t y a r e
ab ras i ve and t h a t , i n t e r a c t i n g w i t h su r face p r o t r u s i o n s , they plough and c u t
fragments f r o m a sur face.
contaminat ion.
Th is wear accounts f o r about 90% o f f a i l u r e s due t o
F a i l u r e s a r i s i n g f rom contaminat ion f a l l i n t o t h r e e categor ies:
13.3.1
valve. For instance, i f a - p a r t i c l e causes a vane t o jam i n a r o t o r s l o t , t he
r e s u l t may w e l l be complete se i zu re o f t h e pump or motor. I n a spool type valve
a l a r g e p a r t i c l e t rapped a t t he r i g h t p lace can s top a spool c l o s i n g complete ly .
Another example of c a t a s t r o p h i c f a i l u r e occurs when t h e p i l o t o r i f i c e of a v a l v e
i s blocked by a l a r g e p a r t i c l e .
f a i l u r e , for instance i f a v a l v e f a i l s t o operate due t o s i l t i n g .
Catast rophic F a i l u r e occurs when a l a r g e p a r t i c l e en te rs a pump o r
F ine p a r t i c l e s can a l s o cause catastrop.hic
13.3.2
va l ve which prevents i t from resea t ing p roper l y . I f t h e seat i s t o o hard t o
a l l o w t h e p a r t i c l e t o be embedded i n t o it, t h e p a r t i c l e may be washed away when
t h e va l ve i s opened again.
I n t e r m i t t e n t F a i l u r e i s caused by contaminant on t h e seat o f a poppet
La te r , another p a r t i c l e may prevent complete c losure
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only t o be washed away when t h e v a l v e opens.
m i t t e n t f a i l u r e occurs.
Thus a very annoying type o f i n t e r -
13.3.3 Degradat ion F a i l u r e f o l l o w s wear, corros ion, and c a v i t a t i o n eros ion.
They cause increased i n t e r n a l leakage i n the system components, bu t t h i s condi-
t i o n i s o f t e n d i f f i c u l t t o de tec t .
The eventual r e s u l t , p a r t i c u l a r l y w i t h pumps, i s l i k e l y t o be ca tas t roph ic
f a i l u r e .
which j u s t pass through the clearances between moving p a r t s (F ig .2) .
The p a r t i c l e s most 1 i k e l y t o cause wear a r e c learance-s ize p a r t i c l e s
8
Particles of similar size to the clearances between moving parts cause the most abrasive wear. Larger particles cannot enter clearance, smaller particles pass through without contact.
Clearancesize particles interact with both faces simultaneously. Extra contamination is generated by the disintegration of the moving parts. Larger particles are'ground up' by the interaction in the clearances.
a 0
Fig.2 I n t e r a c t i o n o f moving pa r t s .
Manufactur ing c learances w i t h i n h y d r a u l i c components can be d i v ided i n t o two
p r i n c i p a l zones, i .e . around 5 micrometres f o r h i g h pressure u n i t s , and 15-40
micrometres f o r low pressure u n i t s . The ac tua l c learance may vary considerably
depending on the type o f u n i t and opera t i ng cond i t i ons i t sees. Good component
des ign i s important, thus m in im is ing t h e e f f e c t o f smal l clearances.
We should t h e r e f o r e l o o k a t t h e f a c t o r s a f f e c t i n g c r i t i c a l c learances and
a l s o a t t he type o f f a i l u r e o c c u r r i n g i n va r ious groups o f components.
13.3.4 Pumps
A l l h y d r a u l i c pumps have component p a r t s which move r e l a t i v e t o one another,
separated by a smal l o i l - f i l l e d c learance. General ly, these components a r e
loaded toward each o t h e r by forces r e l a t e d t o pressure, and the pressure always
tends t o f o r c e f l u i d through t h i s c learance.
As t h e f i n i t e l i f e o f most pumps i s determined by a very smal l q u a n t i t y o f
m a t e r i a l being removed f rom a few surfaces, i t f o l l o w s t h a t i f the f l u i d w i t h i n
298
the c learance i s h e a v i l y contaminated, r a p i d degradat ion and eventua l se i zu re
cou ld occur . W i th low pressure u n i t s t h e des ign pe rm i t s r e l a t i v e l y l a r g e c l e a r -
ances and contaminat ion has less e f f e c t . A l so a t t he lower p ressure the re i s
less f o r c e a v a i l a b l e t o d r i v e p a r t i c l e s i n t o c r i t i c a l c learances . Inc reas ing
t h e pressure t h e r e f o r e i s o f major s i g n i f i c a n c e i n de termin ing the e f f e c t o f
con taminat ion on a pump.
Another f a c t o r a f f e c t i n g c learances i s the o i l f i l m th ickness , which i s a l s o
r e l a t e d t o f l u i d v i s c o s i t y . An optimum v i s c o s i t y i s used f o r des ign which pro-
v ides good f i l m th ickness t o suppor t loads hydrodynamical ly bu t which i s a l s o
low enough t o a l l o w adequate f i l l i n g o f t he pump w i t h o u t c a v i t a t i o n . I t i s gen-
e r a l l y found i n p r a c t i c e t h a t f i l t r a t i o n requirements become less c r i t i c a l where
h igher v i s c o s i t i e s a r e used, and f o r t h i s reason the maximum v i s c o s i t y which i s
compat ib le w i t h the i n l e t cond i t i ons should be chosen. S i m i l a r l y , good tempera-
t u r e c o n t r o l w i l l be o f b e n e f i t i n t h i s respec t .
The areas i n pumps p a r t i c u l a r l y sub jec t t o these c learance problems are : -
Vane pump - Vane t i p t o cam r i n g , r o t o r t o s i d e p l a t e .
Gear pump - Tooth t o housing, gear t o s i d e p l a t e .
A x i a l p i s t o n pump - Shoe t o swashplate, c y l i n d e r b lock .
F igures 3 , 4 and 5 i l l u s t r a t e the c r i t i c a l areas d iag rammat i ca l l y .
Clearance under vane tips depends pressure and viscosity
ote Some back flow occurs from hlgh to
on speed,
Fig.3 C r i t i c a l c learances i n a vane pump.
299
In Low pressure
Minimum clearance & db Maximum clearance
Fig.4 C r i t i c a l c learances i n a gear pump.
Fig.5 C r i t i c a l c learances i n an
i s nomina l l y f i x e d , actua
load and v i s c o s i t y .
-
Valve plate
Where P = viscosity p = pressure
a x i a l p i s t o n pump. Al though p c learance v a r i e s w i t h eccent
ston c r c i t y
earance due t o
I n many o f t h e foregoing cases the clearances a r e e f f e c t i v e l y s e l f - a d j u s t i n g
under ope ra t i ng cond i t i ons , i . e . w i t h i nc reas ing pressure c learances become
smal ler . Under adverse cond i t i ons , and p a r t i c u l a r l y where t h e r e i s shock load-
ing, t h i s increases the v u l n e r a b i l i t y t o smal ler contaminant p a r t i c l e s . Even
where clearances a r e nomina l l y f ixed,under h igh loads components may take up
e c c e n t r i c p o s i t i o n s which again makes them vu lne rab le t o sma l le r p a r t i c l e s . I t
i s extremely d i f f i c u l t t o be p r e c i s e about e i t h e r the magnitude o f these c l e a r -
ances, p a r t i c u l a r l y under dynamic loading, o r t he e f f e c t o f d i f f e r e n t s i z e pa r t -
i c l e s i n the gaps.
However, f rom the data we do have and from f i e l d exper ience gained t o date,
we a r e ab le t o suggest contaminat ion l e v e l s which, i f achieved, w i l l r e s u l t i n
an acceptable l i f e f o r most pumps. These a r e presented i n the nex t chapter ,
which deals w i t h t h e s p e c i f i c a t i o n o f contaminat ion l e v e l s .
The use fu l l i f e o f a pump should end when i t no longer d e l i v e r s the requi red
ou tpu t a t a g iven s h a f t speed, d ischarge pressure, and f l u i d temperature. As a
guide, 15-20% l o s s o f f l o w would i n d i c a t e the end o f t h e use fu l l i f e . A l l too
o f t e n degradat ion goes undetected until,finally,catastrophic f a i l u r e occurs
w i t h vas t q u a n t i t i e s o f contaminat ion being re leased i n t o the system. I f , f o l l -
owing such a f a i l u r e , t he system i s no t then p roper l y cleaned, the l i f e o f the
replacement pump w i 1 1 be reduced.
I n the i n t e r e s t o f t he end user, t he system designer should s p e c i f y the m in i -
mum acceptable f l o w r a t e f rom t h e pump t o achieve s a t i s f a c t o r y machine perform-
ance. Means should be prov ided f o r mon i to r i ng pump ou tpu t by i n s e r t i n g s u i t a b l e
inst rumentat ion, e i t h e r tempora r i l y o r permanent ly, so t h a t r o u t i n e checks can
be c a r r i e d o u t t o reduce the r i s k o f c a t a s t r o p h i c f a i l u r e s . With p i s t o n u n i t s ,
i t i s u s u a l l y a s imple ma t te r t o measure case leakage, which can be a usefu l
guide t o pump cond i t i on .
Remember, t o the end user i t i s t o t a l cos ts t h a t a r e impor tant ; t he f a i l u r e
o f a low-cost pump may w e l l r e s u l t i n expensive downtime. If, by the i n c l u s i o n
o f a f l o w meter, such a f a i l u r e can be avoided, the i n i t i a l investment i n a f low
meter would be f u l l y j u s t i f i e d .
13.3.5 Motors
What has been w r i t t e n about pumps app l i es g e n e r a l l y t o motors o f s i m i l a r
des i gn.
I t must be remembered t h a t much o f the, contaminant passing through the pump
may be reaching the motor a l s o , where the re w i l l be a s i m l l a r performance degrad-
a t i o n .
o f i t s o r i g i n a l va lue and the vo lumet r i c e f f i c i e n c y o f t he motor has f a l l e n to ,
say, 90% o f o r i g i n a l , then the o v e r a l l vo lumet r i c e f f i c i e n c y o f t he pump and
motor w i l l be down t o 0.85 x 0.9 76.5% o f t he o r i g i n a l value. For t h i s reason
I f , due t o wear, t he vo lumet r i c e f f i c i e n c y o f the pump has f a l l e n t o 85%
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contaminat ion c o n t r o l i s p a r t i c u l a r l y impor tant i n h y d r o s t a t i c t ransmissions t o
prov ide the necessary l e v e l o f f l u i d c lean l i ness .
13 .3 .6 D i r e c t i o n a l Valves
The r a d i a l c learance s p e c i f i e d between bore and spool i n most d i r e c t i o n a l
valves i s i n the range o f 5 t o 13 micrometres. As i s w e l l know, the p roduc t i on
o f p e r f e c t l y round and s t r a i g h t bores i s e x c e p t i o n a l l y d i f f i c u l t , so i t i s un-
l i k e l y t h a t any spool w i l l l i e e x a c t l y c e n t r a l i n the c learance band. I n a nom-
i na l & i n . va lve, a good spool i s l i k e l y t o have less than 2.5 micrometres
c 1 ea rance.
I n an e l e c t r i c a l l y operated va lve, t he forces a c t i n g on the so lenoid a r e
shown i n F ig .6. They are: Flow forces + Spr ing f o r c e + F r i c t i o n + I n e r t i a .
Tank Cylinder Pressure Cylinder Tank port Return port pressure port
Port Port
r--- 1 I
IIRll a m l---
r--
Spring lorce r--- 0 Flow force
F a A x A P
Leakage carrying particles
1 r--- L---
Q MI Spring force
AP = Pressure drop across spool
Dotted lines show position of spool lands when closed. . .
Fig.6 Valve spool c r i t i c a l c learances ( w i t h f lows and forces)
Flow, sp r ing , and i n e r t i a forces a r e inherent f a c t o r s bu t f r i c t i o n forces are,
t o a g rea t ex ten t , dependent on f i l t r a t i o n .
a ted w i t h p a r t i c l e s s i m i l a r i n s i z e t o the r a d i a l and d iametra l c learances,
h ighe r forces w i l l be needed t o move the spool .
I f the system i s h e a v i l y contamin-
An even worse s i t u a t i o n r e s u l t s f rom s i l t i n g , where contaminant i s forced
i n t o the c learances under pressure, even tua l l y leading t o breakdown o f the o i l
f i l m and spool hang-up (F ig .7 ) .
Th is s i t u a t i o n occurs where va lves subjected t o cont inuous pressure a r e oper-
a ted i n f r e q u e n t l y . Such va lves should p r e f e r a b l y have l o c a l f i l t r a t i o n of a
very h i g h o rde r i n the adjacent pressure l i n e bu t due account should be taken
o f p o s s i b l e pressure surges generated du r ing component operat ion. The use o f
h igh e f f i c i e n c y f i l t e r s as a spec ia l p r o t e c t i o n f o r s i n g l e u n i t s o r groups o f
302
u n i t s can r e s u l t i n the need f o r a ve ry h i g h d i r t capac i t y i f the general l eve l
o f f i l t r a t i o n i n the system i s much lower.
1 One large particle can bridge gap
Working pressure
Break in oil film due / to dirt concentration Build-up
Silt
F ig .7 Valve spool c r i t i c a l c learance. E c c e n t r i c i t y i s t he normal c o n d i t i o n i n many cases.
Some idea o f t he fo rces needed t o break t h i s spool hang-up, compared w i t h
the forces a v a i l a b l e f rom the so lenoid, can be gained from the example o f a
nominal & in . va l ve opera t i ng a t 210 bar. I f a v a l v e o f t h i s type remains s e l -
ected i n the sp r ing o f f s e t o r energised p o s i t i o n f o r a lengthy p e r i o d o f t ime,
s i l t i n g takes p lace between spool and bore t o produce t o t a l immob i l i t y . The
force needed t o overcome t h i s s t a t e has been found by experiment t o be o f t he
order o f 135 Newtons, bu t bo th s p r i n g and so leno id cou ld e x e r t o n l y 45 Newtons.
Thus the e f f e c t o f t he s i l t i s t o cause t o t a l system f a i l u r e .
13.3.7 Pressure Contro ls
H igh ly ab ras i ve p a r t i c l e s i n h i g h v e l o c i t y streams o f o i l erode the surfaces
w i t h which they come i n t o contact . Th i s s i t u a t i o n i s common t o pressure con-
t r o l l e r s , p a r t i c u l a r l y r e l i e f va lves which a r e subjected t o maximum system
pressure drop and v e l o c i t i e s o f t he o rde r o f 30 m/s.
e r a l l y see low volumes a t h i g h v e l o c i t i e s and heavy contaminat ion a f f e c t s both
t h e i r s t a b i l i t y and r e p e a t a b i l i t y .
P i l o t c o n t r o l stages gen-
13.3.8 Flow Contro ls
The contaminat ion to le rance o f f l o w c o n t r o l va lves w i l l depend very much on
the o r i f i c e c o n f i g u r a t i o n .
303
Figure 8, f o r example, shows two o r i f i c e s which a r e o f en't
shape, a l though having equal areas. The groove type (a) w i l l
contaminat ion l e v e l except when used a t low s e t t i n g , whereas
more prone t o s i l t i n g a t a l l s e t t i n g s .
r e l y d i f f e r e n t
t o l e r a t e a h igh
ype (b) i s much
With a l l types o f pressure-compensated f l o w c o n t r o l s , t he performance o f t he
pressure reduc ing element can be cons ide rab ly a f f e c t e d by contaminat ion , i r r e s -
pec t i ve o f v a l v e s e t t i n g . Damage t o t h e meter ing o r i f i c e can a l s o occur, which
w i l l become p a r t i c u l a r l y apparent a t lower s e t t i n g s .
Note: Throttle profile gives orifice segments of equal area
/ \
Fig.8 Flow c o n t r o l va l ve t h r o t t l e sec t i ons . P r o f i l e (b) i s more prone t o s i l t i n g .
General ly speaking, a l l spoo l - type c o n t r o l va lves a r e a f f e c t e d by contamina-
The e f f e c t s a r e l i k e l y t o be t i o n i n t h e system, e s p e c i a l l y a t h i g h pressures .
magnif ied i f p r e c i s e a x i a l p o s i t i o n i n g o f t he spool i s necessary as, f o r example,
i n pressure reduc ing va lves where l i m i t e d fo rces a r e a v a i l a b l e t o opera te the
spool.
contaminat ion, tend t o be f a r more t o l e r a n t o f s i l t due t o the s e l f - c l e a r i n g
ac t i on o f t he sea t , However, e ros ion i s s t i l l l i k e l y t o occur .
On t h e o t h e r hand, poppet va lves , though a f f e c t e d by l a r g e p a r t i c l e s o f
13.3.9 Summary
I t can be seen from the fo rego ing t h a t an i n d i v i d u a l l a rge p a r t i c l e a r r i v i n g
a t the wrong p lace a t t he wrong t ime can cause c a t a s t r o p h i c f a i l u r e .
Surfaces w i t h i n components should be separated by an o i l f i l m , the th ickness
o f which may be c o n t i n u a l l y changing.
wear w i l l occur thereby genera t i ng f u r t h e r p a r t i c l e s which may w e l l be ground
i n t o many more sma l le r p a r t i c l e s . F ine p a r t i c l e s i n d i v i d u a l l y o r i n smal l
When t h i s gap i s b r idged by contaminants,
304
Fig.9 Suggested acceptab le contaminat ion l e v e l s f o r va r ious h y d r a u l i c systems
Target Con- Suggested taminat ion Maximum Class t o P a r t i c l e Suggested CETOP RP70H Level S e n s i t i v i t y Type o f System F i l t r a t i o n
Ra t ing Bx > 75 5um 15 vm 5vm 15vm
1 3
15
16
18
19
21
9 4 000 250 Super S i l t s e n s i t i v e c o n t r o l 1-2 c r i t i c a l system w i t h very h i g h
r e l i a b i l i t y . Labora tory o r aerospace.
and h i g h pressure long l i f e systems, i .e. a i r - c r a f t , machine t o o l s , e t c .
11 16 000 1 000 C r i t i c a l High performance servo 3-5
13 32 000 4 000 Very imp- High q u a l i t y r e l i a b l e 10-12 o r t a n t systems. General machine
requirements
systems. Medium pressure , medium capac i t y .
15 250 000 16 000 Average Low pressure heavy indus- t r i a l systems, or a p p l i c a - t i o n s where l ong l i f e i s n o t c r i t i c a l .
14 130 000 8 000 Important General machinery and mob i l e 2-15
5-25
17 1 000 000 64 000 Hain Low pressure systems w i t h 25-40 l a r g e c 1 earances.
Note: This graph assumes viscosity to be within recommended range.
I !I\,.- - 0 200
f UI
* .
I -. Average hydraulic components .-. including most pumps. ‘=-..I_ -. -----
‘,Very sensitive i * =.. I
t components
I I
7 .--------
p,yery tolerant components.
I ’.
Figures relate to cumulative particle concentrations above5prnand 15pm respectively (see chapter 8).
Fig.10 Suggested c l e a n l i n e s s l e v e l f o r good l i f e
305
q u a n t i t i e s may n o t cause damage, bu t i f present i n l a r g e r concentrat ions may
lead t o f a i l u r e through s i l t i n g .
The o b j e c t i v e must be t o o b t a i n the most economic combinat ion o f contamin-
a t i o n c o n t r o l and contaminat ion to le rance f o r a g iven r e l i a b l e system l i f e under
known performance and environmental cond i t i ons .
13.4 SPECIFYING CONTAMINATION LEVELS
As s t a t e d p rev ious l y , 25 micrometres i s a t y p i c a l general l e v e l o f f i l t r a t i o n ,
s p e c i f i e d w i thou t regard t o working pressure, l o c a l environment o r duty cyc le .
We know f rom exper ience t h a t under s i m i l a r environmental and operat ina cond i t i ons
the e f f e c t i v e n e s s o f a 25 micrometre f i l t e r w i l l vary depending on i t s l o c a t i o n
i n the system. Also, under steady f l o w cond i t i ons as the pores tend t o c l o g the
f i l t r a t i o n performance may a c t u a l l y improve. Under va ry ing o r i n t e r m i t t e n t
f low, however, the r e s u l t can be very d i f f e r e n t because contaminant p a r t i c l e s
a re d is lodged from the pores a l l o w i n g more f i n e p a r t i c l e s t o pass through.
I t i s obvious t h a t the cond i t i ons o f use have the g rea tes t e f f e c t on the
contaminat ion l e v e l obta ined w i t h a s p e c i f i c f i l t e r , which means t h a t i t i s gen-
e r a l l y unwise t o o f f e r w i thou t q u a l i f i c a t i o n o r accept w i thou t ques t i on a
b lanke t recommendation o f , say the use o f a 25 micrometre f i l t e r . For the user
i t means p u t t i n g h i s investment a t r i s k and f o r the manufacturer i t makes the
a d m i n i s t r a t i o n o f war ran t i es extremely d i f f i c u l t .
From a wide spectrum o f f i e l d data, Fig.9 summarises the l e v e l s o f contamina-
t i o n which a r e considered acceptable f o r most h y d r a u l i c systems.
The dec i s ion o f whether a system i s c r i t i c a l w i l l depend l a r g e l y on the type
o f components used and the system design pressure. F igure 10 has the re fo re been
produced t o g i v e the recommended contaminat ion l e v e l s w i t h knowledge o f the
system pressure and hav ing analysed the d i r t s e n s i t i v i t y o f the system com-
ponen t s.
13.5 SELECTING THE FILTER
Before a choice o f f i l t e r may be made the va r ious r a t i n g s used by manufactur-
e r s must be examined.
13.5.1 Nominal Rat ing
S p e c i f i c a t i o n s MIL-F5504A and MIL-F5504B were es tab l i shed for determin ing
nominal r a t i n g s . Version A def ines a 10 micrometre f i l t e r as be ing ab le t o
remove 98% by weight o f a l l p a r t i c l e s o f the e l e c t e d contaminant (AC f i n e t e s t
dust ) l a r g e r than 10 micrometres a t a c e r t a i n h i g h concentrat ion.
de f i nes a 10 micrometre f i l t e r as be ing a b l e t o remove 95% by weight o f 10-20
micrometre g lass beads a t a h i g h concen t ra t i on . Al though l i t t l e use has been
made o f these p a r t i c u l a r s p e c i f i c a t i o n s , many manufacturers use s i m i l a r t e s t s
t o p rov ide nominal r a t i n g s f o r t h e i r f i l t e r s .
Version B
306
Such t e s t s have two major l i m i t a t i o n s . F i r s t l y , they do n o t l i m i t t he maxi-
mum s i z e o f p a r t i c l e a l l owed t o pass through t h e element and from t e s t s i t has
been found t h a t f i l t e r s meet ing these requirements can pass p a r t i c l e s up t o 200
micrometres. Secondly, t h e h i g h concen t ra t i on o f contaminant added i s n o t t y p i -
c a l o f c o n d i t i o n s exper ienced i n a normal system. I n p r a c t i c e , t h e p a r t i c l e s
approach the f i l t e r i n smal l concen t ra t i ons and those p a r t i c l e s t h a t a r e smal le r
than the mean pore s i z e pass r e a d i l y through t h e f i l t e r as long as t h e f i l t e r
medium remains reasonably c lean.
For the reasons g i ven above the re i s a good case f o r d i s c o n t i n u i n g the use
o f nominal r a t i n g s .
13.5.2 Abso lu te Ra t ing
The NFPA F l u i d Power Glossary of Terms d e f i n e s t h e abso lu te r a t i n g as be ing
t h e diameter o f t he l a r g e s t hard spher i ca l p a r t i c l e t h a t w i l l pass through a
f i l t e r under s p e c i f i e d t e s t c o n d i t i o n s . Th is i s an i n d i c a t i o n o f t h e l a r g e s t
opening i n t h e f i 1 t e r - e w n t .
13.5.3 The Bubble Tes t
T h i s i s a t e s t used by manufacturers t o de termine the area o f g rea tes t poro-
s i t y . I t i s achieved by a p p l y i n g a i r p ressure t o the i n s i d e o f t h e f i l t e r e l e -
ment, which i s submerged i n a l i q u i d such as a l c o h o l , which wets t h e f i l t e r
media. The opera to r r o t a t e s the f i l t e r element a t each pressure l e v e l and rec-
o rds the pressure a t which the f i r s t stream o f bubbles emi t ted f rom the f i l t e r
element.
t h i r d , f o u r t h , e t c . l a r g e s t ho le . By c o n t i n u i n g t o s low ly inc rease t h e pressure ,
a p o i n t i s reached c a l l e d t h e 'open bubb le p o i n t ' a t which a i r bubbles appear
over t h e e n t i r e su r face o f t h e f i l t e r element;
measuring the mean pore s i ze .
The t e s t can be cont inued t o measure t h e pressures o f t he second,
t h i s i s a s i m p l i f i e d method o f
I t i s c la imed t h a t p r e c i s e r e s u l t s a r e n o t o b t a i n a b l e f r o m t h e bubb le t e s t ,
which i s un fo r tuna te because such a s imp le t e s t would be inva luab le . As i t i s ,
i t s main use i s i n the q u a l i t y c o n t r o l o f elements t o ensure t h e r e i s no damage
t o the media o r a bad sea l .
13.5.4 Mean F i l t r a t i n g Ra t ing
Th is i s a measurement o f t h e average s i z e of t h e pores o f t h e f i l t e r media.
Th is i s a ve ry s i g n i f i c a n t r a t i n g , s i n c e i t i s a measure o f t h e p a r t i c l e s i z e
above which the f i l t e r s t a r t s be ing e f f e c t i v e .
'open bubble p o i n t ' method j u s t descr ibed.
I t can be measured us ing t h e
307
13.5.5 Mul t ipass F i l t e r Test
This t e s t , designed t o p rov ide a means o f desc r ib ing the performance charac-
t e r i s t i c s o f a f i l t e r , i nvo l ves the cont inuous i n j e c t i o n o f a c o n t r o l l e d conta-
minant i n t o a t e s t system. As the contaminant can be removed o n l y by the t e s t
f i l t e r , i t w i l l con t i nue t o c i r c u l a t e i n the system unless i t i s captured. The
separat ion c a p a b i l i t y o f t he t e s t f i l t e r i s monitored by analys ing upstream and
downstream f l u i d samples. The d i r t ho ld ing capac i t y i s measured by the amount
i n grammes o f t e s t contaminant which can be added t o the system before a speci -
f i e d terminal pressure drop across the f i l t e r i s reached.
The mathematical r e l a t i o n s h i p which descr ibes the t e s t i s developed from the
fo l l ow ing express ion:
Number o f p a r t i c l e s Number o f p a r t i c l e s Number o f Number o f downstream o f s i z e = o r i g i n a l l y o f s i z e + p a r t i c l e s
s i z e >xpm
- p a r t i c l e s removed > xpm > xpm i n j e c t e d o f o f s i z e >xpm
13.5.6 Beta R a t i o
The separat ion c h a r a c t e r i s t i c s a r e g i ven by the Beta r a t i o , which i s def ined
as fo l l ows :
o.. - Number o f upstream p a r t i c l e s l a r g e r than xpm ~~
Number o f downstream p a r t i c l e s l a r g e r than xpm
A Beta r a t i o o f 1 i nd i ca tes t h a t no p a r t i c l e contaminat ion i s removed. A
f i g u r e o f less than 1 i s c l e a r l y impossib le unless the f i l t e r i s un loading con-
taminants.
For a f i l t e r e x h i b i t i n g a Beta r a t i o g rea te r than 1 , t he downstream concen-
t r a t i o n o f p a r t i c l e s above a g iven s i z e w i l l s t a b i l i z e t o g i v e an almost con-
s tan t contaminat ion l e v e l .
13.5.7 P r a c t i c a l C l a s s i f i c a t i o n o f F i l t e r Performance
Whatever format i s used by the f i l t e r manufacturer t o g i v e performance i n f o r -
mation on h i s product, the degree o f f i l t r a t i o n prov ided w i l l b a s i c a l l y f a l l
i n t o one o f t h ree categor ies, depending on the degree o f s i l t c o n t r o l . Typica l
data corresponding t o these categor ies a r e g i ven i n Fig.12 though i t should be
the manufacturer 's r e s p o n s i b i l i t y t o s t a t e i n t o which o f the th ree c l a s s i f i c a -
t i ons h i s products f a l l . A t present t he re i s no u n i v e r s a l l y recognised s t
c l a s s i f i c a t i o n bu t work being c a r r i e d o u t by var ious bodies should eventua
lead t o an i n t e r n a t i o n a l l y agreed d e f i n i t i o n .
Two adverse f a c t o r s a f f e c t t he ac tua l performance o f f i l t e r s i n se rv i ce
namely p u l s a t i n g f l o w , and the sometimes unce r ta in performance o f i n t e r n a l
seals and bypass va lves.
idard
1 Y
308
Fig.12 D e f i n i t i o n o f p r a c t i c a l c l a s s i f i c a t i o n ca tegor ies Category Nomi na 1 Absolute Beta Ra t io
Rat ing pm Rat ing
S i l t Contro l f t o 1 3 t o 5 B3-5 ’75 P a r t i a l S i l t Contro l 3 t o 5 10 t o 15 B10-15>75
10 t o 15 No S i l t Contro l (Chip removal)
25 t o 40 625-40’75
13.5.7.1 Pu lsa t i ng Flow forces trirough the media those f i n e p a r t i c l e s which
would otherwise lodge among the f i b r e s and between l a r g e r p a r t i c l e s a l ready
in tercepted.
stones, some l a r g e r and some sma l le r than the openings i n the mesh; when t h e
s ieve i s s t a t i o n a r y , many o f t he smal l stones a r e re ta ined b u t they f a l l through
when the s ieve i s shaken. Pu lsa t i ng f l o w t h e r e f o r e increases the p r o p o r t i o n o f
s i l t p a r t i c l e s i n the system downstream o f t he f i l t e r , and t h i s i s r e f l e c t e d i n
the performance curve o f t he f i l t e r (F ig .13) .
The e f f e c t may be compared w i t h a s ieve ho ld ing a m i x t u r e o f
13.5.7.2 Bypassing by i n t e r n a l o r ex te rna l v a l v i n g i s acceptable f o r many
systems s ince, i n a number o f passes, a l l t he f l u i d even tua l l y goes through the
f i 1 t e r .
For f i l t e r s o t h e r than those designed f o r permanent bypassing i t would nor-
ma l l y be expected t h a t t he bypass be o p e r a t i v e o n l y when the element i s reaching
the end o f i t s useful l i f e .
bypass va l ve o r a f a u l t y i n t e r n a l seal need t o be considered. Since bypassing,
whatever the cause, does n o t d i s c r i m i n a t e between f i n e and coarse p a r t i c l e s ,
the e f f e c t i s t o weigh t h e contaminat ion p r o f i l e h e a v i l y a t t h e coarse end.
F igure 14 shows the e f f e c t o f i nc reas ing percentages o f bypass f lows, and i t
w i l l be noted t h a t even a t 0.1% bypass t h e maximum p a r t i c l e s i z e has almost
doubled.
t o r o f f i v e .
Therefore the e f f e c t s o f premature opening o f t he
A t 1% i t has mare than t reb led , and a t 10% i t has increased by a fac -
The p r a c t i c a l s i g n i f i c a n c e o f bypassing and the need t o make p r o v i s i o n f o r
For example, on i n l e t i t depend on the l o c a t i o n o f t he f i l t e r i n t h e system.
f i l t e r s a bypass va l ve i s mandatory i n order t o p r o t e c t t h e pump from c a v i t a t i o n
as the f i l t e r becomes blocked.
Pressure f i l t e r s a re intended t o p r o t e c t t h e system i f the re i s ca tas t roph ic
f a i l u r e o f the pump.
then the p r o t e c t i o n i s non-ex is tent .
bypass pressure f i l t e r s where t h e need f o r t h i s type i s estab l ished.
must be f i t t e d t h a t w i l l w i ths tand the t o t a l system pressure, which increases
the cos t o f the f i l t e r .
f a l l o f f and a r e l i a b l e i n d i c a t o r i s essen t ia l t o g i v e e a r l y warning o f t h i s .
I f such a f a i l u r e occurs when t h e f i l t e r i s bypassing,
There i s obv ious l y a case f o r f i t t i n g non-
An element
Clogging o f the element causes system Performance t o
309
-
lnterupted flow performance.
Steady flow
Q filter r=-
106 1
105 Q filter
w)'
103
102
10
1
1 0 2
1 0 3
50 70 90 120 m m Particle size - micmmetns.
Fig.13 F i l t e r performance de te r io ra tes Fig.14 Inf luence o f by-pass under i n t e r m i t t e n t o r pu l sa t i ng leakage on the f i l t e r f low. The e f f e c t i s more marked performance. w i t h f i n e r p a r t i c l e s which are forced through the media.
Where a bypass i s judged t o be permissible, the system designer must weigh
c a r e f u l l y the e f f e c t s o f premature opening due t o co ld s t a r t s and surge flows;
the e l i m i n a t i o n o f surges, even those a t low pressure, w i l l reduce the tendency
fo r the bypass t o open under normal operat ion.
A t present t he re i s l i t t l e in format ion ava i l ab le on the extent o f the adverse
e f f e c t t ha t pu l sa t i ng f low has on f i l t e r performance. Hopeful ly, Beta r a t i o s
w i l l eventual ly be quoted which do r e l a t e t o condi t ions which l i n e f i l t e r s a re
subjected t o i n actual p rac t i ce . Na tu ra l l y , where stea,dy f l ow e x i s t s r e l a t i n g
more c l o s e l y t o laboratory t e s t condi t ions, more accurate p red ic t i ons o f f i l t e r
performance can be made. However, using the l i m i t e d knowledge a t our d
l i k e l y performance leve ls f o r each of our categories have been produced
Fig. 15.
On each graph a cumulative count o f both 5 and 15 micrometre s i z e pa
sposal,
i n
t i c l e s
has been shown. The reason f o r using these f i gu res i s because o f the standard
set by the CETOP RP70H.
For ease i n r e l a t i n g t o t h i s document,the range numbers are a l s o quoted. I n
a d d i t i o n t o f i x e d f i gu res a t the two values the l i nes a re extended t o show expected trends f o r t y p i c a l f i l t e r s .
310
SILT REMOVER
CUMULATIVE COUNT SIZE pm
16.000(14) 64.000 (161
N.B. Range number as per CETOP RP7OH shown in brackets.
22 lor= ^.
105 I E h
m a
e i i
3 10‘ a 0 10’
- 0
a
- z
1 0 2
10
1 2 5 K) 1 5 2 0 2 5
P*rtlCk .I20
PARTIAL SILT REMOVER
CUMIJLATIVE COUNT SIZE pm
25,000 (18)
N. B. Range number as per CETOP RP7OH shown in brackets.
Fig.15A Assumed mean pore r a t i n g 3 F ig .159 Assumed mean pore r a t i n g 7 micro- micrometre. Curves show de- metre. Curves show d e t e r i o r a t i o n t e r i o r a t i o n o f f i l t e r p e r f o r - o f f i l t e r performance w i t h res- mance w i t h respect t o par- pect t o p a r t i c l e s below t h i s s i z e t i c l e s below t h i s s i z e under under p u l s a t i n g f low. p u l s a t i n g f low.
311
CHIP REMOVER
CUMULATIVE COUNT SIZE r m
IS 64.000 (16)
N.B. Range number as per CETOP RP70H shown in-brackets
Fig.15C Assumed mean pore r a t i n g 15 micrometre. Curves show d e t e r i o r a t i o n o f f i l t e r performance w i t h respec t t o p a r t i c l e s below t h i s s i z e under p u l - s a t i n g f l ow . There i s l i t t l e o r no c o n t r o l o f 5 micrometre p a r t i c l e s .
312
Assuming these graphs r e l a t e t o performance o f t he f i l t e r suppl ied by the
p a r t i c u l a r manufacturer chosen i t i s necessary t o show t h a t t h e actua requ i re -
ment when p l o t t e d s tays below the l i n e drawn f o r steady o r p u l s a t i n g low.
As an.example, i f we have a system w i t h a working pressure o f , say 150 bar
us ing components w i t h average d i r t to lerance, our c lean l i ness l e v e l graph
(Fig.10) t e l l s us t h a t the des i red contaminat ion l e v e l should be somewhere
between 15/11 and 18/14.
Re fe r r i ng now t o Fig.15a and b, we see t h a t under steady f l o w cond i t i ons
t h i s could be achieved by our p a r t i a l s i l t c o n t r o l f i l t e r , b u t i f p u l s a t i n g
f l ow i s present a s i l t removal f i l t e r i s requi red.
I t can be assumed t h a t most pressures and r e t u r n l i n e f i l t e r s a r e sub jec t t o
p u l s a t i n g f l o w cond i t i ons . The reasons a r e g i ven i n g r e a t e r d e t a i l i n t h e sec-
t i o n on f i l t e r l oca t i on .
of a steady f l o w r a t e cond i t i on .
On t h e o t h e r hand, o f f - l i n e systems have the b e n e f i t
I t must be emphasised t h a t the onus i s ve ry much on the f i l t e r manufacturer
t o i d e n t i f y the l i k e l y performance o f h i s p a r t i c u l a r media.
13.5.8 F i l t e r S i z i n g
I t i s cu r ren t p r a c t i c e f o r manufacturers t o s t a t e a f l o w r a t i n g a t a s p e c i f i c
c lean pressure drop (see Fig.16). Whi le t h i s i s a gu ide t o capaci ty , i t may
w e l l be necessary f o r t he system designer t o p r o v i d e a d d i t i o n a l d i r t capac i t y so
07 c I 07 -
Flow (I/mln)
Oil vlscoslty 30 cSt. Element pressure drop Is directly proportional to fluld viscosity.
Fig.16 Typ ica l pressure drops f o r c lean s i l t c o n t r o l and c h i p c o n t r o l elements o f s i m i l a r s i ze .
313
as t o ensure t h a t t he end user ob ta ins an acceptable element l i f e . A l l too
of ten f i l t e r s s i zed p u r e l y on f l o w r a t e have a s h o r t element l i f e . In choosing
l a rge r f i l t e r c a p a c i t i e s g rea te r i n i t i a l expense may be incurred, b u t t h i s i s
almost c e r t a i n t o be recovered i n reduced running costs , i .e. fewer element
changes, reduced labour cos ts and reduced downtime.
Correct f i l t e r s i z i n g necess i ta tes r e l a t i n g the d i r t e n t e r i n g the f i l t e r t o
the e f f e c t i v e element area and the maximum a l l owab le pressure drop.
ionship o f area t o pressure drop i s n o t simple, however, and f i l t e r i n l e t d i r t
l e v e l s a r e r a r e l y known.
The r e l a t -
There i s a l a b o r a t o r y t e s t ( t he 'comparat ive l i f e ' or ' d i r t c a p a c i t y ' t e s t )
which i s designed t o compare the d i r t ho ld ing c a p a c i t i e s o f h y d r a u l i c f i l t e r s .
An a r t i f i c i a l contaminant i s added a t a constant r a t e t o a cont inuously r e c i r c -
l a t i n g o i l system and the r e s u l t a n t increase i n d i f f e r e n t i a l pressure i s p l o t t e d
against t he weight o f contaminant added, as shown i n Fig.17. The r e s u l t i n g
curve has a c h a r a c t e r i s t i c form which i s constant f o r a g iven f i l t e r media.
Total A.C. flne test dust added (grams).
Fig.17 Typ ica l d i r t capac i t y curve f o r hydraul i c f i l t e r element.
6 7[ "I
I I I 1 0 25 50 75 100
Percent dirt capacity.
Fig.18 Comparison o f . d i r t c a p a c i t i e s and d i f f e r e n t i a l pressures f o r two d i f f e r e n t f i l t e r s .
314
I n the f i r s t stages o f d i r t a d d i t i o n i t can be seen t h a t the pressure drop
increases s lowly , whereas l a t e r t h e pressure drop increases very r a p i d l y . Th i s
i l l u s t r a t e s why very l i t t l e i s gained i n terms o f element l i f e by a l l o w i n g the
f i l t e r t o opera te a t a h i g h d i f f e r e n t i a l p ressure a f t e r the ' knee ' o f the curve
has been passed. The curve also i l l u s t r a t e s the i r r e l e v a n c y o f a system
designer concern ing h imse l f s o l e l y w i t h c lean pressure drops; the more s i g n i f -
i c a n t f a c t o r i s t h e d i f f e r e n t i a l p ressure across the f i l t e r a f t e r a s p e c i f i e d
amount o f d i r t has been added.
Except w i t h non-bypass f i l t e r s , t h e maximum pressure drop across the element
i s u s u a l l y determined by the bypass va l ve s e t t i n g .
also check t h a t the system performance i s ma in ta ined w i t h the pressure drop a t
i t s maximum value. I f f i l t e r s a r e overs ized, t h e bypass may pass a s u b s t a n t i a l
f l o w w i t h o u t i n d i c a t i n g .
The system des igner must
I t i s commonly thought t h a t t o o b t a i n longer s e r v i c e l i f e f rom a g iven f i l t e r
element w i t h o u t s a c r i f i c i n g f i l t e r e f f i c i e n c y , i t i s o n l y necessary t o pack more
media area i n t o the f i l t e r envelope. Th is i s n o t so s ince an optimum area
e x i s t s f o r a g i ven envelope and i t i s de t r imen ta l t o exceed t h i s a rea .
The comparat ive l i f e t e s t p r e v i o u s l y descr ibed i s used t o compare the d i r t
ho ld ing c a p a c i t y o f d i f f e r e n t f i l t e r elements, and i n Fig.18 we compare the d i r t
c a p a c i t y o f two f i l t e r s o f i d e n t i c a l envelope s i ze . The f i l t e r A has a lower
c lean pressure drop than f i l t e r B because A has more area.
area has been exceeded and t h e r e f o r e f i l t e r A would have a s h o r t e r l i f e than B
f o r a g i ven pressure d i f f e r e n t i a l .
However, t he optimum
How c l o s e pack ing reduces the e f f e c t i v e area i s i l l u s t r a t e d i n Fig.19 where
the p l e a t s c l o s e up under p ressure and the smal l angles between them c l o g
r a p i d l y . Fa t igue f a i l u r e s can a l s o occur when pressure i s app l i ed . As da ta on
the r e s u l t s o f d i r t c a p a c i t y t e s t s a r e no t r e a d i l y a v a i l a b l e , we must r e v e r t t o
the manu fac tu re r ' s f l o w r a t i n g a t m s p e c i f i c c lean pressure drop and use t h i s a s a b a s i s f o r assess ing d i r t capac i t y .
Fluid flow Fluid flow
t + Effective areas
t
under high flow medium
J t Pleats close up under
pressure and small
angles between them effective filter area high flow conditions clog rapidly, reducing the effectlve area
Wider spacing gives smaller but more
High stress under Flow fatigue failure
Fig.19A Increas ing f i l t e r area w i t h i n Fig.198 Changes i n f l o w and pressure a g iven package may a c t u a l l y drop cause the s ides o f element reduce the d i r t h o l d i n g co r ruga t ions t o f l e x and the capac i t y . r o o t t o s t r e t c h , g i v i n g r i s e t o
f a t i g u e s t resses .
315
Recapping, our o b j e c t i v e i s t o achieve a ' d i r t i n equals d i r t o u t ' cond i t i on .
The des i red d i r t capac i t y o f t h e f i l t r a t i o n system w i l l t h e r e f o r e depend t o a
large extent on our a b i l i t y t o c o n t r o l t he d i r t i npu t . The d i r t i npu t i s a
product o f i n b u i l t contaminat ion and ingressed contaminat ion which, i n tu rn ,
produces system generated contaminat ion. Le t us f i r s t examine each source i n
d e t a i l and the f a c t o r s c o n t r o l l i n g i t s i npu t (F ig .20) .
Contamination source C o n t r o l l e r
Dirt input = I n b u i l t : i n components pipes, mani fo lds,etc .
p lus
Present i n i n i t i a l charge o f f l u i d
p lus
lngressed through a i r b rea the r
p l u s
lngressed du r ing f l u i d replenishment
p l u s
lngressed d u r i n g main- tenance
p lus
lngressed through c y l i n d e r rod seals
p l u s
Fu r the r generated conta- m ina t i on produced as a r e s u l t o f t he above and the s e v e r i t y o f the du ty cyc le .
Good f l u s h i n g procedures, system no t operated on load u n t i l acceptable con- tam ina t ion l e v e l obtained.
I n t e g r i t y o f s u p p l i e r . F l u i d s tored under c o r r e c t cond i t i ons (exc lus ion o f d i r t , condensat ion,etc.) du r ing f i l l i n g .
F1 u i d f i 1 tered
An e f f e c t i v e a i r breather w i t h r a t i n g compat ib le w i t h degree o f f l u i d f i l t r a - t i o n .
S u i t a b l e f i l l i n g p o i n t s which ensure some f i l t r a t i o n o f f l u i d before en te r ing r e s e r v o i r . Th is task undertaken by responsib le personnel . Design should minimise the e f f e c t s .
E f f e c t i v e wiper seals o r , i f a i rbo rne contaminat ion, rods p ro tec ted by s u i t a b l e g a i t e r s .
Correct f l u i d s e l e c t i o n and p r o p e r t i e s ( v i s c o s i t y and a d d i t i v e s ) maintained. Good system design min imis ing e f f e c t s o f contaminat ion present on system compo- nents.
Fig.20 The p r a c t i c a l steps which c o n t r o l Contaminat ion i n h y d r a u l i c systems.
Based on Fig.20 we w i l l now grade the c lean l i ness l e v e l o f a system between
1 and 7. An example o f a grade 1 c lean system would be a c lean workshop w i t h
e f f e c t i v e c o n t r o l over a l l contaminat ion ingress. A grade 7 d i r t y system would
be, say, a foundry w i t h l i t t l e o r no c o n t r o l over contaminat ion ingress and a
system opera t i ng several exposed c y l i n d e r s .
316
Figure 21 w i l l a s s i s t i n making a numerical assessment between these values
based on the environment and the degree o f con t ro l over contamination.
Environmental Conditions
Good Average Bad Degree of Control
3 6 7 L i t t l e or no con t ro l over contamination ingression (many exposed c y l inders)
2 4 5 Some con t ro l over contamination ingression (few c y l inders).
1 2 3 Good con t ro l over contamination ingression (ga i tered c y l inders).
Fig.21 Assessment and c l a s s i f i c a t i o n o f system c leanl iness l eve l i n t o 7 grades.
We must now r e l a t e t h i s t o the e f f e c t i v e element area and the maximum al low-
ab le pressure drop. The r e l a t i o n s h i p between area and pressure drop i s not
simple, but by using very broad approximations o f these values and assuming
tha t the manufacturers f l ow r a t i n g a t a s p e c i f i c c lean pressure drop i s a good
guide t o d i r c t capacity, the fo l l ow ing se lec t i on guides can be used:
10 To use this graph:
9 1. Decide on maximum acceptable pressure 2. Assess cleanliness grade in SyStem. from drop, this will depend on system require- Fig. 22. Draw vertical line through inter. ments or bypass pressure. Draw section of pressure drop and cleanliness t
30 horizontal line through this value. grade line.
u 7 B t
i 6 a a 5 ?
3. Read off multiplication factor where vertical llne crosses horizontal axis. Multiply this factor by the actual flow rate at chosen pressure line location. Now select a filter element to handle this revlsed flow rate at a pressure drop of 1 bar (According to manufacturers clean rating).
Actual flow x multiplication factor = recommended filter capacity.
- !I4 2 3 i
2
1
1 2 3 4 Multipiiulion faclor.
Fig.22 Pressure l i n e f i l t e r se lec t i on guide.
317
3'0 4 \ \ \ \ Tousethisgraph:
1. Decide on maximum acceptable pressure 2. Assess cleanliness grade in system, from drop, this will depend on system require Fig. 22. Draw vertical line through inter- ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.
3. Read off multiplication factorwhere vertical line crosses horizontal axis. Multiply this factor by theactual flow rateat chosen return line location. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(According to manufacturers clean rating).
Actual flow x multiplication factor = recommended filter capacity.
2 3 4 Mulliplhlion faclor.
Fig.23 Return l i n e f i l t e r s e l e c t i o n guide.
To use this graph:
1. Decide on maximum acceptable pressure 2. Assess cleanlinessgrade in system, from drop, this will depend on system require Fig. 22. Draw vertical line through inter- ments or bypass pressure. Draw section of pressure drop and cleanliness horizontal line through this value. grade lines.
3. Read off multiplication lactor where vertical line crosses horizontal axis. Multiply this factor by theactual offline pump flow rate. Now select a filter element to handle this revised flow rate at a pressure drop of 0.3 bar(acc0rding to manufacturers clean rating).
Note:Toachieve reasonable lifea minimum multiplication factor of 2 is recommended.
Actual flow x multiplication factor = recommended filtercapacity.
2 - 4 6 Multlpliution factor.
8
Fig.24 O f f l i n e f i l t e r s e l e c t i o n guide.
318
13.6 LOCATING THE FILTER
13.6.1 Pump I n l e t F i l t r a t i o n
Correct f i l l i n g o f t he pump i s v i t a l i f s a t i s f a c t o r y ope ra t i on o f the hydrau-
l i c system i s t o be achieved. A l l t o o o f t e n i n s u f f i c i e n t a t t e n t i o n i s pa id by
the designer t o the t o t a l pump i n l e t c o n f i g u r a t i o n and, as a r e s u l t , c a v i t a t i o n
remains a major cause o f pump f a i l u r e .
For t h i s reason i t i s wor th l ook ing once more a t t he bas i c p r i n c i p l e s o f
pump f i l l i n g . By f a r t he most common method i s t o use atmospher ic pressure
a c t i n g on the f l u i d su r face o f t he r e s e r v o i r t o f o r c e f l u i d i n t o t h e pump i n l e t
chambers. For convenience, t he pump i s o f t e n s i t e d above the f l u i d l e v e l
(F ig .25) .
From bas ic h y d r a u l i c p r i n c i p l e s we know t h a t f o r f l o w t o take p lace we must
have a pressure d i f f e r e n c e .
a c t i o n o f t he pump t o c r e a t e a depression a t i t s i n l e t . I t i s usual f o r manu-
f a c t u r e r s t o quote a maximum a l l owab le depression a t t h e pump i n l e t , which i s
l i k e l y t o be o f t he o rde r o f 0.17 bar .
ted f o r , o n l y a very smal l pressure drop can be t o l e r a t e d across the f i l t e r .
For t h i s reason the s i z e and cos t o f i n l e t f i l t e r s i s o f t e n g rea te r than, say,
f i l t e r s i n the r e t u r n l i n e . Furthermore, such low pressure drops make s i l t
removal v i r t u a l l y impossible.
W i th t h i s arrangement we r e l y on t h e mechanical
Thus w i t h normal pressure drops accoun-
F i r e - r e s i s t a n t f l u i d s a r e very s e n s i t i v e t o s u c t i o n pressures. They have
h igher s p e c i f i c g r a v i t i e s than minera l o i l s , p a r t i c u l a r l y t he s y n t h e t i c types.
This increases the pressure drop t o the pump and a t t he same t ime demands a
h igher pressure t o acce le ra te the f l u i d i n t o t h e pump.
i n - o i l emulsions have a h igh vapour pressure and the pump i n l e t depression
should be l i m i t e d wherever p o s s i b l e t o h a l f t h e va lue f o r minera l o i l s , even
when temperature i s l i m i t e d t o 5OoC.
u s u a l l y e s s e n t i a l t o p r o v i d e a p o s i t i v e head a t t he pump i n l e t when us ing these
f l u i d s (F ig .26) .
Water g l y c o l s and water-
Wi th o r w i t h o u t i n l e t l i n e f i l t e r s i t i s
Atmospheric pressure 'There must beadepression here to'lift'theoil.
Atmospheric pressure
\
lL+ Pump is charged at positive pressure.
Fig.25 Negat ive head tank. Fig.26 P o s i t i v e head tank.
319
I r r e s p e c t i v e o f t h e type o f f l u i d , a p o s i t i v e head w i l l improve i n l e t cond i -
t i o n s because i t increases the f o r c e a v a i l a b l e t o c r e a t e the requ i red f l ow .
For bo th nega t i ve and p o s i t i v e head i n l e t arrangements, when s i z i n g i n l e t
f i l t e r s we must ensure t h a t any f i l t e r (housing and element) and assoc ia ted
pipework should: 1 ) pass the f u l l pump volume w i t h i n t h e pe rm i t ted i n l e t depr-
ession f o r t h a t pump, and 2) pe rm i t a bypass f l o w t h a t i s s t i l l w i t h i n t h a t
l i m i t when t h e f i l t e r element i s blocked. (Th is requirement o f t e n necess i ta tes
the opera t i on o f t he bypass f e a t u r e a t pressures as low as 0.085 bar , a l e v e l
a t which o p e r a t i o n i s seldom c o n s i s t e n t . )
A l l c a l c u l a t i o n s should take i n t o account t h e e f f e c t o f h ighe r v i s c o s i t y
f l u i d , e.g. a t c o l d s t a r t s , o the rw ise c a v i t a t i o n w i l l occur.
The usual micrometre r a t i n g f o r i n l e t f i l t e r s i s 75 o r 150. However, u n i t s
a re a v a i l a b l e i n c o r p o r a t i n g elements down t o 10 micrometre. Th is means l a r g e
housings and probab le o v e r s i z i n g o f t h e i n l e t c o n f i g u r a t i o n .
The 75 and 150 micrometre mesh elements w i l l remove most o f t he p a r t i c l e s
above t h e i r r a t i n g bu t a r e r e l a t i v e l y i n e f f e c t i v e i n removing any th ing sma l le r .
The a d d i t i o n o f magnets w i l l remove some o f t h e f i n e m e t a l i i c p a r t i c l e s bu t t he
l o c a t i o n o f t h e magnets w i t h i n the f i l t e r must be such t h a t under no circum-
stances can accumulated contaminant break away, thereby passing a conglomerate
i n t o t h e pump.
An advantage o f t e n c la imed f o r i n l e t f i l t e r s i s ease o f s e r v i c i n g . However,
i n c o r r e c t re-assembly o f access covers can r e s u l t i n a i r ingress , which o f t e n
goes undetected and can be harmful t o the system as d i r t .
, I n l e t f i l t e r s a r e g e n e r a l l y used i n systems where maintenance procedures do
no t p revent q u a n t i t i e s o f l a r g e p a r t i c l e s e n t e r i n g the r e s e r v o i r , f o r ins tance
d u r i n g topping-up. P r o v i d i n g bypassing i s no t occu r r i ng , they p r o t e c t t h e pump
from t h i s type o f i ng ress ion . However, good r e s e r v o i r design t h a t i nc udes a
mesh screen o r b a f f l e can p r o v i d e an e q u a l l y acceptab le contaminat ion eve1 a t
t h e pump i n l e t . Under these circumstances, s t r a i n e r s and f i l t e r s and h e i r
assoc ia ted f i t t i n g s can be omi t ted f rom the i n l e t l i n e , thus improving pump
f i l l i n g cond i t i ons .
I t i s an encouraging s i g n t h a t more des igners a r e p r o v i d i n g separa te f i l l i n g
arrangements, u s u a l l y through some fo rm o f coarse f i l t e r t o reduce the need f o r
las t -chance pump i n l e t p r o t e c t i o n .
I f t h e f i l l i n g requirements o f t h e pump a r e c r i t i c a l and supercharging i s
necessary, as i s t h e case q u i t e o f t e n w i t h l a r g e var iab le -d isp lacement p i s t o n
pumps, i t i s common t o s i t e a f i l t e r between t h e supercharger and main pump.
For such a p p l i c a t i o n s , t h e same g u i d e l i n e s g i ven f o r t he s i z i n g o f p ressure o r
r e t u r n l i n e f i l t e r s can be used. However, depressions can occur when a v a r i a b l e
pump moves f rom zero t o f u l l d isplacement, and t h e l i k e l y e f f e c t on f i l t e r per -
formance must be c a r e f u l l y considered.
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13.6.2 Pressure L i n e F i l t r a t i o n
Let us s t a r t by d iscuss ing the l o c a t i o n o f t h e pressure l i n e f i l t e r i n r e l a -
t i o n t o pump and r e l i e f va lve.
F igure 27 shows the pressure f i l t e r located downstream o f t he r e l i e f va lve.
For the non-bypass type the arrangement shown i n Fig.28 i s mandatory.
ac tua l f l o w seen by t h e f i l t e r d u r i n g t h e o p e r a t i n g c y c l e depends on t h e system
demand and du r ing o f f - l oad per iods the re i s leakage f l o w o n l y i f b lock -cen t re
d i r e c t i o n a l va lves a r e employed. N a t u r a l l y , i f o f f - l o a d i n g i s achieved through
open-centre d i r e c t i o n a l va lves the f i l t e r w i l l see f u l l pump ou tpu t f o r t h i s
per iod.
The
Pressure Bypass
Rellef ‘c filter
Tank
Strainer
Tank
Fig.27 Pressure l i n e f i l t r a t i o n w i t h bypass f i l t e r .
Pressure Q line filler
Relief
.-+--A
Pump Tank
Y Strainer
ie_l Tank
Fig.28 Pressure l i n e f i l t r a t i o n w i t h non-bypass f i l t e r .
To increase the f l o w across the f i l t e r i t has become common p r a c t i c e t o
l oca te the pressure f i 1 t e r between pump and re1 i e f va l ve (Fig.29).
cates o f t h i s arrangement p o i n t t o the f a c t t h a t t h e r e l i e f va l ve i s p ro tec ted
from pump generated d i r t . A v a l i d p o i n t , however, i s t h a t t h i s generated d i r t
i s caused by pump wear which, i n tu rn , i s d i r e c t l y p r o p o r t i o n a l t o t h e conta-
m ina t i on l e v e l o f t h e f l u i d e n t e r i n g the pump i n l e t .
component and we should t h e r e f o r e d i r e c t more a t t e n t i o n t o reducing the conta-
m ina t i on e n t e r i n g t h e pump to a l e v e l which w i l l min imize wear.
arrangement, a bypass i s mandatory and t h e r e must be an assurance from the
f i l t e r manufacturer t h a t any f i l t e r ma l func t i on w i l l no t r e s u l t i n excessive
The advo-
The pump i s o f t e n a c o s t l y
For t h i s
321
pressure a t the pump o u t l e t .
Relief
Tank
,$!, Strainer
Tank
Fig.29 Loca t ing pressure f i l t e r be fo re r e l i e f va l ve g i ves constant f l o w through f i l t e r .
Where var iab le-d isp lacement pumps a r e employed, c a r e f u l ana lys i s o f t he f l o w
seen by the f i l t e r must be made.
pump which i s compensating ( i . e . low displacement a t maximum system opera t i ng
pressure), where a low f l o w r a t e e x i s t s a l though t h e pump i s s t i l l ope ra t i ng a t
pressure. With a pressure l i n e f i l t e r d i r t removal i s l i m i t e d . Even when f l o w
demands a r e made these a r e 1 im i ted t o c y l i n d e r displacement.
Take t h e c l a s s i c case o f a pressure compensated
As i l l u s t r a t e d by Fig.15, t h e f i l t e r e f f i c i e n c y depends upon t h e type o f f l o w
i t sees, a l though c u r r e n t l y no standard t e s t e x i s t s f o r eva lua t i ng f i l t e r s when
they a re subjected t o abrupt f l o w and pressure changes and mechanical v i b r a t i o n .
P rac t i ce has shown t h e r e i s a reduc t i on i n e f f i c i e n c y bu t i t s ex ten t depends no t
on ly on the q u a l i t y o f t h e ac tua l f i l t e r media b u t a l s o on how w e l l t h i s media
i s supported.
Beta r a t i o quoted which r e l a t e s more c l o s e l y t o cond i t i ons seen by a f i l t e r i n
real h y d r a u l i c systems. I n t h e f u t u r e i t w i l l a l s o be e s s e n t i a l t h a t t h e system
designer has i n fo rma t ion on the performance o f i n t e g r a l bypass va lves under
system opera t i ng cond i t i ons .
We must hope t h a t standard t e s t parameters can be agreed and a
I n a d d i t i o n t o mechanical v i b r a t i o n and abrupt f l o w and pressure changes when
valves a r e operated, a pressure l i n e f i l t e r i s a l s o sub jec t t o pump pu lsa t i ons .
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These e f f e c t s were demonstrated r e c e n t l y on a h y d r a u l i c component t e s t r i g where
the so -ca l l ed 10 micrometre nominal f i l t e r proved almost t o t a l l y i n e f f e c t i v e i n
removing sub-5 micrometre contaminant.
Because pressure f i l t e r s have t o w i ths tand t h e f u l l system pressure w i t h
adequate margin o f s a f e t y , t h e r e i s a tendency t o make these sma l l , thus l i m i t i n
t h e i r d i r t h o l d i n g capac i t y . It i s usual f o r manufacturers t o quote a ra ted
f low a t around 1 bar f o r bypass types and a t a s l i g h t l y h ighe r p ressure f o r non-
bypass types which i nco rpo ra te h i g h pressure d i f f e r e n t i a l c a p a b i l i t y elements.
I n summary, i t can be s a i d t h a t p ressure f i l t e r s a r e sub jec ted t o cond i t i ons
f a r removed from t h a t found i n a l a b o r a t o r y f i l t e r t e s t r i g , and because o f t h i s
the des igne r ' s t ask i n assess ing t h e r e s u l t a n t con taminat ion i s ex t remely d i f f i -
c u l t . The l e v e l s g i ven i n F ig .15 a r e based on f i e l d exper ience us ing good
q u a l i t y elements.
Pressure f i l t e r s may w e l l be used i n c e r t a i n a p p l i c a t i o n s t o p r o t e c t t h e
system should c a t a s t r o p h i c f a i l u r e s o f t h e pump occur, o r t o p rov ide spec ia l
p r o t e c t i o n t o a s i n g l e u n i t o r group o f u n i t s . An example o f t h e l a t t e r would
be a servo va l ve , where f a i l u r e migh t be ex t remely expensive, though i t should
be noted t h a t a p ressure f i l t e r does n o t p r o t e c t a servo v a l v e f rom d i r t i n -
gressed th rough c y l i n d e r rod sea ls . I n such cases a non-bypass f i l t e r should be
considered w i t h t h e assurance t h a t t o t a l element c o l l a p s e cannot occur .
pass types a re employed, some means o f i n d i c a t i o n should be p rov ided t o g i v e
warning o f a p a r t i a l l y b locked element.
I f by-
From a maintenance p o i n t o f view, changing elements i nvo l ves s topp ing t h e
system un less e x t e r n a l bypass v a l v i n g i s p rov ided. T h i s o p e r a t i o n o f t e n a l l ows
f r e e a i r i n t o the system which must be c lea red b e f o r e s a t i s f a c t o r y machine per -
formance can be ob ta ined.
13.6.3 Return L i n e F i l t r a t i o n
The usual r e t u r n l i n e f i l t r a t i o n arrangement shown i n F ig .30 has a l l r e t u r n
l i n e s passing through t h e f i l t e r . D ra in l i n e s f rom pumps, motors, and c e r t a i n
va lves should no t be sub jec ted t o p ressure surges emanating f rom the system
r e t u r n l i n e s and should r e t u r n separa te l y t o tank.
Where t h e r e a reh igh surges (e.g. due t o u n c o n t r o l l e d decompression o r the
r a p i d a c c e l e r a t i o n o f t he f l u i d column i n t h e r e l i e f va l ve tank l i n e when t h i s
va l ve opera tes) i t may be undes i rab le t o pass these th rough t h e f i l t e r .
p revent c o l l a p s e o f t h e element due t o h i g h - v i s c o s i t y o i l , e.g. a t a c o l d s t a r t ,
o r when t h e element i s loaded w i t h d i r t , an i n t e r n a l bypass should be provided.
When bypassing occurs under m i n i m u m f l o w and surge c o n d i t i o n s t h e c i r c u l a t e d
To
f l u i d should no t be contaminated by d i r t a l ready r e t a i n e d i n t h e f i l t e r . The
t ype o f surge exper ienced i n most r e t u r n l i n e s w i l l reduce t h e f i l t e r e f f i c i e n c y
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and i n the absence o f p rec i se data the f i l t e r should be se lected us ing the
pu lsa t i ng f l o w c o n d i t i o n f rom Fig.15.
F u l l f l o w r e t u r n f i l t r a t i o n should be o f s u f f i c i e n t capac i t y t o handle the
maximum r e t u r n f l o w ( i n c l u d i n g any i n excess o f pump f l o w where, f o r example,
unbalanced c y l i n d e r s a r e used) w i thou t t he bypass opening.
Linear actuator
I I Directional
valve
I valve
Strainer
Tank
$0 Return line
Bypass filter
LLI Tank
Fig.30 Basic arrangement o f r e t u r n 1 ine f i l t r a t i o n .
Careful assessment o f the f l o w across the f i l t e r i s necessary. The comments
r e l a t i n g t o the use o f v a r i a b l e displacement pumps w i t h a pressure f i l t e r again
apply, s ince w i t h pressure compensated pumps the f i l t e r sees o n l y the volume
d isp laced by t h e c y l i n d e r .
For c e r t a i n systems p a r t i a l r e t u r n l i n e f i l t r a t i o n i s acceptable w i t h the
With bypass ( i n t e r n a l o r e x t e r n a l ) always pass ing a percentage o f the f low.
t h i s arrangement much depends on the cont inuous r a t e o f f l o w which we can ge t
across the f i l t e r . S a t i s f a c t o r y performance can sometimes be achieved w i t h as
l i t t l e as 1 0 % passing across the ac tua l element.
To summarize, a r e t u r n l i n e f i l t e r does n o t p r o t e c t the system f rom envi ron-
mental d i r t e n t e r i n g the r e s e r v o i r v i a b rea the rs o r du r ing topping-up. However,
i f we s t a r t o f f w i t h a c lean r e s e r v o i r and take precaut ions t o prevent the
ingress o f environmental contaminat ion, then exper ience has shown t h a t e f f e c t i v e
f i l t r a t i o n can be achieved economical ly w i t h r e t u r n l i n e f i l t e r s .
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13.6.4 O f f -L ine F i t r a t i o n
I t has been s t a t e d t h a t t h e e f fec t i veness o f f i l t e r s s ted i n bo th pressure
and r e t u r n l i n e s i s reduced by shocks, surges, p u l s a t i o n s v i b r a t i o n s , e t c . t o
an ex ten t which depends on t h e type o f media and how w e l l i t i s supported.
Steady f l o w r e l a t i v e l y f r e e o f pressure f l u c t u a t i o n s prov ides optimum f i l t e r
performance. The s imp les t way o f ach iev ing t h i s i s t o r e m v e the f i l t e r from
the main system and p lace i t i n an independent ly powered c i r c u l a t i n g u n i t where
i t s performance i s more p r e d i c t a b l e .
Though connected t o t h e system r e s e r v o i r , a t y p i c a l arrangement i s shown i n
Fig.31. The l i k e l y contaminat ion l e v e l f rom our th ree f i l t e r r a t i n g s under
steady f l o w cond i t i ons i s g i ven i n F ig .15, .andaour a b i l i t y t o p rov ide t h e c o r r -
e c t f l o w r a t e w i l l determine whether o r n o t these l e v e l s can be achieved and
ma i n t a i ned .
Fig.31 Layout o f o f f - l i n e f i l t r a t i o n system. Valves (a) and (b) can be pos i t i oned so t h a t the u n i t w i l l p rov ide a f i l t e r e d f i l l or discharge.
Assuming reasonable standards o f engineer ing i n the design and b u i l d o f t h e
system, the most re levan t f a c t o r s i n s e l e c t i n g the f l o w r a t e through t h e o f f -
l i n e f i l t r a t i o n a r e l i k e l y t o be environment and tank s i ze . From f i e l d exper-
ience t o date the g u i d e l i n e s g i ven i n Fig.32 can be app l i ed and the f l o w r a t e
325
der ived can be used t o determine t h e s i z e o f t h e f i l t e r by t h e procedure prev-
i ous l y o u t l i n e d .
Environment Flow r a t e ( l /m in ) as percentage o f t ank capac i t y ( I i t r e s )
Good 5%
Average 10%
Bad 20%
Fig.32 Suggested f l o w r a t e s f o r o f f - 1 i n e f i l t r a t i o n .
With o f f - l i n e f i l t r a t i o n t h e des igner 's t ask i s s i m p l i f i e d because he i s not
governed by t h e f l o w and pressure c h a r a c t e r i s t i c s o f t h e main system.
enables him t o s e l e c t t h e best f i l t e r and the best f l o w through i t and then
s e l e c t t h e s i z e t o achieve the des i red frequency o f maintenance.
I t
Should t h e des i red contaminat ion l e v e l n o t be achieved, then co r rec t i ons a r e
e a s i l y made t o the f l o w r a t e o r type o f f i l t e r w i t h o u t i n any way a f f e c t i n g the
design o f t h e main system. Furthermore, t he o f f - l i n e i n s t a l l a t i o n can be run
p r i o r t o s t a r t i n g t h e main system i n o rde r t o c lean t h e o i l i n t he tank and
reduce t h e contaminat ion l e v e l t h e pump w i l l see a t s ta r t -up . By t h e a d d i t i o n
o f s imple v a l v e gear i t can be employed t o f i l t e r t h e i n i t i a l charge o f f l u i d
and any used subsequent ly i n topping-up. I d e a l l y , i t should be l e f t running
con t inuous ly t o p rov ide a complete tank o f c lean f l u i d ready f o r every s ta r t -up .
U n l i k e l i n e f i l t e r s , t he o f f - l i n e i n s t a l l a t i o n w i l l con t i nue t o c lean up the
f l u i d when v a r i a b l e d e l i v e r y pumps a r e running a t minimum displacement.
Being independent o f t he main system o f f - l i n e f i l t e r s can be p laced where
they a r e most convenient f o r s e r v i c i n g .
main system i s n o t a f f e c t e d ;
w i t h o u t s topping o r i n t roduc ing a i r i n t o the main system, thus making the very
minimum o f s e r v i c i n g s k i l l s acceptable.
When element changes a r e necessary the
the opera t i on can be c a r r i e d o u t a t any t ime
Whether o r n o t o f f - l i n e f i l t r a t i o n can be the s o l e means o f f i l t r a t i o n
depends on many f a c t o r s r e l a t i n g t o t h e cha rac te r , q u a n t i t y and o r i g i n o f the
contaminat ion. As i t i s p a r t i a l f i l t r a t i o n we must decide i f i t i s necessary
t o p r o t e c t i n d i v i d u a l o r groups o f components from s t r a y p a r t i c l e s l i k e l y t o
cause a c a t a s t r o p h i c f a i l u r e .
The foregoing has at tempted t o show t h a t t he re i s a p lace f o r i n l e t , pressure,
r e t u r n and o f f - l i n e f i l t r a t i o n , which i s borne o u t by f i l t e r manufacturers who
once t o l d us t h a t pressure f i l t e r s answered a l l our problems and who now o f f e r
a range o f r e t u r n l i n e f i l t e r s , and manufacturers o f r e t u r n f i l t e r s who now
o f f e r ranges o f pressure f i l t e r s .
ee r ing , system f i l t r a t i o n engineer ing i nvo l ves t a k i n g a r i s k . Sure ly what i s most impor tant i s t o r e a l i s e t h a t l i k e a l l branches o f engin-
A s i n g l e p a r t i c l e
326
Fig. 3 3 A pictorial representation 01 the bask liltration W4ualiOn.
327
say, i n the 5 t o 1 0 micrometre s i z e range, cou ld cause system ma l func t i on i f i t
a r r i v e d a t t he wrong p lace a t a c r i t i c a l t ime. The machine t o o l designer may
we l l have the b e n e f i t o f a p ro to type on which t o eva lua te t h e f i l t r a t i o n system
performance and make changes p r i o r t o the f i r s t p roduc t i on ba tch . The designer
o f heavy equipment, say, f o r s tee lworks i s a f f o r d e d no such o p p o r t u n i t y ; he
must be r i g h t f i r s t t ime and may w e l l have t o use a l l t he f i l t e r s e l e c t i o n pro-
cedures a v a i l a b l e t o him i n o rde r t o min imize the r i s k o f expensive stoppages
l a t e r . That r i s k w i l l be w i t h us u n t i l (a) we have more r e a l i s t i c da ta on f i l -
t e r performance, and (b) maintenance p r a c t i c e s a r e g r e a t l y improved.
The message o f the preced ing chapters can bes t be summarised d iagrammat ica l l y
(Fig.33).
by app ly ing good i n s t a l l a t i o n p r a c t i c e s and us ing a l l means a v a i l a b l e t o l i m i t
the amount o f ingressed contaminat ion , enables us t o c o n t r o l t h e system gener-
ated contaminat ion t o an acceptab le l e v e l .
Th is h igh1 i g h t s the f a c t t h a t c o n t r o l over the i n b u i l t con taminat ion
The l e v e l o f con taminat ion e n t e r i n g the pump i s a c r i t i c a l f a c t o r and should
be so c o n t r o l l e d t h a t i t prevents the s o r t o f a c t i o n dep ic ted by a mangle. The
d i f f i c u l t y i n c o n t r o l l i n g t h i s l e v e l by i n l e t f i l t e r s has been c l e a r l y s ta ted .
I n the main, they should be cons idered o n l y f o r p reven t ing l a r g e p a r t i c l e s en te r -
ing the pump and causing c a t a s t r o p h i c f a i l u r e .
Downstream o f t h e pump t h e a b i l i t y o f a pressure f i l t e r t o t r a p p a r t i c l e s
w i l l be i n f l uenced t o a l a r g e ex ten t by any ab rup t changes o f f l o w and pressure ,
which have the e f f e c t o f d r i v i n g p a r t i c l e s th rough the f i l t e r i n g media. We
know from p r a c t i c e t h a t bypass va lves can ma l func t i on under c e r t a i n c i rcumstan-
ces, and i t i s e s s e n t i a l t h a t t h e des ign o f t h e f i l t e r p revents any m i g r a t i o n
o f con taminat ion i f t h i s should happen.
Ingress through c y l i n d e r rod sea ls , a l t hough i n t h e sma l le r p a r t i c l e range,
can never the less add up t o s i g n i f i c a n t q u a n t i t i e s .
r e t u r n l i n e f i l t e r w i l l , aga in , depend upon the magnitude o f changes i t sees i n
terms o f f l o w and pressure .
The performance o f any
O f f - l i n e f i l t r a t i o n enables optimum f i l t e r performance t o be accomplished,
thus enab l i ng us t o c o n t r o l con taminat ion l e v e l s more e f f e c t i v e l y . Our a b i l i t y
t o remove contaminat ion depends n o t o n l y on the f i l t e r micrometre r a t i n g bu t
a l so on the f l o w r a t e across i t , and we must p r o v i d e s u f f i c i e n t d i r t h o l d i n g
capac i ty t o ensure an element l i f e acceptab le t o t h e end user .
13.7 SUMMARY
Any examinat ion o f the sub jec t o f con taminat ion i nvo l ves f o u r groups o f
peop 1 e :
( i )
( i i )
( i i i ) The manufacturer o f t he machinery which uses t h e h y d r a u l i c equipment.
( i v )
The f l u i d manufacturer o r s u p p l i e r .
The h y d r a u l i c equipment and f i l t e r manufacturers.
The end user o f t h e machinery.
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Each o f these has a commercial r e s p o n s i b i l i t y t o supply equipment which w i l l
perform i t s duty s a t i s f a c t o r i l y a t a reasonable p r i ce , and each must have some
knowledge o f the c leanl iness o f the working f l u i d .
The f l u i d suppl ier w i l l supply f l u i d as c lean as requi red and w i l l charge
accordingly.
The hydraul ic equipment manufacturer must advise the user to whom he i s
s e l l i n g h i s equipment on the type o f f l u i d and i t s degree o f c leanl iness best
su i ted t o the hyd rau l i c equipment he i s recommending f o r a s p e c i f i c app l i ca t i on .
The machinery manufacturer, who i s responsible t o the end user f o r supplying
equipment, must be aware o f a l l the condi t ions t o be met , inc lud ing supply of
f l u i d s , se rv i c ing f a c i l i t i e s , and the type o f r e l i a b i l i t y which has t o be main-
tained. I t i s h i s prerogat ive t o o f f e r h i s customer the best commercial propo-
s i t i o n and t h i s could mean, f o r example, e i t h e r an inexpensive throwaway u n i t
which must be replaced r e l a t i v e l y f requent ly , o r a more expensive i tem f o r which
the on ly service needed i s the occasional exchange o f a r e l a t i v e l y inexpensive
component, such as a f i l t e r ca r t r i dge .
The end user has t o make the f i n a l judgement. He needs t o be able t o compare
the rea l value o f each i n s t a l l a t i o n o f fe red t o him; each user w i l l place d i f f e r
ent emphasis on the value o f each type o f equipment o f fered.
the value o f a pump i s i t s f i t n e s s f o r the purpose, how long i t w i l l perform i t s
duty s a t i s f a c t o r i l y , and what the cost o f se rv i c ing w i l l be. He i s not i n t e r -
ested i n how many 10 micrometre p a r t i c l e s are contained i n 100 m l o f hyd rau l i c
f l u i d .
To the end user
The use r ' s i n t e r e s t i s i n the l eas t expensive f i l t e r t ha t w i l l p rov ide the
required degree o f c leanl iness. In making t h i s assessment, the o r i g i n a l o r
cap i ta l cost o f the equipment has t o be balanced against the cost o f service.
He may have t o compare, f o r example, the advantages o f fe red by a very expensive
pump tha t w i l l operate on ' d i r t y ' o i l w i t h those o f a low-cost pump p lus f i l t e r .
I f i t i s t o mainta in i t s present h igh i n t e g r i t y , the hydraul ic equipment
industry needs t o e s t a b l i s h more meaningful spec i f i ca t i ons f o r d i f f e r e n t types
of f i l t e r .
out necessari ly having t o know how t h i s performance i s achieved.
who need t o w r i t e these spec i f i ca t i ons a re those who know the actual requi re-
ments. The r e s p o n s i b i l i t y must be d i v ided between manufacturers o f f i l t e r
elements, who should know what i s poss ib le , and the manufacturers o f hydraul ic
equipment, who should know what i s needed. These two groups should be able t o
communicate i n meaningful terms.
The spec i f i ca t i ons must a l l ow the end user t o buy performance wi th-
The people
A t present, there are no adequate techniques covering a l l aspects o f contam-
i n a t i o n measurement i n hydraul ic f l u i d s .
a s t a r t i n the r i g h t d i rec t i on . The c leanl iness l eve l cha r t , f o r example, w i l l
not be 100% r i g h t f i r s t time; there w i l l be a cont inual need t o review i t i n
This should not deter us from making
329
the l i g h t o f new exper ience.
The e s c a l a t i n g c o s t o f equipment downtime and maintenance must encourage the
end user t o consider running cos ts more c a v e f u l l y , f o r bo th new and e x i s t i n g
p lan t .
contaminat ion c o n t r o l p lays i n determin ing those cos ts f o r the hyd rau l i c
i n s t a l l a t i o n . I t i s hoped t h a t t h i s volume has g iven a use fu l i n s i g h t t o the
rea l na tu re o f the problem and has po in ted the way t o a systemat ic approach and
more c o s t - e f f e c t i v e so lu t i ons .
To be ab le t o do t h i s he must app rec ia te more f u l l y the p a r t t h a t
This chapter has been e d i t e d by k i n d permission o f Sperry Vickers f rom a
more comprehensivb paper " E f f e c t i v e Contamination Contro l i n F l u i d Power
Systems", w r i t t e n by J.B. Spencer and C. Balmer, publ ished by Sperry Vickers.
330
1 L 1 SEALS FOR FLUID POWER EQUIPMENT PART ONE
B.D.HALLIGAN, C Eng,MIMechE AMPRI, Techn ica l Manager (Produc t A p p l i c a t i o n s ) James Walker & Co.Ltd.
14 .1 INTRODUCTION
The f a c t t h a t a sea l manufac tur ing i n d u s t r y e x i s t s a t a l l o r i g i n a t e s f rom
the commercial o r p h y s i c a l i m p r a c t i c a b i l i t y o f a c h i e v i n g p e r f e c t and permanent
m u t u a l i t y o f a d j a c e n t sur faces - be they i n dynamic o r s t a t i c r e l a t i o n s h i p .
S e a l i n g dev ices , which a r e u s u a l l y o f an e s s e n t i a l l y de formab le n a t u r e , per-
m i t t h e f l u i d power des igner t o work w i t h i n economica l l y s e n s i b l e c o n d i t i o n s o f
f i t , s u r f a c e f i n i s h , t o l e r a n c e and f l u i d c l e a n l i n e s s across t h e range o f temper-
a t u r e s and pressure c o n d i t i o n s w i t h wh ich he i s l i k e l y t o contend.
The des igner o f f l u i d power equipment has, i n f a c t , i n f l u e n c e d t r e n d s i n the
s e a l i n g i n d u s t r y by go ing beyond t h e fundamental requ i rement o f i n t r o d u c i n g an
e n v i r o n m e n t a l l y t o l e r a n t component f o r p r e v e n t i n g leakage. The ques t t o m i n i -
mise p r o d u c t i o n c o s t s has a c c e l e r a t e d t h e movement towards sea l des igns embody-
i n g i n t e g r a l bear ings and a n t i - e x t r u s i o n dev ices capable o f s u p p o r t i n g s e a l s
a g a i n s t s u b s t a n t i a l e x t r u s i o n gaps a t s i g n i f i c a n t p ressures . O v e r a l l i n t h e
c o n t e x t o f those s e a l s s p e c i f i e d f o r l i n e a r dynamic mot ion , a much a x i a l l y
s h o r t e r sea l has e v o l v e d which a l l o w s h x d r a u l i c c y l i n d e r s , f o r example, t o be
produced t o more compact p r o p o r t i o n s w i t h t h e consequent s a v i n g i n raw m a t e r i a l
cos ts and i n t h e b u l k o f f l u i d power sub-assembl ies.
None o f t h i s i s bad. However i t does mean t h a t a p r o l i f e r a t i o n o f sea l
designs and m a t e r i a l s e x i s t s and those r e s p o n s i b l e f o r s e l e c t i o n a r e r e q u i r e d
t o make t h e i r cho ice w i t h d i s c r e t i o n t o ensure t h a t t h e most c o s t - e f f e c t i v e
s o l u t i o n i s found. P i c k i n g a sea l which f a c i l i t a t e s t h e achievement o f t h e
cheapest p r o d u c t i o n c o s t s f o r t h e component i n t o which i t f i t s i s seldom t h e most
reasonable b a s i s f o r guarantee ing minimum leakage and maximum s e r v i c e a b i l i t y .
331
I t i s an u n f o r t u n a t e f a c t t h a t most f l u i d leakages a r e a t t r i b u t e d t o a f a u l t y
seal , whereas i n v e s t i g a t i o n w i l l f r e q u e n t l y show t h a t t he number o f occasions o f
ma l func t i on due t o a f a u l t y p roduc t a r e few.
usua l l y stems from i n c o r r e c t seal s e l e c t i o n , work ing c o n d i t i o n s which d i f f e r
from those considered t o app ly , seal containments o u t s i d e s p e c i f i e d to le rances
i n terms o f d imension o r f i n i s h , f a u l t y f i t t i n g , contaminants i n the f l u i d t o
be sealed o r , impor tan t l y , persona l i n t e r p r e t a t i o n o f ' l eakage ' i n q u a n t i t a t i v e
terms.
The r e a l source o f d i f f i c u l t y
To understand t h e l i m i t a t i o n s o f f l u i d seal performance i t i s necessary t o
know a l i t t l e o f t h e m a t e r i a l s most t y p i c a l l y used, t he o r i g i n s o f fundamental
seal designs, how they f u n c t i o n , and t h e i r re f inement t o t h e present s t a t e o f
the a r t . Fo r tuna te l y , t he sea l i s no longer an a f t e r - t h o u g h t b u t more o f t e n
receives the a t t e n t i o n i t p r o p e r l y deserves as a v i t a l i n t e r a c t i v e element i n
any complete f l u i d power system.
14.2 MATERIALS
W i t h i n t h e scope o f t h i s chapter , a t t e n t i o n w i l l be cent red on s o l i d e l a s t o -
meric sea ls , t y p i f i e d by the '0' r i n g f o r s t a t i c connect ions, e lastomer con ta in -
ing sea ls such as c y l i n d e r packings based on proo fed t e x t i l e , and those designs
which employ p l a s t i c components f o r va r ious f u n c t i o n s .
The compounding o f e las tomers i s regarded by many as be ing a b l a c k a r t and
i t i s t r u e t o say t h a t w i d e l y d i f f e r i n g p r o p e r t i e s can be developed by va ry ing
the q u a n t i t y and t ype o f rubber chemicals added t o the s tock rubber i n e i t h e r
i n t e r n a l m ixe rs o r on m ix ing m i l l s . The raw m a t e r i a l supp l i ed t o the seal man-
u f a c t u r e r must u s u a l l y be processed t o t a i l o r t he phys i ca l c h a r a c t e r i s t i c s re -
qu i red f o r t h e s e a l i n g f u n c t i o n . Parameters such as t e n s i l e s t reng th , e longa t ion
a t break, compression s e t , b r i t t l e p o i n t , and behaviour i n c o n t r o l f l u i d s a re
t y p i c a l o f those reviewed when assessing performance requirements and l a y i n g down
compound s p e c i f i c a t i o n s .
i n a d d i t i o n t o a p p l i c a t i o n s u i t a b i l i t y , p r o c e s s a b i l i t y i n mix ing , e x t r u s i o n
and moulding sequences must be assured as must s a t i s f a c t o r y post-moulding opera-
t i o n s such as de - f l ash ing .
To m a i n t a i n cons is tency o f seal e lastomers requ i res ex tens i ve l abo ra to ry
suppor t and cont inuous s tandard t e s t i n g o f t he compound i t s e l f on a ba tch- to -
ba tch bas i s .
Tables 14.1 - 1 4 . 3 a r e taken from t h e B r i t i s h Rubber Manufac turers ' Associa-
t i o n 'Guide t o E las tomer ic Sea ls ' t o g i v e broad re fe rence t o most commonly used
elastomers, t h e i r p r o p e r t i e s , f l u i d c o m p a t i b i l i t y and temperature res i s tance .
332
TABLE 14.1
Polymers-types and General Proper t ies
NATURAL POLY I SOPRENE SYNTHETIC POLYISOPRENE Very good
Poor res is tance t o
CHLOROPRENE (NEOPRENE-CR) Very good res i stance t o
STYRENE-BUTADIENE (SBR) Very good res is tance t o
Poor res is tance t o
ACRYLONITRILE-BUTADIENE ( N i tri le-NBR)
Very good res is tance t o
(Natural Rubber-NR) (Pol y i soprene- I R) General physical proper t ies, i n p a r t i c u l a r tear resistance, res is tance t o g lyco l -e ther brake f l u i d s and vegetable o i l s .
Heat, weather, ozone and mineral o i l .
Ozone, weathering and abrasion
Abrasion, g lyco l -e ther brake f l u i d s and vegetable o i l s .
Mineral o i l and ozone.
Low, medium, and h igh n i t r i l e s are ava i l ab le based on increasing acry lo- n i t r i l e content which s i g n i f i c a n t l y a f f e c t s low temperature and f l u i d swell proper t ies.
Mineral o i l , compression set and abrasion.
ISOBUTYLENE-ISOPRENE (Bu ty l - l I R ) CHLORO-ISOBUTYLENE-ISOPRENE (Chloro-Butyl) Very good res is tance t o Tear, weather and gas permeation. Poor Tens i l e p roper t i es and mineral o i l
resistance.
POLYBUTADI ENE (Butad iene-BR) Very good res is tance t o Tear, abrasion, low temperature,
g lyco l -e ther brake f l u i d s and vegetable o i l s . Of fers h igh r e s i l i e n c e .
Poor resistance t o Water, ozone and mineral o i l .
POLYSULPHIDE (Thiokol-T) Very good res is tance t o Ozone, mineral o i l s , pet-roleum f u e l s
and weather.
Poor A l l o ther p roper t i es .
ETHYLENE PROPYLENE (EPR-EPM) ETHYLENE PROPYLENE TERPOLY MER (EPT-EPDM) Very good resistance t o Weather, ozone, heat, water, steam,
g l yco l -e the r brake f l u i d s and vegetable o i l s .
?oor resistance t o Mineral o i l .
CHLOROSULPHONATED POLYETHYLENE (Hypa lon-CSM) Very good resistance t o Water, ozone, abrasion, ac id and
weather.
333
TABLE 14.1 (contd.)
METHYL-VINYL SILOXANE ( S i l icone-VMQ) PHENYL-METHYL-VINYL SILOXANE (S i 1 icone-PVMQ) Very good r e s i s t a n c e t o H igh and low temperature.
Poor res i s tance t o Abrasion, t e a r and tens ion .
TRI FLUOROPROPYL S I LOXANE ( F l u o r o s i 1 icone-FMQ) Very good M ine ra l o i l and f u e l res i s tance .
A l l o t h e r p r o p e r t i e s as VMQ.
POLYURETHANE D I - I SOCYANATE (Urethane-AU) Very good res i s tance t o M ine ra l o i l , abras ion , t e a r ,
ozone and weather. O f f e r s h i g h modulus and t e n s i l e p r o p e r t i e s .
Poor Mo is t heat res i s tance .
FLUORINATED HYDROCARBON (F I uorocarbon-FPM) Very good r e s i s t a n c e t o High temperature ( i n a i r and most
Poor Tear s t r e n g t h .
o i l s ) , weather and pe t ro leum f u e l .
POLYACRYLATE (Ac ry l ic-ACM) Very good Heat, weather, m inera l o i l and
ozone res i s tance .
Po0 r Water res i s tance .
O f major i n t e r e s t t o the f l u i d power i n d u s t r y a re : -
A c r y l o n i t r i l e - b u t a d i e n e ( N i t r i l e - NBR)
Probably upwards o f 80% o f sea ls supp l i ed t o the f l u i d power ndus t r y a r e
based on n i t r i l e composi t ions.
terms o f good compression s e t q u a l i t i e s , abras ion res i s tance , m i era1 o i l
c o m p a t i b i l i t y , ease o f p rocess ing and low i n i t i a l cos t favour t h i s cho ice .
The balance o f p r o p e r t i e s a v a i l a b e from NBR i n
N i t r i l e composi t ions a r e no t , however, compat ib le w i t h f i r e - r e s i s t a n t f l u i d s
o f t he phosphate e s t e r type .
Isobuty lene- isoprene (Bu ty l - I IR)
S a t i s f a c t o r y i n s e r v i c e w i t h phosphate e s t e r f l u i d s b u t r a t h e r l i f e l e s s from
a phys i ca l p o i n t o f v iew. Not s u i t a b l e f o r minera l o i l se rv i ce , n e i t h e r should
b u t y l sea ls be smeared w i t h minera l o i l o r grease on f i t t i n g .
E thy lene propy lene (EPR - EPM)
or Ethy lene propy lene te rpo lymer (EPT - EPDM)
Same s e r v i c e f u n c t i o n and l i m i t a t i o n s as b u t y l bu t g e n e r a l l y b e t t e r phys i ca l
p r o p e r t i e s .
F l u o r i n a t e d Hydrocarbon (Fluorocarbon - FPM)
The o n l y usual s e l e c t i o n where c o m p a t i b i l i t y w i t h minera l o i l and phosphate
e s t e r i s requ i red .
334
Polyurethane (Urethane - Au)
Widely favoured as a dynamic seal m a t e r i a l i n the US and i n Germany, po l y -
urethane e x h i b i t s e x c e l l e n t abras ion res i s tance and i s very du rab le i n s i t u a t i o n s
which might adverse ly a f f e c t n i t r i l e composi t ions o r even proofed f a b r i c s , e.g.
passage o f seal l i p s over p o r t s i n a c y l i n d e r w a l l . Poor low temperature f l e x i -
b i l i t y , h y d r o l y s i s i n ho t water and a lower ope ra t i ona l temperature c e i l i n g a re
l i m i t i n g f a c t o r s .
Other seal component m a t e r i a l s wor thy o f ment ion are:-
T e x t i l e s - woven c l o t h such as cot ton, asbestos, t e r y l e n e and ny lon
a re used as the substratum i n elastomer proofed f a b r i c
packings f o r medium and h i g h pressure d u t i e s .
Non-woven m a t e r i a l s such as po lyes te r and polyamide a r e
a l s o a v a i l a b l e .
The p r o o f i n g elastomer can be v a r i e d t o s u i t b u t w i l l
f r equen t l y be based on NBR and/or ch loroprene (Neoprene-CR) . PTFE - employed as a back-up r i n g m a t e r i a l f o r ' 0 ' r i n g s . Poor
creep p r o p e r t i e s r e s t r i c t i t s use as a seal ma te r ia l i n i t s
own r i g h t unless energ ised by an e las tomer i c component o r
sp r ing .
Nylon and ace ta l - main f u n c t i o n i s heel support f o r dynamic sea ls o r as com-
ponent bea r ing r i n g m a t e r i a l .
H y t r e l - the r e g i s t e r e d t rade name o f a Du Pont range o f thermo-
p l a s t i c p o l y e s t e r m a t e r i a l s which form a
elastomers and thermoplast ics . Has s i m i l a r p r o p e r t i e s t o
po lyurethane b u t e x h i b i t s much b e t t e r low temperature f l e x -
i b i l i t y . Used c u r r e n t l y as an a n t i - e x t r u s i o n element i n
c e r t a i n spool - type p i s t o n head sea ls .
b r i d g e between
When r e f e r r i n g t o rubber technology as a ' b l a c k a r t ' t he re i s a two- fo ld
i n t e r p r e t a t i o n . On the one hand i t i s c e r t a i n l y a manufactur ing area in f luenced
by many va r iab les . On the o the r , the end product used i n a s e a l i n g con tex t i s
i n v a r i a b l y b lack i n co lou r due t o the use o f carbon b lack as a f i l l e r necessary
f o r toughening the f i n i s h e d a r t i c l e . Seal i d e n t i f i c a t i o n i s t he re fo
problem unless housekeeping i s o f a h i g h o rde r w i t h seals p o s i t i v e l y
and bagged i n smal l numbers hav ing f u l l s p e c i f i c a t i o n o f m a t e r i a l , s
p a r t number c l e a r l y shown.
Colour coding i s one s o l u t i o n bu t the rea l answer l i e s i n the use
co loured elastomers. A t t he moment a good deal o f development work
e a major
separated
ze, and
o f s e l f -
s being
335
TABLE 14.2 R-Recommended A-Acceptable N-Not recommended
R A
N N N N N N N N
N N N R N R
T A B L E 1 4 . 3
OC Temperature Range OC 250 Any statement of temperature range fo r a given
polymer must be qualified by consideration fo r the environment in which i t i s t o work. For example, service temperatures recommended fo r 1 iquid applications are l ike ly t o be s ign i f icant ly higher than those which would be appropriate t o dry heat s i tua t ions . The values given in Table 3 r e l a t e to typical f l u id conditions. N.B. These a re not continuous ra t ings .
200
150
W W Q,
T A B i E 14.4 TYPES OF SEALS FOR RECIPROCATING, ROTARY AND STATIC APPLICATIONS
RECIPROCATING AVAILABLE STANDARD APPLICATIONS APPLICATIONS MATERIALS
L I P TYPE U R i n g (A lso k n o w n
- as U P a c k i n g )
Fabric-for pressures up to 350 b a l I I though rohs't Conitructions of leis sensi t ive D r o f i l e * ? I 1 extend t h i s range. (as shorn i n the aise&ly sketcher) or an in ternal l i p spvcadcr.
Frequently f i t t e d d t h an adapt& r i n g
Spring loaded adaptors arc available
W W Q,
TABLE 14.4 TYPES OF SEALS FOR RECIPROCATING, ROTARY AND STATIC APPLICATIONS
RECIPROCATING AVAILABLE STANDARD APPLICATIONS APPLICATIONS MATERIALS
L I P TYPE U R i n g (A lso k n o w n as U P a c k i n g )
50116 e l a i t m - f o r p r e s ~ u r e i up to 10 bar--gmd f i l m wrp i ty and lo* pressure sealing characterlst3cs- ideal f o r a i r cy l inder . Ymm to abrasive *car. e x t r u ~ l ~ n and f l u i d t v t t l n g a t hlgher hydraulic p r e s i u ~ e i . Can be f i t t e d in recerrrr requi r ing no sep*r*te dCLCII.
l o l a d e l a i t m i w t h P l i i t i C heel-for OT~IIYTCI UP to 100 b a r d s 10lld rubbcr but hiqher oressure Capabil ity due to enhanced CItlYIlOn r e l l r t m c e .
Fabric-for p r e i i u r e l up to 350 b i i 1 1 though r o h i t Conitructions of leis sensi t ive p r o f i l e w11 eitend t h i s range. Frequently f i t t e d wi th an adaptor r i n g (as shorn i n the d 5 l m h l y sketcher) 01 an in ternal l i p sp~cadcr . Spring loaded l d l p t o r r arc available t o aupnent low QrCIIYlt real,ng I b 9 l l t y . Usually rcqulrr separate housing access such I S remyable cover plate but I p c c i a I fabric U-nngr f o r indCcesItble 9movc1 d m available. include wlyurethdne. P l F t and leather.
Other materials
None o f t W I e designs are n o m l l y r e c m n d c d fOT shock loading. rams subject to l a t e r a l thrust . abrasive condit30nr o r high speed recIpToCation. only. A double-acting a r m n g m n t M i l l *quire tw ~ e d l i to be r i g ? d l y scpamted and each fac ing the p r c s ~ u r c IOUTCC.
U n n q 1 seal ~n one di rect ion
K
0 L
>
0)
"T
ow
ll-m
eU
L
v
L
e
n
337
I
V Ring (When i n s e t form, more commonly known as Chevron
L x t r e n r l y wide range of h y d r a d i c rervices-mst types of hydrau l i c Dress, rec ip roca t ing steam
N o r m l l y pmafed f a b r i c . Others inc lude eiastMcr ,
punpr, sludge p w s . hydrau l i c cyl inder; Oil and water expanlion glandr, hydrau l i c valves.
'lFC Or leather * Oil
Heavy Duty L i p
Packi ngs (b ) Two-Lips (a ) S ing le L i p
fa1 U p - r t m k i m l IlMl omving press rams. rec ip roca t ing Dmp l a m . l i f t s , cranes. hydrau l i c acCuMlators.
Proofed fabr ic
l a ) Large diameter ram. up-s t rok ing or d o n - s t r o k i n g (*ere f r i c t i o n a l res is tance i s no t o f paralmunt importance) Wen where subjected t o Shock loads. e.g. i n fo rg ing prei ies' hor izon ta l vams of any diameter inc lud ing tho%; o f e x t r w i o h ~PCIICI r a m sub jec t to I d t e m l t h r u s t : h igh pressure' r e c i p i o c d t i n g pulps; t b s e heavy duty app l i ca t ions where F i s t c l a l 5 cond i t ion .
packing housing and buIneI are no t i n
w w -2
338
c a r r i e d o u t t o achieve t h i s end w i thou t any s i g n i f i c a n t l oss i n phys i ca l proper-
t i e s . Coloured f luorocarbon rubbers a r e a l ready a v a i l a b l e .
14.3 SEAL DESIGNS
Wi th in the boundaries se t by f l u i d power systems t h e m a j o r i t y o f seals empl-
oyed can be categor ised as l i p o r squeeze types.
F igure 1 i l l u s t r a t e s the s implest forms u t i l i s e d f o r s t a t i c o r dynamic r e c i -
p roca t i ng s i t u a t i o n s - the ' U ' r i n g and the '0' seal - and ind i ca tes how both
r e l y on i n t e r f e r e n c e s t ress f o r a t l e a s t a component o f t h e i r f u n c t i o n a l oper-
a t ion.
- INT
P
r--.
I.--' P
Fig.1 L i p and Squeeze Seals
Both a r e responsive t o system pressure and w i l l generate a r a d i a l s t ress
Ne i the r r e l y on e x t e r n a l l y app l i ed g rea te r than t h e pressure t o be sealed.
compression t o any r e a l degree as i n the case o f the s o f t packed gland. Th is
i s p a r t i c u l a r l y re levan t t o squeeze seals which a re u s u a l l y intended t o f l o a t
a x i a l l y i n t h e i r housings.
The q u a l i t y o f the sea l i ng con tac t area and the i n t e r f e r e n c e s t ress i n the
same zone w i l l c o n t r o l low pressure s e a l i n g e f f i c i e n c y . These f a c t o r s p l u s
o v e r a l l seal geometry and composi t ion w i l l se t t he p o i n t a t which t ime h y d r a u l i c
response takes over f rom the manufactured i n t e r f e r e n c e cond i t i on . The h y d r a u l i c
component may become s i g n i f i c a n t a t pressures o f 40 bar f o r an e las tomer i c ' 0 '
r i n g o r as h igh as 200 bar f o r a r i g i d proofed f a b r i c packing.
Seal ing s lack o i l and low pressures i s g e n e r a l l y a f u n c t i o n o f seal i n t e -
g r i t y and i s the more d i f f i c u l t cond i t i on . Prevent ing leakage a t h i g h press-
ures i s a f e a t u r e o f seal containment, i .e . p reven t ing e x t r u s i o n o r r a p i d wear
i f i n a dynamic duty .
Tables 14.4 and 14.5 l i s t t y p i c a l l i p and squeeze types i n common and t r a d i -
t i o n a l usage. (Courtesy o f ERMA).
339
Some p a r t i c u l a r p o i n t s t o note on each category:
( i ) L i p seals - a r e s i n g l e - a c t i n g t o app l i ed pressure and must be separated
by a f i x e d component i n double-act ing d u t i e s so t h a t t he h y d r a u l i c load
from the element under pressure i s no t passed on t o t h e t r a i l i n g element.
For maximum s e r v i c e l i f e m u l t i - r i n g packing sets a re p r e f e r r e d where the
succession o f sea l i ng edges ensures t h a t breakdown i s n o t sudden. The use
o f s p l i t r i n g s i s a l s o e n t i r e l y f e a s i b l e w i t h most designs o f t h i s s o r t
w i t h o u t s a c r i f i c i n g s e a l i n g performance t o any c r i t i c a l degree. Savings i n
downtime w i l l be obvious.
( i i ) Squeeze seals - have the advantage o f be ing double-act ing and a r e u s u a l l y
housed more economical ly than t h e i r corresponding l i p brethren. The l i n e -
age from the humble '0' r i n g t o the u n i t seals i n contemporary use can be
seen from Table 14.5.
Most seals i n t h i s category use a s i n g l e s e a l i n g zone and damage i n se rv i ce
w i l l be fo l l owed by more immediate breakdown than i n the case o f m u l t i - l i p
packings.
The squeeze seals a r e r a r e l y used i n s p l i t form a l though they can be engin-
eered so t o do.
Tables 14.6 and 14.7 d e p i c t the extens ion o f l i p and squeeze seals t o
r o t a r y and s t a t i c f unc t i ons .
More advanced seal designs a r e d e t a i l e d i n the nex t - sec t i on .
14.4 TR I BOLOG I CAL CONS I DERAT I ONS
I t i s hear ten ing t o f i n d the s tudy o f seal behaviour an essen t ia l p a r t o f
t r i b o l o g y seminars, r e c e i v i n g as much a t t e n t i o n , indeed, as bear ings, l u b r i c a n t s
and su r face topography. I n recent years much company research and independent
study by o rgan isa t i ons such as BHRA has been expended on d e f i n i n g s e a l i n g mech-
anisms - p a r t i c u l a r l y i n r e l a t i o n s h i p t o r e c i p r o c a t i n g motion.
14.4.1 F i l m Condi t ions
A l l dynamic seals r e l y on a coherent f l u i d f i l m under t h e i r con tac t area i f
they a r e t o f u n c t i o n c o n s i s t e n t l y and p r e d i c t a b l y . Such f i l m s may stem from
boundary l u b r i c a t i o n i n some modes and be t r u l y hydrodynamic i n o the rs . The
f i l m w i l l va ry i n th ickness accord ing t o seal p r o f i l e , i n t e r f e r e n c e s t ress ,
pressure, speed, su r face f i n i s h , type o f f l u i d employed and i t s temperature.
On r e c i p r o c a t i n g d u t i e s i t might t y p i c a l l y vary from 0.25 - 3.0 microns. On
r o t a r y sha f t s values o f 0.6 - 1.0 micron would u s u a l l y apply .
Leakage from r o t a r y s h a f t l i p seals i s seldom ev ident un less seal wear down
o r u n d e r - l i p c rack ing has taken p lace - a l l o t h e r fea tu res being equal . However,
340
TABLE 14.5
RECIPROCATING AVAILABLE STANDARD APPLICATIONS APPLICATIONS MATERIALS
SQUEEZE TYPE
0-Seal L i g h t and medium duty, P r i n c i p a l l y elastomer b u t polyurethane and PTFE are pneumatic and hydrau l i c
rec ip roca t ing services, a lso manufactured. e.g. small cy l inders , valve spools and stems, rod f i l m wiping.
Lobed Seal
As 0-seal b u t b e t t e r Elastomer. res is tance t o s p i r a l t w i s t .
Energized Sleeve Seal Pneumatic and hydrau l i c Elastomer and PTFE ( p l a i n o r
r e i n f o r c e d ) o r elastomer and cy l inders p a r t i c u l a r l y where s e n s i t i v i t y i s essent ia l , proofed f a b r i c . e.g. weighing machines, t e s t i n g equipment, e tc . P r i m a r i l y f o r p i s t o n heads b u t a lso a v a i l a b l e f o r g land du t ies .
Supported S i ngle-Act ing Sea 1
B a s i c a l l y developed f o r Elastomer/Plast ic. hydrau l i c c y l i n d e r r o d Elastomer/Fabric, and p i s t o n head du ty Elastomer/Fabric/Plastic p a r t i c u l a r l y i n mobi le and i n d u s t r i a l hydrau l i c polyurethane.
can a l s o be made i n
environments. m Supported Double-Acting Seal
Developed f o r p i s t o n head Elastomer/Fabric, duty i n hydrau l i c cy l inders Elastomer/Fabric/Plastic, associated w i t h mobi le/ Elastomer/Plast ic. i ndus tr i a 1 hydraul i c i n d u s t r i e s .
341
TABLE 14.6
ROTARY AVAILABLE STANDARD APPLICATIONS APPLICATIONS MATERIALS
LIP TYPE
Retention o f l ub r i can t and Elastomer/metal, exclusion o f fo re ign matter Elastomer/ fabr ic. from bearings and c r i t i c a l surfaces.
Spring loading t o l i p normally provided by to ro ida l spr ing o r f i nge r spr ing.
SQUEEZE TYPE The app l i ca t i on o f squeeze-type seals t o ro ta ry du t ies requires special ised a t ten t ion . Consul tat ion w i t h the seal manufacturer i s recommended.
TABLE 14.7 AVAILABLE STANDARD
STATIC SEALS APPLICATIONS MATERIALS
GASKETS AND JOINTINGS
The most widely used form o f s t a t i c connection where external mechanical load i s ava i lab le .
Elastomer, re in fo rced elastomer, proofed fab r i c , elastomer bonded cork, compressed asbestos f i b r e and a wide va r ie t y o f non- elastomer based compositions.
LIP TYPE
U Ring (Also known as U-Packinal
.,I _. . _.
A wide range o f s t a t i c sea l ing dut ies. Cyl inder leather, Polyurethane and end caps, autoclave doors, PTFE. pressure vessels, couplings. etc.
Elastomer. proofed fab r i c ,
from comnon o r separate source
SQUEEZE TYPE A l l s t a t i c sea l ing du t ies which w i l l permit the use o f the selected 0-seal mater ia l and which are o f a design which w i l l contain t h i s mater ia l w i t h i n the conf ines o f the seal recess a t operat ing pressure.
Elastomer and PTFE.
342
i n r e c i p r o c a t i n g a p p l i c a t i o n s passage o f t he s h a f t through the g land w i l l c a r r y
the o i l f i l m t o the atmosphere s i d e o f t h e s e a l .
ROD EMERGING- t h i c k o i l f i l m
Fig.2 Cond
F igu re 2 i s a diagrammatic
ROD RETRACTING- seal energised prevent ing t o t a l f i l m
t ion o f Maximum C o l l e c t e d 0
rep resen ta t i on o f t h e wors t
iturn
1 Fi lm.
f i l m t r a n s p o r t s i t u a -
t i o n i n v o l v i n g an emerging c y l i n d e r rod w i t h s l a c k pressure t o t h e g land which
r e t r a c t s w i t h the s e a l i n g element under load. I n t h i s mode t h e hee l o f t h e sea l
w i l l be energ ised and can p reven t the r e t u r n o f the t o t a l q u a n t i t y o f f i l m
c a r r i e d by t h e rod.
t i o n .
The f i t t i n g o f an e f f e c t i v e w iper can aggravate t h e cond i -
To combat the emergent f i l m , c a r e f u l a t t e n t i o n i s requ i red t o p ressure s i d e
sea l geometry. Designs such as t h a t shown i n F i g . 3 w i t h k n i f e - c u t s e a l i n g
edges and a s p e c i f i c r e l a t i o n s h i p between con tac t edge and groove he igh ts have
proved ve ry success fu l d e s p i t e t h e i r s h o r t a x i a l l eng th .
Fig.3 Minimum F i l m - Shal low Design
Equa l l y e f f i c i e n t and hav ing t h e advantage o f more than one s e a l i n g edge i s
the concept shown i n F ig .4 wh ich i s a mar r i age o f l i p and squeeze s e a l i n g
p r i n c i p l e s .
343
STANDARD FABRIC CHEVRON GLAND RING
F A B R I C CHEVRON I N T E W l E D I A T E RING - SPECIAL SYNTHETIC RUBBER HEADER RING
F ig .4 Minimum F i l m - M u l t i - L i p Design
F igu re 5 i l l u s t r a t e s t y p i c a l improvement i n performance aga ins t standard
V - r i ng packing under o f f s e t load c o n d i t i o n s and shows i t s a d a p t a b i l i t y even i n
s p l i t form.
LEAKAGE - cc/hr
4 -
COMPARISON OF F A B R I C MULTI-LIP PACKING VERSUS S P L I T AN0 ENDLESS L O F I L M CHEVRON WHEN SUBJECTED TO EXAGGERATED ROD KNUCKLING S I Z E : 44.5mm x 60.3mm
(12" x 2:")
GLAND BUSH CLEARANCE 0.5mm(.D20") TEST DURATION 500 HOURS
OIAMETRAL
( 2 ) S P L I T L O F I L M CHEVRON PACKIHG
(31 ENDLESS L O F I L M CHEVRON PACKING
F ig .5 Comparative Seal Performance
14.4.2 Sur face F i n i s h
I t i s n o t o n l y t h e numerical va lue o f su r face roughness which i s re levan t t o
seal performance bu t a l s o the manner i n which such a f i n i s h i s achieved.
I n the con tex t o f f l u i d power equipment, honed o r r o l l e r burn ished b a r r e l
f i n i s h e s a r e recommended w i t h an average va lue o f between 0 .4 to 0.8 p m Ra.
Larger c y l i n d e r s should be f i n i s h e d t o 0.8 urn Ra o r b e t t e r , i f poss ib le , bu t i n
any event shou ld be no worse than 1.6 urn Ra.
Rods should i n a l l c i rcumstances be o f a su r face roughness 0.8 p m Ra o r
b e t t e r .
p l i e r s t o a s tandard f i n i s h o f 0.2 pm Ra.
i a l , f u n c t i o n , and the des igne r ' s exper ience o f s i m i l a r equipment.
Many o f t h e sma l le r s i z e s w i l l be a v a i l a b l e from p r o p r i e t a r y rod sup-
Type o f f i n i s h w i l l depend on mater-
A l l s t a t i c housing areas may be f i n i s h e d i n t h e range 0.8 t o 1.6 um Ra f o r
p roo fed f a b r i c packings.and, p r e f e r a b l y , 0.4 to 0.8 pm Ra For sma l le r housings
approp r ia te t o '0' r i n g s , e t c .
344
Any long i tud ina l marking on rods o r cy l i nde rs w i l l promote leakage and manu-
fac tu r i ng processes posing such r i sks , e.g. as-drawn tube, r e t r a c t i o n marks on
ro l l e r -bu rn i sh ing heads, should be most c a r e f u l l y monitored.
Equally, the achievement o f too f i n e a f i n i s h - perhaps i n the area 0.01 - 0.05 I.cm Ra - can prevent the establishment o f a coherent f l u i d f i l m under the
seal.
t a i l e d . Several p r a c t i c a l cases have been demonstrated where the de l i be ra te
i n t roduc t i on o f a coarser f i n i s h has restored an adequate q u a l i t y o f seal ing.
I n any event, on most hyd rau l i c i n s t a l l a t i o n s the advantages t o be gained by
improving f i n i shes below 0.2/0.3 pm Ra are d i sp ropor t i ona te l y c o s t l y wi thout
o f f e r i n g tang ib le performance improvement.
An e r r a t i c performance can f requen t l y f o l l o w and packing l i f e can be cur-
14.4.3 Seal F r i c t i o n
In est imat ing seal drag loads f o r c r i t i c a l app l i ca t i ons , designers are faced
w i t h the p r a c t i c a l problem o f f i n d i n g even a general idea o f actua l values from
seal manufacturers.
There are rea l d i f f i c u l t i e s i n ex t rapo la t i ng r e s u l t s based on laboratory
equipment and applying values t o much la rge r p lan t . Equally, type o f l ub r i ca -
t i on , choice o f packing, and degree o f l i p in ter ference, whether as-moulded o r
as a r e s u l t o f compression, a re a l l q u a n t i t i e s which w i l l in f luence the f i n a l
r e s u l t .
Work ca r r i ed out i n t h i s area w i t h a view t o g i v i n g a no t i ona l allowance fo r
f r i c t i o n takes due account of seal length and expresses seal drag load as :
where R = f l u
D z sea
L = sea
DL 25.8
F r i c t i o n load = - x [142 + (0.8 X R)] kgf
d pressure (bar)
contact diameter (2) contact length (2)
I t i s o f t e n des i rab le t o add a contingency allownace o f 15%. As a r e s u l t , i t
may be seen tha t a m u l t i - l i p packing o f 1000 mm contact diameter and 50 m deep
would requi re 0.2% approx. o f the t h r u s t developed on t h i s f u l l area diameter a t
a pressure o f 200 bar.
Figure 6 makes the p o i n t t h a t any elastomeric and therefore f l e x i b l e seal
const ruct ion w i l l s u f f e r an increase i n contact band w id th w i t h increasing
system pressure. Even i n the case o f r e l a t i v e l y tough, h igh modulus ma te r ia l s
such as proofed f a b r i c o r polyurethane, pressures o f 200 bar o r over may be
s u f f i c i e n t t o promote t o t a l a x i a l contact o f the seal w i t h the dynamic wear
s u r f ace.
345
LOW PRESSURE
CONTACT BAND AREA
I I I I I
I 1
HIGH PRESSURE
izsi
is:! CONTACT BAND AREA
Fig.6 Seal Contact Band Width
Interposing a loaded PTFE sleeve is an effective means of limiting seal drag
dramatically despite substantial hydraulic pressure increases.
one form of the composite principle utilising a rectangular section elastomeric
Figure 7 shows
energiser.
LOADED PTFE SLEEVE
RUBBER ENERGISER
Fig.7 Energised PTFE Sleeve Seal
346
The order o f f r i c t i o n c o n t r o l a v a i l a b l e by comparison t o comparably tes ted
'0' r i n g s and ' U ' r i n g s i s g iven i n Fig.8.
500
u- .? 400 v
0 < 0
z A 300
0 I-
$ 200 LL
-I
5 VI 100
- COMPARISON OF MOVING F R I C T I O N LOAD AT VARYING F L U I D PRESSURES
' C '
0 50 100 150 200 250 300 350
O I L PRESSURE (BAR)
Fig. 8 Comparative F r i c t i o n C h a r a c t e r i s t i c s
The patented des ign shown i n Fig.9 combines f r i c t i o n c o n t r o l w i t h maximum
f l l m w ip ing a b i l i t y f o r t he rod s i t u a t i o n .
F i g . 9
E n e r g i s e d PTFE S l e e v e Seal w i t h P r o f i l e d F i l m W i p i n g Edge
1 4 . 4 . 4 Type o f F l u i d
As f a r as dynamic seals a r e concerned, the vas t m a j o r i t y o f mineral-based o r
phosphate e s t e r f l u i d s do not present l u b r i c i t y problems.
One o f t he most demanding types o f duty f o r any dynamic seal invo lves p l a i n
water a t h igh pressure whether by des ign o r by t o t a l l o s s o f s o l u b l e o i l content
341
i n a nominal ly l u b r i c a t e d system. High s p e e d s a r e p a r t i c u l a r l y d i f f i c u l t t o
sus ta in un less a minimum so lub le o i l l e v e l o f 2% i s guaranteed.
Tests conducted on three- throw min ing pumps opera t i ng a t 250 bar , 0.6 m/s,
have shown t h a t f o r even a 2% s o l u b l e o i l content , average seal l i f e w i l l be
extended by a f a c t o r o f f o u r by comparison w i t h un t rea ted water. Equal ly , a
separate l u b r i c a n t feed w i l l produce s i m i l a r l y dramat ic improvements.
The temperature o f h y d r a u l i c f l u i d should n o t exceed 60°C i f a t a l l poss ib le ,
as s i g n i f i c a n t l y f a s t e r swel l and so f ten ing o f proofed f a b r i c s and s t r a i g h t
polymers w i l l occur above t h i s va lue. To i l l u s t r a t e t h e p o i n t , t h e f o l l o w i n g
1 T e l l u s 27 minera l o i l . data i s based on immersion t e s t i n g f o r seven days i n She
N i t r i l e proofed f a b r i c - room temperature + 0.7%
9OoC 2.6% 6OoC 1.2%
1 2O0C 3.3%
High n i t r i l e elastomer - 6OoC 0.8%
90°C 2.3% 1 2O0C 3.8%
Sof ten ing o f moulded f a b r i c m a t e r i a l u s u a l l y reduces
w i l l normal ly d i m i n i s h s e r v i c e l i f e .
volume increase
i n t r i n s i c s t reng th and
14.4 .5 F i l t r a t i o n
For t h e b u l k o f e lastomer proofed f a b r i c and s o l i d elastomer seals f l u i d
f i l t r a t i o n o f 25 microns should be p e r f e c t l y adequate f rom the sea l i ng pe r fo r -
mance aspect . Special sea ls such as those embodying PTFE wear faces w i l l bene-
f i t f rom 10 micron f i l t r a t i o n or b e t t e r . I n e i t h e r event, f i l t r a t i o n equipment
o f f e r i n g a f i n e r c u t - o f f w i l l p robably be s p e c i f i e d t o s u i t c o n t r o l va l ve func-
t i o n s on a g i ven press o r h y d r a u l i c component.
The m a j o r i t y o f convent ional h y d r a u l i c seals which a r e tes ted in-house work
i n con junc t i on w i t h no more than a coarse w i r e s t r a i n e r i n any h y d r a u l i c system.
However, modern f i l t r a t i o n a i d s must be considered an advantage, p a r t i c u l a r l y i f
the opera t i on o f t he p l a n t invo lves produces aggress ive res idues.
14.4.6 A i r Entrainment
As c y c l e speeds become f a s t e r due t o i nc reas ing work demands, system press-
ures must f l u c t u a t e more q u i c k l y . I n many s i t u a t i o n s , f u l l working pressure
must be exhausted i n m i l l i seconds , e.g. d ie -cas t i ng machines, p l a s t i c s i n j e c t i o n
moulding presses, e t c . I f a i r i s en t ra ined i n the h y d r a u l i c f l u i d such rap id
decompressions can be exceedingly dangerous i f no automat ic ven t ing i s avai lab le.
I f one considers t h a t i n 1O;l o f h y d r a u l i c o i l a t 200 bar and 10°C i t i s
p o s s i b l e t o d i s s o l v e n e a r l y 200 1 o f a i r , some i n d i c a t i o n o f t he magnitude of
348
r i s k w i l l be apparent.
The main problem r e l a t e s t o p i s t o n head s i t u a t i o n s where f l u i d c o l l e c t s
between two opposed seals.
in ter ference polyurethane cup r i ngs are o f t e n suspect i n t h i s d i r e c t i o n as they
do not react s u f f i c i e n t l y qu i ck l y .
L i p packings are l a r g e l y se l f - ven t i ng but heavy
Under no circumstances should two squeeze type seals, e.g. '0' r ings, be
employed on a p i s ton head as the a i r entrainment contingency can be aggravated
by a proven phenomenon known as in ter -seal pressure whereby three o r f ou r times
system pressure can be b u i l t up i n the annular clearance between the seals.
Extrusion o f such seals into the appl ied pressure can be noted i n t y p i c a l cases.
PRESSURE - 3 TO 4 P
P P
Fig . 10
I n the gland s i t u a t i o n , the r i s k s are no t o f the same degree unless there i s
r e s t r i c t e d access o f working f l u i d t o the packing v i a a s i n g l e p o r t i n the neck
bush o r some s i m i l a r feature.
assured unless de l i be ra te attempts are being made t o reduce the actual pressure
a t t h i s p o i n t o r t o dampen pressure v a r i a t i o n s .
Adequate f l u i d access t o a l l seals should be
14.5 SELECTION
As so many seal designs w i l l apparently meet given condi t ions and ye t be
s i g n i f i c a n t l y d i f f e r e n t i n ma te r ia l , s ize, and p r i ce , the f l u i d power equipment
designer can be fo rg i ven f o r being confused.
Table 14.8 i l l u s t r a t e s the v a r i a t i o n s t h a t could apply t o a given actuator
o f f i x e d rod, ba r re l and s t roke dimensions and r e f l e c t s a survey made several
years ago when eighteen quotations
double cushioning and a s t roke length o f 18".
va r ied between €175 and €24.50, y e t a l l purported t o do the same job .
were sought f o r a 3" d ia . cy l i nde r w i t h
Pr ices received a t t h a t t ime
Much w i l l depend on the use r ' s own experience and preference based on h i s
knowledge o f the app l i ca t i on .
consequence o f leakage, i n i t i a l cost and a v a i l a b i l i t y w i l l a l l p lay t h e i r p a r t
i n s tee r ing the dec is ion towards u l t i m a t e secu r i t y o r some other l eve l o f cost-
e f fect iveness.
Schedule maintenance periods, a c c e s s i b i l i t y ,
349
TABLE 14.8
UPPER
LOWER
UPPER
LOWER
UPPER
LOWER
350
I t i s s i g n i f i c a n t t h a t B r i t i s h Steel Corporat ion a r e now s e t t i n g t h e i r own
standards f o r c r i t i c a l c y l i n d e r d u t i e s and w i l l use m u l t i - l i p packing throughout.
Such a s o l u t i o n would be e n t i r e l y unacceptable t o the manufacturer o f ea r th -
moving
s i z e and seal cost .
veh ic les who would seek a more economic s o l u t i o n i n terms o f c y l i n d e r
Most seal manufacturers w i l l e r r on the s i d e o f cau t i on i f they a r e advised
o f intended s e r v i c e c o n d i t i o n s and i f i n doubt t h i s i s t he p r e f e r r e d rou te t h a t
the designer should take.
Equal ly , having made h i s choice and being confronted w i t h a range o f o v e r a l l
s izes f o r a given diameter, t he l a r g e s t seal s e c t i o n a v a i l a b l e should be taken - n o t the smal lest - o t h e r requirements a l l ow ing . The sma l le r t h e seal - whether
l i p o r squeeze - the f i n e r i s t he working to le rance band o f i n t e r f e r e n c e and the
lower i s the c a p a b i l i t y f o r absorbing misal ignment, v i b r a t i o n , adverse accumula-
t i v e to lerances, e t c . T h i s a s s e r t i o n a p p l i e s e q u a l l y t o s t a t i c and dynamic
p o s i t i o n s .
14.6 STORAGE
BS 3754:1963 'Storage o f Vulcanised Rubber' was prepared under the a u t h o r i t y
o f t he Rubber Indus t r y Standards Committee and inc ludes t h e f o l l o w i n g recommen-
da t ions:
"Most vu lcanised rubbers change i n phys i ca l p r o p e r t i e s d u r i n g s torage and
u l t i m a t e l y may become unserv iceable, f o r example, because o f excessive
hardening, so f ten ing , c rack ing , c r a z i n g o r o t h e r su r face degradat ion.
These changes may be the r e s u l t of one p a r t i c u l a r f a c t o r o r a combinat ion
o f f a c t o r s , namely, t h e a c t i o n o f oxygen, ozone, l i g h t , heat and humidi ty."
f u
14
The d e l e t e r i o u s e f f e c t s o f these f a c t o r s may, however, be minimised by care-
choice o f s torage cond i t i ons .
6.1 Recommendations
14.6.1.1 Temperature
The s torage temperature should be below 25OC and p r e f e r a b l y below 15OC.
temperatures exceeding 25OC c e r t a i n forms o f d e t e r i o r a t i o n may be acce le ra ted
s u f f i c i e n t l y t o a f f e c t t he u l t i m a t e s e r v i c e l i f e . Sources o f heat i n s torage
rooms should be so arranged t h a t t he temperature o f no s to red a r t i c l e exceeds
25OC.
canised rubber a r t i c l e s bu t they may become s t i f f e r i f s to red a t low tempera-
tu res and care should be taken t o avo id d i s t o r t i n g them dur ing hand l i ng a t t h a t
temperature. When a r t i c l e s a r e taken from low temperature s torage f o r immediate
use t h e i r temperature should be ra i sed t o approx imate ly 3OoC throughout be fo re
they a r e pu t i n t o s e r v i c e .
A t
The e f f e c t s o f low temperature a r e n o t permanently d e l e t e r i o u s t o v u l -
351
14.6.1.2 Humidi ty
Mois t cond i t i ons should be avoided; s torage cond i t i ons should be such t h a t
condensation does not occur.
14.6.1.3 L i g h t
Vulcanised rubber should be p ro tec ted from l i g h t , i n p a r t i c u l a r d i r e c t sun-
l i g h t and s t rong a r t i f i c i a l l i g h t w i t h a h i g h u l t r a - v i o l e t content . Unless the
a r t i c l e s a re packed i n opaque con ta ine rs , i t i s adv i sab le t o cover any windows
o f storage rooms w i t h a red o r orange coa t ing o r screen.
14.6.1.4 Oxygen and Ozone
Where poss ib le , vu lcanised rubber should be p ro tec ted from c i r c u l a t i n g a i r
by wrapping, s torage i n a i r - t i g h t conta iners, o r o t h e r s u i t a b l e means; t h i s
p a r t i c u l a r l y a p p l i e s t o a r t i c l e s w i t h l a rge su r face area t o volume r a t i o s , e.g.
proofed f a b r i c , c e l l u l a r rubber.
As ozone i s p a r t i c u l a r l y d e l e t e r i o u s , s torage rooms should n o t con ta in any
equipment t h a t i s capable o f generat ing ozone, such as mercury vapour lamps,
h igh vo l tage e l e c t r i c a l equipment, e l e c t r i c motors, o r o t h e r equipment which may
g i v e r i s e t o e l e c t r i c sparks o r s i l e n t e l e c t r i c a l d ischarges.
14.6.1.5 Deformat ion
Vulcanised rubber should, wherever poss ib le , be s to red i n a re laxed cond i t i on
f r e e from tens ion, compression o r o t h e r deformat ion.
High q u a l i t y requirements f o r s torage and p e r i o d i c i nspec t i on such as those
s p e c i f i e d by the Aero-Space Indus t r y a re ob ta inab le f rom BS 2F.68:1963 "Recom-
mendations f o r the s torage and inspec t i on i n s t o r e o f vu lcanised rubber items".
14.7 ASSEMBLY
Although i n d i v i d u a l a p p l i c a t i o n s w i l l i nvo l ve p a r t i c u l a r f i t t i n g problems
the re a re a number o f bas i c p o i n t s o f good p r a c t i c e which, i f observed, w i l l
c o n t r i b u t e t o optimum seal performance:
( i ) Check t h a t seal i s o f c o r r e c t type, p a r t number or s i z e and m a t e r i a l .
( i i ) Ensure t h a t seal i s i n undamaged c o n d i t i o n and clean.
( i i i ) Where pe rm iss ib le , smear the s e a l i n g edge o f dynamic sea ls w i t h c lean
grease. Consul t any f i t t i n g i n s t r u c t i o n l a b e l prov ided by the manufac-
t u r e r t o a s c e r t a i n whether f u r t h e r grease a p p l i c a t i o n t o i n t e r - s e a l cav i -
t i e s e t c . i s recommended.
Do n o t t r e a t f l a t gasket surfaces w i t h any form o f j o i n t i n g paste o r lub-
r i c a n t unless i n s t r u c t e d so t o do, o therwise the a b i l i t y o f the gasket t o
g r i p the adjacent sea l i ng faces may be impaired.
( i v )
352
( v ) Clean a1 seal housing o r gasket sea t ing areas. Check t h a t o t h e r sur faces
adjacent t o the passage o f t he seal on f i t t i n g a re a l s o f r e e o f d i r t ,
swarf or o the r contaminants.
( v i ) Check seal housing dimensions and su r face f i n i s h t o design recommendations.
( v i i ) I f a seal i s l i k e l y t o con tac t threads, sharp corners, p o r t s , c i r c l i p s o r
s i m i l a r cont ingencies du r ing the assembly ope ra t i on then s u i t a b l e f i t t i n g
a ids must be provided. The s l i g h t e s t n i c k o r t e a r on a c r i t i c a l edge o f
an e las tomer i c component w i l l reduce s e a l i n g i n t e g r i t y . I f f requent use
i s envisaged, non -meta l l i c f i t t i n g s leeves can be o f advantage s ince dam-
age t o a s i m i l a r m e t a l l i c dev ice can d u p l i c a t e the hazard t o the sea l .
( v i i i ) D o n o t leave seal i n par t 'assembly f o r any l eng th o f t ime i f s e a l i n g edges
a re sub jec t t o misa l igned loads; f o r example rod seal f i t t e d t o c y l i n d e r
w i t h rod i n p o s i t i o n b u t no g land bush f i t t e d .
I f approp r ia te , apply any post-assembly ope ra t i on recommended by seal man-
u f a c t u r e r , f o r example compression o f the p resc r ibed amount i n an a d j u s t -
ab le gland housing; running a t ha l f -speed t o a s s i s t seal bedding- in on a
r o t a r y duty; f o l l o w i n g up f l ange b o l t s a f t e r a p e r i o d a t temperature,. . . . and so on.
( i x )
353
15 SEALS FOR FLUID POWER EQUIPMENT PART TWO ROTARY SHAFT LIP SEALS
E.D.HALLIGAN, CEng, MIMechE, A M P R I . ,
Techn ica l Manager (Product A p p l i c a t i o n s ) James Walker G Co.Ltd.
15.1 INTRODUCTION
For t h e purpose o f s e a l i n g l u b r i c a n t w i t h i n a bea r ing o r exc lud ing f o r e i g n
m a t t e r f rom bear ing surfaces, t h e spr ing- loaded r o t a r y s h a f t l i p sea l , as shown
t y p i c a l l y i n Fig.1, i s w ide l y accepted.
t he use o f t h i s type o f seal f rom o p e r a t i n g a t s i g n i f i c a n t p ressures un less the
s e a l i n g l i p i s adequately supported by a shaped p l a t e . For p ressure c o n d i t i o n s
much i n excess o f 2'.0 - 3.0 bar , combined w i t h r o t a r y movement i t would be pre-
f e r a b l e t o cons ider e i t h e r a compression pack ing o r a r a d i a l face mechanical
sea l as f i r s t cho ice .
The n a t u r e o f most designs prec ludes
F ig .1 Standard f a b r i c back sea l w i t h r e t a i n i n g p l a t e .
3 54
15.2 D E S I G N
A wide v a r i e t y o f l i p sea l designs i s a v a i l a b l e i n terms o f o v e r a l l cons t ruc-
t i o n , m a t e r i a l , and l i p p r o f i l e . Some a r e in tended f o r f i t t i n g i n housings
which have no separa te cove r -p la te and may be supp l i ed w i t h a r i g i d metal case
t o which the sea l i s bonded, t h e u n i t be ing a f o r c e - f i t i n t he housing, (F ig .2 ) .
F ig .2 Typ ica l metal-encased s e a l .
Others have t h e advantage o f f l e x i b i l i t y by v i r t u e o f hav ing a p roo fed f a b r i c
back (F ig .3a) w h i l s t r e q u i r i n g t h e p r o v i s i o n o f a r e t a i n i n g p la ' te t o n i p a x i a l l y
the back o f t he seal t o p revent leakage and o b v i a t e r o t a t i o n . More recent dev-
elopments combine the m e r i t s o f f l e x i b i l i t y and s e l f - r e t e n t i o n i n a meta l -
supported a l l - r u b b e r c o n f i g u r a t i o n (F ig .3b ) .
Fig.3a. Standard f a b r i c back s e a l . Fig.3b. Moulded- in f l e x i b l e s t e e l band s e l f - r e t a i n i n g sea l .
One o f the severa l advantages o f the f a b r i c back seal i s t he f a c i l i t y w i t h
which s p l i t sea ls can be f i t t e d w i t h o u t reduc ing performance which, i n many
cases, i s o f the same o rde r o f e f f i c i e n c y as an endless r i n g . A rubber i n l a y
i s f r e q u e n t l y moulded i n t o the back o f such a seal th rough which the s p l i t i s
e f f e c t e d . The rubber abutment which r e s u l t s ensures good s e a l i n g across the
s p l i t p o r t i o n .
The p r o f i l e o f t he l i p con tac t area i s sub jec ted t o each manu fac tu re r ' s
design ph i losophy. Some havea knife-edge con tac t band and r e l y on heavy as-
355
moulded l i p i n t e r f e r e n c e . Others depend on the s p r i n g tens ion t o urge the l i p
i n t o i n t i m a t e s h a f t con tac t . I n p r a c t i c e , a c a r e f u l balance o f i n te r fe rence ,
sp r ing c h a r a c t e r i s t i c s and con tac t band w i d t h must be a l l i e d t o knowledge o f
the type o f m a t e r i a l be ing used, bea r ing type, and c o n d i t i o n and o t h e r env i ron-
mental cons ide ra t i ons such as temperature, f l u i d and, i f any, pressure.
A t y p i c a l range o f seal p r o f i l e s i s shown i n F ig .4 which a l s o i l l u s t r a t e s
expedient used when i n s u f f i c i e n t space i s a v a i l a b l e t o accommodate a p a i r o f
seals, i . e . t he so -ca l l ed dust l i p design.
Fig.4 Typ ica l seal p r o f i l e s .
15.3 MATERIAL
N i t r i l e - b a s e s y n t h e t i c rubber compounds a r e w ide ly employed as standard mat-
e r i a l s by reason o f t h e i r c o m p a t i b i l i t y w i t h most l u b r i c a t i n g o i l s and greases
and t h e i r res i s tance t o wear. There are, however, c e r t a i n r o l l i n g o i l s and
coolants which may cause excess ive swe l l o r shr inkage o f such polymers and a
f luorocarbon rubber, e.g. V i ton , may be a necessary s e l e c t i o n . This i s p a r t i -
c u l a r l y t r u e o f some palm o i l s o l u t i o n s and o t h e r s o l u b l e types o f o i l .
Considerat ion f o r ope ra t i ona l temperature i s important and, i n t h i s respect,
i t i s the c o n d i t i o n a t t he l i p o f the seal which i s paramount - n o t necessa r i l y
the environmental temperature. However, i t may be s a i d t h a t n i t r i l e s a r e f r e -
quen t l y used w i t h success f o r i n t e r m i t t e n t se rv i ce temperatures o f 15OoC and
a r e con t inuous ly ra ted f o r 120°C i n l u b r i c a t e d cond i t i ons .
e i t h e r a c r y l i c rubbers, f luorocarbon o r s i l i c o n compounds would r e q u i r e i n v e s t i -
ga t i on .
Above these values
Many s i l i c o n e and n i t r i l e rubber s h a f t seals a re f i t t e d i n automot ive a p p l i -
ca t i ons f o r crankshaf t and gearbox sea l i ng , i n which areas much use i s be ing
made o f grooved s e a l i n g sur faces which a re designed t o produce a hydrodynamic
356
e f f e c t i n the o i l f i l m be ing sealed, e f f e c t i v e l y causing the o i l t o be pumped
away from the seal l i p (see F ig .5 ) .
Fig.5 Metal i n s e r t seal w i t h hydrodynamic a id .
De ta i l ed m a t e r i a l recommendations a r e g iven i n Appendix 1 .
15.4 SEAL LUBRICATION
I n common w i t h most o t h e r forms o f dynamic seal , r o t a r y s h a f t l i p seals
depend f o r t h e i r e f f i c i e n t and cons is ten t performance upon the presence o f a
s t a b l e f l u i d f i l m . The establ ishment o f t h i s f i l m , which w i l l t y p i c a l l y be i n
the order o f a few microns thickness, may n o t occur immediately, and i t i s no t
uncommon f o r a h igh percentage o f wear t o occur on r o t a r y s h a f t seal l i p s dur ing
t h i s pe r iod be fo re s teady-state cond i t i ons a re achieved. For t h i s reason, i t
i s always good p r a c t i c e t o apply a l u b r i c a n t t o the seal l i p and i n the seal
c a v i t y be fo re f i t t i n g .
Where s i n g l e seals a r e housed a t each end o f the housing, t he re i s normal ly
s u f f i c i e n t bear ing l u b r i c a n t i n con tac t t o p rov ide adequate l u b r i c a t i o n . Again,
where two sea ls a r e housed together , i t i s o f t e n found t h a t w i t h bear ing l u b r i -
cant on one seal and e i t h e r r o l l i n g f l u i d o r r o l l coo lan t on the o the r , f u r t h e r
l u b r i c a t i o n i s unnecessary. I n a l l cases the l i b e r a l a p p l i c a t i o n o f grease t o
the seals on assembly w i l l ensure l u b r i c a t i o n from the beginning and, i n some
instances, t h i s w i l l be found s u f f i c i e n t t o l a s t f rom one f i t t i n g t o the next .
However, t he re a r e many s e a l i n g arrangements, e s p e c i a l l y on r o l l i n g m i l l
bear ings, i n which two o r more sea ls a r e f i t t e d together i n the same housing
and the re i s the danger t h a t a t l e a s t one w i l l r un d r y un less l u b r i c a n t i s
supp l i ed from an ex te rna l source.
ho le through the chock connect ing w i t h an annular groove i n the back o f the
seal housing, as shown i n Fig.6, and us ing a spec ia l type o f seal which has a
s e r i e s o f r a d i a l p o r t s i n the base f o r passage o f l u b r i c a n t t o the seal l i p .
When i t i s d i f f i c u l t o r impossib le t o machine an annular groove i n the back o f
the seal housing, t h i s groove may be incorporated i n the seals themselves, as
shown i n F ig .7.
This can bes t be accomplished by d r i l l i n g a
357
Fig.6 Seal l u b r i c a t i o n v i a p o r t s i n seal base.
F ig .7 Seal l u b r i c a t i o n v i a p o r t s and annular groove i n seal base.
I n genera l , a good q u a l i t y minera l o i l o r grease i s s u i t a b l e f o r seal l u b r i -
ca t i on , b u t molybdenised l u b r i c a n t s may be used t o advantage where a p p l i c a t i o n
i s i n f requen t . Care should be taken t o ensure t h a t t he grease o r l u b r i c a n t w i t h
which the seal i s t o come i n t o con tac t i s compat ib le. For example, where seals
a re be ing used w i t h d i u t e minera l ac ids ,bu ty l composi t ions may w e l l be used.
This m a t e r i a l i s l i k e l y t o swe l l i n con tac t w i t h minera l o i l o r grease and an
a l t e r n a t i v e l u b r i c a n t w 1 1 be e s s e n t i a l .
15.5 SHAFT SURFACES
The s e a l i n g area o f t he s h a f t should be a f i n e ground f i n i s h o f 0.4 t o 0.8 pm
(16 - 32 p i n . ) CLA or Ra f o r most a p p l i c a t i o n s , b u t f o r t he h ighe r speed range
i t i s recommended t h a t the sur face f i n i s h be improved t o 0.2 t o 0 . h pm ( 8 - 1 6 p I n )
CLA o r Ra. I n a l l cases i t i s impor tant t h a t t he s h a f t s e a l i n g area be f r e e
from machining marks, dents, b u r r s and scratches.
Where f luorocarbon o r a c r y l i c rubber seals a r e be ing employed, i t i s a l s o
adv i sab le t o use the f i n e r l e v e l o f f i n i s h i n d i c a t e d above i n o rde r t o e l i m i n a t e
p ick-up o f t he seal m a t e r i a l .
I f l u b r i c a t i o n i s adequate and f r e e from ab ras i ve con tac t , unhardened m i l d
s t e e l s h a f t s w i l l g e n e r a l l y g i v e s a t i s f a c t o r y r e s u l t s under normal ope ra t i ng
cond i t i ons . However, a harder s h a f t m a t e r i a l i s t o be p r e f e r r e d f o r a p p l i c a t i o n s
where l u b r i c a t i o n i s poor, abras ives a re present, o r speed and pressure condi-
t i o n s a re p a r t i c u l a r l y arduous.
A f i n e machined f i n i s h i s s u i t a b l e f o r t he housing bore.
358
15.6 FRICTION
The rubb ing f r i c t i o n o f t he seal l i p on t he s h a f t m a t e r i a l i n e v i t a b l y causes
a h ighe r l o c a l temperature than i s present i n the f l u i d be ing sea led . Th is
e f f e c t w i l l be due t o the i n t e r f e r e n c e o f the seal l i p on the s h a f t m a t e r i a l
and may be aggravated by inadequate l u b r i c a t i o n o r speeds i n excess o f t h a t f o r
which a
w e l l w i t h i n t h e bounds o f t h e m a t e r i a l c a p a b i l i t y , a h i g h d i f f e r e n t i a l between
l i p t i p temperature and f l u i d temperature w i l l be man i fes ted i n many cases by
h a i r - l i n e scores i n the seal l i p co -ax ia l w i t h the s h a f t . Local c a r b o n i s a t i o n
o f the rubber compound w i l l a l s o be ev iden t .
p a r t i c u l a r seal was designed. Assuming the system temperature t o be
F igu re 8 i n d i c a t e s a se t o f curves de r i ved by exper iment, p r o j e c t i n g f r i c -
t i o n a l horse power aga ins t sea l d iameter f o r a convent iona l r o t a r y l i p seal
design o p e r a t i n g a t 500 r.p.m. i n m ine ra l l u b r i c a t i n g o i l a t d i f f e r e n t system
pressures.
300 LOO 500 600 700 800 900
Shaft diameter [mml
F ig .8 F r i c t i o n a l horse power absorbed by r o t a r y s h a f t l i p s sea ls , having convent iona l l i p i n t e r f e r e n c e o p e r a t i n g a t 500 r e v h i n
359
15.7 SPEEDS
Many o f t h e fabr ic -backed type o f sea ls toge the r w i t h those o f t he meta l -
supported type a r e work ing s a t i s f a c t o r i l y on 400 mm r o l l - n e c k d iameters a t
speeds o f up t o 25 m/s (5000 f t / m i n ) over sus ta ined work ing pe r iods .
however, many f a c t o r s such as su r face f i n i s h , e c c e n t r i c i t y , and l u b r i c a t i o n
which can l i m i t the maximum speed f o r which any seal i s s u i t a b l e , and manufac-
t u r e r s should be consu l ted i n cases o f doubt.
There are,
15.8 ECCENTRICITY
Where p l a i n metal o r s y n t h e t i c compos i t ion bear ings a r e be ing employed, i t
i s n o t uncommon t o f i n d t h a t t h e s h a f t i s n o t t r u l y concen t r i c w i t h t he seal
housing, owing t o bea r ing c learance and subsequent wear. I n t h i s event, i t i s
e s s e n t i a l f o r e f f i c i e n t s e a l i n g t h a t t he seal l i p be capable o f f o l l o w i n g a l l
s h a f t movement and, indeed, on many l a r g e compara t ive ly slow-moving s h a f t s
e c c e n t r i c i t y values o f 2 .5 mm have been s a t i s f a c t o r i l y accommodated. N a t u r a l l y ,
t h e sea l performance i n terms o f e c c e n t r i c i t y c a p a b i l i t y w i l l be speed and sha f t
d iameter dependent.
Where s p l i t sea ls a r e f i t t e d , then p a r t i c u l a r c o n s i d e r a t i o n i s necessary t o
the problem o f s h a f t e c c e n t r i c i t y , s i n c e t h e r e may be a tendency f o r a s p l i t
seal t o open a t t he j o i n .
15.9 PRESSURE
A l though few r o t a r y s h a f t l i p sea ls a r e spec
compqnents t o accept s i g n i f i c a n t pressures, t he
w i l l extend the use fu lness o f t h i s t ype o f sea l
F ig .9 .
f i c a l l y designed as s tandard
use o f meta l -suppor t ing p l a t e s
A t y p i c a l p r o f i l e i s shown ' i n
F ig .9 Seal w i t h shaped suppor t p l a t e f o r p ressures up t o 3.0 ba r .
As a r e s u l t o f exper imenta l work on t h e s e a l i n g o f o i l - f i l l e d marine s t e r n
glands and manoeuvring t h r u s t e r s , a seal l i p p r o f i l e has been developed which
s a t i s f a c t o r i l y sus ta ins pressures up t o 4 . 0 bar w i t h o u t the use o f a shaped
support p l a t e . The base must be f u l l y supported as i n d i c a t e d i n Fig.10.
360
Q Fig.10 New seal development f o r pressures up t o 4.0 ba r w i t h o u t
shaped support p l a t e .
Where constant app l i ed pressure i s n o t a n t i c i p a t e d , i t i s f r e q u e n t l y sugges-
ted t h a t grease- lubr icated bear ings a r e equipped w i t h r e l i e f ho les and t h a t o i l -
l ub r i ca ted bear ings have d r a i n s o f adequate s i z e . Drains taken f rom the ends
o f the bear ing near the seals w i l l he lp t o d i s s i p a t e any l o c a l i s e d pressure
bui ld-up. Where poss ib le , steps should be taken i n bea r ing des ign t o prevent
escaping h i g h pressure o i l impinging d i r e c t l y upon the seals .
I n some cases where the loss o f a smal l amount o f grease i s unimportant, a
simple r e l i e f system may be formed by f a c i n g sea ls away f rom the bear ing and
a l l ow ing the seal l i p t o be l i f t e d under the i n f l u e n c e o f the l u b r i c a n t pressure.
15.10 CARE AND HANDLING
F l u i d seals should be handled a t a l l t imes w i t h extreme care s ince t h e l i f e
o f bear ings o r o t h e r c o s t l y machine p a r t s may depend upon t h e i r e f f i c i e n c y .
A t t e n t i o n t o the f o l l o w i n g v i t a l p o i n t s w i l l a s s i s t i n ensur ing t r o u b l e - f r e e
operat ion du r ing se rv i ce .
15.10.1 Storage
( i ) The s t o r e should have a coo l , c lean and d r y atmosphere, f r e e f rom
dust and g r i t .
( i i ) Whenever poss ib le , seals should n o t be removed from the wrapping i n
which they were suppl ied, as t h i s prov ides p r o t e c t i o n and i d e n t i f i c a t i o n
( i i i ) Avoid u n t i d y s tack ing as the weight may d i s t o r t t he seals a t t he bottom
o f the s tack.
Seals should never be threaded on w i r e o r s t r i n g as t h i s w i l l damage
the l i p s .
( i v )
361
15.10.2 Handl ing
( i ) I t must be remembered t h a t seal l i p s a re extremely vu lne rab le t o damage
and the smal lest n i c k prov ides a p o t e n t i a l leak path.
Seal r e i n f o r c i n g i n s e r t s , a l though adequate f o r t h e i r duty , may deform
under adverse hand l i ng o r s tack ing.
( i i i ) Seals having metal ou ts ide surfaces may damage o t h e r seals , e s p e c i a l l y
i f the metal edges con tac t the rubber p a r t s o f ne ighbour ing seals.
( i i )
15.10.3 F i t t i n g
A h igh p r o p o r t i o n o f f a i l u r e s and leakage o f o i l seals i s due t o i n c o r r e c t
f i t t i n g r e s u l t i n g i n damage t o both seal and s e a l i n g sur face. S t r i c t a t t e n t i o n
t o the f o l l o w i n g ma t te rs i s e s s e n t i a l i f best performance i s t o be obtained.
( i ) Before f i t t i n g , the seal should be examined t o ensure t h a t i t i s c lean
and undamaged.
The s e a l i n g l i p should be smeared w i t h s u i t a b l e c lean l u b r i c a n t . Seals
used as dust exc luders should be packed w i t h a compat ib le grease.
( i i )
( i i i ) The s e a l i n g l i p , normal ly , should face the f l u i d t o be sealed.
When f i t t i n g , i t i s impor tant t o ensure t h a t the s e a l i n g l i p i s no t
damaged even by the s l i g h t e s t n i c k , t h a t the s p r i n g i s c o r r e c t l y
located when i n p o s i t i o n , and t h a t the seal i s p roper l y pressed home
i n t o the housing recess.
Examine the s h a f t which should be f ree from a l l roughness and sharp
edges and avo id passing the s e a l i n g l i p over keyways, screw threads,
o r shoulders. Shaf t edges o r shoulders should be w e l l rounded o r
chamfered, and where t h i s i s n o t p r a c t i c a b l e a f i t t i n g s leeve s l i g h t l y
l a r g e r than the
According t o the type o f assembly, i t may be necessary e i t h e r t o f i r s t
press the seal i n t o the housing and subsequently on to the s h a f t o r ,
a l t e r n a t i v e l y , t o pass the seal over the s h a f t and then press i t i n t o
the housing. I t i s p re fe rab le f i r s t t o mount the seal on the s h a f t
where circumstances permi t , s i nce t h i s a l l ows observat ion o f the l i p
d u r i n g assembly.
s h a f t w i t h a l ead - in taper should be used.
( v i i ) The assembly should no t be al lowed t o r e s t f o r any l e n g t h o f t ime a t
an incomplete s t a t e o f f i t t i n g , where the weight o f the s h a f t o r housing
may be borne by the seal , r e s u l t i n g i n damage o r d i s t o r t i o n t o the
l a t t e r .
( v i i i )When p ress ing the seal i n t o the housing, a un i fo rm pressure should be
exer ted, p r e f e r a b l y by means o f an arbor press i n combinat ion w i t h a
s u i t a b l e t o o l . The diameter o f the t o o l should be s l i g h t l y smal ler
than the diameter o f the housing by 0.1 t o 0.4 mm. The ou ts ide surface
362
of the seal can be smeared with a suitable lubricant in order to facili-
tate fitting. Care must be taken to ensure that the seal does not enter
the housing recess in a tilted position, since this will cause damage
to the outer surface.
15.11 SERVICE PROBLEMS AND THEIR SOLUTIONS
A number of fault-finding procedures follow which, if taken in sequence,
should analyse the reason for a given difficulty with rotary shaft lip seals.
15.11.1 Unacceptable Leakage
This is almost always associated with oil lubricated bearings, since grease
is not a difficult lubricant to seal. The term "unacceptable" can have wide
interpretation since an occasional drop of oil might be disastrous if it resul-
ted in contamination of the product being handled by the machine concerned in
such spheres as the textile, paper, or food industries, whereas it would pro-
bably remain unnoticed in a heavy industrial environment.
Since the vast increase in oil prices there is, however, a greater sensiti-
vity to oil losses in any form and leakage rates that hitherto have been ignored are now becoming regarded as unacceptable.
When dealing with such complaints it is essential to discover the history of
equipment concerned, and this broadly falls into three categories:
(I) New equipment recently commissioned, where sealing has been regarded
as unsatisfactory from the start.
( 1 1 ) Equipment that has been in operation for a period of time and only
recently has developed leakage problems.
( I l l ) Equipment that was satisfactory during its first term of operation but
leakage has occurred after fitting replacement seals during routine
maintenance or overhaul.
Since trouble tracing is basically a process of eliminating of substantia-
ting faults, the sequence of checks required would vary with each of the above
categories.
In order to simp1 ify the procedures and'avoid irrelevant investigations, the
recommended sequences for each of the above categories is defined by letter
symbols to be used in conjunction with the attached fault-finding chart.
Category (
Full checks
Category ( I
A , B , C ( 1 ) ,
tain Deriod of
)
in order as A, B, C, D and E until fault is discovered.
1 D ( 2 ) , and D ( 3 ) . If faults as C ( 1 ) o r D(2) are exhibited, ascer-
service with seals. This should be calculated in terms of hours
363
o f runn ing and r e l a t e d t o speed, temperature, and o t h e r environmental cond i t i ons
I f sea ls
Category
A , B, C(
A moderate speed w i t h good c lean l u b r i c a t i o n c o n d i t i o n s and ambient tempera-
tu res would no rma l l y a n t i c i p a t e a seal l i f e o f around 10,000 hours.
High speeds, poor l u b r i c a t i o n , e leva ted temperatures, o r p a r t i a l l y ab ras i ve
media cou ld reduce t h i s t o as l i t t l e as 2,000 hours .
The problem may t h e r e f o r e be s imp ly t h a t o f be ing due f o r seal replacement.
I f f a u l t s as i n (D3) a r e i n evidence, o b t a i n d e t a i l s o f a l l media i n con tac t
w i t h seal ( i n c l u d i n g any c l e a n i n g f l u i d s ) as a change o f l i p m a t e r i a l may be
necessary t o o b t a i n c o m p a t a b i l i t y .
d i s p l a y no f a u l t s , check f o r mechanical de fec ts as E(1) and E(2 ) .
F a u l t - f i n d i n g Chart
( A ) I s leakage a c t u a l l y o c c u r r i n g f rom the seal o r does i t stem from such
sources as bear ing cover f l anges and i s merely " co l l ec ted " by the seal
housing, g i v i n g a f a l s e impression? Check by w ip ing c lean a l l app rop r ia te
areas and run machine t o a s c e r t a i n leakage source.
( B ) I s leakage from around seal back o r f rom the l i p a long the s h a f t ? Check by
w ip ing bo th c lean and observ ing w h i l e machine i s runn ing .
(C) I f O / D leakage - check the f o l l o w i n g :
( 1 ) Is seal a good f i t i n housing o r i s i t s l a c k ? (On s p l i t sea ls a s lack
sea l w i l 1 d i s p l a y a gap between sea l ends) .
( 2 ) I f housing bore i s c o r r e c t s i z e then seal O / D dimension i s suspect i f
s l a c k i n housing.
(3) I f seal i s good f i t i n housing, check f o r damage on housing bore.
(4) Check housing depth t o ensure seal i s be ing a x i a l l y compressed - i f
a p p l i c a b l e .
(0) I f leakage a long s h a f t , check the f o l l o w i n g :
( 1 ) Sha f t s i ze , su r face f i n i s h , s h a f t damage a t con tac t a rea .
(2) If ( 1 ) O . K . , check c o n d i t i o n o f s e a l i n g l i p f o r harden ing and/or crack-
i ng . I f e i t h e r i n evidence, then speed o r temperature c o n d i t i o n s a re
probab ly incompat ib le w i t h seal m a t e r i a l .
( 3 ) I f l i p i s s o f t o r swo l len t h i s i s u s u a l l y an i n d i c a t i o n o f chemical
i n c o m p a t i b i l i t y w i t h the media i n con tac t w i t h the s e a l .
(4) I f (2) and (3) O.K. , check s e c t i o n w i d t h o f seal w i t h s p r i n g f i t t e d .
364
This should be a t l e a s t nominal s e c t i o n +1% immediately on removal and
inc reas ing t o nominal +3% a f t e r one hour i n f r e e s t a t e . Spr ing may be
shortened by up t o 5% o f i t s o r i g i n a l l eng th i f s e c t i o n appears inade-
quate. Sect ion measurement should be average o f 4 e q u i - d i s t a n t readings.
(E) I f checks (C) and (D) do n o t reveal any f a u l t s the problem may be due t o
mechanical cond i t i ons and the f o l l o w i n g should be checked:
( 1 ) Shaf t t o housing concentr c i t y - check by means o f c a l i p e r s between
s h a f t and housing bore a t 4 p o i n t s around per iphery. V a r i a t i o n s o f more
than 0.3 mm r e q u i r e f u r t h e r i n v e s t i g a t i o n .
(2) I f sma l les t c a l i p e r measurement occurs between bottom o f s h a f t and
housing, t h i s may i n d i c a t e bear ing wear w i t h r e s u l t a n t dynamic e c c e n t r i -
c i t y . (App l i cab le t o h o r i z o n t a l s h a f t s o n l y ) .
( 3 ) I f bear ing i s O.K. then housing o f f s e t may be responsib le . Seal housings
a re no rma l l y c e n t r a l i s e d w i t h the s h a f t by means o f a machined r e g i s t e r
w i t h the bear ing housing. Where t h i s f e a t u r e i s n o t incorporated then
i t may-be p o s s i b l e t o c e n t r a l i s e the housing by s lackening the b o l t s and
repos i t
(4) I f (11,
ga t i on :
I f bear
prevent
oning.
(2 ) and (3 ) a r e blameless then the f o l l o w i n g requ i res i n v e s t i -
ngs a re o i l pressure l u b r i c a t e d i s t he re adequate drainage t o
pressure bu i l d -up aga ins t t he sea l?
I f equipment operates on a constant o i l l e v e l p r i n c i p l e , a re the re gear-
wheels o r b a l l - j o u r n a l bear ings i n c lose p r o x i m i t y t o the seal causing
o i l t u rbu lence o r f l o o d i n g ?
Where the l a t t e r s i t u a t i o n e x i s t s , t he housing lands should be o n l y
marg ina l l y l a r g e r than s h a f t d iameter i n o rde r t o form a b a f f l e o r ,
a l t e r n a t i v e l y , a b a f f l e p l a t e f i t t e d between bear ing and seal housing.
A temporary remedy can be made by us ing a 2.5 mm CAF gasket a t the bottom o f
the housing, the I / D o f which should be s h a f t d iameter p l u s 0.5 nnn maximum. A
f u r t h e r gasket o f equal th ickness should be f i t t e d underneath the r e t a i n i n g
p l a t e t o r e s t o r e the c o r r e c t amount o f a x i a l compression i n the case o f re ta ined
seals (see Fig.11 and F ig.12) .
365
Fig.11 Showing how t o o l a r g e bore diameter o f seal housing land pe rm i t s h i g h v e l o c i t y o i l impingement on s e a l .
B a f f l e p l a t e between bear ing and sea l o r c l o s e - f i t t i n g seal would reduce r i s k o f o i l leakage.
.A.F gasket .5mm thick
F ig .12 Showing how f i t t i n g o f C.A.F. gaskets can p rov ide a temporary remedy f o r s i t u a t i o n i n F ig .11 .
(With acknowledgement t o the l a t e Mr.Ray Bladwin - Senior Adv isory Engineer,
r o t a r y s h a f t l i p sea ls - who compi led much o f t he da ta presented he re ) .
366
APPENDIX 1
ROTARY SHAFT L IP SEAL F(ATERIAL RECOMMENDATION CHARTS
The accompanying cha r t s show va r ious l u b r i c a n t s and r o l l i n g f l u i d s t h a t a r e
commonly used i n the metal r o l l i n g i n d u s t r y . Some o f these a re no t chemica l l y
compat ib le w i t h the standard n i t r i l e rubbers used i n r o t a r y s h a f t l i p seal p ro-
duc t i on , and i t i s t h e r e f o r e necessary t o employ an a l t e r n a t i v e rubber compound
t o achieve s a t i s f a c t o r y r e s u l t s i n terms o f sea l e f f i c i e n c y and l i f e .
Seal L i p s
The rubber compound i n the column headed "1s t choice" i s t h e grade l e a s t
a f f e c t e d by the media concerned and should be se lec ted wherever p o s s i b l e .
most ins tances a second cho ice i s g iven and t h i s rubber can be used w i t h o u t
se r ious de t r imen t t o the sea l .
I n
Where f l uo roca rbon rubber (denoted by the l e t t e r 'A') appears as the o n l y
cho ice , p lease n o t e t h a t none o f the convent iona l commercial grade rubbers can
be used as an a l t e r n a t i v e . I n ins tances where f l uo roca rbon rubber appears as
the second cho ice i t i s u s u a l l y f o r economic reasons, i n d i c a t i n g t h a t i t i s on l y
m a r g i n a l l y super io r t o the commercial grade compound g i ven as f i r s t cho ice , and
the smal l advantage would o f t e n n o t j u s t i f y t he a d d i t i o n a l c o s t .
Seal Backs
Al though w i t h some media the f a b r i c m a t e r i a l s used f o r c o n s t r u c t i o n o f the
seal back s u f f e r l i m i t e d v o l u m e t r i c change, t h e e f f e c t s a r e n o t d e t r i m e n t a l t o
s e a l i n g e f f i c i e n c y as t h i s p a r t o f t h e sea l i s f u l l y housed and no rma l l y a x i a l l y
r e s t r a i n e d .
Se lec t i on o f M a t e r i a l s
I n r o l l i n g - m i l l a p p l i c a t i o n s , r o t a r y s h a f t l i p sea ls a r e no rma l l y requ i red t o
seal no t o n l y aga ins t bea r ing l u b r i c a n t s b u t a l s o r o l l i n g f l u i d s , and ca re should
be taken t o s e l e c t a m a t e r i a l which i s compat ib le w i t h both. Th is may i n v o l v e
some compromise by s e l e c t i n g a second cho ice m a t e r i a l f o r e i t h e r t h e l u b r i c a n t
o r the r o l l i n g f l u i d , i n o rde r t o o b t a i n a reasonable degree o f c o m p a t i b i l i t y
w i t h bo th .
I n dua l s e a l i n g arrangements employing back-to-back sea ls , should o n l y one o f
t he media n e c e s s i t a t e f l uo roca rbon rubber , t h e opposing sea l cou ld be made w i t h
a l ess c o s t l y compat ib le grade o f rubber p r o v i d i n g t h a t care i s taken i n i d e n t i -
f i c a t i o n and f i t t i n g .
For convenience, l e t t e r symbols a r e used i n the cha r t s t o i n d i c a t e va r ious
rubbers, and t h e key t o these i s g iven :
367
KEY TO MATERIAL CODES
Code Base rubber compound
F I uorocarbon High n i t r i l e Medium h i g h n i t r i l e N i t r i le N i t r i l e w i t h g r a p h i t e N i t r i l e w i t h p a r a f f i n wax
RECOMMENDED SEAL LIP MATERIALS FOR USE WITH BEARING OILS AND GREASES
Seal l i p m a t e r i a l s
1 s t Choice 2nd Choice Brand Name o r Number
B.P. HCT 80 C E
B .P . Sperno 350 HB 'I 450 HB
B . P . Energol GR 125-XP
Cas t ro l 98
CLP 114
Ca lyso l Grease
Duckhams Zero O i l
EP 69 O i l
EP 80 Gear Lub r i can t
Esso E s t i c 65
Esso Nuray 146
Esso Pen-0-Led EP3
Fuchs MR 40
Mobi 1 Exu 66/25
" Vacuol i ne ' A A ' II ' I 25 x 25R
C
C
C
C
C
D
C
D
C
C
D
C
C
C
C
D
0
E
E
A
C
D
C
A
A
C
F
D
F
E
I, I ' Heavy C E
I EE" C E
I ' Mobi lube H.D.90 C E
" Mobi l e x EP2 Grease C F
OM 100 O i l C A
Regal Gear O i l C B
She l l O i l 1624 and 1611 B C
" Carnea C B
" Faunus ' B ' B A
I ' Macoma D C
" Nassa 78 C A
II ,I
Seal l i p m a t e r i a l s f o r bear ing o i l s and greases (contd.)
Seal l i p ma te r ia Brand Name o r Number
S
1 s t Choice 2nd C Dice
C A She1 1 Telona 945
' I Teressa
" T i v e l a 75
" V i t r e a 75
" Ret inox Grease
'I Donax 17
I ' Aeroshel 1 Grease No.7
Ucon O i l
A lo-J i dac
Caster O i l ( w i t h water ) I , I ' ( w i t h methyl a ted
s p i r i t )
C i mcoo 1
Cirncool E 5
Cas t ro l Coolage SL
Croda D4 A
'I Lubro tex 1; 7804
Dasco 900
Esso Somentor N35
' I w20
I' 33 and N60
I,
,I
I' U n i v i s ~ 5 8
Emulsion KF 81
Germ K i n e t r o l FR3A
Gu l f Mineral Seal
I' Cut O i l
" 93
Houghton Permasol
" Su l fona ED
H u i l e de Laminage 102 P & 982 R
Hydr i t
KF 61 R o l l i n g Oil
Lubr i co r ' T I
C A
C D
C B
A C
C B
C A
C D
A
A B
C E
E A
B C
A B
B A
A
B A
C A
A 6
A C
C B
A
A
C D
C D
C D
B A
k B
B C
B D
A D
B E
- -
369
Seal l i p m a t e r i a l s f o r r o l l coo lan ts and r o l l i n g o i l s (con td . )
Brand Name o r Number Seal l i p m a t e r i a l s
1 s t Choice 2nd Choice ~~~ ~
Mobi l Generex 56, 57 and 404D A B
" 22 and 24 C B
1 1 322 C D
Proso l 66 A B
44 B A
33 A - 'I Solvac 11 A - 1 1 A 1 1 a00 B
1 1
1 1
1 1
I I
I ' Vac t ra "HH" and "BB" C B
Mi robo 415A B A
Ocut G C A
Palm O i l B A
Quaker T i n n o l 12 E D
I 1 109 B D
B A
1 1
' I Quakerol 41 ( w i t h S h e l l Carnea 31)
I, 43, 82, a7 M and
88-182 M B A
I' Qwerl 506 B D
Rol l u p 200 B A
She l l Dromus B B A
Ste rno l PL106 and PL107 C B
' I 1270/1307 B A
I ' 1076 D C
Tayol 316 Emulsion B A
Texaco Texatherm 320 A C
T r e l l u b 12 A B A
Wyrol H40 C A
370
I(; SEALS FOR FLUID POWER EQUIPMENT PART THREE COMPRESSION PACKINGS
B.D. HALLIGAN, C.Eng, MIMechE, AMPRI Technical Manager (Product Appl i c a t i o n s ) James Walker and Co. L td .
16.1 THE PACKED GLAND
Compared t o the f i n i t e q u a l i t i e s o f f e r r o u s meta ls f o r example, t he essent-
i a l l y deformable na tu re o f sea l i ng m a t e r i a l s has in t roduced a measure o f
v a r i a b i l i t y t h a t causes many commentators t o look on f l u i d s e a l i n g technology
as an a r t r a t h e r than a science. I f t h i s i s t r u e , and manufacturers o f mechan
i c a l face sea ls would be bu t one area o f v a l i d o b j e c t i o n , then the f i e l d o f
compression packings i s , a rguab ly , t he b lackes t area o f t h a t a r t .
Regarded as an anachronism i n a p e r i o d o f h i g h techno log ica l achievement,
compression packings show no s igns o f l o s i n g s i g n i f i c a n t ground i n terms o f
p roduc t i on q u a n t i t i e s as new and improved types p r o l i f e r a t e bo th i n Europe and
elsewhere. To understand t h i s s i t u a t i o n requ i res some a p p r e c i a t i o n o f t he
fundamental mode o f o p e r a t i o n o f t he a d j u s t a b l e g land o r s t u f f i n g box shown i n
F ig . 1 .
SYSTEM __c
PRESSURE
COMPRESSIVE FORCE
SPIGOT &THROUGH GLAND
f h t Y
Fig.1 Compression Packing
371
This may be f i l l e d w i t h s p l i t packing r i n g s chosen from a v a r i e t y o f
m a t e r i a l s and cons t ruc t i ons , descr ibed elsewhere, which a r e persuaded to reac t
against a s h a f t , whether r o t a r y o r r e c i p r o c a t i n g , t o the ex ten t t h a t the r a d i a l
f o rce developed exceeds the pressure t o be sealed.
Such a p r i n c i p l e cou ld n o t be m r e elementary and i t s i n t r i n s i c value could
be f u r t h e r quest ioned as packings i n t h i s category used f o r r o t a t i n g o r rec ip -
roca t i ng equipment r e l y on a c o n t r o l l e d leakage f o r long- term l u b r i c a t i o n pur-
poses, i f they a r e t o su rv i ve f o r an adequate pe r iod .
The cont inued j u s t i f i c a t i o n for the compression packing might appear obscure
against such a background b u t t he re can be no doubt t h a t c e r t a i n areas o f app-
l i c a t i o n e x i s t where no reasonable s u b s t i t u t e i s a v a i l a b l e .
16.1.1 Pumps
Many reasoned and wel l - researched papers have been publ ished t o support
mechanical seals against s o f t packing and v i c e versa. There i s no doubt t h a t
the former have supplanted packed glands as o r i g i n a l equipment on the m a j o r i t y
o f rotodynamic pumps f o r a v a r i e t y o f process and se rv i ce f l u i d s bu t they a re
operat ing parameters and cos t cons ide ra t i ons which w i l l f r e q u e n t l y d i c t a t e the
choice o f s o f t packing.
Table 16.1 compares the r e l a t i v e a t t r i b u t e s o f the two contenders i n bas ic
terms.
I n general i t may be s a i d t h a t , un less zero leakage i s an absolute p r i o r i t y ,
compression packings w i l l r e t a i n an impor tant p o s i t i o n wherever regu la r mainten-
ance i s a v a i l a b l e and the f o l l o w i n g cons ide ra t i ons apply :
- s i m p l i c i t y i n g land design and a n c i l l a r y equipment
- ease o f f i t t i n g
- f l e x i b i l i t y of supply and spares f o r p l a n t u t i l i z i n g many d i f f e r e n t
types and s i zes o f pump handl ing a wide v a r i e t y o f f l u i d s
- f requent a b i l i t y to c a t e r f o r adverse cond i t i ons w i thou t e labo ra te
precaut ions
16.1.2 Valves
If any doubt e x i s t s regard ing s e l e c t i o n on pumps then a much more obvious
choice o f s o f t packing a p p l i e s t o the va lve scene.
ment, ease o f f i t t i n g and, i n t h i s case, l a c k o f leakage requirement f o r lub-
r i c a t i o n purposes p l u s the most dec i s i ve advantage o f low cost , a re f a c t o r s
which i d e a l l y r e l a t e t o compression packings.
The r e l a t i v e l ack o f move-
There a r e areas where moulded e las tomer i c seals present a reasonable a l t e r n -
a t i v e but even the most e x o t i c compounds would seldom be used above 250°C - unless r e i n f o r c e d by asbestos f a b r i c .
w 4 N
TABLE 16.1
Compa r i son So f t Packing Mechan i ca 1 Sea 1
I n i t i a l Cost o f the order o f 1 O : l i n favour o f s o f t packing depending on s i z e and a p p l i c a t i o n
APPROXIMATELY EQUAL Re1 i a b i 1 i t y ample warning o f impending f a i l u r e l i t t l e or no warning o f
w i t h p o s s i b i l i t i e s f o r c o r r e c t i o n end o f use fu l l i f e w i t h p o s s i b i l i t y o f sudden complete f a i l u r e
I n s t a l l a t i o n
Maintenance
Spares
Shaft Wear
Operat ing Costs
e s s e n t i a l l y s imple - r e q u i r i n g s k i l l e d f i t t i n g requ i red - no specia l s k i l l s i f c o r r e c t p r e c i s e l y de f i ned env i ron- procedure adopted ment and assembly
regu la r and r e q u i r i n g exper ience zero
f a c i l i t y f o r s tock ing l eng th form spare seal components must m a t e r i a l o r complete pre-formed be a v a i l a b l e - cos t can be sets a t r e l a t i v e l y low cos t s u b s t a n t i a l
can be considerable; s h a f t sleeves n i l reduce replacement cos ts
f r i c t i o n losses s l i g h t l y h ighe r w i t h s o f t packing
leakage losses zero w i t h mechanical seals bu t p o s i t i v e w i t h s o f t packing as l u b r i c a t i o n o f sea l i ng r i n g s i s essen t ia l
373
16.2 OPERATING PRINCIPLES
By comparison t o the seal types descr ibed i n the o t h e r papers - p a r t i c u l a r l y
e lastomer ic l i p and squeeze seals - compression packings respond t o app l i ed
pressure i n inverse p r o p o r t i o n to the hardness o f t h e i r c o n s t r u c t i o n and r e l y
on an ex te rna l f o r c e t o produce the r a d i a l pressure requ i red f o r e f f e c t i v e
seal ing.
ab l y ) invo lves a b o l t e d q land sp igo t as shown i n Fig.1 where c o n t r o l l e d a x i a l
movement i s e a s i l y achieved by adjustment o f the r e t a i n i n g nu ts o r studs.
Spring l oad ing i s sometimes used i n i naccess ib le s i t u a t i o n s bu t such a pro-
v i s i o n lacks the f i n e c o n t r o l demanded by some packing types and has a l i m i t e d
range o f load c a p a b i l i t y .
The method o f generat ing t h a t f o rce can vary b u t u s u a l l y (and p r e f e r -
Wh i l s t t he s e a l i n g f o r c e can be ad jus ted t o c a t e r f o r se rv i ce wear, care must
be taken t o avo id over compression which w i l l lead t o excessive f r i c t i o n , s h a f t
wear and premature packing f a i l u r e .
To increase d e n s i t y and d i s s i p a t e heat, s o f t packings i n v a r i a b l y contain.
l u b r i c a n t s , l o s s o f which, through excessive compression o r over-heat ing i n
serv ice, w i l l r e s u l t i n packing volume l o s s w i t h subsequent reduc t i on i n the
e f f e c t i v e sea l i nq r e a c t i o n and correspondingly i nc reas ing leakage ra tes . By
l i m i t i n g compression t o a p o i n t where s l i g h t c o n t r o l l e d leakage i s obtained,
adequate l u b r i c a t i o n o f t he dynamic sur faces i s ensured and over-compression o f
the packing avoided. However, where l u b r i c a t i o n i s a problem - o r a degree o f
g land c o o l i n g i s requ i red - a l a n t e r n r i n g can be incorporated i n t o the gland
area f o r t he d i s t r i b u t i o n o f a d d i t i o n a l l u b r i c a n t / c o o l a n t (Fig.2a). The p o s i t i o n
o f a l a n t e r n r i n g w i l l depend on the na tu re o f the a p p l i c a t i o n b u t , s ince the
packing r i n g s nearest the g land sp igo t do most o f t he work, t he a d d i t i o n a l f l u i d
should u s u a l l y be in t roduced near t o t h a t area.
ADDITIONAL LUBRICANT/COOLANT
I
DISTRIBUTION RING PORTS
BARRIER FLUID
374
I f i t i s e s s e n t i a l t h a t the f l u i d be ing pumped does n o t escape t o atmosphere
(e.g. a t o x i c medium) the l a n t e r n r i n g may serve t o i n t roduce a b a r r i e r f l u i d
a t a p ressure of 0.5 t o 1 bar above t h a t t o be sealed (F ig .2b) . S i m i l a r l y ,
where the re i s a r i s k o f severe ab ras i ve wear t o the packing, a f l ush in ! f l u i d
may be in t roduced throuoh the l a n t e r n r i n g (F ig .2c) .
For a p p l i c a t i o n w i t h neca t i ve pump pressures ( i . e . suc t i on ) a supply o f t he
medium be ing sealed can be made through the l a n t e r n r i n g t o p revent a i r -d raw ing
(Fig.2d).
FLUSH I NG FLU I D I
SUPPLY OF SEALED MEDIUM TO PREVENT AIR-DRAWING
I
I f extreme temperatures a r e t o be encountered, i t i s u n l i k e l y t h a t c o o l i n g
through the l a n t e r n r i n g w i l l be s u f f i c i e n t and recourse must be made t o i n -
t e r n a l c o o l i n g o f t he g land housing and s h a f t t o reduce the temperature a t t h e
gland t o a va lue w i t h i n the pack ing ' s c a p a b i l i t i e s . Conversely, when d e a l i n g
w i t h media which c r y s t a l l i z e o r congeal when coo l (e.g. sugars, t a r s , e t c . ) ,
the pack ing w i l l f ace r a p i d d e s t r u c t i o n un less g land hea te rs o r a steam jacke ted
arrangement a r e employed t o r e s t o r e t h e f l u i d s t a t e be fo re s t a r t i n g up.
I t should always be remembered t h a t the i n c l u s i o n o f a l a n t e r n r i n g i n t o the
gland area i n v a r i a b l y compl ica tes assembly and can p r o v i d e a p o s s i b l e source o f
sha f t scor ing ; they should, t h e r e f o r e , o n l y be cons idered when the n a t u r e o f
the a p p l i c a t i o n a b s o l u t e l y demands t h e i r presence.
16.3 GLAND DESIGN
A t t h i s j unc tu re , few i n t e r n a t i o n a l standards e x i s t t o d e f i n e housing design
f o r s o f t packings bu t t he dimensions shown i n Table 16.2 should be s a t i s f a c t o r y
f o r most a p p l i c a t i o n s . Housing depths w i l l va ry w i t h i n d i v i d u a l c i rcumstances,
such as the i n c l u s i o n o f a l a n t e r n r i n g , bu t f i v e r i n g s o f square s e c t i o n
packings a re u s u a l l y recommended f o r t he average, uncompl icated du ty .
375
TABLE 16.2 Suggested housing widths i n r e l a t i o n t o s h a f t diameters. A l l dimenZions i n mm.
A l l packings except expanded g raph i te Expanded g raph i te
Shaft Diameter Housing Width Shaft Diameter Housing Width
up t o 12 3 up t o 18 3
above 12 t o 18 5 above 18 t o 75 5 18 t o 25 6.5 75 t o 150 7.5 25 t o 50 8 150 and above 1 0
50 t o 90 10
90 t o 150 12.5
150 15
Other design cons ide ra t i ons worthy o f note, b u t o f t e n over looked, can be
summarized as:-
( i ) The p r o v i s i o n o f an adequate tapered ' l e a d i n ' a t the mouth o f the
g land t o f a c i l i t a t e e n t r y o f the packina and t o obv ia te the r i s k o f
damage i n the assembly ope ra t i on . A minimum o f 15" x 6.5mm u s u a l l y
represents good p r a c t i c e .
The p r o v i s i o n o f a reasonable sur face f i n i s h on adjacent metal p a r t s - p a r t i c u l a r l y the dynamic surface. The b e t t e r the f i n i s h the l ess wear
w i l l occur; 0.4um (16 p i n ) CLA o r Ra on the s h a f t and 1.6 pm
(54 u i n ) CLA o r Ra on the s t u f f i n g box bore should be ideal f o r most
a p p l i c a t i o n s . The use o f s h a f t sleeves can g i v e considerable main-
tenance advantage when cons ide r ing the quest ion o f sur face f i n i s h .
The danger o f extreme running clearances a t the g land - p a r t i c u l a r l y
on the sp igo t s ide. In those except ional cases where excessive
c learance i s unavoidable, the packing should be p ro tec ted by an
independent r i n g o f s u i t a b l y robust ma te r ia l o r cons t ruc t i on which
reduces the c learance t o a minimum.
An al lowance for e n t r y o f the g land sp igo t w e l l i n t o the gland area;
c e r t a i n l y t o an e x t e n t t h a t exceeds s u b s t a n t i a l l y the depth o f the
tapered lead in . The leng th o f sp igo t se lected must a l s o c a t e r f o r
packing compression, r e s u l t i n g f rom gland adjustment. Typica l e n t r y
lengths should be a t l e a s t two t imes packing sect ion. For packings
o f s o f t e r cons t ruc t i on , maximum leng th should be provided. With
modern packing m a t e r i a l s , beve l l ed glands a r e seldom an advantage
and can a c t u a l l y promote movement o f t he s e a l i n g r i n g on the sp igo t
s ide i n t o the l i v e c learance.
( i i )
( i i i )
( i v )
376
(v ) The need t o avo id excess ive s h a f t misal ignment o r whip.
( v i ) The p r o v i s i o n o f adequate s h a f t support . The pack ing must
no t be used as a bear ing .
16.4 PACKING CONSTRUCTION AND MATERIALS
16.4.1 F i b r e Ma te r ia l
M inera l asbestos
Vegetable c o t t o n
f l a x
j u t e
ramic
s i s a l
16.4.2 Lub r i can ts
Dry g r a p h i t e
mica
t a l c
molybdenum d i s u l p h i t e
Wet t a l l o w
c a s t o r o i l
s t r a i g h t minera l
pe t ro la tum
sol i d f r a c t i o n s
p a r a f f i n wax
soaps
s i l i c o n e grease
PTFE d i spe rs ions
l u b r i c a t i n g o i l
Syn the t i c aramid
g lass
g r a p h i t e f i l a m e n t
g r a p h i t e f o i l
ny lon
p o l y t e t r a f l u o r o e t h y l e n e
rayon
(PTFE)
Meta ls lead f o i l and w i r e
aluminium f o i l
copper f o i l and w i r e
b rass w i r e
monel w i r e
incone l w i r e
s t a i n l e s s s t e e l w i r e
Elastomers n a t u r a l and s y n t h e t i c
16.4.3 Cons t ruc t i on
The p r i n c i p a l forms o f c o n s t r u c t i o n s f o r f i b r o u s compression packings a re :
( i ) Braided I n d i v i d u a l yarns a r e b ra ided tube over tube and squared o f f .
The d e n s i t y o f t h i s type o f c o n s t r u c t i o n i s h i g h and idea l f o r many
va l ve a p p l i c a t i o n s .
( i i ) P l a i t e d M u l t i p l e yarns a r e interwoven i n p l a i t e d bundles i n such a
fash ion t h a t the d i r e c t i o n o f f i b r e f o l l o w s the pe r iphe ry o f t he
packing r i n g . The n a t u r a l c h a r a c t e r i s t i c o f t h i s c o n s t r u c t i o n i s more
s u i t e d t o c e n t r i f u g a l pump a p p l i c a t i o n s than va l ve s e r v i c e a l though the
inherent f l e x i b i l i t y o f t he fo rm i s popu lar w i t h some users.
Fig.3
( i i i )
( i v )
A l 1
Basic pack ing c o n s t r u c t i o n s
Cross P l a i t A l l t he yarns a r e i n t e r l o c k i n g and pass d i a g o n a l l y through
the pack ing t o p rov ide a f i r m c o n s t r u c t i o n o f c o n s i s t e n t d e n s i t y and
shape.
pumps.
Composite asbestos p l a s t i c Th is pack ing ca tegory i s o f f a i r l y recent
o r i g i n and inc ludes those types based upon a b ra ided and r e i n f o r c e d
asbestos j a c k e t enc los ing a ' p l a s t i c ' core . Al though inaccura te i n
t h e s c i e n t i f i c sense, t he te rm ' p l a s t i c ' conven ien t l y descr ibes those
many m ix tu res o f asbestos f i b r e and l u b r i c a n t , bo th minera l and s o l i d ,
f rom which r e a d i l y deformable pack ing m a t e r i a l may be made. Th is
pack ing i s w ide l y accepted f o r d i f f i c u l t va l ve sea l i ng d u t i e s .
t he f i b re -based cons t ruc t i ons descr ibed here a r e f r e q u e n t l y r e i n f o r c e d
Used e x t e n s i v e l y f o r s y n t h e t i c yarn packings f o r va lves and
w i t h metal .
o f metal w i r e i n the ya rn can ex tend t h e s e r v i c e c a p a b i l i t y o f t he packing t o
800°C and beyond.
asbestos yarn pack ing would be r e s t r i c t e d t o temperatures o f about 315°C
maximum.
T h i s a p p l i e s p a r t i c u l a r l y t o asbestos'based produc ts where the use
But f o r t h i s fea tu re , even t h e bes t q u a l i t y non -meta l l i c
A l l o f t he l u b r i c a n t s shown a r e used i n c o n j u n c t i o n w i t h f i b r e packings o f
d i f f e r e n t s o r t s and a r e a p p l i e d by d ipp ing , coa t ing , soaking, vacuum impreg-
na t i on , dus t i ng , e t c . The pr ime o b j e c t i s maximum l u b r i c a n t r e t e n t i o n . Fre-
quen t l y , severa l t rea tments and repeat processes a r e employed t o achieve t h i s
end.
318
Foi l wrapped deformable asbestos core. F o i l cr ink led, twisted and
folded upon i t s e l f .
Corrugated f o i l , concertina wound.
F ig .4 Typical metal f o i l based packing construct ion
F ig .5 Double-bevel led, e lastomer proofed f a b r i c packing f o r abrasive d u t i e s
379
I n the f i e l d o f compression packings, e las tomers a r e no t w i d e l y used a l though
some bra ided packings do employ yarns t h a t a r e t r e a t e d w i t h a rubber p r o o f i n g t o
render them more s u i t e d t o d i f f i c u l t wet a p p l i c a t i o n s such as condensate du ty .
Rings o f square o r rec tangu la r sec t i on compression packing, manufactured from
folded, r o l l e d o r laminated e las tomer p roo fed c l o t h , a r e s t i l l popu lar f o r
r e l a t i v e l y slow moving, lower p ressure r e c i p r o c a t i n g pumps hand l i ng water o f LP
steam.
One p a r t i c u l a r design, w i t h a moulded, doub le-beve l led sec t i on , made f rom
semi-metal l ic rubber ised yarn , i s p a r t i c u l a r l y e f f e c t i v e on r o t a r y a p p l i c a t i o n s
deal ing w i t h v iscous media which s o l i d i f y when the pump i s i d l e and cause damage
t o convent ional p l a i t e d packings on r e s t a r t i n ? f rom co ld .
i s a l so s u i t a b l e f o r d u t i e s i n v o l v i n g s o l i d s and abras ives (See F i0 .5 ) .
Th i s moulded pack ing
16.4.4 A d d i t i o n a l M a t e r i a l s
In h igh performance r o t a r y and r e c i p r o c a t i n g packings, t h r e e m a t e r i a l s which
may be regarded as impor tan t a d d i t i o n s t o the seal manufac turers ' armoury have
become a v a i l a b l e i n the l a s t two decades and p l a y a h i q h l y s i g n i f i c a n t p a r t i n
extending the f r o n t i e r s o f performance o f t he t r a d i t i o n a l s o f t pack ing :
( i ) PTFE P o l y t e t r a f l u o r o e t h y l e n e yarns p rov ide s o f t packings f o r se rv i ces
where c o r r o s i v e media a r e be ing handled o r freedom f rom contaminat ion
i s an e s s e n t i a l requirement.
A s e m i - r i g i d f l uo roca rbon p l a s t i c , PTFE i s unique i n possessing almost
complete chemical res i s tance w i t h i n i t s temperature range which, i n t h i s
f i e l d o f a p p l i c a t i o n , spans t h e c ryogen ic area t o 250°C.
advantage r e f e r s t o i t s very low c o e f f i c i e n t o f f r i c t i o n .
Lub r i ca ted p l a i t e d PTFE yarn packings a r e s u i t a b l e f o r r o t a r y sur face
speeds up t o 8 m/s and a r e a l s o f i n d i n g i nc reas ing acceptance on h i g h
speed, h i g h pressure, mu l t i - ram r e c i p r o c a t i n g pumps.
In s o l i d fo rm t h i s m a t e r i a l i s n o t acceptab le as a compression pack ing
due t o poor creep p r o p e r t i e s and lack o f r e s i l i e n c e . However, s o l i d
junk r i n g s o r spacers i n PTFE a r e o f t e n used t o enhance pack ing per -
formance on arduous pump d u t i e s .
Another major
( i i ) Aramid f i b r e C r o s s p l a i t yarn packings made f rom aramid f i b r e , u s u a l l y
o f a d i s t i n c t i v e y e l l o w c o l o u r , a r e becoming i n c r e a s i n g l y popu lar f o r
a v a r i e t y o f pump and va l ve se rv i ces h i t h e r t o s a t i s f i e d by PTFE
l u b r i c a t e d asbestos packings.
380
The yarn has h i g h t e n s i l e s t rength, e x c e l l e n t r e s i l i e n c e , thermal
s t a b i l i t y up t o 250°C and i s r e s l s t a n t t o a wide range o f chemicals.
Aramid f i b r e packings a re s u i t a b l e f o r speeds up t o 15 m/s and g i v e
impressive r e s u l t s w i t h abras ive media.
( i i i) Graphi te
(a) Yarn packings i n t h i s m a t e r i a l a r e a development f o r r o t a r y pump
a p p l i c a t i o n s and p rov ide p o s s i b i l i t i e s f o r extending the range o f
the packed g land beyond boundar ies h i t h e r t o es tab l i shed .
A h igh c o e f f i c i e n t o f thermal c o n d u c t i v i t y , low f r i c t i o n and res i s tance
t o chemical a t t a c k a re the use fu l c h a r a c t e r i s t i c s o f t h i s m a t e r i a l .
Temperatures up t o 400°C may be considered.
I f a good performance i s t o be obta ined, then c lose a t t e n t i o n must be
p a i d t o mechanical c o n d i t i o n s such as s h a f t run o u t and f i n i s h . Care
i n f i t t i n g and runn ing - in i s a l s o mandatory.
(b) Expanded g raph i te f o i l i s the recent and dramat ic a p p l i c a t i o n o f
g raph i te , p a r t i c u l a r l y i n the con tex t o f va l ve a p p l i c a t i o n s . Expanded
g raph i te m a t e r i a l s combine the we l l -es tab l i shed thermal and f r i c t i o n
c h a r a c t e r i s t i c s , long associated w i t h the c o r r e c t l y developed use o f
carbon based products , w i t h a unique f l e x i b i l i t y and r e s i l i e n c e . The
a t t r i b u t e s o f t h i s e x f o l i a t e d form o f g r a p h i t e bear record ing.
- e x c e l l e n t res i s tance t o compression se t r e s u l t i n g i n l i t t l e l o s s o f
r a d i a l g land f o r c e o r f l ange sea t ing s t r e s s over long pe r iods
(see Table 16.3)
- no loss o f v o l a t i l e s even a t h i g h temperature thus m in im is ing
frequency o f g land adjustment
- h i g h temperature c a p a b i l i t y p a r t i c u l a r l y i n non -ox id i s ing
environments
- h igh thermal c o n d u c t i v i t y
- low f r i c t i o n p r o p e r t i e s - s e l f l u b r i c a t i n g
- e x c e p t i o n a l l y low c h l o r i d e content
- no adhesion o r co r ros ion problems
- f i r e - s a f e
381
TABLE 16.3
Compress i b i 1 i ty / recove ry Expanded Expanded ASTM F36-66 Procedure H g r a p h i t e g raph i te
(Major load 7/mm2) (1000 l b f / i n Z ) l m m t h i c k 2mm t h i c k 1 .o g/cc 1.0 g/cc
compress ib i l i t y 39% 33%
recovery 20% 25%
Stress r e l a x a t i o n BS 1832:1972
Temperature 300°C f o r 16 h r s I n i t i a l s t ress o f 40 N/mm2
Residual s t r e s s 40 N/mm2 40 N/mm2
N.B. 10 N=l kg f 1 N/mmz=lO kgf/cm2 approx.
Fig.6 shows an i n t e r e s t i n g comparison o f performance on a t e s t g land between
h a l f r i n g s o f expanded g r a p h i t e and a l u b r i c a t e d asbestos yarn packing. Not
only d i d the former r e q u i r e fewer g land adjustments du r ing the p e r i o d o f t e s t i n g
but the average leakage r a t e was much l e s s - t o the p o i n t o f running v i r t u a l l y
dry f o r p r o t r a c t e d per iods.
Rather than use the tape form o f expanded g r a p h i t e which i s p r i m a r i l y a use-
f u l maintenance expedient , moulded r i n g s t o a se lec ted and c o n t r o l l e d dens i t y
should be the f i r s t choice f o r pump and va l ve glands.
Al though more c o s t l y than convent ional packing m a t e r i a l s , economies o f r a d i a l
w id th and number o f r i n g s used a r e f e a s i b l e q u i t e a p a r t f rom the performance
advantage l i k e l y t o be de r i ved from the use o f expanded g raph i te .
One cannot leave m a t e r i a l s w i thou t spec ia l re ference t o the v i t a l r o l e played
by asbestos - a much denigrated minera l f i b r e w i thou t which economic and prac-
t i c a l s o l u t i o n s t o many s e a l i n g problems would n o t be f e a s i b l e . Al though under-
standable, i n an age o f c o r r e c t awareness o f h e a l t h and s a f e t y mat ters , the
over- react ion aga ins t asbestos has revealed many incons is tenc ies . Motor manuf-
ac tu re rs may p r o h i b i t i t s use as a p l a n t maintenance m a t e r i a l b u t cont inue t o
use asbestos i n a b r a k e - l i n i n g and c l u t c h - f a c i n g r o l e where res idua l dust i s
ev ident . Some users may seek t o l i m i t i t s a p p l i c a t i o n i n a safe form as a va lve
packing o r gasket b u t perpetuate i t s s p e c i f i c a t i o n f o r f i r e - p r o o f p o s i t i o n s and
r o o f i n g where the minera l i s c u t i n a d r y form.
Hazards e x i s t b u t adherence t o bas ic adv ice on hand l i ng asbestos w i l l r e s u l t
i n a sensib le balance between p r o h i b i t i o n and p r a c t i c a l i t y . In t h i s con tex t ,
users o f asbestos based pump and va l ve packings, gaskets o r a l l i e d components
might heed, t o advantage, the statement issued by the Asbestos In format ion
Committee t o the B r i t i s h Valve Manufacturers ' Associat ion, a copy o f which i s
appended t o these notes.
I000
900
800
700
600
500
400
300
200
100
0
2.5
2.0
I . 5
1 .o
0 . 5
- 0
D I S T I L L E D WATER TEST CONDITIONS: TEMPERATURE OF MEDIUM - 9OoC
2.5 m/s
PRESSURE OF MEDIUM
U x p a n d e d graphite rings ( i n halves)
- 3.45 bar
‘9 5 m/s
- Lubricated asbestos yarn rings ( s p l i t )
L 7.5 m/s
SHAFT SURFACE SPEED
I - - - - -
& I - - - -
. J
7.5 m/s
10 m/s P
10 m/s
12.5 m/s
12.5 m/s
SHAFT SURFACE SPEED
I 1
5 10 15 2 0 25 30
TOTAL TEST TIME-HOURS
383
16.4.5 Select ion
The most vex ing quest ion as, f o r many du t i es , so many reasonable a l t e r n a t i v e s
e x i s t . Much w i l l depend on personal exper ience, frequency o f maintenance,
o r i g i n a l cos t l e v e l , contaminat ion cons ide ra t i ons , s i ze , e t c .
I n the case o f a manufacturer producing l a rge q u a n t i t i e s o f va lves o r pumps
t o standard dimensions, t he re i s much t o be sa id f o r purchasing sets o r r i n g s
rather than leng th form packing. With the techniques a v a i l a b l e , packing can be
suppl ied ready f o r immediate f i t t i n g w i t h s u b s t a n t i a l reduc t i on i n t h a t o v e r a l l
cost represented by r e c e i v i n g l eng th form t h a t must be c u t t o s i z e by s k i l l e d
personnel. Th is economy i s n o t con f ined t o the l a rge manufacturers bu t i t i s
they who w i l l en joy the g rea te r advantage.
On the o t h e r hand, i n many instances, the problem o f s tock ing r i n g s o r sets
ta i lor-made f o r an assortment o f va lves va ry ing i n o r i g i n , type and dimensions
can prove i n t o l e r a b l e . For these cases, t he re i s a c l e a r need f o r the ve rsa t -
i l i t y o f packing i n l eng th form. Comparable w i t h t h i s s o l u t i o n i s the expedient
provided by those packings o f p l a s t i c na tu re t h a t a r e a v a i l a b l e i n loose form
but t h i s advantage must be weighed aga ins t the labour cos t i n the c a r e f u l
f i t t i n g requi red.
The q u a n t i t y o f packing t o use.and i t s s i z e f o r a g i ven a p p l i c a t i o n r e l i e s
l a rge ly on the exper ience o f the user/manufacturer i n the type o f duty being
performed, o r on l i a i s o n w i t h a packing s u p p l i e r a t the design stage. The
l a t t e r course o f a c t i o n i s always favoured i f any doubt e x i s t s , s ince an exact
knowledge o f the c a p a b i l i t i e s and l i m i t a t i o n s o f the m a t e r i a l employed can be
found o n l y w i t h those s p e c i a l i s t s responsib le f o r compounding and product ion.
Five r i n g s o f square sec t i on packing a r e o f t e n accepted as a s u f f i c i e n t
number f o r the average uncompl icated duty bu t t he re a re many pump a p p l i c a t i o n s
where the presence o f a l a n t e r n r i n g o r s i m i l a r cons ide ra t i on may d i c t a t e a
greater q u a n t i t y .
The approp r ia te packing s e c t i o n t o use i n r e l a t i o n t o diameter i s open t o a
degree o f i n d i v i d u a l preference bu t broad recommendations a re shown i n
Table 16 .2 .
To g i v e an idea o f the c a p a b i l i t i e s o f the var ious m a t e r i a l s and const ruc-
t i ons o f s o f t packings which a r e r e a d i l y a v a i l a b l e , re ference may be made t o :
Table 16.4 - s u i t a b i l i t y i n d i f f e r e n t media/speed and temperature l i m i t s
Table 16.5 - comparat ive speed performance
Table 16.6 - comparat ive temperature performance
Table 16 .7 - comparat ive cos t i n d i c a t i o n .
N . B . The statement o f speed and temperature l i m i t s f o r a g iven m a t e r i a l should
no t be const rued as meaning t h a t a packing w i l l be s u i t a b l e f o r d u t i e s
where such maxima a re j o i n t l y encountered.
384
TABLE 16.4
SYMBOLS 0 Recommended
Consult -* Corrosion inhibitor included
RECIPROCATING, ROTARY PUMPS & VALVES
Lubricated aluminium foil. 1 540 I 7.5 I Lubricated braided asbestos I 350 I CI lo Lubricated plaited asbestos. I 315 1 20 10
290 8
290 ;“t Plaited, lubricated asbestos impregnated with PTFE dispersion.
Plaited. lubricated asbestos impregnated with PTFE disoersion but with no additional lubricant.
PTFE impregnated asbestos and glass fibre yarns with suitable lubricant. I 2 9 0 1 7 5 I
250 8 PTFE yarn impregnated with PTFE dispersion and inen lubricant.
Soft lead based foil wrapped round lubricated asbestos COW. 260 12
Hydrocarbon resistant lubricated plaited asbestos. 1 200 I 7 I Lubricated plaited cotton. ( 9 0 1 7 I Cross plan aramd hbre yams I 250 I 15 10
~
Puregraphite foil with no volatile additives n D O
RECIPROCATING PUMPS & VALVES
Monel wire reinforced asbestoscoverwith plastic core.* I 480 I 10
0 Synthetic rubber bonded braided asbestos with brass wire reinforcement. 310
Lubricated plaited flax. 9 0
VALVES ONLY I I T Constructed from a jacket of asbestos reinforced with inconel wire braided ouer a resilient asbestos core.
Lubricated braided asbestos with monel
Self-lubricating fibrous asbestos with flake Qraphite or mica. 540
Lubricated braided asbestosand brasswirereinforced. 1 510 I 10
2 50
250
PTFE yarn impregnated with PTFE dispersion and inen lubricant but with no additional lubricant.
Unsintered PTFE cord gland seal for rapid valve packing. 0
solvents
T A B L E 16.5
MAXIMUM ROTARY SPEEDS FOR PUMP PACKINGS
MATERIAL I ROTARY SPEED I d s ) I
0 5 10 15 20 25 30
m/s
T A B L E 16.6
MAXIYUM SERVICE TEMPERATURES OF PUMP PACKINGS
MATERIAL
TABLE 16.7
TYPICAL RELATIVE COSTS OF PUMP PACKINGS
MATERIAL I RELATIVE COST I
388
16.4.6 F i t t i n g
I t i s o f t e n assumed t h a t u n s k i l l e d labour can be used t o repack pump glands
bu t t h i s i s t r u e o n l y so l ong as u n s k i l l e d i s n o t equated w i t h unaware. Fam-
i l i a r i t y w i t h the f o l l o w i n g i dea l procedure w i l l be more than repa id i n terms
o f t r o u b l e f r e e pack ing performance.
Where l eng th fo rm i s used:
( i ) S p i r a l l y wrap the m a t e r i a l around a rod o f d iameter e q u i v a l e n t t o the
pump s h a f t .
( i i ) Cut t he requ i red number o f r i n g s c l e a n l y t o o b t a i n good b u t t - j o i n s .
See Fig.7.
( i i i ) Proceed as f o r pre-formed s p l i t pack ing r i n g s .
Fig.7
Where pre-formed s p l i t pack ing r i n g s a r e used:
( i ) C a r e f u l l y remove o l d pack ing ( i n c l u d i n g where a p p r o p r i a t e the pack ing
on the f a r s i d e o f a l a n t e r n r i n g ) .
( i i ) Thoroughly c l e a n a l l sur faces t h a t w i l l con tac t t h e pack ing and, where
pe rm i t ted , smear w i t h o i l . Gland and neck bushes, s h a f t su r face and
bear ings shou ld a l s o be checked f o r s igns o f wear and r e c t i f i e d as
necessary.
( i i i ) Place f i r s t r i n g over the s h a f t by opening t o an ' S ' c o n f i g u r a t i o n t o
ensure t h a t bending e f f e c t s a r e spread over t h e whole r i n g . See Fig.8.
Fig.8
389
( i v ) I nse r t f i r s t r i n g i n t o s t u f f i n g box and l i g h t l y bed i n w i t h a s p l i t
(wooden) d i s tance p iece and g land sp igo t .
format ion on the ou ts ide diameter o f t he r i n g should be p o i n t i n g i n
the d i r e c t i o n o f s h a f t r o t a t i o n . See Fig.9.
With p l a i t e d packing the ' V '
Fig .9 (a)
(v) Repeat ( i
i s f i r m l y
N.B. The
P l a i t e d packing Eig.g(b) F o i l wrapped packing
i ) and ( i v ) w i t h remainder o f r i n g s ensur ing t h a t each r i n g
seated and t h a t the b u t t j o i n s a r e staggered by a t l e a s t 90".
ings must be f i t t e d i n d i v i d u a l l y and under no circumstances
should complete se ts be f i t t e d as a u n i t .
( v i ) When the r e q u i s i t e number o f r i n g s have been f i t t e d , t i g h t e n g land nu ts
u n t i l the s h a f t o r sp ind le torque increases. Then s lack o f f g land and
p u l l up t o f i n g e r t i gh tness on ly . ( I f pump i s t o be s to red be fo re use
leave g land s lack so t h a t packing r e s i l i e n c e i s n o t impaired).
( v i i ) Running i n pumps Prime c a s t i n g and run pump up t o ope ra t i ng speed f o r
10 t o 15 minutes. I f pump i s n o t f i t t e d w i t h g land coo l i ng , a c o l d
water spray over the g land housing w i l l avo id excess ive heat bu i l d -up
dur ing t h i s stage. I f no leakage occurs, s top pump, vent cas ing
pressure and s lacken g land f u r t h e r . Repeat u n t i l leakage s t a r t s .
The c o n t r o l l e d leakage, e s s e n t i a l f o r l u b r i c a t i o n purposes, can then
be obta ined by running the pump and evenly t i g h t e n i n g the g land nu ts
i n increments o f 2 f l a t s u n t i l approximately one drop every few seconds
i s obtained. Approximately 15 minutes should be l e f t between successive
adjustments. DO NOT OVERTIGHTEN.
( v i i i ) Where loose form m a t e r i a l i s used f o r va lves proceed as ( i ) and ( i i ) and
tamp packing i n t o a dense homogeneous mass p rog ress i ve l y f i l l i n g the
housing t o the requ i red degree.
390
( i x ) Because o f t h e danger o f c o r r o s i o n th rough e l e c t r o l y t i c a c t i o n , packings
c o n t a i n i n g g r a p h i t e should be avoided on va l ves o r pumps w i t h s t a i n l e s s
o r chrome s t e e l stems. Th is r i s k i s most acu te when the pack ing remains
i n the g land du r ing s to rage and i s p a r t i c u l a r l y aggravated by the
presence o f mo is tu re .
16.4.7 F a u l t F ind ing
A ma jor advantage o f compression packings i s t h a t breakdown i s r a r e l y sudden
o r c a t a s t r o p h i c b u t r a t h e r a ma t te r o f a g radua l b u i l d up o f leakage u n t i l an
unacceptable l e v e l i s reached. Normal ly, cons ide rab le l i f e can be achieved by
c o n t r o l l i n g leakage w i t h f u r t h e r t i g h t e n i n g o f t h e g land n u t s (N.B. t he seepage
o f f l u i d which a c t s as a l u b r i c a n t f o r the pack ing on r o t a r y a p p l i c a t i o n s
should n o t be confused w i t h leakage and the r a t e o f one drop every few seconds
should be main ta ined) . However, i f o t h e r than r o u t i n e maintenance o r j u s t p l a i n
' f a i r wear and t e a r ' a r e suspected as the cause o f leakage and the need f o r
re-packing, then the f o l l o w i n g h i n t s cou ld w e l l prove u s e f u l .
( i ) Conf i rm t h a t the pack ing i s r a t e d as s u i t a b l e f o r t he a p p l i c a t i o n .
( i i ) I f one o r more r i n g s a re m iss ing f rom the se t , check f o r excess ive
neck bush c learance a l l o w i n g e x t r u s i o n o f r i n g s i n t o the system.
I f the top r i n g has ex t ruded between the g land f o l l o w e r , a n t i - e x t r u s i o n
r i n g s cou ld a v o i d replacement o f metal p a r t s .
( i i i ) I f the pack ing ' s r a d i a l t h i ckness appears d imin ished i n one o r more
p laces , check f o r an unders ize s h a f t o r bad ly worn bear ings which
cou ld cause s h a f t whip o r s p i n d l e wobble.
( i v ) I f r a d i a l sec t i on o f pack ing d i r e c t l y beneath the sha f t i s reduced or
premature leakage occurs a long the top o f t he s h a f t , check f o r mis -
a l ignment o f s h a f t c e n t r e t o s t u f f i n g box bore.
( v ) I f pack ing i s worn on the o u t e r diameter, check f o r loose r i n g s or
r i n g s r o t a t i n g w i t h the s h a f t due t o i n s u f f i c i e n t g land load.
( v i ) I f pack ing r i n g s have bu lges on t h e i r r a d i a l faces, the ad jacent r i n g
was probab ly c u t t o o sho r t , caus ing pack ing under p ressure t o be
fo rced i n t o the gap a t t he j o i n t .
( v i i ) I f pack ing neares t g land s p i g o t shows excess ive de format ion w h i l s t
o t h e r r i n g s a r e i n f a i r c o n d i t i o n , the se t was probab ly i n c o r r e c t l y
i n s t a l l e d and sub jec ted t o excess ive g land t ightening..
N.B. Over - t i gh ten ing i s u s u a l l y t he g r e a t e s t s i n g l e cause
o f premature pack ing f a i l u r e .
391
( v i i i ) I f the cause o f your p a r t i c u l a r problem i s s t i l l n o t apparent, g i v e
equal a t t e n t i o n t o I .D . and O.D. leakage and check f o r a rough s t u f f i n g
box bore be fo re seeking s p e c i a l i s t adv ice .
16.4.8 S tandard i za t i on
In the i n t e r e s t s o f s tock c o n t r o l i t i s c l e a r l y an advantage t o r a t i o n a l i s e
the v a r i e t y o f packings used i n any p l a n t to t h a t minimum number which w i l l
e f f e c t i v e l y c a t e r f o r a l l the c o n d i t i o n s l i k e l y t o be encountered. I f cos t i s
no ob jec t , then the re a re s i n g l e , s o p h i s t i c a t e d m a t e r i a l s and cons t ruc t i ons
tha t w i l l go some way towards s a t i s f y i n g most demands, bu t i t i s doub t fu l i f
cost e f fec t i veness cou ld be j u s t i f i e d . Far b e t t e r t o compromise on a small
number o f repu tab le p roduc ts developed f o r t he areas i n ques t i on , e.g. pumps,
valves, e t c .
There a r e few standards app ly ing t o compression packings on a n a t i o n a l o r
i n te rna t i ona l bas is , a l though many i n d i v i d u a l companies and o r g a n i z a t i o n s have
domestic standards which have, i n many cases, been t h e sub jec t o f c o l l a b o r a t i o n
between user and packing manufacturer.
BS 4371 : 1968 s p e c i f i e s minimum standards f o r l u b r i c a t e d p l a i t e d c o t t o n ,
l ub r i ca ted p l a i t e d f l a x , l u b r i c a t e d p l a i t e d o r b ra ided asbestos, d ry w h i t e non-
m e t a l l i c p l a i t e d o r b ra ided asbestos, p l a i t e d o r b ra ided asbestos, m e t a l l i c
wire re in fo rced , indura ted asbestos, and l u b r i c a t e d f i b r o u s asbestos and g i ves
guidance on l i m i t i n g o p e r a t i n g parameters f o r these cons t ruc t i ons .
Where packings a r e requ i red f o r s e r v i c e w i t h po tab le water i n the Water
Au tho r i t y d i s t r i b u t i o n system (which covers r e s e r v o i r t o t a p ) , o n l y those mat-
e r i a l s which have gained a Nat iona l Water Counci l Approval may be used. Such
products have been t e s t e d t o e s t a b l i s h t h a t they produce no c o l o u r , t a s t e o r
t u r b i d i t y , a re non - tox i c and w i l l n o t support m i c r o b i a l growth.
S ta tu to ry Instruments 1978 No. 1927 "The M a t e r i a l s and A r t i c l e s i n Contact
w i th Food Regu la t ions 1978" requ i red t h a t compression pack ing m a t e r i a l s , f o r
example, I ' ... do no t t r a n s f e r t h e i r c o n s t i t u e n t s t o foods w i t h which they are ,
o r l i k e l y t o be, i n con tac t , i n q u a n t i t i e s which cou ld -
(i) endanger human h e a l t h o r
( i i ) b r i n g about a d e t e r i o r a t i o n i n the o r g a n o l e p t i c (sensory q u a l i t y ) o f
such food o r an unacceptable change i n i t s na tu re , substance, o r
qua1 i t y . "
Such r e g u l a t i o n s i n e v i t a b l y r e s t r i c t t he range o f a v a i l a b l e m a t e r i a l s and
lubr ican ts . Consu l ta t i on w i t h t h e s u p p l i e r i s recommended t o e s t a b l i s h p re-
fe r red grades.
392
16.4.9 The Future
The research c u r r e n t l y be ing undertaken as a d i r e c t r e s u l t o f t he a n t i -
asbestos lobby may produce glass/PTFE/graphi te o r ceramic c o n s t r u c t i o n s which
match e x i s t i n g m a t e r i a l s . However, many problems remain t o be so lved i n p ro -
ducing a general purpose produc t t h a t can compete w i t h asbestos i n terms o f
l u b r i c a n t r e t e n t i o n , absorbency o r d u r a b i l i t y a t comparable cos t .
There can be no doubt t h a t g r a p h i t e f o i l p roduc ts w i l l , because o f t h i s
s i t u a t i o n , see g r e a t e r acceptance d e s p i t e h i g h i n i t i a l c o s t . Performance
r e p o r t s a re ex t remely encouraging and, i r r e s p e c t i v e o f c u r r e n t market con-
s i d e r a t i o n s , the m a t e r i a l deserves t o succeed on i t s own m e r i t s .
393
APPENDIX 1
COPY OF STATEMENT ISSUED BY THE ASBESTOS INFORMATION CENTRE,
40 PICCADILLY, LONDON W1V 9PA, TO THE BRITISH VALVE MANUFACTURERS
ASSOCIATION, 3 PANNEL COURT, CHERTSEY STREET, GUILDFORD, SURREY, GU1 4EU
ON 30 th APRIL 1980.
SAFETY OF ASBESTOS GLAND PACKINGS AND GASKETS
C r y s o t i l e (wh i te ) asbestos f i b r e i s a bas i c c o n s t i t u e n t o f va l ve packings and
gaskets because i t combines i n one m a t e r i a l so f tness , r e s i l i e n c e , abso rp t i on
p r o p e r t i e s , s t r e n g t h as a re in fo rcement and, where requ i red , heat res is tance.
Asbestos i s o n l y a r i s k t o h e a l t h i f i t s dus t i s inha led . Valve packings and
gaskets based on c h r y s o t i l e (wh i te ) asbestos a r e sa fe t o handle by va l ve users
and by maintenance eng ineers . A good s tandard o f i n d u s t r i a l hygiene should be
observed when hand l i ng o r us ing produc ts which c o n t a i n asbestos.
The Adv isory Committee on Asbestos i n t h e i r i n t e r i m statement and f i n a l r e p o r t
pub l i shed i n October 1979 recommend t h a t asbestos dus t should be kept t o the
lowest p r a c t i c a b l e l e v e l and c a l l f o r a 1 f i b r e / m l c o n t r o l l i m i t . The r e p o r t
f u r t h e r s t a t e s t h a t the presence o f c h r y s o t i l e i s u n l i k e l y t o have produced
any m a t e r i a l inc rease i n the r i s k o f l ung cancer i n the general popu la t i on o r
any apprec iab le number o f cases o f mesothelioma, and the same i s c e r t a i n l y t r u e
o f asbes tos is . Gland packings and gaskets w i l l n o t c r e a t e dus t l e v e l s i n
excess o f t h e c o n t r o l l i m i t , g i ven normal usage and maintenance.
' L u b r i c a t e d ' packings a r e vacuum impregnated w i t h minera l o i l s and greases,
g r a p h i t e and o t h e r l u b r i c a n t s , and do no t em i t dus t .
' D r y ' pack ings a r e no rma l l y t r e a t e d w i t h a dust-suppressant which s i g n i f i c a n t l y
reduces dus t emiss ion so t h a t i n normal use, i n c l u d i n g maintenance, they would
no t be expected t o p resent a hazard.
'Hard' packings and gaskets (compressed asbestos f i b r e ) a r e made f rom a com-
b i n a t i o n o f asbestos, rubber and o t h e r f i l l e r s . The asbestos f i b r e i s locked
i n t o the rubber m a t r i x and w i l l n o t no rma l l y c r e a t e dus t i n use o r maintenance.
I f however, these m a t e r i a l s a r e sub jec ted t o g r i n d i n g r o r o t h e r ab ras i ve pro-
cesses, p recau t ions should be taken t o a v o i d i n h a l i n g any dust which may be
emi t ted .
' S o f t ' o r moulded packings a r e no rma l l y made f rom rubber ised asbestos c l o t h ,
and the asbestos i s sealed w i t h i n the rubber coa t ing , so t h a t they a l s o a re
sa fe i n use and maintenance.
They a r e n o t commonly used i n va l ve glands.
394
C r o c i d o l i t e (b lue ) asbestos
No c r o c i d o l i t e (b lue ) asbestos has been used i n packings and gaskets manuf-
ac tu red i n the UK f o r severa l years. Some imported packings and gaskets may
however, i nco rpo ra te b l u e asbestos. When l u b r i c a t e d these packings a r e sa fe
t o handle bu t r e s p i r a t o r y p r o t e c t i o n w i l l be needed when they a r e i n a d ry
s t a t e .
Remova 1
Care should be taken when removing o l d packings which may have l o s t t h e i r
l u b r i c a n t s . They should be damped and then removed w i t h the c o r r e c t t o o l s ,
i .e . packing e x t r a c t o r s .
Label 1 i ng
Any packings o r gaskets which r e q u i r e spec ia l hand l i ng p recau t ions c a r r y
a warning l a b e l .
395
17 CENTRALISED LUBRICATION SYSTEMS DESIGN
J.G.MERRETT, Managing D i r e c t o r , Eng ineer ing and General Equipment L td .
17.1 INTRODUCTION
Whi ls t we a r e kept compara t ive ly w e l l informed o f the l a t e s t developments i n
o i l and grease technology and the v i t a l r o l e i t p lays i n i n d u s t r i a l and commer-
c i a l app l i ca t i ons , ve ry l i t t l e has been w r i t t e n about t h e e q u a l l y v i t a l "Centra-
I i sed L u b r i c a t i o n Systems" and some o f t he methods a v a i l a b l e t o Engineers by
which grease and o i l may be t r a n s m i t t e d t o the p o i n t o f l u b r i c a t i o n .
I n our s o c i e t y where energy, machinery and labour a r e now (1981) expensive
commodities, i t i s c l e a r t h a t i n the pas t i n s u f f i c i e n t a t t e n t i o n has been pa id
to the d i r e c t and i n d i r e c t losses o f energy, occasioned by wear and f r i c t i o n
and t o the savings o f m a t e r i a l s . However, i n 1977 a government f inanced Ameri-
can Report suggested t h a t $16.25 b i l l i o n p.a. ( a t 1976 va lues) cou ld be saved
by a "Strategy f o r Energy Conservat ion through T r ibo logy " [ l ] . Converted i n t o
U . K . (1980) va lues , t h i s i n d i c a t e s an equ iva len t sav ing o f energy through t r i -
bology i n excess o f €1; b i l l i o n p.a.
I n sho r t , c o r r e c t l y se lec ted l u b r i c a n t s and t h e i r methods o f a p p l i c a t i o n by
Cent ra l i sed Systems can e f f e c t s i g n i f i c a n t savings, e.g. machine t o o l s , convey-
ors, cranes, r o l l i n g m i l l s , b l a s t furnaces, b a l l m i l l s , sugar machinery, paper
m i l l s , heavy mob i l e p l a n t , e t c . t o v a r y i n g degrees, a l l r e q u i r e the a p p l i c a t i o n
o f l ub r i can ts . A c o r r e c t l y designed and i n s t a l l e d Cen t ra l i sed L u b r i c a t i o n
System i s t he eng ineer ' s insurance aga ins t some o f t he severe t r i b o l o g i c a l
problems, i . e . f r i c t i o n and wear, which would o the rw ise occur i f the p l a n t and
machinery were inadequate ly l u b r i c a t e d .
Un fo r tuna te l y , a l l t oo o f t e n i n t h e supply of p l a n t and machinery the se lec-
t i o n o f t he l u b r i c a n t and Cen t ra l i sed L u b r i c a t i o n equipment a r e considered a t
a l a t e stage i n the manufacture o f t he p l a n t which, coupled w i t h the c o n f l i c t i n g
i n te res ts o f machinery b u i l d e r s , can r e s u l t i n u n s u i t a b l e l u b r i c a t i o n equipment
being se lec ted . L ikewise , t h e l u b r i c a t i o n equipment supply companies have a
r e s p o n s i b i l i t y , n o t o n l y t o know t h e i r own produc t , bu t a l s o t o app rec ia te the
t r i b o l o g i c a l requirements o f t h e p l a n t and machinery which requ i res t o be
396
l ub r i ca ted . F a i l u r e i n t h i s a rea i n v a r i a b l y r e s u l t s i n t h e p l a n t user be ing
p laced i n the unenv iab le p o s i t i o n o f hav ing t o app ly , a t an e a r l y stage, f o r
a d d i t i o n a l c a p i t a l t o r e c t i f y new p l a n t o r , more o f t e n than n o t , t o l i v e w i t h
the problems and r e c t i f y as and when th rough a maintenance budget - bo th o f
which the p l a n t user would be the f i r s t t o agree i s e n t i r e l y u n s a t i s f a c t o r y .
Th i s paper endeavours t o deal w i t h some o f these f a c t o r s a f f e c t i n g the cho ice
o f l u b r i c a n t and l u b r i c a t i n g equipment, t he b a s i c elements o f the machinery t o
be l u b r i c a t e d , and t h e c o n d i t i o n s under which i t opera tes .
17.2 POINTS OF LUBRICATION
Bearings, t h e e s s e n t i a l components o f p l a n t and machinery, may be g e n e r a l l y
grouped i n t o j o u r n a l , t h r u s t , conveyor cha in p i n and l i n k , a n t i - f r i c t i o n , s l i d e -
ways and crane r a i l s .
t o l u b r i c a n t and l u b r i c a t i o n techn ique.
Each wear su r face must be t r e a t e d separa te l y w i t h regard
The l u b r i c a t i o n requirement o f a p l a i n j o u r n a l bear ing i s t h e p r o v i s i o n o f
an adequate and cons tan t f l o w o f l u b r i c a n t o f s p e c i f i e d v i s c o s i t y t o g i v e a
f l u i d f i l m o f h igh - load bear ing capac i t y . The j o u r n a l bea r ing has i n h e r e n t l y a
convergence between t h e s h a f t and t h e bear ing . When r e l a t i v e mot ion takes p lace
a f i l m o f l u b r i c a n t i s induced between the sur faces , e f f e c t i v e l y separa t i ng them.
Bear ings employed t o absorb t h r u s t and p reven t misal ignment of s h a f t s vary
g r e a t l y i n type and l u b r i c a t i o n requirement, w h i l s t a n t i - f r i c t i o n bear ings
r e q u i r e l ess l u b r i c a n t than p l a i n bear ings . Most a v a i l a b l e formulae d e a l i n g
w i t h the a p p l i c a t i o n o f grease t o these bear ings t r e a t speed as an impor tan t
f a c t o r . For smal l a n t i - f r i c t i o n bear ings such as those employed i n l i g h t l y
loaded f r a c t i o n a l horse-power motors, t o o much grease can be damaging. I n such
cases, recommended l u b r i c a t i o n i n t e r v a l s o f up t o severa l years have been es ta-
b l i s h e d . Chain p i n s and l i n k s p resent ma jor c r i t i c a l wear p o i n s on f l o o r and
overhead ( i n c l u d i n g Power and Free) conveyors. I n the automobi e i n d u s t r y ,
cha in lengths o f severa l hundred metres, hav ing thousands o f PO n t s r e q u i r i n g
l u b r i c a t i o n , a r e commonplace ( F i g . 1 ) .
Slideway and crane r a i l l u b r i c a t i o n requ i res the r i g h t l u b r i c a n t and the
r i g h t a p p l i c a t o r (see Fig.10, Sec t i on 17.6.1). Too l i t t l e l u b r i c a n t r e s u l t s i n
r a p i d wear; excess ive l u b r i c a n t can be a hazard t o l i f e o r l imb. Every case i s
d i f f e r e n t , y e t i n every case i t i s c r i t i c a l t h a t t he l u b r i c a n t i s a p p l i e d i n
l i n e w i t h the requirements o f t h a t component, bo th w i t h regard t o mechanical
wear and t o energy conservancy.
1 7 . 3 SELECTING THE LUBRICANT - O I L OR GREASE?
I n modern machinery l u b r i c a t i o n , l u b r i c a n t s and t h e means o f t he app
must be considered toge the r .
The bes t l u b r i c a n t w i l l se rve no u s e f u l purpose i f i t i s n o t a p p l i e d
i c a t i on
a t the
397
F ig .1 Excessive p i n and l i n k wear i n a conveyor cha in .
A 3mm p i n wear per p i n on a 732111 conveyor increases t h e cha in l e n g t h by 23 metres.
r i g h t t ime, a t the r i g h t p lace , and i n the c o r r e c t q u a n t i t y . Conversely, t he
best l u b r i c a t i o n equipment, app ly ing l u b r i c a n t q u i t e c o r r e c t l y , w i l l be o f l i t t l e
use i f the l u b r i c a n t i t feeds i s u n s u i t a b l e f o r t he du ty i t has t o perform.
Lub r i can ts g e n e r a l l y , e i t h e r o i l s o r greases, a r e supp l i ed by o i l companies
supported by s p e c i a l i s t o i l and grease manufac tur ing companies, who w i l l p rov ide
advice on any p a r t i c u l a r l u b r i c a n t and a p p l i c a t i o n as w e l l as on the cho ice o f
l u b r i c a n t s i n genera l . The s u p p l i e r o f t he l u b r i c a t i o n equipment w i l l a l s o
o f t e n be ab le t o adv ise , e s p e c i a l l y on l u b r i c a t i o n des ign problems, o r a c t as an
in te rmed iary . The s u b j e c t o f l u b r i c a n t s i s t r e a t e d here o n l y i n i t s very broadest terms
under th ree headings:-
"OIL" o r GREASE"; when t o use one and when the o the r .
LUBRICATING GREASE; t he types and how t o s e l e c t them.
LUBRICATING OILS; types and how t o s e l e c t them.
17.3.1 O i l o r Grease? When t o Use One and When the Other
Movement between two d ry sur faces causes heat and wear. The purpose o f
i n t roduc ing a l u b r i c a n t between t h e two sur faces i s t o reduce f r i c t i o n , heat,
and wear.
O i l has t h e f o l l o w i n g advantages: i t f lows, i t penet ra tes , i t removes heat,
and i t scavenges. A t t h e same t ime , i t has l i m i t e d s e a l i n g q u a l i t i e s and poor
' s t a y i n g ' power, t h e r e f o r e requ i res more f requent r e p l e n i s h i n g than grease.
398
L u b r i c a t i n g grease, a semi -so l i d p l a s t i c - l i k e m a t e r i a l , has e x c e l l e n t sea l -
ing c h a r a c t e r i s t i c s , possesses good ' s t a y i n g ' power, i . e . i t adheres t o surfaces
more r e a d i l y and longer than o i l and i t can be an e x c e l l e n t c o r r o s i o n preventa-
t i v e . U n l i k e o i l , i t i s a poor conductor o f heat and a poor scavenger; i t does
no t f l o w o r spread eas i
where contaminat ion by
and the t e x t i l e i n d u s t r
Where an a p p l i c a t i o n
choice. I t can be appl
y - t h e l a t t e r p r o p e r t y can, however, be an advantage
u b r i c a n t s must be avoided such as i n food and b o t t l i n g
es and where t h e l u b r i c a n t i s used as a s e a l i n g medium.
e n t a i l s HEAT REMOVAL, o i l i s t h e r e f o r e g e n e r a l l y t he
ed i n t h e fo rm o f a l i q u i d o r as an o i l m i s t , i . e .
m ic ro- fog compris ing an o i l - a i r m ix tu re . I n severe cases o f heat removal, o i l
can be r e c i r c u l a t e d and, du r ing t h e c i r c u l a t i o n , i t can be coo led and cleaned.
Large t u r b i n e bear ings and f a s t gear t r a i n s a r e t y p i c a l examples where o i l c i r -
c u l a t i n g systems a r e used and l a r g e amounts o f o i l a r e c i r c u l a t e d .
For a p p l i c a t i o n s where heat removal i s no t a p rob lem,but LUBRICATION o r the
PREVENTION OF INGRESS OF DIRT, WATER, DUST and OTHER EXTRANEOUS MATTER i s - l u b r i c a t i n g grease can be used. I t s a p p l i c a t i o n ranges from heavy m i l l bear ings
and s l i d e s t o t e x t i l e machinery, e t c . I t i s a l s o t h e most s u i t a b l e where a p p l i -
c a t i o n o f t h e l u b r i c a n t i s requ i red a t LONG INTERVALS.
Near ly a l l B a l l and R o l l e r Bear ings, except those i n respec t o f which heat
removal i s e s s e n t i a l , a r e g e n e r a l l y grease l u b r i c a t e d . Where DIRTY/DUSTY and
WET CONDITIONS e x i s t , grease
l u b r i c a t i o n makes grease the
r o l l i n g bear ings .
Genera l l y , o i l l u b r i c a t i o n
i s no t p o s s i b l e t o take advan
1 u b r i c a n t .
s g e n e r a l l y p re fe rab le .
p r e f e r r e d ' l u b r i c a n t f o r t h e vas t m a j o r i t y o f
The convenience o f grease
i s employed i n t h e r e l a t i v e l y few cases where i t
age o f t h e m e r i t s o f grease as a r o l l i n g bear ing
Where mot ion i s INTERMITTENT o r OSCILLATING, grease i s o f t e n the more s u i t -
ab le l u b r i c a n t .
Summarising, where heat conveyance away f rom rubb ing sur faces o r p e n e t r a t i o n
o f the l u b r i c a n t i s o f importance, o r t h e scavenging f u n c t i o n i s necessary, o i l
i s p re fe rab le , w h i l s t l u b r i c a t i n g grease can and, i n most cases, should be
considered as the p r e f e r r e d l u b r i c a n t f o r s low moving machinery, l ong l i f e
l u b r i c a t i o n , and where d i r t and dus t has t o he kept o u t o f t he bear ings , s l i d i n g
sur faces , e t c .
17 .3 .2 L u b r i c a t i n g Grease - The Types and How t o Se lec t Them
Greases designed f o r l u b r i c a t i o n a r e e s s e n t i a l l y a m i x t u r e o f minera l o i l
and th i ckener , accord ing to t he a p p l i c a t i o n requirement. I n t h e most w i d e l y
used modern grease the th i ckener i s a m e t a l l i c soap, u s u a l l y o f l i t h i u m o r
calc ium, w i t h the q u a n t i t y o f o i l ad jus ted t o g i v e a s o l i d , semi -so l i d o r semi
f 1 u i d cons is tency .
399
The soap f i b r e s form a s t r u c t u r e t h a t r e t a i n s the o i l , t he dimensions and
arrangement o f t h e f i b r e s v a r y i n g accord ing t o the metal and the f a t t y a c i d
from which the soap was made. The q u a n t i t y , dimensions and d i s t r i b u t i o n o f t h e
f i b r e s a r e t h e main parameters c o n t r o l l i n g t h e s t a b i l i t y and f l o w p r o p e r t i e s o f
t h i s l u b r i c a t i n g m a t e r i a l .
One o f t h e most impor tan t phys i ca l f ea tu res o f grease i s s t i f f n e s s ( f o r
s i s tency ) which i s i n d i c a t e d by a t e s t t h a t measures t h e depth t h a t t h e cone
s inks i n t o a sample o f grease. The depth measured i s a t e n t h o f a m i l l i m e t r e ,
and r e f e r r e d t o as t h e degree o f p e n e t r a t i o n . F igu re 2 shows one c l a s s i f i c a -
t i o n system f o r greases.
Grade Number. Worked p e n e t r a t i o n a t 25'C D e s c r i p t i o n
000 445 - 475 Very f l u i d
0 355 - 385 Semi - f 1 u i d 1 310 - 340 Very s o f t 2 265 - 295 S o f t 3 220 - 250 Semi - f i rm 4 175 - 205 F i r m 5 130 - 160 Very f i r m 6 85 - 115 Hard
OD 400 - 430 F l u i d
Fig.2 NLGl c l a s s i f i c a t i o n o f t h e cons is tency o f greases.
Al though l i th ium-based greases today s a t i s f y a vas t number o f l u b r i c a t i o n
requirements f o r cranes, conveyors, f o r g i n g presses, cont inuous c a s t i n g p l a n t s ,
r o l l i n g m i l l s , e t c . , t he re a r e a number o f s i t u a t i o n s r e q u i r i n g l u b r i c a n t s w i t h
spec ia l p r o p e r t i e s , f o r example, t he a b i l i t y t o w i ths tand h i g h temperatures such
as those encountered i n power s t a t i o n tu rb ines , bakery oven conveyors, e t c .
These l a t t e r greases i nc lude the c lay - th i ckened and o t h e r so l i d - th i ckened com-
pounds which w i ths tand cons iderab ly h ighe r temperatures, e s p e c i a l l y when employed
w i t h h igh temperature s y n t h e t i c l u b r i c a n t s , e.g. p o l y g l y c o l , s y n t h e t i c es te rs ,
and s i l i c o n e s .
The upper temperature l i m i t a t which any grease may be used i s dependent
p a r t l y on the type o f t h i ckener , p a r t l y on the f l u i d and i t s requ i red s e r v i c e
l i f e . Higher ope ra t i ng temperatures have t h e e f f e c t o f sho r ten ing the l u b r i -
c a n t ' s s e r v i c e l i f e and reduc ing pe rm iss ib le o p e r a t i n g speeds. I t i s t h e r e f o r e
convenient t o express the working l i m i t s o f a grease i n terms o f bear ing speeds
and temperatures, as shown i n Fig.3. Th is diagram shows t h a t l i t h i u m , sodium
and calcium-based greases have upper temperature l i m i t s o f 130, 110 and 7OoC
r e s p e c t i v e l y , and t h a t c lay-based and calcium-complex greases can be used a t
up t o 150°C.
400
F ig .3 Working l i m i t s o f m inera l o i l greases
1 7 . 3 . 3 L u b r i c a t i n g 0 1 1 s - Types and How t o Se lec t Them
Vegetable and animal o i l s a r e e x c e l l e n t l u b r i c a n t s , bu t have s h o r t l i f e as
they o x i d i s e and tend t o go ranc id ; as a r e s u l t , t h e i r main use i s f o r 'one sho t '
a p p l i c a t i o n s such as f o r f o r g i n g o r as a d d i t i v e s f o r minera l o i l s .
Genera l l y , t he o r i g i n o f t he o i l determines i t s use (F ig .4 )
O i l O r i g i n ADD1 i c a t i o n
Minera l O i l e.g. pe t ro leum base f o r general l u b r i c a t i o n o f mechanical
Vegetable O i l e.g. cas to r , palm, and rape seed o i l s f o r spec ia l a p p l i c a -
p a r t s such as engines, gears and genera l eng ineer ing p l a n t .
t i o n s where h i g h l u b r i c i t y i s d e s i r a b l e such as k i l n s , bakery ovens.
t i o n s such as k i l n s , bakery ovens. Animal O i l e.g. sperm o r o t h e r f i s h o i l s f rom sheep wool f o r a p p l i c a -
Syn the t i c O i l e.g. glycol d e r i v a t i v e s and d i e s t e r f o r extreme h i g h temp- e r a t u r e .
F ig .4 O r i g i n o f O i l s
401
F igu re 5 i l l u s t r a t e s how v i s c o s i t y o f o i l s change w i t h temperature, becoming
t h i n n e r when they a r e heated, bu t they do no t change v i s c o s i t y a t the same r a t e .
The r a t e o f v i s c o s i t y change w i t h temperature i s r e f e r r e d t o as the ' v i s c o s i t y
i ndex ' .
-40-30-20 -10 0 10 20 30 $0 50 60 70 80 90XX)11O1201X)14015MM)710
degrees fahrenheit
F ig .5 V i s c o s i t y vs Temperature f o r two o i l s hav ing v i s c o s i t y indexes o f 0 and 100 r e s p e c t i v e l y .
17.3.3.1
a l u b r i c a t i o n system, e.g. conveyor chains passing through a pre- t rea tment oven
may reach a temperature o f 180/200°C.
g r a p h i t e o r molybdenum d i s u l p h i d e i n g l y c o l as used i n bakery ovens must have
p r o v i s i o n f o r a g i t a t i o n o r r e c i r c u l a t i o n w i t h i n the l u b r i c a t o r s to rage tank t o
prevent s e t t l i n g o u t o f t he g r a p h i t e o r molybdenum.
The p r o p e r t i e s o f t he o i l must be c a r e f u l l y considered when des ign ing
O i l s w i t h spec ia l i n c l u s i o n s such as
Bakery ovens g e n e r a l l y use molybdenum d i s u l p h i d e i n g l y c o l w h i l s t f o r extreme
pressure l oad ing c o n d i t i o n s on Power and Free t r o l l e y s , as i n the ca r i ndus t r y ,
chains and t r o l l e y wheels r e q u i r e spec ia l l u b r i c a n t s , hav ing a h i g h adhesion
c h a r a c t e r i s t i c - a s w e l l as impar t i ng r u s t - p r o o f i n g and wa te r -p roo f i ng .
402
17.3 .3 .2 A f u r t h e r s p e c i a l i s t l u b r i c a n t i s rape seed o i l ; t h i s i s a low a c i d
f a t t y vegetab le o i l . Because o f i t s r e l a t i v e l y h i g h f l a s h - p o i n t i t i s s u i t a b l e
f o r h igh temperature work when r e f i n e d and g i ven a g r a p h i t e i n c l u s i o n . Typ ica l
uses a re f o r cont inuous c a s t i n g moulds, f o rg ing , and as a c u t t i n g o i l f o r harder
metals. Here aga in , i t s spec ia l p r o p e r t i e s must be considered when p r o v i d i n g
the l u b r i c a t i n g means.
17.3 .4 Summarising Lubr i can t Se lec t i on
For the f i n a l s a t i s f a c t o r y s e l e c t i o n o f l u b r i c a n t t he f o l l o w i n g f a c t o r s must
t h e r e f o r e be determined. The c o n s t r u c t i o n and m a t e r i a l s employed i n the compon-
en ts t o be l u b r i c a t e d , ope ra t i ng speeds and load ing , l i f e requirement o f l u b r i -
can t and machine, ope ra t i ng environment, e.g. t h e presence o f water , steam,
chemicals o r ab ras i ve m a t e r i a l s i n t h e atmosphere; and l a s t , bu t n o t l e a s t , t he
method o f a p p l i c a t i o n , t h e type o f system t o be employed, the diameter of p i p e
runs, e t c .
17.4 PIPE DIAMETER vs FLOW CHARACTERISTICS
To determine the l u b r i c a n t f l o w r a t e and volume, the l eng th and diameter o f
t he p i p e l i n e s should be c a r e f u l l y c a l c u l a t e d t o ensure t h a t t he l u b r i c a n t can
s a t i s f a c t o r i l y reach t h e wear p o i n t s a t t h e extreme ends o f t h e p i p e l i n e s . The
f o l l o w i n g t a b l e summarises some p r a c t i c a l r e s u l t s f rom t e s t s t o prove optimum
measurements and e n d - o f - l i n e pressure , us ing a l i th ium-based grease o f No.2
cons is tency a t 15OoC.
Nominal bore App l ied pressure Pressure drop (mm) (kPa) (kPa/m)
50 1100 36.1 38 1 a75 6 1 . 5 25 4410 144.7 19 6410 210.3
17 .5 LUBRICATION REQUIREMENTS FOR PLAIN BEARINGS
For normal work ing c o n d i t i o n s i t has been found t h a t t he amount o f grease
requ i red f o r p l a i n bear ings i s equ iva len t t o a l a y e r o f O . l m m on the developed
bear ing area (0.1 x d x L) per hour o f bear ing opera t i on . F igu re 6 i l l u s t r a t e s
a c h a r t f o r c a l c u l a t i n g the grease requirements f o r p l a i n bear ings . As an exam-
p l e t o c a l c u l a t e t h e grease requ i red f o r a bear ing o f 75mm diameter by 250mm long, i n t e r s e c t diameter and leng th as a t * f o l l o w l i n e o f a r row and where i t
i n t e r s e c t s the top sca le , t h i s i n d i c a t e s the amount o f grease requ i red , be ing
i n t h i s example 6 g m o r 0.21 oz per hour.
403
D
LENGTH OF BEARING L
F ig .6 Grease requirements f o r a p l a i n bea r ing .
17.6 SELECTING THE RIGHT TYPE OF LUBRICATION SYSTEM
L u b r i c a t i o n systems f o r p l a n t and equipment, whatever the a p p l i c a t i o n , requ i re
i n d i v i d u a l design. They must be f u n c t i o n a l and c o r r e c t l y engineered t o s a t i s f y
the bear ing requirements and t h e des igne rs ' a p p l i c a t i o n s p e c i f i c a t i o n . Wi th
regard t o t h e human element, they must be f o o l - p r o o f . Systems inc luded i n t h i s
chapter deal w i t h : Grease, O i l and Micro-Fog.
17.6.1 Grease L u b r i c a t i o n Systems
A l l Cen t ra l i sed Grease L u b r i c a t i o n Systems a r e o f t he n o n - r e c i r c u l a t i n g type
and opera te on the t o t a l loss p r i n c i p l e . They a r e b a s i c a l l y d i v i d e d i n t o D i r e c t
Feeding systems and I n d i r e c t Feeding systems, o the rw ise r e f e r r e d t o as L ine sys-
tems, as i l l u s t r a t e d i n F ig .7 .
17.6.1.1
ec t Feeding l u b r i c a t i n g pump i s p o s i t i v e ; t he pumping p lungers and means o f
meter ing t h e ou tpu t t o i n d i v i d u a l wear p o i n t s be ing i nco rpo ra ted i n the l u b r i c a -
t i n g pump.
c i p l e , i .e . they i n t roduce a metered volume o f l u b r i c a n t i n t o t h e pipes, and
s ince t h i s volume i s no t a f f e c t e d by pressure i n t h e p ipes , t h e pumps can work con t inuous ly aga ins t h i g h back pressures.
D i r e c t Feeding Systems a r e those where t h e volume ou tpu t o f t he D i r -
The re fo re D i r e c t Feeding systems opera te on a P o s i t i v e Volume P r i n -
I GREASE SYSTEMS I TOTAL LOSS SYSTEMS
DIRECT FEEDSYSTEMS - I PR0GRESSI:E MSTEMS I
INDIRECT FEED M T E M S - I
DUAL L I N E Y
PARALLEL SVSTEMS - FIG.7 FUNDAMENTAL DIVISION OF CENTRALISED GREASE
LUBRICATION SYSTEMS.
405
D i r e c t Feeding systems can be d i v i d e d i n t o those where each o u t l e t has i t s
own meter ing p lunger (Fig.8) and those where a s i n g l e moving plunger d i s t r i b u t e s
p rog ress i ve l y the metered amounts o f grease i n t o the va r ious o u t l e t s (F ig .9 ) .
Normal ly, d i r e c t feeding l u b r i c a t o r s o b t a i n t h e i r pumping a c t i o n e i t h e r by
means o f moving p lungers, combined w i t h a system o f p o r t i n g , o r by the use o f
p lungers i n connect ion w i t h spr ing- loaded b a l l va lves. The l a t t e r type should
be avoided i n the case o f d i r t y o r dusty cond i t i ons as any ingress o f ext ran-
eous ma t te r may lodge i n the sea t ing o f t he b a l l v a l v e and render the l u b r i c a t o r
i nopera t i ve .
I t f o l l o w t h a t where a l u b r i c a t o r i s d r i v e n by a moving p a r t o f a machine, an
amount o f l u b r i c a n t requ i red r e l a t i v e t o t h e movement o f t he s h a f t i n the bearing
being l u b r i c a t e d , a d i r e c t feeding l u b r i c a t o r should be used (Fig.10) w i t h the
l u b r i c a t o r d r i v e n v i a the d i r e c t d r i v i n g elements, e.g. Eccen t r i c Dr ives, Throw
Plates, o r O f f s e t P i n Dr ives.
17.6.1.2
g r a p h i c a l l y separated;
The a c t i o n i s hyd rau l i c , t he pump se rv ing as a pressure c r e a t i n g u n i t f o r pumping
grease i n t o a p i p e l i n e which i s thereby s e t under pressure. As the pressure
increases, so t h e va r ious meter ing elements e j e c t t h e i r charges e i t h e r progress-
i v e l y , o r accord ing t o the back pressures against which they operate.
I n d i r e c t o r L i n e Systems have t h e pumping and meter ing elements geo-
they a r e connected by means o f one o r severa l p ipe l i nes .
Dur ing normal ope ra t i ons each element, having g iven up i t s metered amount,
blocks i t s e l f and w i l l n o t pass any f u r t h e r l u b r i c a n t t o the p o i n t s o f app l i ca -
t i o n . When a l l elements have g iven up t h e i r metered charge, a r a p i d r i s e i n
pressure occurs i n t h e main l i n e . U t i l i z i n g e i t h e r t h i s increase i n pressure a t
the pump o r a t t he end o f the l i n e , a s igna l i s g i ven i n d i c a t i n g t h a t t he l u b r i -
c a t i n g phase i s completed.
Thereupon t h e main l i n e has t o be depressur ized, which i s u s u a l l y e f f e c t e d
by opening i t t o the grease supply u n i t , e.g. t h e r e s e r v o i r . Depending on the
type o r make o f system, the p lungers o f t he meter ing va lves a r e then rese t t o
pe rm i t t h e i r f u r t h e r ope ra t i on , o r they a r e a l ready s e t f o r another a p p l i c a t i o n
phase which moves them back i n t o t h e i r o r i g i n a l p o s i t i o n , thereby complet ing a
system cyc le . The method and mechanics o f r e s e t t i n g depend on the p a r t i c u l a r
type o f system; a l s o whether one complete c y c l e o f t h e system involves one o r
two a p p l i c a t i o n ( l u b r i c a t i o n ) phases.
The fundamental d i v i s i o n o f ' l i n e systems' i s t h a t between progress ive and
p a r a l l e l types o f systems. I n the p rog ress i ve system the l u b r i c a n t must pass
through t h e meter ing elements o r va lves p rog ress i ve l y , i . e . o n l y a f t e r having
actuated t h e f i r s t element t o feed l u b r i c a n t t o the p o i n t o f a p p l i c a t i o n w i l l
the l u b r i c a n t be passed t o the second element, and so f o r t h . Th is i s i n con-
t r a s t t o the opera t i on o f t he p a r a l l e l system, on which the meter ing u n i t s are
YOl>INNO> OlNW YOU3NN03 m a3a~3 311~1s
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NOllV3l11!fll 01Slll11lN33 311VWOlflV 11Oj 1102V3111!111 0334 1331110
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407
408
Introducing central lubrication in stages:
Application: Lubiicat/onoluplo IWp~) in l~ .~ l lhg~ea leoros .
machinery pafl(IcUlar1y on presses. machine lools, packagmg
Design: A m a n pipe lmm a simple fillinp 01 lubricalion pump loads lo the dislrtbulor; the quantities of lubricant delivered are diitiibuled by the. dwibu lo r 10 the outlels in a parlicular pie. arranged sequence If required. the Iubricanl lrom the distributor can be led lo other distrtbulors lor lurlhet redislribulion
1.1 .t.g.: Lubrmicon of not more lhan 20 points by means 01 a grease nipple screwed into the dislributor andsgreasegun whichisoperaled untiltheflow indjcatoc shows tihat the lubr#calion operation has been compleletl.
2nd .lago:
in a specified manner.
A hand pump and dlslrlbUIorS in Series. WhlCh distribute the lubr~csnl lo the lubrication point6
Fig. 11. Examples o f Progressive Systems.
Fig. 12. Progressive Lubrication on a plate shear.
409
a c t u a t e d as p r e s s u r e inc reases i n t h e main l i n e .
Each group i s f u r t h e r d i v i d e d i n t o systems u t i l i z i n g one l i n e o n l y and systems
u t i l i z i n g two l i n e s , t h e former be ing c a l l e d s i n g l e - l i n e systems, t h e l a t t e r
d u a l - l i n e systems. The most p o p u l a r types o f systems used a r e :
P r o g r e s s i v e Systems, o p e r a t e on pressure/volume c y c l e s ; i n t h e i r case t h e
i n c r e a s i n g p r e s s u r e c r e a t e d by t h e pump a c t u a t e s a m e t e r i n g v a l v e which, hav ing
g iven up i t s s e t amount o f l u b r i c a n t . a l l o w s t h e grease t o pass i n t o t h e main
l i n e l e a d i n g t o t h e n e x t m e t e r i n g v a l v e .
b u i l t up, t h e v a l v e i s a c t u a t e d and l u b r i c a n t a l l o w e d t o f l o w t o t h e n e x t meter-
i n g v a l v e , and so on p r o g r e s s i v e l y , u n t i l i t r e t u r n s t o t h e l u b r i c a t o r o r where,
when s u f f i c i e n t p r e s s u r e has been b u i l t up, a r e v e r s i n g v a l v e i s a c t u a t e d which
reverses t h e f l o w o f t h e grease.
dent upon t h e number o f p o i n t s t o be l u b r i c a t e d .
o p t i o n s a v a i l a b l e , and F i g . 1 2 shows a t y p i c a l i n s t a l l a t i o n o f a p r o g r e s s i v e
l u b r i c a t i o n system on a P l a t e Shear.
When s u f f i c i e n t p ressure has been
The s e l e c t i o n o f p r o g r e s s i v e systems i s depen-
F i g u r e 1 1 i l l u s t r a t e s some
Dual L i n e Systems ( P a r a l l e l ) , o p e r a t e on t h e same b a s i c p r i n c i p l e , v i z : t h e
motor ized l u b r i c a t i n g pump (F ig .13) f o r c e s grease i n t o one o f two main feed
l i n e s i n which a r e p laced a number o f d u a l - l i n e m e t e r i n g elements, each o u t l e t
MODE BSA
M U orised BS-A h p
L
SC-A Diffaeraial R c U n
Fig .13 Dual l i n e system.
410
2oo
I I _ I_ Line 1 weuurising Line 2 pressurising Interval
I end of line 1 b 2
Fig .14 Dual l i n e system pressure .
6
6-
Main Line 2 2
Fig.15 Dual l i n e meter ing element.
Dua l - l i ne meter ing elements ope ra te h y d r a u l i c a l l y w i t h o u t t he use o f b a l l s
and sp r ings o r check va lves .
e i t h e r s i d e of a meter ing p i s t o n which i s f i t t e d w i t h an i n d i c a t o r p i n , enab l i ng
easy i nspec t i on o f o p e r a t i o n a t each p o i n t on the system.
f i t t e d t o each i n d i c a t o r housing pe rm i t adjustment down t o 20% o f maximum ou tpu t .
The p o s i t i o n o f t he c o n t r o l p i s t o n (2) and t h e feed p i s t o n (6) a r e shown i n ' A '
a f t e r t he f i r s t p a r t o f t he d u a l - l i n e cyc le .
A c o n t r o l p i s t o n d i r e c t s t h e supply o f grease t o
Regu la t ing screws
Pressur ised l u b r i c a n t f rom main
411
l i n e 1 has moved over c o n t r o l p i s t o n (2) and then a t t h e upper s i d e o f t he feed
p i s t o n ( 6 ) , d i s p l a c i n g i t and d i scha rg ing a measured q u a n t i t y o f l u b r i c a n t v i a
cross p o r t i n g (7) and across spool (1) t o o u t l e t (8 ) . P o s i t i o n ' B ' i s t he second
p a r t o f the d u a l - l i n e c y c l e and p ressu r i sed l u b r i c a n t f rom main l i n e 2 has moved
con t ro l p i s t o n (2 ) and d i sp laced t h e meter ing p i s t o n (6) thus d i scha rg ing a
measured shot o f l u b r i c a n t v i a cross p o r t (5) and across spool ( 3 ) t o o u t l e t ( 4 ) .
For extremely d i r t y and ab ras i ve environments such as those found i n a B las t
Furnace, P i g Caster o r Coal P repara t i on and Washery P l a n t , i t i s adv i sab le t o
have the dual l i n e meter ing elements housed i n toughened g lass - f ron ted protect ion
boxes, s i m i l a r t o t h a t shown i n Fig.16.
Fig.16 P r o t e c t i o n box.
412
17.6.1.3 Comparison o f P a r a l l e l Systems - S i n g l e l i n e and Dua l - l i ne .
Both systems depressur ize t h e l i n e , b u t i n the case o f t h e S i n g l e L i n e System
the r e s e t t i n g o f t he p lungers i n the meter ing elements i s u s u a l l y e f f e c t e d by
sp r ing pressure, whereas i n the Dual L i n e system. when main feed l i n e No.2 i s
pressur ised, a second se r ies o f dual l i n e meter ing elements d ischarge l u b r i c a n t
t o the p o i n t s o f a p p l i c a t i o n . I n do ing s o , they rese t t he dual l i n e meter ing
elements i n main l i n e No.1, w i t h which they form an i n t e g r a l u n i t .
17.6.1.4 Se lec t i ng Grease L u b r i c a t i o n Systems.
Wherever poss ib le , l u b r i c a t i o n systems should be avoided which use equipment
i nco rpo ra t i ng sp r ings and va lves, p a r t i c u l a r l y where t h e l u b r i c a t i o n systems are
requ i red t o operate i n arduous and ab ras i ve environments.
I n general p r a c t i c e , t he m a j o r i t y o f Cen t ra l i sed L u b r i c a t i o n Systems used are
e i t h e r o f the ' D i r e c t Feed' o r 'Pa ra l l e l -Dua l L i n e ' type; both a r e capable o f
being operated 'manual ly ' o r ' a u t o m a t i c a l l y ' . The choice o f Grease Lubr i ca t i on
Systems i s u s u a l l y d i c t a t e d by the number o f p o i n t s o f a p p l i c a t i o n and t h e i r
l oca t i on , always bear ing i n mind t h a t f o r utmost r e l i a b i l i t y and minimal main-
tenance D i r e c t Feed Systems should be se lec ted wherever i t i s p r a c t i c a l . By way
o f explanat ion, a Para l l e l -Dua l L i n e system can c y c l e and y e t f a i l t o d e l i v e r
l u b r i c a n t t o some of t h e p o i n t s o f a p p l i c a t i o n which i s o n l y ev iden t i f t he
operator v i s u a l l y examines the p o s i t i o n o f every dual l i n e element i n d i c a t o r pin.
Qui te o f t e n t h i s i s imprac t i ca l because o f elements be ing l oca ted i n hazardous
areas o n l y being access ib le when the p l a n t and machinery a r e shut down.
This cannot occur w i t h D i r e c t Feed pumps, f o r they w i l l o n l y operate e f f e c t i v e l y
p r o v i d i n g the p o i n t s o f a p p l i c a t i o n a r e r e c e i v i n g l u b r i c a n t ; b a r r i n g , o f
course, broken feed-pipes (which can apply t o both types o f system). I n prac-
t i c e , a D i r e c t Feed pump can u s u a l l y c a t e r f o r up t o 40 p o i n t s o f a p p l i c a t i o n .
17.6.1.5 L u b r i c a t i o n o f P l a i n o r Sleeve Bearings.
These bear ings, p a r t i c u l a r l y those over 4" d iameter, r e q u i r e as near c o n t i n -
uous l u b r i c a t i o n as i s poss ib le . I t i s normal ly recommended t h a t bear ings of
t h i s type be fed by D i r e c t Feeding l u b r i c a t o r s d r i v e n from the moving s h a f t .
Th is ensures l u b r i c a n t i s f ed t o the bear ings when the s h a f t i s i n ope ra t i on and
no l u b r i c a n t i s f ed t o the bear ing when the s h a f t i s i nopera t i ve .
17.6.1.6 L u b r i c a t i o n o f A n t i - F r i c t i o n ( B a l l and R o l l e r ) Bear ings.
A n t i - f r i c t i o n bear ings r e q u i r e considerably sma l le r q u a n t i t i e s o f l u b r i c a n t
than p l a i n bear ings. Except i n t h e case o f l a r g e a n t i - f r i c t i o n bear ings, they
w i l l n o t normal ly be l u b r i c a t e d cont inuously .
Where a n t i - f r i c t i o n bear ings a r e c lose together , a hand-operated o r t ime
Where they a r e spaced over c lock c o n t r o l l e d d i r e c t feeding pump may be used.
413
some d is tance, a l i n e system, e i t h e r hand-operated o r t ime c l o c k c o n t r o l l e d , i s
p re fe r red .
For l a r g e a n t i - f r i c t i o n bear ings and those i n s t a l l a t i o n s where the g rea tes t
r e l i a b i l i t y o f D i r e c t Feeding systems i s d e s i r a b l e bu t t he number o f p ipes
should be kept low, d i r e c t feeding pumps w i t h P o s i t i v e D iv ide rs may be used,
s p l i t t i n g v o l u m e t r i c a l l y metered amounts o f l u b r i c a n t independent o f va ry ing
back pressures.
Very f a s t ope ra t i ng a n t i - f r i c t i o n bear ings such as those running a t 1400 rpm
should n o t be fed cont inuously . However, over-greas ing w i l l do no harm t o l a rge
slow-running a n t i - f r i c t i o n bear ings where q u i t e o f t e n t h e grease i s used as a
sealant , p reven t ing d i r t and o t h e r f o r e i g n ma t te r - the g rea tes t dest royer o f
a n t i - f r i c t i o n bear ings - t o en te r t h e bear ing. Therefore i t i s essen t ia l t h a t
the grease i s kept c lean a t a l l t imes, w i t h the l u b r i c a t i n g pump con ta ine r o r
r e s e r v o i r bot tom f i l l e d v i a a grease keg o r b u l k grease s torage system.
17.6.2 O i l L u b r i c a t i o n Systems
O i l L u b r i c a t i o n Systems serve two purposes: t o l u b r i c a t e and/or cool . On
many a p p l i c a t i o n s . p a r t i c u l a r l y i n t h e absence o f h i g h ambient temperature o r
where the heat generated i n t h e bear ings o r the gears i s no t g rea t . t he removal
o f heat by the o i l need n o t f e a t u r e as a separate cons ide ra t i on i n the s e l e c t i o n
o f the o i l c i r c u l a t i o n system. T h i s can be arranged on the bas is o f l u b r i c a t i o n
considerat ions alone. However, i n the case o f many o t h e r a p p l i c a t i o n s , the
coo l i ng p r o p e r t i e s o f the o i l a r e o f g rea t importance.
O i l systems may t h e r e f o r e be grouped under th ree main headings:-
Group 1 Systems designed f o r l u b r i c a t i o n on a t o t a l loss basis.
Group 2 Systems designed f o r l u b r i c a t i o n and w i t h a smal l
amount o f heat removal.
Group 3 Systems designed f o r l u b r i c a t i o n where an apprec iab le
degree o f c o o l i n g i s a l s o requ i red because o f
ope ra t i ng cond i t i ons .
Group 1 and 2 systems vary accord ing t o t h e type o f machine and i t s l u b r i c a t i o n
requirements. Systems of the t o t a l l oss t ype may be operated e i t h e r manually,
mechanical ly, o r motor ised, whereas systems o f t he type which c o l l e c t t h e used
o i l and r e c i r c u l a t e i t must be automat ic . Var ious combinations o f these systems
can be employed, and t h e f o l l o w i n g a r e some t y p i c a l examples:
17.6.2.1 Group 1 - T o t a l loss systems designed f o r l u b r i c a t i o n purposes on ly .
I n t h i s type o f system t h e l u b r i c a n t , a f t e r l u b r i c a t i n g t h e bear ings o r gears,
i s no t used again. The group c o n s i s t s o f manual, mechanical o r motor operated
pumps. The former genera l l y app l i es on smal l items o f p l a n t , e.g. machine t o o l s ,
414
mechanical handl ing equipment, j i g s and f i x t u r e s , presses, e t c . These systems
may be f u r t h e r sub-d iv ided i n t o D i r e c t P o s i t i v e Systems or P o s i t i v e S p l i t Systems.
D i r e c t P o s i t i v e Systems u s u a l l y comprise one o r more d i f f e r e n t i a l p lunger
type o i l l u b r i c a t o r s , e.g. Sect ions 6.1.1 (Fig.9) shows the opera t i on o f a man-
u a l l y operated 8 - o u t l e t grease pump which i s a l s o adaptable f o r o i l , w i t h Fig.17
i l l u s t r a t i n g a t y p i c a l a p p l i c a t i o n on a press l u b r i c a t i n g the s l ideways and
crosshead. For a p p l i c a t i o n s where automat ic l u b r i c a t i o n i s requ i red , mechanical
l u b r i c a t o r s having u s u a l l y 28 t o 32 pumping u n i t s may be f i t t e d (F ig .18) . These
l u b r i c a t o r s may be d r i v e n e i t h e r mechanica l ly through a r a t c h e t f rom the machine
being l u b r i c a t e d o r by geared e l e c t r i c motor. Each pump u n i t can be regulated
from zero t o maximum, t o feed minute p r e c i s e q u a n t i t i e s o f o i l t o t h e p o i n t s o f
l u b r i c a t i o n a p p l i c a t i o n .
Fig.17 D i r e c t p o s i t i v e system.
415
Fig.18 D i r e c t p o s i t i v e system.
P o s i t i v e S p l i t System. T h i s t ype o f system i s used where q u a n t i t i e s o f o i l
per a p p l i c a t i o n p o i n t a r e g r e a t e r than can be supp l i ed by t h e p r e v i o u s l y descr-
ibed d i r e c t feed ing p lunger system.
p ressure pumps f i t t e d w i t h i n t e g r a l r e l i e f va lves , and supp l i es l u b r i c a n t t o
the bear ings through volume d i v i d e r s . The d i v i d e r s may be used e i t h e r t o i n -
crease t h e number o f p o i n t s or t o mod i fy t h e q u a n t i t i e s f e d t o t h e l u b r i c a t i o n
p o i n t s .
I t comprises o f one o r more smal l h igh
416
Fig.19 P o s i t i v e s p l i t system.
17.6.2.2 Group 2 - Systems designed f o r l u b r i c a t i o n w i t h smal l amount o f
heat removal. Th i s t ype o f system supp l i es the l u b r i c a n t t o t h e bear ings and
re tu rns i t under g r a v i t y through t h e r e t u r n p ipes and/or drainways t o the supp ly
tank f o r r e c i r c u l a t i o n . Th is group can a l s o be sub-d iv ided i n t o D i r e c t P o s i t i v e
Systems o r P o s i t i v e S p l i t Systems.
operated d i f f e r e n t i a l p lunger t ype o i l l u b r i c a t o r as descr ibed i n Group 1.
The former c o n s i s t s o f t h e mechan ica l l y
417
Typ ica l a p p l i c a t i o n s inc lude paper machines, l a r g e k i l n s , o r genera l l y where
a l a r g e number o f bear ings a r e t o be fed p o s i t i v e l y w i t h an ad jus tab le feed.
Systems o f t h i s type r e q u i r e a minimum amount o f maintenance and a t t e n t i o n . The
l u b r i c a t i o n r e s e r v o i r s o f conta iners can be kept f i l l e d e i t h e r by a header tank
supply ing severa l l u b r i c a t o r s , o r each l u b r i c a t o r can be f i t t e d w i t h spec ia l
b u i l t - i n s u c t i o n pumps which supply the conta iner w i t h l u b r i c a n t f rom the main
supply tank. Each l u b r i c a t o r pump u n i t ( o u t l e t ) can be connected d i r e c t t o the
l u b r i c a t i o n p o i n t s o r t o a p o s i t i v e volume d i v i d e r , depending on the number o f
feeds and the l u b r i c a t i o n requirements o f t h e po in ts .
The l u b r i c a n t i s re tu rned under g r a v i t y t o t h e main supply tank through d r a i n -
ways o r r e t u r n p ipes (which can be arranged w i t h bear ing sump l e v e l c o n t r o l
devices) f o r r e c i r c u l a t i n g t o the l u b r i c a t o r conta iners o r header tank by means
of s u c t i o n pumps, as descr ibed, o r t o t h e tank by a f l o a t - c o n t r o l l e d gear pump
(F ig.20) .
Discharge f i l t e r -
I I & 6 6 1 b b \ / b O b d L b d b b b l b l b
b b b b d b d b b
Reservoir
Fig.20 System w i t h smal l amount o f heat removal
As i n t he case o f the prev ious group, t h e l u b r i c a t o r can be d r i v e n e i t h e r
d i r e c t f rom the machine being l u b r i c a t e d o r f i t t e d w i t h i t s own motor, Each
pump can be fed e i t h e r f rom t h e machine sump or from a separate d r a i n and supply
tank. The system i s u s u a l l y i n s t a l l e d on machine t o o l s , sugar machinery, gear
boxes, p r i n t i n g machines, and special-purpose machinery.
17.6.2.3 Group 3 - Systems designed f o r l u b r i c a t i o n coo l i ng .
Where the re i s cons iderable ambient heat o r where the power t ransmi t ted by
the p a r t being l u b r i c a t e d creates a h i g h degree o f heat , t he c o o l i n g f u n c t i o n
o f t he l u b r i c a n t assumes g rea t importance. I t i s necessary i n such cases t o
apply s u f f i c i e n t l u b r i c a n t t o e x t r a c t t h e heat and t o ma in ta in the bear ings or
gears a t an optimum temperature. Arrangements must a l s o be made f o r r e t u r n i n g
the l u b r i c a n t t o a supply tank f o r coo l i ng , f i l t r a t i o n , and r e c i r c u l a t i o n ,
between the l u b r i c a t i o n equipment manufacturer, t he p l a n t designers, and the
operators .
Such a system u s u a l l y comprises a l a r g e o i l r e s e r v o i r or s torage tank; motor
d r i v e n pump (normal ly a gear t y p e pump adjacent t o o r on t h e s to rage tank) ;
coolers ; f i l t e r s ; pressure gauges; a larm and f l o w c o n t r o l equipment; together
w i t h the necessary va lves and in te rconnec t ing pipework.
and se l f - con ta ined w i t h a capac i t y o f 38cc/sec t o 750cc/sec per minute, o r a
complex system capable o f d e l i v e r i n g severa l l i t r e s / s e c . These systems can be
prov ided w i t h s imple o r e labo ra te f l o w c o n t r o l , warning dev ices, and o t h e r
inst rumentat ion accord ing t o t h e needs o f t h e i n s t a l l a t i o n (Fig.21).
The system may be simple
Fig.21 Typ ica l example o f l u b r i c a t i n g and c o o l i n g system packaged u n i t f o r the l u b r i c a t i o n o f rubber machinery.
419
17.6.3 Micro-Fog L u b r i c a t i o n Systems
Aerosol l u b r i c a t i o n i s t h e gener i c term f o r o i l m i s t o r Mic ro- fog systems
which have been used s u c c e s s f u l l y f o r over twenty years . Compared w i t h Centra-
l i s t e d Grease o r O i l l u b r i c a t i o n , a M ic ro - fog system, t o per fo rm t h e same task ,
requ i res l e s s l u b r i c a n t and energy and t h e i n i t i a l cos t i s r e l a t i v e l y low. I t
i s a l s o a h i g h l y f l e x i b l e system, r e a d i l y i n s t a l l e d o n t o e x i s t i n g p l a n t as w e l l
as a t t h e new machine stage.
17.6.3.1
elements o f t h e M ic ro - fog system and p rov ides a gu ide t o some o f t h e more common
areas o f a p p l i c a t i o n .
concen t ra t i on of mic ro p a r t i c l e s o f o i l which a r e conveyed i n a ' d r y ' f og i n a
low pressure d i s t r i b u t i o n system.
' d r y ' f og i s passed th rough r e c l a s s i f i e r s , which a r e r e a l l y me te r ing and conden-
s ing o r i f i c e s , so as t o a c c u r a t e l y feed an exac t q u a n t i t y o f l u b r i c a n t t o s u i t
the o p e r a t i n g cond i t i ons .
Working P r i n c i p l e s (Fig.22) ou t1 ines t h e e s s e n t i a l and aux i 1 i a r y
Dur ing opera t i on , t h e system produces con t inuous ly a dense
On reach ing t h e p o i n t of a p p l i c a t i o n t h e
M i cro-foq
l u b r i c a t i o n 1 ubr i ca t ion
1 ub r i ca t i on
F ig .22 Mic ro- fog system.
420
To ensure t h a t the fog reaches the r e c l a s s i f i e r dry , t he d i s t r i b u t i o n p i p i n g
i s s i zed t o a l l o w o i l p a r t i c l e s t o t r a v e l a long the p i p i n g a t a v e l o c i t y l ess
than 7.3 metres/sec, which i s s low enough t o prevent condensat ion. The turbu-
lence i n the r e c l a s s i f i e r s causes t h e o i l p a r t i c l e s t o 'wet o u t ' i n t o the l i n e
leading t o o r d i r e c t onto the bear ing sur faces where they then form a p r o t e c t i v e
f i l m o f o i l .
In o rde r t o c rea te ' d r y ' fog, o i l i s f i r s t drawn i n t o a compressed a i r stream
as i t passes through a Ven tu r i located on top o f t he l u b r i c a t i o n c o n t r o l u n i t
(F ig .23) . O i l p a r t i c l e s o f approx imate ly 0.002mm i n diameter a r e c o l l e c t e d i n
the a i r stream and can be t ranspor ted long d is tances i n the d r y cond i t i on .
Lubricator head
A i r bypass a d j u s t i n
Low pressure w i t c h
lligh pressure swi tch
E l e c t r i c a l condui t ent
Auxi l iary requlatcr
Supply t o pressur j e t r e c l a s s i f i e r
Waste p ipe e x i t /
Fig.23 L u b r i c a t i o n c o n t r o l u n i t .
Since Micro- fog systems r e q u i r e no r e t u r n l i n e s they can be designed t o easy
i n s t a l l a t i o n and assembly;
assoc iated w i t h s i n g l e - l i n e s e r i e s o i l c i r c u l a t i n g systems which i nco rpo ra te
q u i t e soph is t i ca ted and r e l a t i v e l y expensive v a l v e b locks t h a t have a b u i l t - i n
s e l f - r e v e r s i n g opera t i on . Al though such systems may r e q u i r e no separate reverse
and recyc le va l ve o r ven t ing phase du r ing the l ube cyc le , they do have, however,
a d i s t i n c t d isadvantage because they r e q u i r e a m u l t i p l i c i t y o f system tub ing
hence, l o w cos t i n s t a l l a t i o n , w i t h o u t t h e problems
(F ig.20) .
Moreover, Once a Micro- fog system has been i n s t a
i ng than o t h e r automat ic l ube systems.
b u i l d i n g b lock approach, which i s designed f o r easy
general maintenance.
The system
led i t i s much less demand-
s e a s i l y assembled, us ing a
s e r v i c i n g , r e p a i r s and
421
17.6.3.2 O i l Q u a l i t y .
Correct o i l s e l e c t i o n i s impor tant as some o i l s i nco rpo ra te polymers which
suppress aerosol p r o p e r t i e s , w h i l e heavier grade o i l s may r e q u i r e hea t ing up t o
between 4Oo/45OC t o a t t a i n the v i s c o s i t y f o r maximum ou tpu t . For a l l normal
purposes t h e c o n t r o l u n i t s w i l l per form w e l l when working w i t h o i l s up t o 700
cen t i s tokes a t 20°C.
Summarised l u b r i c a t i n g o i l requirements f o r a s a t i s f a c t o r y Micro- fog system
a re : -
( i ) Good aerosol p r o p e r t i e s .
( i i ) Low r a t e o f condensat ion through p ipes.
( i i i ) Low l e v e l o f s t r a y i n g by the p a r t i c l e s .
( i v ) A h i g h degree o f r u s t i n h i b i t i o n .
( v ) Absence o f c logg ing tendencies a t t h e v e n t u r i nozz le o r any polymer p r e c i p i t a t i o n .
17.6.3.3 Compressed A i r 2 Accept ing t h a t most i n d u s t r i a l compressed a i r i s supp l i ed a t 7 bar (100 l b / i n )
i n a Micro- fog l u b r i c a t i o n system i t has t o be reduced t o about 2 bar (30 l b / i n 1. Dur ing i t s passage through the v e n t u r i o r i f i c e on t h e c o n t r o l u n i t , a pressure
drop o f 0.7 bar (10 I b / i n ) takes place.
2
2
17.6.3.4 System Design Considerat ions
To c a l c u l a t e the l u b r i c a t i o n requirements o f bear ings, an emp i r i ca l f a c t o r
r e f e r r e d t o as a ' l u b r i c a t i o n u n i t ' (L.U.) has been evolved, enabl ing a l l moving
sur faces r e q u i r i n g l u b r i c a n t t o be converted t o t h e i r equ iva len t L.U. r a t i n g .
The amount o f l u b r i c a n t main and branch l i n e p i p e bores and r e c l a s s i f i e r nozzles
may then be s i zed t o p r o v i d e the c o r r e c t amount o f l u b r i c a n t a t each l u b r i c a t i n g
p o i n t . I n t h i s manner, a n t i - f r i c t i o n bear ings, j o u r n a l bear ings, s l i d e s , gears,
chains, and o t h e r wear ing sur faces r e q u i r i n g l u b r i c a t i o n can a l l be converted t o
equ iva len t L.U. r a t i n g s and served by a p p r o p r i a t e l y s i z e d Micro- fog l u b r i c a t i o n
systems.
17.6.3.5 Some Typ ica l App l i ca t i ons
F igu re 24 i l l u s t r a t e s a t h r e e Strand Aluminium F o i l M i l l o p e r a t i n g a t 1000 t o
1500 metres/minute w i t h two 1000 L.U. generat ing heads ( t h i r d a c t s as a standby)
se rv ing the m i l l s tack and e x i t a n c i l l a r i e s , w i t h a separate 300 L.U. generat ing
u n i t se rv ing the e n t r y a n c i l l a r i e s . The t o t a l amount o f o i l used i s l ess than
2.5 l i t r e s per work ing hour. F igure 25 i l l u s t r a t e s the l u b r i c a t i o n o f v i b r a t o r
motor oears.
422
423
I i "i" 'i' 0-
I
Fig.25 Lubrication o f vibrator motor gears.
17.6.4 Check List
Based on the foregoing, prior to actual selection of the appropr
cant and associated equipment, it is advisable to draw up a check 1
facts and requirements. The following list, in simplified form, is
purposes only and can naturally be elaborated upon to suit the spec
the designer or plant engineer.
Specification of Plant to be lubricated:
Type of plant and machinery to be lubricated.
ate lubri-
st of known
for guide
fic needs o f
Industrial Application, including operating environmental conditions, e.g. is i t dirty, abrasive, wet, hot, etc.?
Surfaces, sizes and speeds to be lubricated, e.g. Bearings (plain), Bearings (roller), Grease (type), Slideways, Chains, etc.
Number and Location of lubrication Points (Fixed) and (Moving) and
proposed site (if known) for lubricator enabling assessment of pipe and flexes:
Frequency plant and machinery operators and whether lubrication needs to be continuous, semi-continuous, or intermittent.
424
17
Lubr icant type and spec i f c a t i o n - Grease, O i l , o r Micro- fog, e t c
Services a v a i l a b l e i n the P lan t - e l e c t r i c , pneumatic.
Heal th and Safety aspects e.g. normal, f lameproof, a c c e s s i b i l i t y any h i s t o r y o f p rev ious l y t r i e d lubr icants /equipment .
Method proposed f o r f i l l i n g the l u b r i c a t i o n tank / rese rvo i r , e.g. manual, semi-automatic, b u l k storage.
Recommended L u b r i c a t i o n Equipment:
Why?
Est imated performance w i t h any known Case Studies and References.
Econom i cs . Spares and Service.
Sometimes equipment s e l e c t i o n i s an i n e v i t a b l e compromise as a r e s u l t
of c o n f l i c t i n g l u b r i c a t i o n requirements; in such circumstances i t i s
important t o app rec ia te a l l the f a c t s and t o subsequently gauge p l a n t
performance acco rd ing l y .
7 SUMMARY
This chapter has at tempted t o c l a r i f y the more g e n e r a l l y accepted methods o f
l u b r i c a t i o n , where necessary i l l u s t r a t i n g ac tua l examples o f p l a n t and l u b r i c a -
t i o n equipment. I t i s n o t i n any way intended t o i n f e r t h a t t h i s i s t he o n l y
l u b r i c a t i o n equipment a v a i l a b l e ; o f course, t he re a re o the rs r e a d i l y determined
from Trade Journals , e t c . L ikewise, t he re a r e many more s e l e c t items o f l u b r i -
c a t i o n equipment ta i lor -made t o meet the needs o f s p e c i a l i s t p l a n t .
For example, Overhead and F loo r Conveyors which can t r a v e l up t o 50 metres/
minute, un less e f f e c t i v e l y l u b r i c a t e d (and cleaned where the environment
demands) can wear and se ize, r e s u l t i n g i n c o s t l y stoppages.
l n i t i a
and o i l - n o t o n l y
page w i t h
l y , convent ional s t a t i c l u b r i c a t o r s were used where a m ix tu re o f a i r
and now more r e c e n t l y j u s t o i l - was shot over a gap, which r e s u l t e d
n the f a i l u r e t o adequately l u b r i c a t e (Fig.1) bu t a l s o caused d r i p -
consequent ia l product contaminat ion and h e a l t h hazards.
These problems were complete ly overcome by i n t roduc ing a range o f spec ia l -
purpose l u b r i c a t o r s ; (Fig.26) shows one such example.
The conclus ion i s t o determine a l l the f a c t s regard ing the p l a n t t o be
l u b r i c a t e d and t o then evaluate the l u b r i c a n t s and l u b r i c a t i o n equipment
ava i 1 able.
425
Fig.26 A dog cha in assembly engages t h e conveyor cha in which moves t h e o i l - d i s p e n s i n g nozz le p l a t e s i n and o u t over t h e p i n l i n k s . O i l i s f o rced th rough each nozz le .
REFERENCES
1 Mechanical L u h r i c a t i o n o f E.O.T. Cranes by Dr .H . Pe ter Jos t and
2
3
4
5
Peter W. Murray. A F u l l y Automat ic Bu lk Hand l ing L u b r i c a t i o n System f o r a S i n t e r P lan t by G. Wil l iams. An Eng ineer ing Approach t o the s e l e c t i o n o f Cen t ra l i sed Grease L u b r i c a t i o n Systems by D r . H. Peter Jos t . Modern B r i t i s h and European Steelworks L u b r i c a t i o n Developments by D r . H. Peter Jos t . E!ew M i s t L u b r i c a t i o n Concepts f o r Tapered R o l l e r Bear inos used on High Speed R o l l i n g M i l l Back-up by R o l l s by \!.E.McCoy, C.H. West and P.E. Wi lks .
426
6 Aerosol L u b r i c a t i o n Systems - t h e i r c o n t r i b u t i o n t o savings i n o p e r a t i n g
7 Micro-Fog L u b r i c a t i o n f o r bear ing e f f i c i e n c y by J.G. M e r r e t t . 8 Automat ic L u b r i c a t o r s and Cleaners Increase Conveyor L i f e by
9 Automat ic L u b r i c a t i o n o f Chain and Conveyor Systems by R.M. Dombroski.
and maintenance cos ts by R.E. Kn igh t and J.G. M e r r e t t .
J.G. M e r r e t t .
10 The James C lay ton Lec ture - "Energy Saving through T r ibo logy " by D r . H. Peter Jos t and D r . J. Schof ie ld .
421
18 ON CONDITION MAINTENANCE
R.A. COLLACOTT, Ph.D., B.Sc(Eng) , F. I .Mar. E . , F. I .Mech.E.
D i r e c t o r - UK Mechanical Hea l th Mon i to r i ng Group
Head - F a u l t D iagnos is Centre, Le i ces te r Po ly techn ic .
18.1 INTRODUCTION
Maintenance c a r r i e d o u t when requ i red a f t e r a s i g n i f i c a n t d e t e r i o r a t i o n i n a
component as i n d i c a t e d by a sensor o r mon i to red parameter i s c a l l e d on -cond i t i on
ma i n tenance.
I f a person, when v i s i t i n g the doc tor f o r a hea l th -check was i n v i t e d t o have
t h e i r body opened up t o see whether ' eve ry th ing was c o r r e c t ' a most unsa t i s -
f a c t o r y s t a t e o f a f f a i r s would e x i s t . S i m i l a r l y , t o open up a machine i n o rde r
t o check t h a t i t i s a l r i g h t i s j u s t as u n s a t i s f a c t o r y - much more harm than
good can be done t o a machine i n t h i s way. Accord ing ly i t i s l o g i c a l t o use
d iagnos t i c techniques t o assess the ' h e a l t h ' o r c o n d i t i o n o f p l a n t and mach-
i n e r y i n j u s t t he same way as a medical doc tor uses symptoms and a i d s t o assess
the c o n d i t i o n o f human beings - and very o f t e n s i m i l a r o r i d e n t i c a l equipment
i s used bo th by the medical doc to r and d iagnos t i c engineer.
18.2 BACKGROUND
On-cond i t ion mon i to r i ng i s a l ready a c t i v e l y and e f f e c t i v e l y used i n many
i n d u s t r i e s - i n a i r c r a f t , nuc lea r reac to rs , s t e e l m i l l s , pe t ro leum r e f i n e r i e s ,
sh ips , power genera t i on e t c . I t i s a technology which has r a p i d l y evo lved
du r ing the pas t 5 t o 10 years by us ing methods and techniques which have been
developed s ince 1945.
H i s t o r i c a l l y , t he cha in o f e v o l u t i o n i s shown i n Fig.1.
1750 ... steam engines - s imp le con ten ts gauges, s imple governor
1915 ... steam t u r b i n e s - pressure , temperature, con ten ts gauges e t c .
1940 ... steam t u r b i n e s - automat ic c o n t r o l s
1950 ... nuc lea r reac to rs - r o b o t r y
1960 ... space v e h i c l e s - m i n u t i a r i s a t i o n , remote te lemet ry
1970 ... ... ... ... ... computer m i n u t i a r i s a t i o n , microprocessors
F igure 1 H i s t o r i c a l cha in o f e v o l u t i o n o f c o n d i t i o n mon i to r i nq
428
18.3 MANAGEMENT OF CONDITION MONITORING
There a r e th ree e f f e c t i v e stages i n the management o f an on -cond i t i on main-
tenance system which go hand-in-hand w i t h a change i n o r g a n i s a t i o n whereby
maintenance invo lves t h e runn ing o f ( i ) a d iagnos t i c c a p a b i l i t y , ( i i ) a r e p a i r
team.
The th ree stages i n d i a g n o s t i c management a re :
1 ) F a i l u r e Modes and E f f e c t s Ana lys i s - whereby the whole/complete p l a n t i s analysed s y s t e m a t i c a l l y t o determine which p a r t s a r e c r i t i c a l and need t o be mon i to red , a l s o t o app ra i se t h e i r t y p i c a l f a i l u r e cause
2)
3 ) L i m i t Dec is ion - whereby the amount o f d e t e r i o r a t i o n
Mon i to r i ng Technique Se lec t i on and - whereby the most e f f e c t i v e method Sensor Appra isa l i s chosen
which can be pe rm i t ted i s decided upon
A f u l l account o f a l l t h ree stages i s g i ven i n re fe rence [ l ] .
18.4 FAILURE MODES AND EFFECTS ANALYSIS
A t y p i c a l example o f t he way i n which t h i s can be done was exp la ined by
Venton and Harvey [ 2 ] , Br idges [ 3 ] and mentioned by Davies [ 4 ] , i n essence i t
i s t he use o f i n fo rma t ion t o p repare a numerical assessment o f the o rde r o f
e s s e n t i a l i t y o f va r ious components (sub-systems) w i t h i n a p l a n t o r machinery.
A t i t s most elementary t h i s may be prepared by a s imp le ' D e l p h i ' a n a l y s i s ; a t
i t s more complex i t may invo lve an i n t e g r a t e d da ta -appra i sa l method us ing
f a i l u r e r a t e data which can be ob ta ined f o r d i f f e r e n t components - such f a i l u r e
r a t e data can be ob ta ined f rom a number o f e s t a b l i s h e d da ta banks [ 5 ] .
A t y p i c a l ' s t a r t i n g ' app ra i sa l f o r a marine power p l a n t migh t be the i n t e r -
dependency c h a r t f o r a l l sub-systems as shown i n Fig.2. Th is a l ready shows t h a t
f o r t he p a r t i c u l a r m iss ion f o r which t h i s app ra i sa l was made, h i g h r e l i a b i l i t y
o f e l e c t r i c a l genera t i on was impor tan t - a s t a t e o f a f f a i r s which was conf i rmed
by a bas i c PHASE CRITICALITY ANALYSIS and remained unchanged when al lowance was
made f o r the i nhe ren t s a f e t y th rough a HAZARDS AND R I S K S ANALYSIS.
The causes o f p o t e n t i a l f a i l u r e need t o be es tab l i shed . Again, t h i s must
be de r i ved from h i s t o r i c a l records - i t i s impor tan t t o make, ma in ta in and
analyse records i n o rde r t o p rov ide adequate i n fo rma t ion upon which t o choose
the c o r r e c t mon i to r i ng methods.
K i l b o u r n [61 the f a i l u r e r a t e da ta o f Table 18.1 was used t o determine the best
arrangement o f an e l e c t r i c a l genera t i ng standby p l a n t t o choose the most
re1 i a b l e system.
In a t y p i c a l a n a l y s i s by Wi lk inson and
429
x x
I nter-dependen t System
System ( 1 ) (2 ) (3) (4 ) ( 5 ) ( 6 ) (7) ( 8 ) (9) ( 1 0 ) ( 1 1 ) (12) (13) 1 ) Main eng ine x x X x x x
3) o i l f u e l X
4 ) Compressed X
2) Transmission X
a i r
5) E l e c t r i c a l genera t i o n
6) S t e e r i n g X
7) Deck X
8 ) Sea water X
9 ) B i I g e and X
1 0 ) V e n t i l a t i o n X
12) C o n t r o l s X
13) Steam X x x
gear
machinery
b a l l a s t
11) Exhaust
X
x x
X
Fig.2 System inter-dependency A n a l y s i s
As a consequence o f p a s t o p e r a t i o n a l exper ience - e i t h e r by an i n d i v i d u a l
user o r f rom a manufac turer o r some data c e n t r e such as t h a t p r o v i d e d by L loyds
R e g i s t e r o f Sh ipp ing o r t h e UK Atomic Energy A u t h o r i t y (Systems Re1 i a b i l i t y
Serv ice) - i t i s p o s s i b l e t o p repare FAULT TREES such as t h a t shown i n
F I ~ . ~ [ 7 ] . D e t a i l e d examinat ion o f such f a u l t t r e e s w i l l i d e n t i f y t h e poss-
i b l e p r i m a r y and secondary causes o f f a i l u r e s and t h e l i k e l y symptoms they w i l l
produce.
F a i l u r e symptoms a r e t h e p h y s i c a l consequences o f a m a l f u n c t i o n s i t u a t i o n .
I f f o r example one c o n s i d e r s b e a r i n g f a i l u r e s t h e r e a r e two q u i t e d i f f e r e n t
p h y s i c a l e f f e c t s between r o l l e r element b e a r i n g s and p l a i n bear ings . Wi th
r o l l e r e lement bear ings exper ience shows t h a t f a i l u r e s a r e a s s o c i a t e d w i t h l o c a l
sur face d e f e c t s , consequent ly o p e r a t i o n o f d e f e c t i v e bear ings s e t s up u l t r a -
h i g h f requency shock waves whi& can be most e f f e c t i v e l y diagnosed by shock
pu lse methods p o s s i b l y a i d e d by s t a t i s t i c a l a n a l y s i s by a method c a l l e d
' k u r t o s i s ' . On t h e o t h e r hand, p l a i n b e a r i n g s wear i n a u n i f o r m whole-surface
method w i t h t h e r e s u l t t h a t c learances a r e changed and some i n t e r f e r e n c e
( rubb ing) may ensue;
i n d u c t i v e p ick -ups or v i b r a t i o n measurement or even d e b r i s a n a l y s i s may be
a p p r o p r i a t e . For t h e two c l a s s e s o f bear ings d i f f e r e n t sensor systems w i l l be
f o r such c o n d i t i o n s , methods o f c l e a r a n c e measurement by
430
L
u m
aao
FIGURE 3 Typical FAULT TREE for a bearing failure analysis
c PROBLEM IIEFINITION
f FAlLlJRE DATA COLLECTION
EVALUATION OF RMA OBJECTIVES, REOUIREMENTS AN0
.1 t FAULT TREE ANALYSIS
I I EVALUATION OF FUhCTlONAL BLOC DIALRAMS OF ALTERNATIVE SYSTEMS
I ENVIRONMEIITAL A N A l YSI5 I
I FORMULATION Of A N A I Y Y , PROGHAM I
JI I CORRECTIVE MAINTENANCE ANALYSIS I
f e L MAINTENANCE DATA COLLEC
FIGURE 4 Reliability analysis
- I
I ANALYSIS REVIEW 1
ti1 T R A D E - O F F STUDIES
OUANTITATIVE RMA - ANALYSIS - FORMULATION OF RELIABILITY BLOC DIAGRAMS
CHARACTERISTICS - I +
I FORMUI.ATION OF MATHEMATICAL MODELS I J
PREDICTION OF R M A - CHARACTERISTICS 1 I ANALYSIS REVIEW 1
r I TRADE-OFF STUDIES I
sequence diagram
432
ava i 1 a b I e . Thus t h e who le i n i t i a l a n a l y t i c a l s t a g e can be r e p r e s e n t e d b y a number o f
p rocedures wh ich have been f u l l y s e t o u t i n F i g . 4 [ 7 ] .
TABLE 18.1 F a i l u r e / r e p a i r d a t a
R e p a i r Ra te , Mean Time t o Components Repa i r s / h R e p a i r , h
( 2 men)
Tu rbo -a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end 0.08197 12.2 t u r b i n e end 0.122 8.2
D i e s e l a l t e r n a t o r and c o n t r o l s : a l t e r n a t o r end 0.08197 12.2 d i e s e l end 0.05051 19.8
T u r b o - a l t e r n a t o r system sea w a t e r c i r c u l a t i n g pump 0.1 10.0
D i e s e l a l t e r n a t o r sea w a t e r c i r c u l a t i n g pump 0.1493 6.7
Condenser 0.1667 6.0
A i r e j e c t o r 0.1136 8.8
E x t r a c t i o n pump 0.09434 10.6
B o i l e r f e e d pump 0.1429 7.0
Waste h e a t economise r - t ype b o i l e r 0.03636 27.5
O i l f i r e d b o i l e r 0.05+: 20 . O ”
Composite b o i l e r c i r c u l a t i n g pump 0.09259 10.8
:l: E s t i m a t e d
To ensu re t h a t t h e c h a r a c t e r i s t i c s o f each f a i l u r e / d e f e c t m a l f u n c t i o n i t i s
necessa ry t o be a b l e t o r e c o g n i s e t h e c h a r a c t e r i s t i c s - w h i c h i s o n l y o b t a i n e d
t h r o u g h e x t e n s i v e d e f e c t r e c o g n i t i o n e x p e r i e n c e .
18 .5 MONITORING TECHNIQUE SELECTION
There a r e b a s i c a l l y f o u r t e c h n i q u e s f o r t h e m o n i t o r i n g o f p l a n t and
mach ine ry d e t e r i o r a t i o n :
1 . dynamic methods - i n p a r t i c u l a r v i b r a t i o n m o n i t o r i n g b u t a l s o
i n c l u d i n g t h e use o f a i r - b o r n e sounds
433
2. i n s p e c t i o n / i n t e g r i t y - which o r i g i n a t e d w i t h non -des t ruc t i ve t e s t i n g
s u r v e i l l a n c e methods techniques bu t have now extended t o i nc lude
l e a k - t e s t i n g ; odour i d e n t i f i c a t i o n ; c o r r o s i o n
mon i to r i ng and s t r e s s wave emission
3. contaminant i nspec t i on - as a means o f i d e n t i f y i n g wear deb r i s and
r e l a t i n g i t bo th q u a n t i t a t i v e l y and q u a l i t a t i v e l y
t o i t s source
4 . t r ends a n a l y s i s - e f f e c t i v e l y data l ogg ing e i t h e r as s t r a i g h t
sensor ou tpu ts as f o r example temperatures,
pressures and speeds o r i n a coord ina ted form
us ing such parameters as s p e c i f i c f u e l con-
sumpt ion r a t e o r even ' d e l t a s ' o f v a r i a t i o n s
f rom the norm
18.5 .1 V i b r a t i o n Mon i to r i ng
Th is i s a w e l l e s t a b l i s h e d technique rang ing i n s o p h i s t i c a t i o n f rom the use
o f j u d i c i o u s l y p laced d i a l gauges i n con junc t i on w i t h stroboscopes t o broad-
band a n a l y s i s , narrow band a n a l y s i s , a u t o - c o r r e l a t i o n , s igna l averag ing and
o t h e r h i g h l y instrumented techniques [8] . Most mon i to r i ng a p p l i c a t i o n s a r e
s a t i s f a c t o r i l y d e a l t w i t h by means o f :
( i ) p r o x i m i t y probes and pick-ups w i t h p o s s i b l y a cathode ray tube ( c r t )
d i s p l a y
( i i ) seismic v e l o c i t y t ransducers o r p i e z o - e l e c t r i c accelerometers o u t p u t t i n g
t o e i t h e r
(a) broad-band (overa l I ) v i b r a t i o n meters
(b) v i b r a t i o n spectrum ana lysers (par t -oc tave o r narrow-band
w id th )
( i i i ) waveform ana lysers
18 .5 .1 .1 Prox im i t y Probes
The o r b i t moved through a s h a f t which i s loose i n i t s bear ings y e t sub jec ted
t o the i n f l u e n c e o f va r ious fo rces can be observed by us ing two p r o x i m i t y
probes p laced a t a 90" r e l a t i v e ang le and t h e i r ou tpu ts l e d t o an X - Y p l o t t e r o r
a CRT. The r e s u l t i n g d i s p l a y , Fig.5, can be used to measure the ac tua l amount
o f o r b i t a l e c c e n t r i c i t y (and thus t o determine whether ' bea r ing w ipe ' i s 1 i k e l y
t o occu r ) a l s o the shape o f t he o r b i t i s r e l a t e d t o the type o f de fec t fo rce
90 that the source o f troub/e cdn be estab/ished.
434
PICKUP
Fig.5 Shaft o r b i t a l ana lys i s
18.5.1.2 Seismic V e l o c i t y Transducers/Accelerometers/Vibration Meters/Spectrum Analysers
This i s the developing area o f i n t e r e s t i n ' s t r a i g h t ' v i b r a t i o n ana lys i s .
V e l o c i t y t ransducers a r e most a p p l i c a b l e a t the lower frequency ranges;
erometers a r e e f f e c t i v e a t the h ighe r f requency ranges; a t in termediate
f requencies (around 500 Hz = 500 x 60 = 3000 cpm) e i t h e r t ransducer i s a p p l i c -
able.
accel -
With o v e r a l l (wide) bandwidth v i b r a t i o n a n a l y s i s a t ransducer p i cks up a
s ignal de r i ved from a l l the c o n s t i t u e n t f requencies and t h i s i s measured by the
meter - which acco rd ing l y t e l l s whether the v i b r a t i o n i s increas ing i n
s t rength, ie . the system i s d e t e r i o r a t i n g .
To t e l l what i s d e t e r i o r a t i n g i n a machine i t i s necessary t o measure the
v i b r a t i o n s igna l produced by each c o n s t i t u e n t component. Th is i s done by
record ing the frequency 'spectrum' f o r the machine. Thus each component w i l l
generate v i b r a t i o n s a t a p a r t i c u l a r f requency ( i t s ' d i s c r e t e ' frequency) and
when p l o t t e d as i n Fig.6 produce a ' s p i k e ' on the graph a t t h a t frequency.
I f the ' s p i k e ' increases w i t h succeeding spect ra i t w i l l mean t h a t a defect i s
developing i n t h a t p a r t i c u l a r component. To know the values f o r the d i s c r e t e
f requencies f o r d i f f e r e n t components i t i s necessary t o make frequency c a l c u l -
a t i o n s o f va r ious k inds - i n any event machine designers make these c a l c u l a t i o n s
when machines a r e designed and such in fo rma t ion can be obta ined when the
machines a re being purchased. Typica l f requencies a re g i ven i n the f o l l o w i n g
Table.
435
5
FREQUENCY
Fig.6 Typica l frequency spectrum
TABLE 18.2 D isc re te Frequency Ca lcu la t i ons
V i b r a t i o n Type Frequency Equat ion
Simple harmonic
Pendu 1 um ( s imp1 e )
Pendulum (compound)
Bar, un i fo rm ly loaded, f i x e d both ends
Shaft, t o r s i o n a l o s c i l l a t i o n s , s i n g l e f lywheel
Bal l bear ing - ma l func t i on o f o u t e r race
Bal l bear ing - ma l func t i on o f inner race
1 A f = - - 2n M
2n L
1 2n (k2 i h2)
L2 w
f = 1 2 271 I
nN d 2 D
nN d 2 D
f = L 9
f = - gh
f - 3.57 L!.
f = - ( 1 - - cos B )
f = - ( 1 + - cos B )
Bal l bear ing - d e f e c t i v e b a l l
Gear t e e t h - t o o t h de fec t
where, A = s t i f f n e s s o f system M = mass o f system g = acce le ra t i on o f g r a v i t y L = length o f pendulum h = distance,c.g. t o p i v o t ,
compound pendulum k = radius of g y r a t i o n about c.g.,
compound pendulum !i = length o f bar E = modulus o f E l a s t i c i t y (Young’s
Modulus) I = second mment of area o f
sect ion about n e u t r a l a x i s w = load per u n i t l eng th a p p l i e d
t o bar
D d f = N(- - - C O S ~ 8 ) d D
f = N t 1
G = shear modulus J = p o l a r second moment of area per s h a f t I = p o l a r moment o f i n e r t i a o f f lywheel n = number o f b a l l s i n bear ing N = s h a f t speed (rev/min) d = b a l l d iameter D = b a l l p i t c h c i r c l e diameter N1= gearwheel speed (rev/min) t = number o f t e e t h on gearwheel.
T = to rs iona l s t i f f n e s s o f s h a f t CJ 9.
436
When us ing v i b r a t i o n s sensors p a r t i c u l a r ca re must be taken i n s e l e c t i n g
t h e i r mounting p o s i t i o n and i n methods by means o f which they a r e a t tached.
Some v i b r a t i o n de fec ts o n l y show up i n the r a d i a l d i r e c t i o n , o t h e r s o n l y p ro -
duce major e f f e c t s i n the a x i a l d i r e c t i o n . I t i s t h e r e f o r e t o be recommended
t h a t 3 sensors be used a t each l o c a t i o n so t h a t t he v i b r a t i o n s can be es tab-
l i s h e d i n two pe rpend icu la r r a d i a l p o s i t i o n s as w e l l as the a x i a l d i r e c t i o n .
A study o f t he e f f e c t s o f va r ious de fec ts i n d i c a t e s the f o l l o w i n g v ib ra t i ona l
c h a r a c t e r i s t i c s :
TABLE 18.3 V i b r a t i o n C h a r a c t e r i s t i c s o f De fec t i ve Components
unbalance ... occurs a t s h a f t speed ........ i n a r a d i a l d i r e c t i o n
misal ignment .. " a t s h a f t speed (sometimes 3 o r 4 x ) ........ i n a r a d i a l o r more
dominant ly i n an a x i a l d i r e c t ion
p l a i n bear ing loose i n housing .. occurs a t 1 / 2 o r 1 / 3
worn gears ... occurs a t t o o t h mesh
s h a f t speed ........ i n a r a d i a l d i r e c t i o n
f requency = rpm x no. t e e t h .. i n a r a d i a l o r a x i a l d i r e c t ion
f a u l t y d r i v e
mechan
e l e c t r
be1 t
c a l looseness ... 2 x s h a f t speed
c a l induced ... a t synchronous
... occurs a t b e l t f requency ..... i n a r a d i a l d i r e c t i o n
f requency ........ should disappear o f f power
18.5.1.3 Waveform Ana lys i s
The shape o f t he v i b r a t i o n waves and the genera l i n t e r a c t i o n o f super-
imposed v b r a t i o n s can be s tud ied by pass ing the v i b r a t i o n s igna l through a
time-doma n recorder such as an u l t r a - v i o l e t (UV) recorder . Such methods a r e
i n genera l i m i t e d t o very low frequency ( s t r u c t u r a l ) v i b r a t i o n s and some
t r a n s i e n t s tud ies .
18.5.2 I n s p e c t i o n / l n t e g r i t y Su rve i l l ance
Methods t o determine the presence o f f l aws which have been adopted f rom
non-des t ruc t i ve (nd t ) t e s t i n g methods i nc lude the f o l l o w i n g :
( i ) Dye p e n e t r a t i o n - which revea ls c racks a s small as 0.025 pm
t o the naked eye
431
( i i ) Flux t e s t i n g - magnetic m a t e r i a l s magnetised t o revea l
t he presence o f c racks when the sur face
i s spread w i t h magnet ic par t i c les /powder
( i i i ) E l e c t r i c a l res i s tance - by which two probes a r e moved over the
sur face w i t h an e l e c t r i c a l p.d. between
them; c racks a l t e r t he r e s i s t a n c e and
t h e r e f o r e the pass ing c u r r e n t
( i v ) Eddy c u r r e n t t e s t i n g - c u r r e n t s induced i n a m a t e r i a l (no t
n e c e s s a r i l y magnet ic) and c racks l oca ted
by a search c o i l
( v ) U l t r a s o n i c t e s t i n g - whereby c racks a r e l oca ted by the
r e f l e c t i o n o f u l t r a s o n i c waves propagated
i n t o the m a t e r i a l ; t h i s method i s
deve lop ing i n t o one o f the p r imary n d t
techniques
( v i ) Radiographic - by which X-rays pene t ra te the sur face
and l o c a t e hidden c racks .
I f one adds t o t h i s ca ta logue o f techniques the newer ones which have been
deve lopedexc lus ive ly f o r i n s p e c t i o n / s u r v e i l l a n c e purposes and add t o t h i s leak
d e t e c t i o n and c o r r o s i o n mon i to r i ng , a vas t a r r a y o f p o s s i b l e methods i s p re-
sented. An i n d i c a t i o n o f t he range o f these i s g i ven i n the f o l l o w i n g t a b l e :
TABLE 18.4 C l a s s i f i c a t i o n s o f I n t e g r i t y Su rve i l l ance Techniques
A c o u s t i c / u l t r a s o n i c - holography; s t r e s s wave emission; u l t r a s o n i c ;
E l e c t r i c a 1
Magnetic
- capac i tance; corona d ischarge; c o r r o s i o n probe;
- h y s t e r e s i s ; p a r t i c l e s ; p r i n t s
eddy c u r r e n t ; microwave; res i s tance
Radiography - beta- ray backsca t te r ; X-ray; gamma-ray; neut ron
Thermal
V isua l
- i n f r a - r e d ; su r face impedance; thermographic compounds
- borescopes; C.C.T.V.; dye-penetrant; holography pho to -e lec t ron emission;
438
TABLE 18.5 Some f u r t h e r d e s c r i p t i o n s
Beta-ray Backscat te r
Capacitance
Corrosion-probe
Dye Penetrant
Eddy Current
F lux Sensors
Magnetic Hys te res i s
Magnetic P a r t i c l e s
Magnetic P I i n t s
Microwave
Op t i ca l Aids
Photoe lec t ron Emission
Radiography
R e s i s t i v i t y
S t ress Wave Emission
U1 t r a son i cs
t o determine l a y e r th ickness ; assess the compos i t ion o f aggregates
c rack d e t e c t i o n , bond de fec ts i n non -meta l l i c m a t e r i a l s
m a t e r i a l loss by c o r r o s i o n measurement
c rack p e n e t r a t i o n us ing v i s i b l e o r f l u o r e s c e n t dyes
surface de fec ts de tec ted by e lec t romagnet ic i nduc t i on
uses r e s i d u a l o f induced magnet ic f l u x per t u r ba t ions
measures magnetic changes due t o the presence o f f a u l t s
s o l i d magnetic p a r t i c l e m i g r a t i o n i n the presence o f an a p p l i e d magnetic f i e l d l oca tes sur face de fec ts
a s t r i p p a b l e p a i n t f i l m under magnetic f i e l d e f f e c t s which i l l u s t r a t e s sur face de fec ts i n f e r romagne t i c mate r i a 1 s
i n f r a - r e d i nspec t i on o f non-conduct ing m a t e r i a l s
borescopes, f i b r e o p t i c s , CCTV
the d e t e c t i o n o f spontaneous e l e c t r o n emissions f rom p l a s t i c a l l y deformed sur faces
p e n e t r a t i o n t o show up d e f e c t i v e and bu rn t -ou t p a r t s
c rack o r bond t e s t i n g by e l e c t r i c a l res i s tance measurement
a method f o r l o c a t i n g t o p o s i t i o n and s e v e r i t y o f d e f e c t s i n metal sur faces and s t r u c t u r e s
which uses t ransmiss ion c h a r a c t e r i s t i c s o f u l t r a - h i g h frequency son ic waves t o l o c a t e de fec ts .
18.5.3 Contaminant Ana lys i s
Moving con tac t between the m e t a l l i c components o f any mechanical system i s
accompanied by wear, which r e s u l t s i n t h e genera t i on o f wear d e b r i c p a r t i c l e s .
I n a l u b r i c a t e d system these p a r t i c l e s a r e i n suspension i n the c i r c u l a t i n g
o i l . Under normal c o n d i t i o n s the r a t e o f wear i s low and p a r t i c l e s formed a r e
very smal l , The s i z e and r a t e o f genera t i on o f these p a r t i c l e s increase as the
r a t e o f wear increases. By i d e n t i f y i n g and measuring these m e t a l l i c p a r t i c l e s ,
the sur face f rom which t h e p a r t i c l e s were worn can be i d e n t i f i e d and the r a t e
of wear can be determined t o be normal o r abnormal.
439
Techniques f r e q u e n t l y used i n o i l c o n d i t i o n mon i to r i ng a re
( i ) Magnetic p l u g i nspec t i on
( i i ) Spec t romet r ic O i l Ana lys is Procedure (SOAP)
( i i i ) Ferrography
( i v ) P a r t i c l e Count ing ( f o r h y d r a u l i c f l u i d s )
( v ) Patch Tes t i ng
A t y p i c a l magnetic p lug , F ig .7 , incorpora tes a non- re tu rn va l ve so t h a t i t
can be i n s e r t e d i n t o a p i p e - l i n e and/or wi thdrawn w i t h o u t loss o f f l u i d . Debr is
which has been trapped by such a magnetic p lug can be measured w i t h a magnet-
ometer t o determine the amount t h a t has been c o l l e c t e d ; t h i s may then be
recorded t o determine the c o l l e c t i o n t rend . A t t he same t ime, examinat ion under
a s imp le microscope and comparison w i t h Debr is Recogn i t ion Drawings/Photographs
makes i t p o s s i b l e t o t e l l t he component f rom which the major amount o f wear
m a t e r i a l has a r i s e n , as shown i n Fig.9.
When spectroscopy was f i r s t in t roduced as a chemical a n a l y t i c a l ins t rument
100 years ago, i t brought about a r e v o l u t i o n i n chemis t ry . I t s advantages were
no t mere ly t h a t spectroscopy was a s e n s i t i v e de tec to r bu t t h a t the ins t rument
c o u l d d e t e c t and measure the q u a n t i t y o f an element p resent i n the sample i n -
dependent ly o f how the element was incorpora ted i n a compound. The f lame o r
spark o f the emiss ion o r abso rp t i on apparatus broke down the compounds and each
element d i sp layed i t s i n d i v i d u a l se t o f spectrum l i n e s , Fig.10. Nothing was
occluded, t h i s was the fundamental d i f f e r e n c e f rom a l l p r e v i o u s l y e x i s t i n g
methods of chemical a n a l y s i s .
When the advantages o f o i l a n a l y s i s were f i r s t r e a l i s e d a t t he t ime o f t he
i n t r o d u c t i o n o f d i e s e l locomotives t o the r a i l r o a d s , the emission spectrograph
was adapted t o o i l a n a l y s i s , t he theo ry be ing t h a t a r a p i d increase o f a
m e t a l l i c element i n the l u b r i c a t i n g o i l would imply t h a t a p a r t made o f t h a t
element was wearing r a p i d l y . A i r d i r t , assembly o r r e p a i r deb r i s , system wear
metals, c o r r o s i o n produc ts and coo lan t water i n h i b i t o r s a r e some of t he mat-
e r a i l s which may be detected.
E l emen t wavelength (Angstrom u n i t s )
A l umi n ium 3092
Copper 3247
Ch rom i um 3579
I r o n
Lead
Sod i um
3720
2833
589 0
T i n 2354
440
Fig.7 Magnetic p l u g ( ch ip d e t e c t o r )
VALVE OPEN- FLOW VALVE CLOSED
MAGN
O-RING SEALS
BAYONET PINS
Fig.8 Operat ing p r i n c i p l e o f the TEDECO/Muirhead V a c t r i c Magnetic Plug/ Chip Detec tor
B a l l Debr is - rounded ' rose -pe ta l ' r a d i a l l y s p l i t shape
(b) Track Debr is - rounded, su r face break up, c r i s s c ross scra tches
Rol l e r Debr is - (d) Gear Tooth Debr is - g e n e r a l l y c u r l e d and rec tangu la r , p a r a l l e l l i n e s su r face as sp lash o f across w i d t h so lde r Magnetic p l u g d e b r i s source r e c o g n i t i o n
i r r e g u l a r shape, g rey
441
-
30 -
-
20 -
-
10 -
40 1
I d C I
IB
0 L9-J-J
D
1 3800
WAVE- LENGTH (ANGSTRON UNITS 1
Fip.10 Typ ica l spec t roscop ic spectrum
TABLE 18.6 Element de tec ted and f requent source
Element Sources
Aluminium, S i l i c o n Dust and A i rbo rne d i r t
Borax, potassium, sodium Coolant i n h i b i t o r res idues
Calcium, Sodium S a l t water res idues
Chromium, cooper, i r on , l ead t i n , z i n c
Wear, c o r r o s i o n o r res idua l assembly d e b r i s
Barium, calc ium, magnesium, Engine o i l a d d i t i v e s phosphorus, z i n c
Some manufacturers i nco rpo ra te spec ia l elements i n t o d i f f e r e n t p a r t s o f an
engine t o a c t as t r a c e r s so t h a t t h e i r presence i n a sample p rov ides an un-
ambiguous i n d i c a t i o n o f t he source o f t r o u b l e .
SOAP has developed t o the p o i n t where the m a j o r i t y o f m i l i t a r y serv ices ,
r a i l r o a d s and a i r l i n e s opera te spec t rograph ic o i l a n a l y s i s as a s tandard pro-
cedure f o r d e t e c t i n g problem areas, schedule overhau ls and r o u t i n e o i l
mon i t o r ing .
44 2
I t might be thought t h a t wear p a r t i c l e s cou ld be examined by v iewing a
f i l t e r and measuring the number o f p a r t i c l e s .
o the r than h y d r a u l i c systems t h i s i s n o t poss ib le . I f o i l i s f i l t e r e d a mis-
ce l laneous mass o f p a r t i c l e s a re found.
one on the o t h e r so t h a t i t i s n o t poss ib le t o determine t h e i r c h a r a c t e r i s t i c s ,
s i z e d i s t r i b u t i o n e t c . I n f a c t , f i l t e r e d depos i t s g i v e the impression o f being
" j u s t d i r t " .
Un fo r tuna te l y f o r most cases
Large and small p a r t i c l e s a re p i l e d
COLLECTOR RECEPTACLE
01 L SAMPLE
Fig.11 Schematic diagram of 'Ferrograph' ( the s t reng th of the magnetic f i e l d i s g rea tes t a t the bottom o f the s l i d e . )
The mon i to r i ng o f wear p a r t i c l e s on the Ferrograph, Fig.11, on the o the r
hand, i s a technique i n which most th ings a re occluded. One o f i t s most v a l -
uable c h a r a c t e r i s t i c s stems from the f a c t t h a t i t does n o t see every p a r t i c l e
and, i n f a c t , ignores eve ry th ing except wear metal p a r t i c l e s i n the o i l . The
separat ion and a n a l y s i s o f wear p a r t i c l e s f o r s i z e d i s t r i b u t i o n , type o f meta l ,
phys i ca l shape, c r y s t a l s t r u c t u r e e t c . i s o f t e n a much more s e n s i t i v e i n d i c a t o r
of the wear s i t u a t i o n than a d i r e c t v iew o f the worn surface. C h a r a c t e r i s t i c
deb r i s f rom modes o f f a i l u r e have been discussed by Scot t i n a prev ious chapter
on wear.
443
A l l wear deb r i s ana lys i s techniques a r e p a r t i c l e s i z e dependent and any
pa r t i cu la r technique i s s e n s i t i v e o n l y t o a s p e c i f i c range o f p a r t i c l e s izes.
Fig.12 summarises the e f f i c i e n c y o f these techniques as a f u n c t i o n o f p a r t i c l e
size [g] .
ISOA Ferrography Magnetic plug
0 10 100 1000 Particle size (microns)
Fig.12 E f f i c i e n c y o f va r ious sensors as a f u n c t i o n o f p a r t i c l e s i z e
The clean1 iness requirements o f h y d r a u l i c systems have become more c r i t i c a l
in recent years. Systems employing e l e c t r o h y d r a u l i c servo va lves i n numer i ca l l y
con t ro l l ed machine t o o l s , h i g h pressure systems where pump o r va l ve c learances
may be as smal l as 0.5 micrometre a re p a r t i c u l a r examples. Analys ing p a r t i c -
u la te contaminat ion f o r these cases a r e u s u a l l y c a r r i e d ou t by p a r t i c l e count
methods u t i l i s i n g e i t h e r microscopes o r automat ic counters .
Microscope coun t ing methods (ASTM F312, F 3 1 3 , ARP 598A, I P 2751, are pro-
cedures which a re considered unsu i tab le f o r p a r t i c l e s smal ler than 5 microns.
These methods s i z e and count s t a t i s t i c a l l y p a r t i c l e s r e t a i n e d on a membrane
surface a f t e r the f l u i d sample has been f i l t e r e d .
time consuming, t he re can be v a r i a t i o n when d i f f e r e n t people count the same
Apar t f rom coun t ing being very
444
s l i de . Automatic counters a r e however now a v a i l a b l e u t i l i s i n g scanning com-
puters and TV screens.
Other instruments us ing the p r i n c i p l e o f l i g h t i n t e r c e p t i o n count p a r t i c l e s
suspended i n a l i q u i d f rom e i t h e r a sample b o t t l e o r d i r e c t f rom a h y d r a u l i c
system. As these instruments operate on the blockage o f l i g h t p r i n c i p l e , they
measure the p r o j e c t e d area o f a p a r t i c l e and then record the diameter o f a
c i r c l e o f equ iva len t area.
s o l i d p a r t i c l e s and a i r bubbles.
Un fo r tuna te l y they cannot d i f f e r e n t i a t e between
The t y p i c a l co lou r o f contaminat ion i n any g i ven h y d r a u l i c system remains
f a i r l y constant . The darkness o f t he p a r t i c u l a t e d i s c o l o u r a t i o n o f a f i l t e r i s
t he re fo re a rough i n d i c a t i o n o f the c l e a n l i n e s s o f the t e s t f l u i d . Th is Patch
Test procedure i s however o n l y genera l l y a p p l i c a b l e t o gross l e v e l s o f contam-
i na t ion.
18.5.4 Trends Analys is
This i s l i t t l e more than the data logger w i t h a memory and t rend ana lys i s
c a p a b i l i t y . The most elementary form i s the handwr i t t en watch l o g which i s
inspected and analysed. As in p r a c t i c e , r e a l use i s o n l y made o f the l o g a f t e r
a f a i l u r e has occurred, the modern tendency i s t o use sensors t o i npu t t o a
general data system which scans the moni tor p o i n t s and produces a regu la r p r i n t -
out . I n more advanced form, a computer i s capable o f combining many o f the
inputs and e s t a b l i s h i n g t rends which, by mathematical model l ing, can be assoc-
i a ted w i t h the changes which would occur f o l l o w i n g s p e c i f i c de fec ts and con-
sequent ly when the system i s i n te r roga ted , i t can s t a t e ( through a l o g i c
process) the p l a n t and component which i s d e t e r i o r a t i n g ; w i t h preset -1 i m i t s
such a system w i l l even produce a statement o f the 'unexpi red l i f e ' remaining.
Such advanced t rends moni tors a r e more commonly known as 'performance'
monitors.
18.6 DETERIORATION LIMITS
Most c l a s s i f i c a t i o n s o c i e t i e s and bodies o f a s i m i l a r na tu re have estab-
l i s h e d s p e c i f i c a t i o n s and codes o f p r a c t i c e which d e f i n e the d e t e r i o r a t i o n
l i m i t s which should be a l lowed be fo re c o r r e c t i v e a c t i o n i s taken. T y p i c a l l y ,
l i m i t s which have been es tab l i shed f o r d e t e r i o r a t i o n i d e n t i f i e d by v i b r a t i o n
methods include:
V D I Code o f P r a c t i c e 1056 (October 1964)
D I N 45665 (November 1967)
B S 4675: 1971
IS0 2372/3
I t does seem t h a t f o r v i b r a t i o n l i m i t s , most people i n general e r r on the
445
side o f cons iderab le c a u t i o n a l though i n p r a c t i c e , t h e l i m i t must depend upon a
large range o f environmental f a c t o r s s p e c i f i c t o each i n s t a l l a t i o n .
I t i s on l y by exper ience, bo th persona l and t h a t o f o t h e r t h a t a c t i v e
decis ion l i m i t s can be reached. In the whole f i e l d o f c o n d i t i o n mon i to r i ng ,
developments a r e o c c u r r i n g a t such a r a p i d speed t h a t o n l y an o r g a n i s a t i o n such
as the UK Mechanical Hea l th Mon i to r i ng Group through i t s regu la r seminar/
symposia and courses i s i t p o s s i b l e t o acqu i re a l l t he i n fo rma t ion - known t o
consul tants - needed t o implement and manage an on -cond i t i on maintenance system.
REFERENCES
1 Collacott,R.A. 'Mechanical Fau Chapman & H a l l , London 1977.
2 Venton ,A. D. F. and Harvey ,B. F. system des ign ' Proc. 1.Mech.E.
t D iagnos is and Cond i t i on M o n i t o r i n p '
R e l i a b i l i t y assessment i n machinery 973.
3 Bridges,D.C. 'The a p p l i c a t i o n o f r e l i a b i l i t y t o the design o f s h i p ' s machinery' Trans. 1.Mar.E. 86, P a r t 6 1974.
4 Davies,A.E. ' P r i n c i p l e s and p r a c t i c e o f a i r c r a f t powerplant maintenance' Trans. 1.Mar.E. 85 Par t 6 1973.
5 Collacott,R.A. 'Data Sources f o r Re1 i a b i l i t y S t a t i s t i c s ' UKM P u b l i c a t i o n s L td . , 92 London Road, L e i c e s t e r LE2 OQR 1976.
6 Wilkinson,H.C. and Ki1bourn.D.F. 'The des ign o f s h i p ' s machinery i n s t a l l a t i o n s ' Sh ipp ing World and Sh ipbu i l de r , August 1971.
7 Mathieson,Tor-Chr. ' R e l i a b i l i t y eng inee r ing i n sh ip machinery p l a n t design' Report IF/R.12 U n i v e r s i t y o f Trondheim,N.I.T. 1973.
8 Col l a c o t t ,R.A. ' V i b r a t i o n Mon i to r i ng and D iagnos is ' George Godwin L i m i t e d 1978.
9 Pocock,G. ' I n t r o d u c t i o n t o Fer rography ' Symposium on Ferrography, B r i t i s h I n s t . Non D e s t r u c t i v e Tes t ing , London 1979.
446
I!) THE TRIBOLOGY OF METAL CUTTING
E. M. TRENT
Department o f I n d u s t r i a l Me ta l l u rgy , U n i v e r s i t y o f Birmingham, Birmingham 815 2TT
19.1 INTRODUCTION
The e f f i c i e n c y o f metal c u t t i n g opera t i ons i s very l a r g e l y c o n t r o l l e d by the
behaviour o f the work m a t e r i a l and the t o o l m a t e r i a l a t t he i n t e r f a c e between
them near the c u t t i n g edge o f the t o o l . Metal c u t t i n q i s c a r r i e d o u t on all
the major c lasses o f meta ls and a l l o y s produced commercial ly. The machining
operat ions such as bor ing, d r i l l i n g , tapping, t u r n i n q , m i l l i n g , p lan ing and
sawing a r e very v a r i e d i n character . Objects as small as instrument p a r t s o r
as l a r g e as i n d u s t r i a l b o i l e r s a re machined t o remove excess m a t e r i a l and t o
generate the necessary shapes w i t h the requ i red p r e c i s i o n , b u t , however va r ied ,
a l l machining operat ions have c e r t a i n fea tu res i n common.
19.2 METAL CUTTING PHENOMENA
A l l machining opera t i ons i nvo l ve the use o f one o r more t o o l s o f a wedge
shape w i t h a c u t t i n g edge, which remove a t h i n l a y e r f rom the sur face o f a l a r g e r body as shown i n Fig.1. The t o o l s a re always moved asymmetr ica l ly w i t h
respect t o the wedge angle so t h a t t he l a y e r removed - the "chip" - bears
against and moves over one sur face o f the wedge, known as the "rake face" o f
the t o o l . The t o o l i s so shaped t h a t the f r e s h l y c u t metal sur face does n o t rub
against the o the r face o f the wedge - the "c learance face" o r " f l ank " o f the
t o o l . (F ig .1) .
The laye r o f metal removed i s f i r s t p l a s t i c a l l y deformed by a shear ing
a c t i o n , roughly a long a p lane A-B i n F ig. l . , extending f rom the t o o l edge A t o
the p o s i t i o n B where the c h i p separates f rom the undeformed work m a t e r i a l .
invo lves severe shear s t r a i n - u s u a l l y a n a t u r a l s t r a i n o f a t l e a s t 2 bu t o f t e n
very much h ighe r as can be seen f rom Fig.2. which shows a s e c t i o n through the
forming c h i p du r ing the c u t t i n g o f copper. The shear plane i s under h igh com-
p ress i ve s t ress and the t r i - a x i a l s t ress c o n d i t i o n i s such t h a t w i t h most
d u c t i l e meta ls and a l l o y s the h i g h shear s t r a i n can be susta ined w i thou t f r a c -
t u r e so t h a t a cont inuous c h i p i s formed, as w i t h the example o f copper. With
metals and a l l o y s o f low d u c t i l i t y , however, o r under c u t t i n g cond i t i ons where
the compressive s t ress on the shear p lane i s low, the c h i p may be broken i n t o
This
447
CLEARANCE FACE TOOL
Fig.1 Features o f Metal C u t t i n g
Fig.2 Sec t ion through c h i p and bar o f h i g h p u r i t y copper a f t e r "quick-stop". Machined a t 122111 m i " - ' .
448
small fragments. A d iscont inuous c h i p i s formed, f o r example, when c u t t i n g
cas t i r o n o r a f r e e c u t t i n g brass.
The c h i p moves across the rake su r face o f t h e t o o l away f rom the c u t t i n g edge
and breaks con tac t w i t h the t o o l su r face a t some p o s i t i o n C , u s u a l l y r a t h e r i l l-
def ined. Always the con tac t l eng th A-C i s g r e a t e r than the o r i g i n a l th ickness
o f the l a y e r be ing removed ( t l i n Fig.1.) - the "feed" - A-C i s o f t e n 5 o r even
10 t imes the feed tl.
c u t t i n g edge and u s u a l l y f o r a sho r t d i s tance down the c learance face o r f l a n k
o f the t o o l . The l e n g t h o f con tac t i n t h i s reg ion A-D i s u s u a l l y much s h o r t e r
than on the rake face A-C. The c learance ang le (Fig.1.) which i s u s u a l l y about
6" t o 15" r e s t r i c t s the l e n g t h of con tac t on t h i s su r face , bu t du r ing c u t t i n g
the most common fo rm o f wear i s one i n which a sur face i s worn on the t o o l
n e a r l y p a r a l l e l t o the d i r e c t i o n o f c u t t i n g - the " f l a n k wear land" i n Fig.1.
Th is worn sur face , and hence the l eng th o f con tac t A-D, tends t o inc rease w i t h
c u t t i n g t ime bu t , t o avo id t o t a l t o o l f a i l u r e , t he f l a n k wear should no t be
a l lowed t o become t o o l a r g e be fo re the t o o l i s reground o r replaced.
The work and t o o l m a t e r i a l s a r e i n con tac t a t t he
19.3 CONDITIONS AT THE TOOL-WORK INTERFACE
19.3.1 Tool Forces and Stresses
The fo rces a c t i n g on the t o o l a r e ( 1 ) t h a t requ i red t o shear the work mater ia l
over the area o f t he shear p lane A-8 (Fig.1.) and ( 2 ) t h a t requ i red t o move the
c h i p across the t o o l rake face con tac t reg ion A-C. The f o r c e t o move t h e work
m a t e r i a l over the f l a n k A-D i s smal l compared w i t h the o t h e r fo rces and can be
neg lec ted i n a f i r s t approx imat ion . Tool dynamometers have been developed and
used t o measure the fo rces a c t i n g on the t o o l i n two d i r e c t i o n s - i n the
d i r e c t i o n o f c u t t i n g Fc and i n the d i r e c t i o n o f t he feed F f . I n mast c u t t i n g
opera t i ons the fo rces a c t i n g on the t o o l va ry f rom a few k i lograms t o a few
hundred k i lograms. The c u t t i n g f o r c e Fc a c t s n e a r l y normal t o the rake face o f
the t o o l and e x e r t s a l a r g e l y compressive s t r e s s on t h i s sur face . The feed
fo rce F f i s almost always sma l le r than the c u t t i n g f o r c e ( t y p i c a l l y 40-60% o f
the c u t t i n g fo rce ) and e x e r t s a shear ing s t r e s s on the t o o l sur face .
While the fo rces a c t i n g on t h e t o o l can be measured w i t h accuracy, even the
mean va lue o f the s t r e s s a c t i n g on the con tac t area between work m a t e r i a l and
t o o l i s d i f f i c u l t t o de termine because the area o f c o n t a c t i s d i f f i c u l t o r
impossible t o measure e x a c t l y .
con tac t area and i t i s n o t easy t o determine t h e s t r e s s d i s t r i b u t i o n on t h i s
area. The general cha rac te r o f t h e s t r e s s d i s t r i b u t i o n on t h e rake face o f a
c u t t i n g t o o l , however, i s now g e n e r a l l y accepted t o be t h a t suggested by Zorev
111 and shown d iagrammat ica l l y i n Fig.3. The compressive s t r e s s a c t i n g on the
rake face i s a t a maximum a t o r c l o s e t o t h e c u t t i n g edge and d imin ishes t o zero
a t the end o f t he con tac t area.
The s t resses a r e n o t even ly d i s t r i b u t e d on the
The maximum compressive s t r e s s near the edge
449
(COMPRESSIVE STRESS
ul CHIP * SHEAR STRESS
' b-\ (DISTANCE 'FROM 'CUT , , /
Fig .3 S t ress d i s t r ' 2,000
1,500 Z I - 0 0 4-
5 1,000 CJl C
U 0
0, ' 500
.- c
I.?
0
b u t i o n i n c u t t i n g t o o l ( a f t e r Zorev)
I
A,
50 100 150 200
Cutting speed rn min?
Fig.4 Forces a c t i n g on a t o o l as f u n c t i o n o f c u t t i n g speed. Feed 0.25mm/rev. Depth o f c u t 1.25mm.
450
i s o f t e n tw ice t h e mean s t r e s s on the area o f con tac t . The shear s t r e s s on the
rake face i s more u n i f o r m l y d i s t r i b u t e d as shown i n Fig.3.
The va lues o f t he compressive s t r e s s near the edge a r e h i g h r e l a t i v e t o the
y i e l d s t r e s s o f t he m a t e r i a l be ing c u t - the work m a t e r i a l . For example, i n
c u t t i n g s t e e l the compressive s t r e s s near the edge may be o f t h e o rde r o f
1500 N mm . A major requirement o f a s a t i s f a c t o r y t o o l m a t e r i a l i s thus h i g h
y i e l d s t r e s s i n compression, and the V ickers o r Rockwell hardness va lues a r e
u s u a l l y taken as an i n d i c a t i o n o f t h i s p roper t y . The most commonly used c u t t i n g
t o o l m a t e r i a l s a re hardened h i g h speed s t e e l and cemented carb ide . The minimum
hardness o f c u t t i n g t o o l m a t e r i a l s i n common use i s 750 HV (62 Rockwell C).
- 2
Although the re a r e few r e l i a b l e da ta f o r t he s t resses on t h e con tac t area o f
t o o l s i n r e a l c u t t i n g opera t i ons , i t i s c e r t a i n t h a t they a r e r e l a t e d t o the
y i e l d s t r e s s o f t he work m a t e r i a l . Approximate values f o r t he mean s t r e s s
a c t i n g normal t o the rake face o f a t u r n i n g t o o l under a standard se t o f c u t t i n g
c o n d i t i o n s a r e shown i n Table 19.1 f o r d i f f e r e n t work m a t e r i a l s .
TABLE 19.1 Mean compressive s t r e s s on con tac t area
Work m a t e r i a l Compressive St ress -2
N mm
I ron 340 Steel (medium carbon) 770 T i taniurn 570 Copper 310 70/30 brass 420 Lead 14
For the c u t t i n g o f m a t e r i a l s o f very h igh y i e l d s t reng th , p a r t i c u l a r l y heat
t r e a t e d s t e e l s and n i c k e l base a l l o y s , t he usual s t e e l and cemented ca rb ide
t o o l s may be inadequate because t h e s t r e s s imposed by the work m a t e r i a l i s h i g h
enough t o deform the c u t t i n g t o o l edge even a t very low c u t t i n g speed where the
t o o l edge temperature i s low. I t i s g e n e r a l l y cons idered i n a machine shop
t h a t h igh speed s t e e l t o o l s cannot be used t o c u t s t e e l s w i t h hardness h ighe r
than 350 HV (36 Rc) and t h a t the c u t t i n g o f s t e e l w i t h hardness over 550 HV
(53 Rc) becomes very d i f f i c u l t even w i t h cemented ca rb ide t o o l s .
i n i n g o f f u l l y hardened s t e e l i t has been more usual t o r e s o r t t o g r i n d i n g
us ing s i l i c o n ca rb ide , aluminium ox ide o r bonded diamond wheels, o r t o shape
by e l e c t r o d ischarge machining (EDM) o r e l e c t r o chemical machining (ECM).
Recent ly the i n t r o d u c t i o n o f new c u t t i n g t o o l m a t e r i a l s w i t h s t i l l h ighe r y i e l d
s t reng th - i n c l u d i n g compacted p o l y c r y s t a l l i n e diamond and cub ic boron n i t r i d e
t o o l s - has made the c u t t i n g o f f u l l y hardened s t e e l s , h ighe r s t r e n g t h n i c k e l -
based a l l o y s , and o t h e r very hard m a t e r i a l s , a more f e a s i b l e p r o p o s i t i o n f o r
i n d u s t r i a l shaping opera t i ons .
For the mach-
451
19.3.2 C u t t i n g Speed
One o f the most impor tan t parameters i n metal c u t t i n g i s t h e v e l o c i t y a t
which the work m a t e r i a l passes the c u t t i n g edge - the c u t t i n g speed. Th is
var ies g r e a t l y i n i n d u s t r i a l ope ra t i ons f rom almost zero, f o r example near the
centre o f a d r i l l , t o 300 m min-' o r even h ighe r .
research workers measuring t o o l f o r c e s found t h a t , i n genera l , these fo rces do
not inc rease as t h e c u t t i n g speed i s ra i sed . I n many cases the fo rces decrease,
p a r t i c u l a r l y i n the speed range up t o 65 m min
This has been shown t o be t r u e f o r a wide range o f work m a t e r i a l s and c u t t i n g
cond i t i ons . The fo rces decrease ma in l y because the area o f con tac t between t o o l
and work decreases as c u t t i n g speed i s ra i sed . A l though the re have been no very
d e t a i l e d s tud ies o f t he s t r e s s a c t i n g on t h e t o o l su r face as a f u n c t i o n o f
c u t t i n g speed, the re i s no evidence t o i n d i c a t e t h a t t he s t r e s s a c t i n g on the
t o o l i s r a i s e d as c u t t i n g speed i s increased.
Rather t o t h e i r s u r p r i s e
- 1 as shown f o r example i n Fig.4.
C u t t i n g speed i s o f p a r t i c u l a r importance i n r e l a t i o n t o the economics o f
machining. The c o s t o f machining opera t i ons i s reduced by i nc reas ing the r a t e
of metal removal, and t h e main i n c e n t i v e t o the development i n machining i n the
l a s t hundred years has been the reduc t i on o f t he very h i g h cos ts by the use o f
new machines and t o o l s capable o f machining a t increased ra tes . I n the c u t t i n g
o f h i g h m e l t i n g p o i n t meta ls and a l l o y s the l i f e o f the c u t t i n g t o o l becomes
p rog ress i ve l y s h o r t e r as the c u t t i n g speed i s r a i s e d u n t i l t he cos t o f r e p l a c i n q
worn o u t t o o l s more than outweighs the advantages o f h ighe r speed. I t has been
the a b i l i t y o f t he c u t t i n g t o o l t o wi ths tand the c o n d i t i o n s a t the t o o l edge
which has l i m i t e d t h e r a t e o f machining o f s t e e l and c a s t i r o n . The development
and commercial use f i r s t of h i g h speed s t e e l s and then o f cemented ca rb ides has
enabled c u t t i n g speeds t o be r a i s e d by a f a c t o r o f about 20 t imes compared w i t h
carbon s t e e l t o o l s and the re a r e s t i l l many opera t i ons i n which t o o l l i f e i s
the f a c t o r l i m i t i n g the r a t e o f metal removal.
In genera l as t h e c u t t i n g speed i s r a i s e d n e i t h e r the fo rces a c t i n g on the
too l nor the s t resses on t he area o f con tac t a r e increased. The energy expended
i n metal c u t t i n g , however, increases approx imate ly i n p r o p o r t i o n t o the c u t t i n g
speed, i f o t h e r c o n d i t i o n s remain cons tan t . Th i s energy i s conve r ted i n t o heat
near the c u t t i n g edge, and r a i s e s the temperature o f t he t o o l , reduc ing i t s
y i e l d s t r e s s and inc reas ing the r a t e o f t o o l wear. I t i s t h i s r i s e i n temper-
a t u r e which l i m i t s t h e a b i l i t y o f t h e t o o l s t o w i ths tand inc reas ing c u t t i n g
speed. The genera t i on o f temperatures i n metal c u t t i n g must now be considered.
19.3.3 Heat i n Metal C u t t i n g
In metal c u t t i n g energy i s expended i n t o two main reg ions (1) a long the
shear p lane A-B (Fig.1.) where the work m a t e r i a l i s sheared t o form the ch ip ,
and (2) a t the rake surface o f t he t o o l where the c h i p i s moved across the
contact area.
452
The energy expended i n shear ing the work m a t e r i a l t o fo rm the ch p main ly
r e s u l t s i n r a i s i n g the temperature o f t he c h i p and almost a l l o f t h s heat i s
c a r r i e d o u t o f the system when the c h i p breaks c o n t a c t w i t h the too . Since any
one element o f t he c h i p i s i n con tac t w i t h the t o o l f o r o n l y a very sho r t t ime
- t y p i c a l l y a few m i l l i - s e c o n d s - o n l y a smal l p r o p o r t i o n of t h i s heat c o u l d be
conducted i n t o the t o o l under the most favourab le c o n d i t i o n s . I t i s p robab le ,
as w i l l be shown, t h a t a l l t he heat i n the body o f t he c h i p i s c a r r i e d ou t o f
the system i n most cases. A smal l p r o p o r t i o n o f t he heat generated on the
shear p lane i s conducted back i n t o the body o f t he workpiece. The energy ex-
pended on the shear p lane i s no rma l l y t he lar ’gest p a r t o f t he t o t a l energy o f
c u t t i n g - o f t e n o f the o rde r o f 75 t o 80% o f t he t o t a l .
c h i p i s o f t e n r a i s e d t o 200-350°C when c u t t i n g s t e e l o r o t h e r h i g h m e l t i n g
p o i n t m a t e r i a l s .
The temperature o f the
I t i s the smal le r p o r t i o n o f t he t o t a l energy o f c u t t i n g - t h a t expended i n
moving the c h i p over the t o o l - which i s respons ib le f o r the genera t ion o f h igh
temperatures a t t he too l /work i n t e r f a c e and the c o n d i t i o n s a t t h i s i n t e r f a c e
must t h e r e f o r e be considered. The h i g h compressive s t r e s s normal t o the t o o l
rake sur face has a l ready been emphasised. The mean s t r e s s on the con tac t area
i s always much h ighe r than the s t resses no rma l l y encountered a t moving i n t e r -
faces i n eng ineer ing systems. The very h i g h s t resses a lone would r e s u l t i n the
area o f r e a l con tac t between the two sur faces be ing a much h ighe r p o r p o r t i o n o f
the apparent con tac t a rea than is usual f o r s l i d i n g sur faces , and would tend t o
promote se izure . Other f a c t o r s a r e a l s o favourab le t o se i zu re . The t o o l i s
c o n t i n u a l l y c u t t i n g i n t o c lean meta l , be ing brought i n t o con tac t w i t h sur faces
f r e e f rom ox ide o r o t h e r l a y e r s which i n h i b i t t he ex tens ion o f con tac t areas i n
many s l i d i n g s i t u a t i o n s . The c lean work m a t e r i a l f l ows con t inuous ly over the
t o o l sur face i n one d i r e c t i o n , sweeping away ox ide o r o t h e r l a y e r s i n i t i a l l y
p resent on the t o o l , which have l i t t l e chance t o re - fo rm. R e l a t i v e l y h i g h
temperatures generated a t t he i n t e r f a c e inc rease atomic a c t i v i t y and t h i s a l s o
tends t o promote se izure .
19.3.4 Seizure o f t he Tool/Work I n t e r f a c e
When these c o n d i t i o n s a r e considered, i t i s n o t s u r p r i s i n g t o f i n d t h a t
se i zu re between the t o o l and work sur faces i s commonly observed on c u t t i n g
t o o l s [Z]. The process o f f r i c t i o n we ld ing i s o f t e n c a r r i e d o u t under con-
d i t i o n s l e s s severe than those encountered i n metal c u t t i n g . For example, sound
j o i n t s can be made by f r i c t i o n we ld ing a t p ressures o f 75 N mm-’ and pe r iphe ra l
speeds o f 50 m min-’, whereas i n metal c u t t i n g s t resses on the con tac t area o f
750 N mm-’ occur and h i g h e r speeds a r e o f t e n used. That s e i z u r e occurs a t the
too l /work i n t e r f a c e i s conf i rmed by numerous me ta l l og raph ic observa t ions , o f
which th ree examples w i l l be g i ven here.
453
Fig.5. shows a p o l i s h e d and e tched s e c t i o n th rough the rake face o f a
-1 cemented carb ide t o o l and adher ing work m a t e r i a l a f t e r c u t t i n g s t e e l a t 100 m
min . I t shows t h e work m a t e r i a l i n c o n t a c t w i t h the t o o l sur face n o t j u s t a t
the tops o f the a s p e r i t i e s b u t a t a l l t h e h i l l s and v a l l e y s o f t he su r face on a
micro scale. To say t h a t these sur faces a r e se ized toge the r means t h a t s l i d i n g
as normally conceived, w i t h t h e two sur faces separated by a f l u i d f i l m or i n
contact on l y a t t he a s p e r i t i e s , i s n o t poss ib le .
i c a l l y i n te r l ocked and/or m e t a l l u r g i c a l l y bonded over t h e whole o r a l a r g e p a r t
of the i n te r face . That m e t a l l u r g i c a l bonding i s o f t e n i nvo l ved i s shown by
examples, such as those shown i n Figs.6 and 7, i n which, a f t e r s topp ing c u t t i n g
by p rope l l i ng the t o o l r a p i d l y f rom the c u t t i n g p o s i t i o n , t he c h i p remains
f i rm ly adherent t o t h e t o o l o r separates f rom the t o o l a t some p o s i t i o n remote
from the i n te r face , l e a v i n g a l a y e r o f work m a t e r i a l welded t o the t o o l surface.
The two sur faces a r e mechan-
Fig.5 Sec t ion th rough c a r b i d e t o o l w i t h adherent work m a t e r i a l (wh i te ) a f t e r c u t t i n g s t e e l . Shows se i zu re c o n d i t i o n s a t i n t e r f a c e .
Seizure i s no rma l l y thought o f as a c o n d i t i o n where a mechanism ceases t o
func t ion , as when a bear ing se izes , bu t i n meta l c u t t i n g the se ized area i s
small, t he re i s adequate power t o con t inue c u t t i n g and t h e t o o l i s s
t o r e s i s t t he s t resses imposed by se i zu re cond i t i ons . Movement cont
shear in the work m a t e r i a l i n a reg ion ad jacen t t o the t o o l sur face .
r i s e t o two main s o r t s o f c o n d i t i o n s near the i n t e r f a c e . The f i r s t
work m a t e r i a l se ized t o the t o o l a r e severe ly work hardened and shea
rong enough
nues by
Th is g i ves
ayers o f
i s then
t rans fe r red t o t h e nex t l aye rs . I n t h i s way a body o f "dead meta l " may be
b u i l t up, adherent t o t h e t o o l m a t e r i a l , which p e r s i s t s f o r l ong pe r iods o f
cu t t i ng .
The b u i l t - u p edge reaches a s t a b l e s t a t e and the s i z e and shape depending on
the work m a t e r i a l and the c u t t i n g cond i t i ons . I t i s a dynamic s t r u c t u r e w i t h
Fig.8 shows an example o f t h i s f e a t u r e , known as a " b u i l t - u p edge".
454
Fig.6 Sec t ion through h i g h speed s t e e l t o o l and adherent c h i p ( a u s t e n i t i c s t a i n l e s s s t e e l ) a f t e r machining a t 3 0 m min-1.
Fig.7 Sec t ion throuqh h i g h speed s t e e l t o o l and adherent c h i p fragment a f t e r c u t t i n g low carbon s t e e l a t 107111 min- ' .
fragments be ing con t inuous ly added and broken away.
t o c h i p fo rma t ion may take p lace a t a d i s tance o f 300 urn o r more from the t o o l
sur face .
The shear ing a c t i o n l ead ing
A b u i l t - u p edge i s o f t e n formed when c u t t i n g a l l o y s c o n t a i n i n g more than one
phase, such a s s t e e l , cas t i r o n and a lpha-beta brass [ 3 1 . With such m a t e r i a l s
a b u i l t - u p edge occurs when c u t t i n g a t r e l a t i v e l y low speeds, bu t disappears
455
-1 Fig.8 B u i l t - u p edge formed d u r i n g c u t t i n g low carbon s t e e l a t 15m min .
when speed o r feed a r e ra i sed . When c u t t i n g pure meta ls and s o l i d s o l u t i o n s a t
a lmost any speed, and when c u t t i n g two phase a l l o y s i n the h ighe r range o f
speeds, se i zu re c o n d i t i o n s a r e observed t o e x i s t a t most o f t he i n t e r f a c e , bu t
the b u i l t - u p edge i s absent. Movement o f the work m a t e r i a l takes p lace by
shear concent ra ted i n t o a very t h i n l a y e r ad jacent t o t h e t o o l sur face u s u a l l y
o f t h e o rde r o f 25-50 um i n th ickness . An example of such a l a y e r i s seen i n
Fig.2. f o r t he c u t t i n g o f copper and a l a y e r when c u t t i n g a low carbon s t e e l i s
seen a t h i g h m a g n i f i c a t i o n i n Fig.9. I n t h i s l a y e r the work m a t e r i a l behaves
more l i k e a very v iscous l i q u i d than a normal metal and t h e l a y e r i s termed a
"flow-zone". I n the flow-zone the r a t e o f shear s t r a i n i s ex t remely h i g h - l o 4 t o l o 5 per second - and the amount o f s t r a i n i s so extreme t h a t o r i g i n a l
s t r u c t u r a l f ea tu res (such as p e a r l i t e and f e r r i t e i n s t e e l ) a re comple te ly
destroyed. There i s good evidence t h a t , w i t h i n the f low-zone, dynamic recovery
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and/or r e c r y s t a l l i s a t i o n a r e t a k i n g p lace and the behaviour o f t he m a t e r i a l i s
a k i n t o i t s behaviour i n h o t work ing processes.
Fig.9 Flow zone a t under su r face of c h i p , ad jacen t t o rake face of t o o l , formed du r ing c u t t i n g low carbon s t e e l a t 63m min- ' .
19.3.5 C u t t i n g Tool Temperatures
The energy expended i n deforming the work m a t e r i a l i n the flow-zone pe r u n i t
volume o f metal deformed, i s much h ighe r than on the shear p lane and the temp-
e ra tu res generated i n the f low-zone a r e t h e r e f o r e h ighe r . The con tac t between
the f low-zone and the t o o l i s very good. As has been demonstrated t h e r e i s
cont inuous m e t a l l i c con tac t i n many cases and heat f l ows r e a d i l y across the
boundary t o heat t he t o o l . I t i s t he heat generated i n the f low-zone a t t he
i n t e r f a c e between t o o l and work m a t e r i a l which i s t he main heat source r a i s i n g
the temperature o f the t o o l and c r e a t i n g the c o n d i t i o n s under which c u t t i n g
t o o l s a r e worn.
The reg ions o f t he t o o l s which a r e heated t o h i g h temperature a re ve ry
l o c a l i s e d and w i t h i n these reg ions temperature g r a d i e n t s a r e very steep, b u t i t
i s p o s s i b l e t o study the temperature d i s t r i b u t i o n i n some d e t a i l fo r c e r t a i n
cond i t i ons o f c u t t i n g by obse rva t i on o f t he changes i n s t r u c t u r e o r hardness o f
s t e e l t o o l s i n those p a r t s o f t h e t o o l s heated by c u t t i n g a c t i o n above t h e i r
tempering temperature. Fig.10 shows, f o r example, t he temperature g r a d i e n t s i n
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a h i g h speed s t e e l t o o l used t o c u t a l o w carbon s t e e l a t a speed o f 76m m i n - l
a t a f e e d o f 0.25 mm p e r r e v . T h i s i s c h a r a c t e r i s t i c o f t h e t y p e o f tempera ture
d i s t r i b u t i o n found t o o c c u r i n t o o l s used t o c u t s t e e l under c o n d i t i o n s where a
f low-zone o c c u r s a t t h e i n t e r f a c e . F ig .10 shows t h a t t h e tempera ture near t h e
t o o l edge was r e l a t i v e l y low - i n t h i s case under 650°C - b u t t h e r e i s a h i g h
tempera ture r e g i o n j u s t o v e r l m m f r o m t h e edge i n t h e d i r e c t i o n o f c h i p f l o w ,
where, i n t h i s example, t h e tempera ture a t t h e i n t e r f a c e was o v e r 800°C. I t i s
f o r t u n a t e t h a t i n c u t t i n g s t e e l and many o t h e r a l l o y s a t h i g h speed t h e r e g i o n
o f h i g h e s t t e m p e r a t u r e i s a t a d i s t a n c e f r o m t h e edge where t h e compress ive
s t r e s s on t h e t o o l i s a maximum ( F i g . 3 ) .
As t h e c u t t i n g speed i s r a i s e d t h e maximum t e m p e r a t u r e on t h e r a k e f a c e o f
t h e tool i n c r e a s e s r a p i d l y , w h i l e t h e tempera ture n e a r t h e edge i s i n c r e a s e d
more s l o w l y . The y i e l d s t r e s s o f t h e t o o l m a t e r i a l decreases w i t h r i s i n g tem-
p e r a t u r e and, as c u t t i n g speed i s r a i s e d , t h e tempera ture a t t h e edge may reach
a v a l u e where t h e y i e l d s t r e s s o f t h e t o o l i s reduced below t h e compress ive
s t r e s s e x e r t e d by t h e work m a t e r i a l . The t o o l edge i s then p l a s t i c a l l y deformed
and t h i s l e a d s t o a r a p i d r i s e i n t h e r a t e o f h e a t g e n e r a t i o n a t t h e c u t t i n g
edge. The t o o l t h e n f a i l s c a t a s t r o p h i c a l l y u s u a l l y w i t h i n a few seconds. T h i s
i s t h e main mechanism w h i c h s e t s t h e upper l i m i t t o t h e r a t e o f meta l removal
w h i c h can be a c h i e v e d w i t h h i g h speed s t e e l t o o l s (and, a t a h i g h e r l e v e l o f
speeds, w i t h cemented c a r b i d e t o o l s ) when c u t t i n g s t e e l and o t h e r h i g h m e l t i n g
p o i n t a l l o y s .
F ig .10 Temperature g r a d i e n t s i n t o o l used t o c u t low carbon s t e e l a t 76m m i n - l , 0.25mm/rev feed.
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19.3.6 S1 i d i n g a t t he TooI/\,dork I n t e r f a c e
Many o f the p e c u l i a r and c h a r a c t e i i s t i c f ea tu res o f machining opera t i ons
a r i s e from the unusual f e a t u r e o f se i zu re a t t he i n t e r f a c e between t o o l and work
m a t e r i a l , bu t these c o n d i t i o n s do n o t e x i s t under a l l c u t t i n g c o n d i t i o n s and on
a l l p a r t s o f t he con tac t area. The model o f se i zu re which has been g iven i s too
s i m p l i f i e d and must be co r rec ted .
A t ve ry low speeds se i zu re may n o t occur . S i m i l a r l y a t t he pe r iphe ry o f the
con tac t reg ion , even a t h i g h r a t e s o f metal removal, t he re i s good evidence t o
show t h a t s l i d i n g takes p lace a t t h e i n t e r f a c e by a type o f s t i c k - s l i p process.
Thus a sec t i on through t h e o u t e r edge o f a s t e e l c h i p o f t e n shows a segmented
c h i p w i t h a p e r i o d i c s t r u c t u r e a t t he i n t e r f a c e (Fig.11.) i n d i c a t i n g a s t i c k -
s l i p a c t i o n . The c e n t r e o f t he same c h i p shows a f low-zone demonst ra t ing
se i zu re a t t h i s p a r t o f t he i n t e r f a c e (F ig .12) . That s l i d i n g occurs i n these
pe r iphe ra l reg ions may be a t t r i b u t e d t o two main f a c t o r s (1) lower compressive
s t r e s s near a f r e e sur face o f the c h i p and ( 2 ) access o f atmospher ic oxygen t o
the i n t e r f a c e a t t h i s p o s i t i o n , reduc ing the tendency t o m e t a l l i c bonding.
Since the mechanisms o f wear may be very d i f f e r e n t under c o n d i t i o n s o f
se i zu re and s l i d i n g , i t i s u s e f u l t o have i n mind a model o f t he rea ions where
se i zu re and s l i d i n g occur most u s u a l l y . o n a c u t t i n g t o o l . Fig.13 shows such a
map f o r a s imple t u r n i n g t o o l .
Fig.11 Sec t ion through o u t e r edge o f c h i p a f t e r c u t t i n g medium carbon s t e e l a t h i g h speed. Shows s t i c k - s l i p a c t i o n a t i n t e r f a c e .
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Fig.12 Sec t ion th rough c e n t r e o f same c h i p as Fig.11. Shows flow-zone a t i n t e r f a c e c h a r a c t e r i s t i c o f se izure .
19.4 CUTTING TOOL WEAR
Whi le the upper l i m i t t o t he r a t e o f metal removal when c u t t i n g s t e e l o r
o t h e r h i g h m e l t i n g p o i n t a l l o y s i s determined by the a b i l i t y o f the t o o l t o w i th -
stand t h e c u t t i n g s t resses a t e leva ted temperatures, t he l i f e o f t he t o o l de-
creases as the c u t t i n g speed i s r a i s e d be fo re t h i s l i m i t i s reached. A t lower
speeds t h e shape o f t he t o o l i s changed by one o r more o f a number o f d i f f e r e n t
wear mechanisms u n t i l i t can no longer c u t e f f i c i e n t l y . Fig.14 shows diapram-
m a t i c a l l y on a model t u r n i n g t o o l t h e l o c a t i o n o f t he c h i e f wear fea tu res
observed.
"Flank wear" on t h e c learance face o f t he t o o l o f t e n increases s t e a d i l y w i t h
t ime o f c u t t i n g u n t i l , when a c r i t i c a l amount o f wear i s reached, the temper-
a t u r e on t h i s su r face s t a r t s t o r i s e r a p i d l y and t o o l f a i l u r e may be sudden.
The c r i t i c a l amount o f f l a n k wear v a r i e s under d i f f e r e n t c o n d i t i o n s .but i t may
be between 0.4 and 1.5 mm. To avo id complete f a i l u r e , which may be expensive,
t o o l s a r e no rma l l y reground o r rep laced be fo re the c r i t i c a l wear i s reached.
Flank wear may occur a t any c u t t i n g speed bu t t he wear r a t e increases w i t h
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X X
,L I
INCOMPLETE SEIZURE
SECTION X - X
Fig.13 Diagram o f t u r n i n g t o o l showing reg ions o f se i zu re and o f s l i d i n g a t t he too I /work i n t e r f a c e
/ RAKE FACE
DEFORM AT1 ON OR SLIDING
W t A K ACCELERATED WEAR / CRATER IN REGION OF 1.
$4 ACCELERATED WEAR IN REGION OF ffw SLIDING
FLANK OR CLEARANCE FACE
Fig.14 Diagram showing wear fea tu res on t u r n i n g t o o l
461
speed as the u l t i m a t e l i m i t f o r t he t o o l m a t e r i a l i s approached. I n the reg ion
o f h i g h speed c u t t i n p the r a t e o f f l a n k wear and the t o o l l i f e o f t e n f o l l o w the
r e l a t i o n s h i p g i ven by Tay lo r [ 4 ] f o r t o o l l i f e i n r e l a t i n g t o c u t t i n g speed
V t " = c
V - c u t t i n g speed
t - c u t t i n g t ime t o f a i l u r e o r t o some s tandard amount o f wear
n and C - cons tan ts f o r a g i ven t o o l and work m a t e r i a l
"Cra ter wear" i s t h e term used f o r a groove o r c r a t e r worn on the rake face
o f the t o o l , u s u a l l y a t some d i s tance f rom the c u t t i n g edge (F ig .14) . C ra te r ing
wear i s c h a r a c t e r i s t i c a l l y observed on t o o l s used a t h i g h c u t t i n g speeds and the
r a t e o f c r a t e r wear increases as the c u t t i n g speed approaches the u l t i m a t e l i m i t
f o r t he t o o l m a t e r i a l . As the c r a t e r becomes deeper i t weakens the t o o l edge
and may lead t o f r a c t u r e o f t he edge and t o o l f a i l u r e .
"Flank wear" and " c r a t e r wear" a r e d e s c r i p t i v e terms and t h e words do no t
imply d i s t i n c t wear mechanisms.
Where se i zu re c o n d i t i o n s occur a t t he too l /work i n t e r f a c e a t l e a s t f o u r d i f f e ren t
mechanisms o f wear have been observed and these w i l l be cons idered f i r s t . Wear
under c o n d i t i o n s o f s l i d i n g a t t he i n t e r f a c e w i l l be cons idered separa te ly .
The mechanisms o f wear w i l l now be discussed.
19.4.1 Abrasion
The ab ras i ve a c t i o n o f hard phases i n t h e work m a t e r i a l , such as ox ides o r
carb ides may c o n t r i b u t e t o the wear o f c u t t i n g t o o l s . Abrasion i s , however,
p robab ly n o t a ma jor cause o f wear under se i zu re c o n d i t i o n s un less the p a r t i c l e s
o f t he hard phases a r e l a rge , e.g. , g r e a t e r than 40 pm, o r p resen t i n very h i g h
concent ra t ions , as they may be f o r example on the sur face o f cas t i ngs . Even
w i t h h i g h speed s t e e l t o o l s the ab ras i ve a c t i o n o f d ispersed, f i n e hard p a r t i c l e s
i s p robab ly smal l because, under se i zu re c o n d i t i o n s they r a r e l y impinge on the
t o o l su r face i n such a way as t o remove t o o l m a t e r i a l . Wi th harder t o o l mat-
e r i a l s such as carb ides o r diamond, few i f any p a r t i c l e s i n the work m a t e r i a l
a re harder than the t o o l s and ab ras i ve a c t i o n i s l ess l i k e l y . The hardness of
c u t t i n g t o o l m a t e r i a l s i s o f more s i g n i f i c a n c e as a measure o f t h e i r a b i l i t y t o
w i ths tand h i g h compressive s t r e s s than as a measure o f t h e i r res i s tance t o
abrasion.
19.4.2 Surface Shear ing
When c u t t i n g h ighe r m e l t i n g p o i n t meta ls a t h i g h speeds, the i n t e r f a c e tem-
pera ture , p a r t i c u l a r l y on the rake face o f the t o o l
so t h a t t he y i e l d s t r e s s o f t he s t e e l t o o l i s reduced t o a very low value i n a
small volume of metal a t the i n t e r f a c e . Th in l a y e r s o f t he t o o l ma te r ia l may
then be sheared away by the work m a t e r i a l bonded t o the t o o l sur face . Fig.15
(F ig . lO) , may be very h igh ,
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shows an example o f t h i s wear ing a c t i o n i n which a c r a t e r i s be ing worn on the
rake face o f a h i g h speed s t e e l t o o l when c u t t i n g carbon s t e e l a t h i g h speed.
This wear mechanism i s u s u a l l y observed o n l y where the i n t e r f a c e temperature i s
above 800°C on s t e e l t o o l s .
f o r c r a t e r wear on s t e e l t o o l s and a l s o f o r t he f i n a l stages o f f l a n k wear j u s t
before complete t o o l f a i l u r e . Th is mechanism causes r a p i d t o o l wear. I t has
not been observed t o occur on cemented ca rb ide t o o l s .
It i s one o f t he mechanisms o f wear respons ib le
Fig.15 Sec t ion through t o o l used t o c u t low carbon s t e e l a t h i g h speed. Showing fo rma t ion o f c r a t e r on rake face by shear ing o f h i g h speed t o o l i n reg ion of h i g h temperature.
19.4.3 D i f f u s i o n and i n t e r a c t i o n
Under cond i t i ons o f c u t t i n g where t h e t o o l and work sur faces a r e m e t a l l u r -
g i c a l l y bonded, the t o o l shape can be changed by a process o f d i f f u s i o n and
i n t e r a c t i o n between the two m a t e r i a l s . I n the s imp les t s i t u a t i o n atoms from
the t o o l m a t e r i a l may d i f f u s e i n t o the work m a t e r i a l f l o w i n g over the sur face
and be c a r r i e d away by i t - i .e . , t h e t o o l m a t e r i a l i s d i s s o l v e d i n t o t h e work
ma te r ia l by a process o f t he same charac te r as t h a t of a b l o c k o f s a l t be ing
d i sso l ved by a stream o f water runn ing over i t s sur face . D i f f u s i o n i s a h i g h l y
temperature dependent process and d i f f u s i o n wear occurs a t an apprec iab le r a t e
on l y a t r e l a t i v e l y h i g h c u t t i n g speeds where the i n t e r f a c e temperature i s h igh .
When c u t t i n g s t e e l s w i t h h i g h speed s t e e l t o o l s wear by a d i f f u s i o n mechanism
i s p robab ly s i g n i f i c a n t o n l y where the i n t e r f a c e temperature exceeds 650°C.
I t has been shown t h a t , even a t modera te ly h i g h c u t t i n g speeds o f 25-30 m min
when c u t t i n g s t e e l , temperatures o f 790°C and over o f t e n occur a t p a r t s of t he
- 1
463
i n t e r f a c e , and t h e r e i s a r a p i d r i s e i n temperature w i t h f u r t h e r increments
i n speed.
Simple d i f f u s i o n and a v a r i e t y o f i n t e r a c t i o n s depending on the chemical
compos i t ions and m e t a l l u r g i c a l s t r u c t u r e s o f t h e t o o l and work m a t e r i a l s , occur
across the i n t e r f a c e . I t i s p robab le t h a t wear based on atomic i n t e r a c t i o n s i s
t h e most impor tan t mechanism changing the shape o f h i g h speed s t e e l , cemented
ca rb ide o r diamond c u t t i n g tools i n machining the h ighe r m e l t i n g p o i n t meta ls
and a l l o y s a t h i g h speeds.
Fig.16 shows a sec t i on through t h e c r a t e r e d su r face o f a h i g h speed s t e e l
t o o l w i t h adher ing work m a t e r i a l . The wear process was one o f d i f f u s i o n ; t he
t o o l su r face shows no s igns o f p l a s t i c de format ion . There i s good evidence t h a t
wear by d i f f u s i o n i s respons ib le f o r most c r a t e r and f l a n k wear on h igh speed
s t e e l t o o l s where the i n t e r f a c e temperature i s above 650°C but below the tem-
pe ra tu re requ i red f o r the shear ing a c t i o n .
Fig.16 Sec t ion through h i g h speed s t e e l t o o l i n worn c r a t e r a f t e r c u t t i n g low a l l o y s t e e l a t 18m min- ’ f o r 38 minutes. I n t e r f a c e c h a r a c t e r i s t i c
o f d i f f u s i o n wear
464
C u t t i n g t o o l m a t e r i a l s have been deve loped e m p i r i c a l l y and most have been
deve loped f o r t h e m a c h i n i n g o f s t e e l s i n c e t h i s i s t h e main marke t f o r c u t t i n p
t o o l s [ 5 ] . The f i r s t cemented c a r b i d e s p roduced were a l l o y s o f t u n g s t e n c a r b i d e
and c o b a l t (WC-Co). These a r e v e r y s u c c e s s f u l f o r t h e c u t t i n g o f n o n - f e r r o u s
m e t a l s and c a s t i ron a t speeds much h i g h e r t h a n can be a c h i e v e d w i t h s t e e l t o o l s
because t h e WC-Co a l l o y s have h i g h e r y i e l d s t r e s s and can r e s i s t t h e s h e a r i n g
a c t i o n and d e f o r m a t i o n a t h i g h t e m p e r a t u r e s , and a l s o because t h e WC-Co a l l o y s
do n o t r e a c t w i t h t h e s e work m a t e r i a l s a t t h e h i g h c u t t i n g speeds.
For t h e c u t t i n g o f s t e e l however , t h e WC-Co a l l o y s a r e n o t so s u c c e s s f u l ,
s i n c e r a p i d c r a t e r i n g wear in p a r t i c u l a r o n t h e r a k e f a c e of t h e t o o l s g i v e s a
v e r y s h o r t l i f e f o r t o o l s used a t speeds n o t much h i g h e r than t h o s e used w i t h
h i g h speed s t e e l t o o l s . Cemented c a r b i d e t o o l s were s u c c e s s f u l f o r c u t t i n g o f
s t e e l a t h i g h e r speeds o n l y when a sma l l p r o p o r t i o n ( 5 t o 20%) o f T i c o r TaC was
added t o t h e WC-Co a l l o y s . These " s t e e l c u t t i n g g rades" o f cemented c a r b i d e a r e
a b l e t o machine s t e e l a t more than t h r e e t i m e s t h e speed w i t h WC-Co a l l o y s
because t h e r a t e o f wear , p a r t i c u l a r l y t h e c r a t e r wear on t h e r a k e f a c e , i s so
much lower . I t i s now known t h a t t h e i r success must be a t t r i b u t e d t o t h e much
lower r a t e of s o l u t i o n o f TIC o r TaC i n s t e e l a t e l e v a t e d t e m p e r a t u r e s , compared
w i t h t h a t o f W C . The r a t e o f c r a t e r i n g i n p a r t i c u l a r i s a f f e c t e d because i t i s
on t h e r a k e f a c e o f t h e t o o l , away f r o m t h e c u t t i n g edge, t h a t t h e h i g h e s t
t empera tu res a t t h e t o o l / w o r k i n t e r f a c e a r e l o c a t e d ( F i g . 1 0 ) . The r a t e o f f l a n k
wear a l s o i s g e n e r a l l y reduced b y t h e i n c l u s i o n o f TIC and TaC i n t h e t o o l
m a t e r i a l s when c u t t i n g s t e e l [ 5 ] .
I n a more r e c e n t development t h e r a t e o f wear has been dec reased s t i l l
f u r t h e r by c o a t i n g cemented c a r b i d e t o o l s w i t h t h i n l a y e r s o f s o l u t i o n r e s i s t a n t
m a t e r i a l s . The subs tances used f o r t h e c o a t i n a s a r e TIC, T i N o r H f N , w h i c h a r e
d e p o s i t e d on t h e s u r f a c e o f t h e t o o l s a s l a y e r s 5 t o 10 jm t h i c k b y a p r o c e s s
known as chemica l vapour d e p o s i t i o n ("CVD'I). These c a r b i d e s and n i t r i d e s a r e
t o o b r i t t l e t o be used by themse lves as c u t t i n g t o o l s , b u t a s t h i n l a y e r s w i t h
v e r y f i n e g r a i n s i z e s u p p o r t e d by t h e t o u g h e r cemented c a r b i d e s u b s t r a t e t h e y
w i t h s t a n d w e l l t h e s t r e s s e s o f machine shop o p e r a t i o n s , and t h e y a r e l e s s r e a d i l y
d i s s o l v e d i n t h e s t e e l f l o w i n g o v e r t h e t o o l s u r f a c e s . W i t h t h e s e c o a t e d t o o l s
t o o l I i f e may be p r o l o n g e d by a f a c t o r o f 1.5 t o 3 t i m e s compared w i t h uncoa ted
cemeii ted c a r b i d e s , o r c u t t i n g speeds 30 t o 60 m m in
same t o o l l i f e when c u t t i n g s t e e l .
-1 h i g h e r can be used f o r t h e
Even more r e s i s t a n t t o s o l u t i o n i n s t e e l a t h i g h t e m p e r a t u r e s i s a l u m i n i u m
o x i d e . A1 0 ( a l u m i n a ) has a h i g h y i e l d s t r e s s a t h i g h t e m p e r a t u r e and t o o l
t i p s a r e s i n t e r e d o r h o t p r e s s e d f r o m a l u m i n a powder.
as "ceramic t o o l s " ) o f h i g h d e n s i t y and f i n e g r a i n s i z e can be used f o r c u t t i n g
s t e e l and c a s t i r o n a t speeds a s h i g h a s 600-700 m m i n - ' w i t h v e r y l o w r a t e s o f
wear. These t o o l s , however , a r e l a c k i n g i n toughness and t h e i r use i n c u t t i n g
2 3 A l q 0 3 t o o l t i p s (known
465
s t e e l i s r e s t r i c t e d f o r t h i s reason t o a very smal l p r o p o r t i o n o f machine shop
opera t i ons on s t e e l .
h i g h speed. Recent ly cemented ca rb ide t o o l s have been pu t on the market w i t h
t h i n coa t ings o f A1203 depos i ted by CVD and these a re being assessed f o r the
c u t t i n g o f bo th s t e e l and cas t i r o n .
They can be more w ide ly used f o r c u t t i n g cas t i r o n a t very
React ion between t o o l and work m a t e r i a l a l s o l i m i t s the speeds used when
c u t t i n g s t e e l and n icke l -based a l l o y s w i t h diamond t o o l s .
i s a s y n t h e t i c m a t e r i a l made by the same type o f u l t r a h i g h pressure process
used f o r t he p roduc t i on o f s y n t h e t i c diamond. I t has a s i m i l a r s t r u c t u r e t o
diamond and, a l t hough i t i s less hard, i t can be used i n c u t t i n g s t e e l s and
n icke l -based a l l o y s a t h ighe r speeds because i t reac ts w i t h them l e s s r e a d i l y
than does diamond a t e leva ted temperatures 161.
Cubic boron n i t r i d e
Thus, under the se i zu re c o n d i t i o n s which p r e v a i l i n many metal c u t t i n g
opera t i ons , t he l i f e o f t he c u t t i n g t o o l s i s o f t e n c o n t r o l l e d by processes o f
d i f f u s i o n and i n t e r a c t i o n between t o o l and m a t e r i a l a t h igh i n t e r f a c e temper-
a tu res when c u t t i n g m a t e r i a l s o f h i g h m e l t i n g p o i n t a t h i g h speeds.
19.4.4 A t t r i t i o n
I f , under c o n d i t i o n s o f se i zu re , t he temperature-dependent wear processes o f
shear ing and d i f f u s i o n were the o n l y ones respons ib le f o r changing the t o o l
shape, t o o l l i f e might be expected t o be almost i n f i n i t e a t low c u t t i n g speed.
A t low speeds, however, t o o l s a r e f r e q u e n t l y worn by another mechanism which can
be c a l l e d " a t t r i t i o n " . Sect ions through t h e edge o f t o o l s used a t low speeds
o f t e n show t h a t the t o o l has been worn by a mechanism i n v o l v i n g break ing away
from the t o o l su r face o f fragments o f mic roscop ic s i z e - as shown f o r example i n
Fig.17. f o r a h i g h speed s t e e l t o o l . Such a wear mechanism has been observed
w i t h almost a l l c l asses o f t o o l m a t e r i a l a f t e r c u t t i n g a t low speed, and as the
speed i s r a i s e d i t becomes l e s s impor tan t . I n many cases bo th d i f f u s i o n and
a t t r i t i o n wear a r e observed on the same worn sur faces .
A t t r i t i o n wear seems t o be most severe when the machine t o o l lacks r i g i d i t y ,
when v i b r a t i o n occurs o r when the re a r e pronounced i r r e g u l a r i t i e s i n the work
m a t e r i a l . I t i nvo l ves an i n t e r m i t t e n t a c t i o n i n which smal l fragments o f the
t o o l a r e t o r n away t o leave c h a r a c t e r i s t i c a l l y rough worn sur faces . High speed
s t e e l t o o l s a re more r e s i s t a n t t o a t t r i t i o n than a r e cemented carb ides and the
l i f e o f h i g h speed s t e e l t o o l s i s o f t e n longer a t low c u t t i n g speeds than . tha t
o f cemented ca rb ides f o r t h i s reason. Tw is t d r i l l s f o r example a r e most
commonly made f rom h i g h speed steels, n o t o n l y because they a re cheaper but
because, i n many a p p l i c a t i o n s the l i f e i s longer than t h a t o f cemented carb ide
d r i l l s and performance i s more cons is ten t . When cemented ca rb ide t o o l s a re used
466
Fig.17 Sec t ion th rough c u t t i n g edge of h i g h speed s t e e l t o o l a f t e r c u t t i n g s t e e l a t 20m min '. Worn sur face c h a r a c t e r i s t i c o f a t t r i t i o n wear
i n the low speed range where a t t r i t i o n wear i s dominant, the r a t e o f wear i s
very dependent on the ca rb ide g r a i n s i z e . F ine g ra ined cemented ca rb ides a r e
much more r e s i s t a n t t o a t t r i t i o n wear than coarse g ra ined ones, and WC-Co a l l o y s
a re more r e s i s t a n t than the s t e e l c u t t i n g ca rb ide grades and a r e o f t e n used t o
c u t s t e e l a t low speed f o r t h i s reason.
Fig.13 shows d iagrammat ica l l y those p a r t s o f a t u r n i n g t o o l where se i zu re
Rather f r e q u e n t l y more and s l i d i n g a r e most l i k e l y t o occur d u r i n g c u t t i n g .
r a p i d wear i s observed i n t h e s l i d i n g reg ions than a t t he se ized sur faces . For
example, Fig.18 shows a deep tongue o f wear on a t o o l i n the s l i d i n g wear reg ion
where the o u t e r edge o f the c h i p crossed the c u t t i n g edge o f the t o o l . The wear
r a t e was many t imes h ighe r a t t h i s p o s i t i o n than i n the ad jacen t se ized reg ion .
High r a t e s o f wear a re o f t e n observed a l s o a t t h e nose o f the t o o l i n t u r n i n g
opera t ions , where t h e o t h e r edge o f t he c h i p crosses the c u t t i n g edge.
467
Fig.18 Clearance face o f t o o l used t o c u t low carbon s t e e l . Shows adherent work m a t e r i a l and deeply worn qroove a t ou ts ide o f c u t , c h a r a c t e r i s t i c
o f wear by s l i d i n g a c t i o n
Such increased wear r a t e i n the s l i d i n g reg ions does no t always occur, and the
cond i t i ons i n f l u e n c i n g the s e v e r i t y of wear have n o t y e t been s tud ied i n
s u f f i c i e n t d e t a i l . I t seems most probable t h a t the wear i n the s l i d i n g reg ions
invo lves r e a c t i o n between the t o o l and work sur faces and oxyaen o f the atmos-
phere, which has access t o t h i s p a r t o f t he i n t e r f a c e . One poss ib le wear
mechanism i s the format ion of ox ide l aye rs on the tool and the removal o f these
p e r i o d i c a l l y by the s t i c k - s l i p a c t i o n o f t h e s l i d i n g c h i p (Fig.11).
surfaces i n the s l i d i n g reg ions a r e no rma l l y very smooth and t h i s s o r t o f wear
on ca rb ide t o o l s may be almost as r a p i d as on s t e e l t o o l s , so t h a t abras ion
by hard p a r t i c l e s , a l though p o s s i b l e i n t h i s reg ion, i s probably no t the main
mechanism o f wear i n most operat ions. The r a t e o f s l i d i n g wear may’be i n -
f luenced by the use of c u t t i n g l u b r i c a n t s .
The worn
19.5 COOLANTS AND LUBRICANTS
A d i scuss ion o f t r i b o l o g y i n metal c u t t i n g would n o t be complete w i thou t
cons ide r ing the i n f l uence of coo lan ts and l u b r i c a n t s . The c u t t i n g t o o l i s o f t e n
f looded w i t h a f l u i d descr ibed e i t h e r as a coo lan t o r as a l u b r i c a n t . Two main
types a r e used - water-based f l u i d s con ta in ing o i l and o the r a d d i t i v e s i n sus-
pension o r s o l u t i o n and minera l o i l s w i t h o r w i thou t extreme pressure a d d i t i v e s
(mainly su lphur and c h l o r i n e con ta in ing substances). The f u n c t i o n o f the water-
468
based f l u i d s i s ma in l y as a coo lan t w i t h the o i l p resent ma in ly t o p revent
cor ros ion , w h i l e the o i l based f l u i d s have r e l a t i v e l y poor c o o l i n g c a p a c i t y and
a re used ma in l y f o r t h e i r l u b r i c a n t a c t i o n .
Coolants a re o f t e n necessary t o reduce the temperature o f machine, t o o l and
workpiece i n o rde r t o promote e f f i c i e n t ope ra t i on and t o ma in ta in dimensional
to le rances . P r a c t i c a l exper ience shows t h a t a s t rong f l o w o f coo lan t can a c t
t o increase t o o l l i f e when c u t t i n g a t h i g h speeds, o r t o pe rm i t t he use o f h igher
c u t t i n g speeds. The d i r e c t i n f l u e n c e o f a coo lan t on the maximum temperature
generated a t the too l /work i n t e r f a c e i s u s u a l l y r a t h e r s l i g h t [71. The temp-
e r a t u r e i s generated i n the f low-zone a t t he rake face (F ig .10) . Coolant cannot
prevent t he genera t ion o f heat a t t h i s sur face and can a c t o n l y t o steepen the
temperature g rad ien ts and reduce the volume o f t o o l m a t e r i a l heated t o h i g h
temperature, bu t i t can have l i t t l e i n f l uence on the maximum temperature on the
rake face. Ac t i ng near the c u t t i n g edge the coo lan t can be more e f f e c t i v e and
the major c o o l i n g e f f e c t i s p robab ly t h a t o f reduc ing t h e temperature near the
edge, thus i nc reas ing the y i e l d s t r e n g t h o f the t o o l t o p revent l o c a l deform-
a t i o n , and a l s o reduc ing the r a t e o f d i f f u s i o n wear on the f l a n k o f the t o o l .
From what has been sa id about se i zu re a t t he too l /work i n t e r f a c e , i t seems
u n l i k e l y t h a t any l u b r i c a n t , i n e i t h e r gaseous o r l i q u i d form, can pene t ra te t o
t h a t p a r t o f the i n t e r f a c e where se i zu re occurs . Lub r i can ts can, however, a c t
e f f e c t i v e l y i n the p e r i p h e r a l reg ions where s l i d i n g occurs a t the i n t e r f a c e .
By p e n e t r a t i n g f rom the p e r i p h e r a l reg ions they may be ab le t o r e s t r i c t t he area
o f complete se i zu re and thus t o reduce the fo rces a c t i n g on the t o o l . Force
measurements have shown reduc t i ons i n bo th c u t t i n g f o r c e and feed f o r c e caused
by the use o f coo lan ts and l u b r i c a n t s a t low c u t t i n g speed. Many t e s t s have
ind i ca ted t h a t t he i n f l uence o f l u b r i c a n t s i s g rea tes t a t speeds below 30 m min
wh i l e they have very l i t t l e l u b r i c a t i n g e f f e c t over 60 m min . The most
e f f e c t i v e l u b r i c a n t s i n metal c u t t i n g a r e those w i t h extreme pressure a d d i t i v e s ,
suggest ing t h a t success fu l l u b r i c a t i o n i nvo l ves the fo rma t ion o f e a s i l y sheared
sur face l a y e r s when the l u b r i c a n t s come i n t o con tac t w i t h f r e s h l y exposed metal
sur faces on the work m a t e r i a l .
-1
-1
When a b u i l t - u p edge i s formed, coo lan ts and ' l ub r i can ts a r e o f t e n e f f e c t i v e
i n g r e a t l y reducing i t s s ize . A l a r g e b u i l t - u p edge i s o f t e n respons ib le f o r
very poor sur face f i n i s h and one of t h e most impor tan t f u n c t i o n s o f a c u t t i n g
l u b r i c a n t i s t o improve the sur face where t h i s i s a requirement o f the sur face
being machined. Often, i n t h i s respec t , water-based l u b r i c a n t s , and even water
i t s e l f , a re e f f e c t i v e i n reduc ing the s i z e o f the b u i l t - u p edge.
For many c u t t i n g opera t i ons l u b r i c a n t s a r e e s s e n t i a l . A t h i g h speeds t o o l
l i f e may be improved by coo lan t a c t i o n , bu t t he use o f l u b r i c a n t s a t lower
speeds t o improve sur face f i n i s h i s no t always e f f e c t i v e i n reduc ing the r a t e o f
wear. I n f a c t tool wear r a t e i s o f t e n increased by the a c t i o n o f l u b r i c a n t s i n
469
t he s l i d i n g reg ions where i t can penet ra te . Th is a c c e l e r a t i o n o f w e a r may occur
w i t h bo th s t e e l and cemented ca rb ide t o o l s . High r a t e of wear i n reg ions o f
s l i d i n g a t the i n t e r f a c e has a l ready been discussed.
anisms a major f u n c t i o n of l u b r i c a n t s i s t o p revent se i zu re between moving pa r t s .
I n the case o f metal c u t t i n g opera t i ons , t he e l i m i n a t i o n o f se i zu re i s no t the
o b j e c t i v e o f t he use o f l u b r i c a n t s . The e l i m i n a t i o n o f se i zu re i n many cases
cou ld r e s u l t i n a d i s a s t r o u s inc rease i n the r a t e o f t o o l wear. I n most
c u t t i n g opera t i ons se i zu re between t o o l and work m a t e r i a l i s a normal and
d e s i r a b l e c o n d i t i o n , r a t h e r than a hazard t o be avoided. The main u s e f u l fun-
c t i o n s o f c u t t i n g f l u i d s a r e t o reduce temperature and thus increase c u t t i n g
e f f i c i e n c y , t o reduce c u t t i n g fo rces , t o improve sur face f i n i s h , and t o he lp
c l e a r ch ips away f rom the c u t t i n g t o o l .
I n many eng ineer ing mech-
There i s one way i n which se i zu re between tool and work m a t e r i a l can be mod-
i f i e d , i f no t e l i m i n a t e d , t o the advantage o f c e r t a i n c u t t i n g opera t i ons , and
t h i s i s t o i nc lude w i t h i n t h e work m a t e r i a l a phase which may in te rpose i t s e l f
between t o o l and work d u r i n g the c u t t i n g opera t i on . Such a substance a p p l i e d
f rom w i t h i n the work m a t e r i a l can renew i n t e r f a c i a l l aye rs as they a r e swept
away by the f l o w o f t h e work m a t e r i a l over the t o o l . Under c e r t a i n cond i t i ons
such substances can be descr ibed as per fo rming the f u n c t i o n s o f an i n t e r n a l
l u b r i c a n t and a re g e n e r a l l y known as f ree-machining a d d i t i v e s .
Manganese su lph ide i n s t e e l may a c t i n t h i s way and, when the s t e e l c u t t i n g
grades o f ca rb ide a r e used, the su lph ides from the s t e e l w i l l o f t e n form an
in te rmed ia te l a y e r a t t h e se ized too l /work i n t e r f a c e . Cer ta in calc ium-aluminium
s i l i c a t e s i n s t e e l , which l i k e manganese su lph ide a re p l a s t i c a l l y deformed when
t h e s t e e l i s sheared, a l s o f u n c t i o n i n the same way as t h e su lph ides . S i l i c a t e
l a y e r s a r e formed on s t e e l - c u t t i n g ca rb ide t o o l s a t t he i n t e r f a c e when c u t t i n g
a t h i g h speeds and a r e o f t e n ex t remely e f f e c t i v e i n reduc ing t o o l wear r a t e and
p e r m i t t i n g the use o f h i g h e r c u t t i n g speeds.
Sulphides i n copper a l s o tend t o fo rm a t h i n l a y e r a t the too l /work i n t e r f a c e
where t h e i r main f u n c t i o n seems t o be t o reduce the area o f con tac t on the t o o l
rake face and thus g r e a t l y t o reduce the c u t t i n g fo rces which a r e ex t remely h igh
when c u t t i n g h i g h - c o n d u c t i v i t y copper. The a d d i t i o n o f lead t o brass r e s u l t s i n
an accumula t ion o f lead a t t he b rass / too l i n t e r f a c e under many c o n d i t i o n s o f
c u t t i n g . The g r e a t e s t b e n e f i t which the lead con fe rs i s the break ing up o f the
brass ch ips i n t o smal l f ragments which a r e e a s i l y disposed o f i n h i g h speed
au tomat ic machining.
O f a l l t he aspects o f machining the f u n c t i o n s o f c u t t i n g f l u i d s as coo lan ts
and l u b r i c a n t s have probab ly rece ived the l e a s t a t t e n t i o n f rom s c i e n t i f i c
research. There i s much t o be learned concern ing the ways i n which they ac t t o
achieve the r e s u l t s f o r which they a re used by p r a c t i c a l mach in is ts .
470
REFERENCES
1 Zorev,N.N., I n t e r n a t i o n a l Research i n Produc t ion Eng ineer ing (1963),
2 Trent,E.M., I . S . I . Report N0.94., 1967, p. 11 . 3 W i l l iarn5,J.E. and Rollason,E.C., J., I n s t . , Met., (19701, 98, 144. 4 Boothroyd,G., "Fundamentals of Metal Machining and Machine Tools",
5 Trent,E.M., "Metal Cut t ing" , 1977, But te rwor ths . 6 Hibb5,L.E. and Wentorf,R.E. J r . , 8 t h Plansee Seminar (1974), Paper No.42. 7 Smart,E.F. and Trent,E.M., Proc., 15 th I n t . , Conf., M.T.D.R., (1975) 187.
A.S.M.E., P i t t sbu rgh , p.42.
McGraw-Hi 1 1 (1975).
471
v( ) ROLE OF LABORATORY TEST MACHINES U
PROFESSOR F.T. BARWELL, H.H. JONES
Department o f Mechanical Engineer ing, U n i v e r s i t y Co l lege o f Swansea.
20.1 THE EXPERIMENTAL METHOD
The es tab l i shment o f f a c t s and the unders tand ing o f r e l a t i o n s h i p s between
phys i ca l phenomena can o n l y be based on exper ience, the exper imenta l method
should t h e r e f o r e c o n s i s t o f s t r u c t u r i n g exper ience so as t o produce unambiguous
answers t o c e r t a i n ques t ions . For s t r u c t u r e d exper ience o r exper iment t o have
meaning, i t must reproduce the ci rcumstances sur round ing the occurrence of the
phenomena under study. Otherwise the r e s u l t s , though perhaps i n t e r e s t i n g , w i l l
be i r r e l e v a n t t o the purpose o f the i n v e s t i g a t i o n .
T r i b o l o g i c a l i n v e s t i g a t i o n s a r e bes t c a r r i e d o u t under se rv i ce c o n d i t i o n s on
f u l l sca le apparatus bu t t h i s i s seldom poss ib le . I n fo rma t ion may o f t e n be
requ i red i n advance o f t he c o n s t r u c t i o n o f t he machine invo lved, measurements
may n o t be p o s s i b l e i n the opera t i ona l environment ( i n space f o r example) and
f u l l sca le t e s t i n g may be too c o s t l y e s p e c i a l l y when t e s t s t o d e s t r u c t i o n a r e
requ i red . Labora tory t e s t methods have t h e r e f o r e been evo lved which serve a
number o f v i t a l f u n c t i o n s i n r e l a t i o n t o the p r a c t i c e o f eng ineer ing .
The t e s t i n g o f l u b r i c a n t s by measurement o f t h e i r phys i ca l and chemical
p r o p e r t i e s i s w e l l developed (See Standard Methods o f Tes t i ng Petroleum and i t s
Products, I n s t i t u t e o f Petroleum and ASTM) bu t t he complex i ty o f t he requ i re -
ments o f many eng ineer ing a p p l i c a t i o n s i s such t h a t s a t i s f a c t o r y performance
cannot always be p r e d i c t e d f rom such t e s t s and i t i s necessary t o s imu la te
s e r v i c e cond i t i ons . Thus, f o r a p roduc t t o be approved f o r supply t o the U.S .
Army f o r l u b r i c a t i o n o f I.C. engines, i t has t o be submi t ted t o a s e r i e s o f
engine t e s t s and has t o s a t i s f y c e r t a i n s p e c i f i e d c r i t e r i a . Once approval has
been g iven, b u l k supp l i es may be accepted on the bas i s o f s u f f i c i e n t phys i ca l
and chemical t e s t i n g t o ensure cons is tency o f c o n s t i t u t i o n .
Many i n d u s t r i a l processes i nvo l ve a cons iderab le q u a n t i t y o f raw m a t e r i a l
and f u l l sca le t e s t i n g may be inconven ien t o r expensive. I t may n o t always be
p o s s i b l e t o o b t a i n access t o the i n t e r a c t i n g sur faces o f i n t e r e s t and i t may be
des i red t o exp lo re t h e e f f e c t o f a v a r i a b l e over a g rea te r range than would be
p o s s i b l e w i t h e x i s t i n g machinery. I n these circumstances, t he c o n s t r u c t i o n o f
spec ia l l a b o r a t o r y machines may be e s s e n t i a l .
472
T r i b o l o g i c a l s i t u a t i o n s can g e n e r a l l y be reduced t o the cons ide ra t i on o f
i n t e r a c t i n g sur faces o f g iven compos i t ion and shape w i t h app rop r ia te i oad ing
and r e l a t i v e mot ion together w i t h l u b r i c a n t and environment, and i t i s there-
f o r e a t t r a c t i v e t o p rov ide spec ia l t e s t machines which present no apparent
resemblance t o any p r a c t i c a l machine, bu t which reproduce s u f f i c i e n t l y accur-
a t e l y the t r i b o l o g i c a l c o n d i t i o n s invo lved. A number o f machines a r e a v a i l a b l e
commercial ly which p u r p o r t t o do t h i s and prov ided care i s taken t o ensure the
relevance o f the t e s t c o n d i t i o n s t o the a p p l i c a t i o n , they may produce much
use fu l i n fo rma t ion .
F i n a l l y where an e f f o r t i s be ing made t o expand knowledge o f t he fundamentals
under l y ing t r i b o l o g i c a l a c t i o n , spec ia l equipment i s necessary t o ex tend the
range o f obse rva t i on and t o i s o l a t e p a r t i c u l a r v a r i a b l e s . I t i s o f t e n found
t h a t apparatus devised f o r fundamental research i s p a r t i c u l a r l y use fu l f o r
app l i ed i n v e s t i g a t i o n s .
20.2 LUBRICANT TYPE APPROVAL TESTING MACHINES
20.2.1 Engine Tests
A g rea t deal o f l u b r i c a n t t e s t i n g i s c a r r i e d o u t i n o rde r t o s a t i s f y the
re levan t s p e c i f i c a t i o n s f o r engine o i l , t y p i c a l examples a r e DEF-2101-D i n the
Un i ted Kingdom and MIL-L-46152 i n the Un i ted Sta tes .
TABLE 20.1 Summary o f Typ ica l Engine Tes ts
S p e c i f i c a t i o n Engine Tes t ( s ) P roper t i es under t e s t
DEF-2101-D P e t t e r W1 O i l o x i d a t i o n , bear ing co r ros ion , - gaso l i ne lacquer fo rmat i on
P e t t e r AV1 - d i e s e l
Detergency and h i g h temperature s t a b i l i t y
Oldsmobi le V-8 Low temperature r u s t i n g and sequence I I B depos i t s - gaso l i ne
MIL-L-46152 Ford V-8 sequence VC - gaso l i ne
B u i l d up o f depos i t s due t o i n t e r m i t t e n t low temperature opera t ion
Oldsmobi le V-8 High temperature o x i d a t i o n sequence I I I C - gaso l i ne
CRC L-38 (CLR) Bear ing c o r r o s i o n and shear s t a b i l i t y - gaso l i ne o f mu l t i g rade5
Caterp i 1 l a r TH - d i e s e l depos i t s
Ring s t i c k i n g wear and accumulat ion o f
Fig.1 P e t t e r Av l Rig
473
Fig.2 C a t e r p i l l a r r i g
F ig .3 Piston examination
474
The main c r i t e r i o n o f acceptance o f a l u b r i c a n t which has been sub jec ted t o
type approval t e s t i n g i s t h e c o n d i t i o n o f the p i s t o n a f t e r t e s t . Th i s i s r a t e d
by a panel o f exper t s who assess such f.actors as the freedom o f t he r i n g s , a
minimum o f s c u f f i n g o f t he p i s t o n crown and a minimal amount o f carbon i n the
top r i n g groove. There should a l s o be no carbon depos i t i n the lower r i n g
grooves and the p i s t o n s k i r t should be e n t i r e l y f r e e o f lacquer .
As w i l l have been ga thered f rom p rev ious chapters , the a c t i o n o f l u b r i c a n t
a d d i t i v e s i s complex and the re may be s i g n i f i c a n t i n t e r a c t i o n s o f an undes i rab le
na ture . For example, some a d d i t i v e s l ead t o c o r r o s i o n o f copper - lead bear ings
and i n pe t ro l -eng ines leaded- fue ls may a f f e c t t he na tu re o f p i s t o n depos i t s
("grey pa in t " ) . A d d i t i o n a l t e s t s a r e t h e r e f o r e requ i red which were o r i g i n a l l y
c a r r i e d o u t f o r 36 hours i n a f o u r c y l i n d e r Chevro le t engine. The copper- lead
bear ings o f t h e t e s t eng ine were weighed and examined i n o rde r t o assess
c o r r o s i o n and the p i s t o n s examined t o assess any depos i t . A P e t t e r \ / I . spark-
i g n i t i o n engine has been s u b s t i t u t e d f o r t he Chevro le t eng ine i n the U.K. and an
engine, t he CRC L-38, has been s p e c i a l l y developed f o r l u b r i c a n t approva l t e s t s
i n the U . S . A .
Although i t may appear a t f i r s t s i g h t t h a t t he use o f an a c t u a l engine i s
a s t r a i g h t f o r w a r d means f o r assessing the q u a l i t y o f an o i l , t he t e s t cond i t ions
a re very c r i t i c a l . The compos i t ion o f t he f u e l , t h e t i m i n g o f t he v a l v e s , the
c o n d i t i o n s o f t he i n j e c t i o n system a l l may a f f e c t t he performance o f a lub-
r i c a n t . I n p a r t i c u l a r , t he exhaust arrangements can markedly a f f e c t r e s u l t s
[ l a .
20.2.2 Gear Tes ts
The i n t r o d u c t i o n o f hypo id gears i n back a x l e s was made p o s s i b l e o n l y by the
a p p l i c a t i o n o f extreme pressure l u b r i c a n t s and the maintenance o f adequate
l u b r i c a n t q u a l i t y i s e s s e n t i a l t o t h e i r success fu l f u n c t i o n i n g . Tests t o
qua1 i f y l u b r i c a n t s t o t h e U.K. ( C S 3000) and U.S.A. (MIL-L-2105C) s p e c i f i c a t i o n s
a r e c a r r i e d o u t on a c t u a l gears i n a t e s t arrangement o f t he type shown i n
Fig.4.
an ts i n a x l e s under c o n d i t i o n s o f high-speed, low-torque opera t i on , f o l l o w e d
by low-speed, h igh- to rque o p e r a t i o n should s a t i s f a c t o r i l y p revent t he occ-
u r rence o f gear t o o t h r i d g i n g , r i p p l i n g , p i t t i n g , we ld ing , excess ive wear, o r
o t h e r su r face d i s t r e s s o r t he fo rma t ion o f o b j e c t i o n a b l e depos i t s when t e s t e d
on bo th un t rea ted and phosphate t r e a t e d gear assembl ies i n accordance w i t h the
s p e c i f i e d procedures.
The load c a r r y i n g and extreme pressure c h a r a c t e r i s t i c s o f gear l u b r i c -
The p r imary a c t i o n o f t h e l u b r i c a n t a d d i t i v e i s t o con fe r E.P. p r o p e r t i e s
on the l u b r i c a n t . Th i s imp l i es p reven t ion o f s c u f f i n g by a c t i v a t i o n o f the
chemica l l y a c t i v e a d d i t i v e s by the occurrence o f instantaneous temperature
475
Fig.4 Low speed high torque test r i g hypoid gear and dynamometer installation
Fig.5 Ridging t y p e gear failure
416
Fig.5 Examples of gear damage
477
F i a . 6 R i p p l i n g type near f a i l u r e
F i g . 7 I . A . E . h igh speed gear r i g
478
" f lashes" between p a i r s o f gears. The High-speed, Low-torque t e s t i s used t o
assess t h i s behaviour us ing equipment wh ich embodies an au tomobi le rea r -ax le
assembly. Evidence o f f a i l u r e i s p rov ided by the i n i t i a t i o n o f s c u f f i n g .
TABLE 20.2 Summary o f t he F u l l Ax le Test o f MIL-L-2105C and CS 3000
Test Equipment Requ i remen t s
Mo is tu re Cor ros ion CRC L-33 Maximum o f 5% vapour phase 7 days Federal method 5326 c o r r o s i o n o f cover p l a t e , no
c o r r o s i o n o f f u n c t i o n a l p a r t s .
High Speed Shock CRC L-42 Score p reven t ion equal t o or Load Test Federal method 6507 b e t t e r than RG 110 o r CRC
lP.234/69 10/90 re fe rence o i l .
Low Speed High Torque CRC L-37 No gear su r face d i s t r e s s Test Federal method 6506 o r depos i t f o rma t ion
I P 232 Proc. 8. a l l owab le .
When chemica l l y a c t i v e a d d i t i v e s a r e used, t h e r e i s always t h e r i s k t h a t
they w i l l a c t under c o n d i t i o n s and i n a manner which i s n o t des i red . For
example, " r i dg ing " type f a i l u r e may occur as i n F ig .5 and " r i p p l i n g " as i n
Fig.6. The "Low-speed High-torque' ' t e s t i s used t o ensure t h a t l u b r i c a n t s a r e
fo rmula ted t o p revent these types o f f a i l u r e s f rom o c c u r r i n g under s e r v i c e
cond i t i ons .
Lub r i can ts f o r use i n spur gears a r e u s u a l l y t e s t e d f o r t h e i r a n t i - s c u f f i n g
p r o p e r t i e s i n machines which embody means f o r power c i r c u l a t i o n . One o f t he
o r i g i n a l machines i n t h i s c l a s s was designed by Mansion and i s known as the
I.A.E. machine. The gears under t e s t a r e geared together , supported on two
p a r a l l e l s h a f t s , and loaded by t h e " lock ing-up" o f a predetermined t o r s i o n a l
s t r a i n i n the sha f t s . The gear s e t s a r e assembled i n a "back-to-back" r e l a t i o n -
s h i p so t h a t power c i r c u l a t e s con t inuous ly and the d r i v i n g motor i s o n l y
requ i red t o make up the losses o c c u r r i n g w i t h i n the system.
Another machine which employs t h e power c i r c u l a t i n g p r i n c i p l e i s t he "Ryder"
machine. In t h i s des ign one s e t o f gears a c t s as t e s t s e c t i o n and the o t h e r i s
o f t he s i n g l e - h e l i c a l c o n f i g u r a t i o n . Load i s then a p p l i e d by means o f an a x i a l
f o r c e which i s conver ted i n the h e l i c a l - g e a r s t o a to rque tend ing t o t w i s t t he
s h a f t thereby l oad ing the t e s t gears.
The F.Z.G. Machine (Fig.8) a l s o employs i n v o l u t e gears on the power -c i rc -
u l a t i n g p r i n c i p l e .
A l l t he above machines r e q u i r e gears as t e s t p ieces and, because each t e s t
must be taken t o d e s t r u c t i o n , t he t e s t procedures a r e very expensive. There i s
a s t rong tendency t h e r e f o r e t o employ disc-machines wherein the r e l a t i v e amount
419
Test
Lubr
gears
i can tT
F ig .8 FZG Gear o i l t e s t
o f r o l l i n g and s l i d i n g can be ad jus ted t o correspond w i t h the events i n the
meshing c y c l e o f a spur gear.
20.2.3 S imu la t i on o f I n d u s t r i a l S i t u a t i o n s
Many i n d u s t r i a l processes r e q u i r i n g l u b r i c a t i o n do n o t correspond t o the
c o n d i t i o n s o f t he type approval t e s t o f l u b r i c a n t s , and i t i s necessary t o i n -
v e s t i g a t e the e x i s t i n g t r i b o l o g i c a l s i t u a t i o n t a k i n g i n t o account the na tu re o f
the i n t e r a c t i n g m a t e r i a l s , t he i n d u s t r i a l environment and the a p p l i e d fo rces
and n o t i o n s .
I n t h i s c i rcumstance, i t i s f r e q u e n t l y necessary t o c o n s t r u c t a spec ia l
machine t o reproduce w i t h i n the l a b o r a t o r y the s p e c i f i c c o n d i t i o n s surrounding
the i n d u s t r i a l problem.
An example o f such a machine i s shown i n Fig.9 [ 2 ] . Th is c o n s i s t s o f a
machine designed and cons t ruc ted a t t he Swansea T r ibo logy Centre t o s imu la te
the c o n d i t i o n s de termin ing the a c t i o n o f the s idegu ides of r o l l i n g m i l l s . The
s idegu ides , no rma l l y made o f bronze, a r e ac ted on by the edges o f t he s t e e l
s t r i p undergoing c o l d - r o l l i n g which they r e s t r a i n f rom unwanted sideways move-
ment. They a re sub jec ted t o much wear (F ig .gc) , and moreover, t h e non- fe r rous
m a t e r i a l t r a n s f e r r e d f rom the guide t o the s t e e l was o b j e c t i o n a b l e t o the
customers o f t he m a t e r i a l .
Accord ing ly a guide member was f i t t e d w i t h s t ra in -gauges and the rm is to rs and
sub jec ted t o normal o p e r a t i o n so t h a t t he fo rces a c t i n g on t h e guide cou ld be
determined toge the r w i t h the opera t i ona l temperature. The machine i l l u s t r a t e d
480
Fig .g(a) R o l l i n g M i l l w i t h s i d e guides
i n F ig .g (b) was then designed t o embody a cont inuous s t e e l b e l t t o represent the
c o l d s t r i p beimg r o l l e d i n the f u l l sca le apparatus and a member which resembled
a s ide guide and which was fo rced aga ins t t he edge o f the s t r i p w i t h a f o r c e
which was determined on the bas i s o f t he s t ra in -gauge measurements.
A f t e r a s e r i e s o f t e s t s had conf i rmed t h a t t h e wear mode ob ta ined i n the l ab -
o r a t o r y corresponded e x a c t l y w i t h t h a t observed i n p r a c t i c e , a s e r i e s o f a l t e r n -
a t i v e m a t e r i a l s was inves t i ga ted . I t was concluded t h a t the s u b s t i t u t i o n o f
nodu lar c a s t - i r o n f o r t he bronze presented severa l advantages as f o l l o w s :
(a) The r a t e o f wear was reduced
(b) The ma te r ia l was l e s s expensive t o p rocure
(c) The contaminat ion o f t he produc t w i t h non- fe r rous m a t e r i a l was avoided.
Th is example demonstrates the p o s s i b i l i t y o f study o f i n d u s t r i a l problems us ing
1 a bora t o r y met hods.
20 .3 ANALYSIS INTO SYSTEM ELEMENTS AS THE BASIS FOR SELECTION OF LABORATORY TESTS
T r i b o l o g i c a l s i t u a t i o n s encountered i n d i f f e r e n t machines may present c e r t a i n
fea tu res i n common i n s o f a r as they a l l embody sur faces i n r e l a t i v e mot ion.
481
F i g . 9 ( b ) Experimental Rig for Simula t inq Side Guide Wear
F i g . g ( c ) S imula ted worn s i d e gu ide
482
However, i t w i l l be apparent t h a t t he n a t u r e and c o n f i g u r a t i o n of sur faces may
vary w ide ly as between one machine and another as w i l l t he modes o f f o r c e and
mot ion. Therefore the re can be no s i n g l e l abo ra to ry r i g which can represent a l l
t r i b o l o g i c a l s i t u a t i o n s . However, i t i s a l s o t r u e t h a t many machines possess
fea tu res which a r e s u f f i c i e n t l y a l i k e t o j u s t i f y t h e c o n s t r u c t i o n o f t e s t
machines f o r the purpose o f e v a l u a t i n g l u b r i c a n t s o r m a t e r i a l s o f c o n s t r u c t i o n
f o r use t he re in .
Great care i s necessary however t o analyse p r e c i s e l y the n a t u r e o f t h e t r i -
b o l o g i c a l s i t u a t i o n s invo lved i n o rde r t o s e l e c t t he c o r r e c t c o n d i t i o n s t o be
a p p l i e d i n the t e s t machine. A s c i e n t i f i c a t t i t u d e i s necessary and meaningless
j a rgon such as " f i l m - s t r e n g t h ' ' o r " l u b r i c i t y " should be avoided. (An example o f
a l l o w i n g j a rgon t o take over i s g i ven by the i n v e s t i g a t o r who sub jec ted t o an
E.P. t e s t a l u b r i c a n t requ i red f o r a process imposing h i g h pressure a t very low
speed).
The f i r s t s tep i n the a n a l y t i c a l process must be t o c h a r a c t e r i z e the machine
element under study w i t h respec t t o i t s p o s i t i o n i n the k inemat ic cha in ; i s i t
a "Higher Pa i r " o r a "Lower Pa i r "? O f t h e lower p a i r s , t h e s l i d i n g p a i r , F i g .
lO(a) i s encountered on machine sl ideways and the "Revolute Pa i r " F ig . lO(b) i s
the bas i s o f t he m a j o r i t y o f bear ings . Apar t f rom the "screw1' p a i r (which i s a
combinat ion o f t he s l i d i n g and r e v o l u t e p a i r s ) a l l o t h e r p a i r s a r e h ighe r p a i r s
and a r e so cha rac te r i sed because they must accommodate r e l a t i v e mot ion which i s
p a r t l y s l i d i n g and p a r t l y t u r n i n g .
From the p o i n t o f vie!+ o f T r i b o l o g y , t h e impor tan t d i s t i n c t i o n between lower
and h ighe r p a i r s i s t h a t the former a l l o w con tac t t o be made throughout the f u l l
ex ten t o f a su r face , whereas the l a t t e r o n l y a l l o w "po in t " o r "1 ine" c o n t a c t .
Some common examples o f h ighe r p a i r s a r e shown i n F ig . lO(d -9 ) . Thus lower p a i r s
a r e s a i d t o be "conformal" and h ighe r p a i r s a r e "counterformal" . I t w i l l be
apparent t h a t the pro formal d i f f e r e n c e between d ispersed and concent ra ted con-
t a c t s w i l l be r e f l e c t e d i n the design o f the i n t e r a c t i n g components, i n t h e i r
m a t e r i a l s o f c o n s t r u c t i o n and i n the p r o p e r t i e s requ i red o f any l u b r i c a n t s
app l i ed . [ 3 ] . Fig.11 i l l u s t r a t e s the c l a s s i f i c a t i o n o f some common machine
elements i n t o conformal and counter fo rmal c o n f i g u r a t i o n s as w e l l as i n d i c a t i n g
the mode o f damage c h a r a c t e r i s t i c o f each a p p l i c a t i o n .
To be success fu l , l a b o r a t o r y t e s t i n g machines must be s imp le and must employ
They must however reproduce the c o n d i t i o n s of e a s i l y manufactured t e s t p ieces .
thermal and s t r e s s i n t e n s i t y t o which i t i s a n t i c i p a t e d t h a t t he l u b r i c a t e d
system w i l l be sub jec ted i n se rv i ce . The f i r s t broad c l a s s i f i c a t i o n must be
i n t o lower and h igher p a i r s .
There a r e r e l a t i v e l y few simple machines a v a i l a b l e commercial ly f o r t e s t i n g
l u b r i c a n t s i n lower p a i r s because, as f a r as l u b r i c a t i o n proper i s concerned,
483
/
I
v)
.I
U
Q
L
Q, c
9
+z
,
(d) Gear tooth contact lei &I(( bearmg IfJ Taper roller lgJ Wheel on roil bearing
FIGURE 10 Classification of machine elements into higher and lower pairs
Nature of
Contact
Lower pair (Conformal 1
Higher pair (Counterformal 1
Characteristic Mode of Damage
Fixtures subject to
Gears Hertzian
I Wheel on rail Y ~
FIGURE 11 Classification of mode of damage of common machine elements
485
t h i s i s u s u a l l y hydrodynamic and the o n l y p roper t y requ i red o f the l u b r i c a n t i s
i t s v i s c o s i t y which can e a s i l y be measured by phys i ca l methods. The impor tant
p roper t i es o f t he l u b r i c a n t i n these a p p l i c a t i o n s a r e r e l a t e d t o such f a c t o r s as
detergency o r a n t i - c o r r o s i o n and f u l l sca le t e s t s a re necessary t o safeguard
against undes i rab le i n t e r a c t i o n s .
For t e s t i n g m a t e r i a l - l u b r i c a n t combinat ions under c o n d i t i o n s o f pure s l i d i n g ,
a very convenient arrangement c o n s i s t s o f t h ree c y l i n d r i c a l p i n s which a re h e l d
i n an upper member w i t h t h e i r axes p a r a l l e l and the assembly i s loaded aga ins t
the s ide o f an annular t e s t p iece which i s r o t a t e d about i t s generat ing a x i s .
The "Cygnus" machine i s o f t h i s type. Both the Almen (Fig.13) and the Falex
(Fig.13) machines a r e b a s i c a l l y 1 ine-contact machines bu t a c e r t a i n degree o f
conformi ty u s u a l l y a r i s e s from.wear o f the t e s t p ieces. These two machine types
present the common f e a t u r e t h a t two i d e n t i c a l t e s t p ieces a re fo rced aga ins t a
r o t a t i n g member f rom d i a m e t r i c a l l y opposed d i r e c t i o n s , thus balanc ing the f o r c e
on the sp ind le. The Almen t e s t p ieces form segments o f a c y l i n d e r having an
i n te rna l rad ius some 1.5 x 10 m (0.006 i n ) g rea te r than t h a t o f the sp ind le
which i s 0.35 mm (0.25 i n ) i n diameter. The corresponding t e s t p ieces o f the
"Falex" a r e "V" shaped. Loading on the "Almen" i s hydraul i c and the "Falex"
mechanical. Resul ts a r e d i f f i c u l t t o eva lua te because o f t he loading methods
usual ly employed [ 4 ] .
-4
As regards h ighe r p a i r s , t he re i s a wide v a r i e t y of machines which apply
s l i d i n g under counter formal cond i t i ons . These have g e n e r a l l y been developed t o
assess the q u a l i t y o f gear l u b r i c a n t s w i t h p a r t i c u l a r re ference t o the i nh ib -
i t i o n o f s c u f f i n g . One o f t he most successful o f these has been the Shel l
(Stanhope-Seta) f o u r b a l l machine (Fig.14). The problem o f o b t a i n i n g t e s t
pieces which a r e o f un i fo rm f i n i s h and m e t a l l u r g i c a l c o n d i t i o n has been so lved
by us ing b a l l s se lec ted i n batches f rom commercial product ion. Three o f these
b a l l s a re clamped t o form a nes t i n t o which the f o u r t h b a l l ( he ld i n a chuck) i s
forced and ro ta ted . The b a l l s a r e 12.7 mm (0.5 i n ) i n diameter and the r o t a t -
ional speed o f t he sp ind le t o which the chuck i s a t tached i s 1450-1500 r e v min-'.
Load may be a p p l i e d i n increments up t o 800 kg. The machine may be used i n
several procedures, the most common o f which i s t he "Wear-scar diameter method",
[5 ] . A number o f t e s t s (usua l l y 20) a r e each made on a f r e s h se t o f b a l l s , load
being increased between each t e s t t he d u r a t i o n o f which i s one minute. The d i a -
meter o f the wear scar apparent a f t e r each t e s t i s measured and p l o t t e d aga ins t
load on l o g a r i t h m i c paper. Dur ing the t e s t s a t the lower loads wear i s n e g l i g -
i b l e but s u f f i c i e n t rubbing a c t i o n takes p lace t o leave a measurable mark which
when measured and p l o t t e d , g i ves r i s e to a s t r a i g h t l i n e which c l o s e l y p a r a l l e l s
that obta ined by c a l c u l a t i n g the diameter o f the Her t z ian con tac t between the
loaded b a l l s . With most l u b r i c a n t s a l oad i s reached a t which a sharp r i s e i n
486
Specimen arrangement
F I G . 1 2 CYGNUS F R I C T I O N AND WEAR TEST MACHINE
Brass lockina
Almen blocks
m w+#jL
Falex blocks
at
F I G . 13 FALEX LUBRICANT
TEST MACHINE
Exploded view of pin and V blocks
Cutaway view throughout sample pan
487
Male ball chuck
Fourth ball (rotates) -0
Ball pot lock ring -
(stationary )
Ball Dot insert
Ball pot
Calibrated arm
Ball pot __ mounting disc
Top boll rotates
Load
Four ball machine
FIGURE 14
Ball and flats
Cone and cylinder
488
diameter occurs. Th is va lue i s u s u a l l y s u f f i c i e n t l y d e f i n i t e t o c h a r a c t e r i s e a
l u b r i c a n t and i s c a l l e d the i n i t i a l se i zu re load. I n some cases however, r e -
covery may occur a l t hough the b a l l s o f t e n become welded toge the r a t t he h ighe r
loads. Another method o f e v a l u a t i o n i s based on the t ime e l a p s i n g under a con-
s tan t load be fo re the occurrence o f a sudden increase i n f r i c t i o n .
One disadvantage o f the f o u r b a l l machine i s t h a t the m a t e r i a l o f t he t e s t
p ieces , b a l l bear ing s t e e l , i s no t r e p r e s e n t a t i v e o f m a t e r i a l s w i t h which the
l u b r i c a n t may be requ i red t o i n t e r a c t . Accord ing ly the c e n t r a l b a l l may be
rep laced by a c o n i c a l l y ended p iece and t h e f i x e d b a l l s by smal l c y l i n d e r s
arranged t o form a t r i a n g l e i n a h o r i z o n t a l p lane, Fig.14.
The Timkin Machine (Fig.15) embodies a l i n e r a t h e r than a p o i n t con tac t . The
r o t a t i n g element, formed f rom the r i n g o f a taper r o l l e r bea r ing a c t s on a
s t a t i o n a r y rec tangu la r s t e e l b lock . F r i c t i o n can be measured.
Most t e s t i n g machines a r e so arranged t h a t t h e wear ing sur faces a r e i n con-
t i nuousor repeated con tac t so as t o o b l i t e r a t e the i n i t i a l man i fes ta t i ons o f
sur face f a i l u r e .
which cons is ted o f two c y l i n d e r s which, i n a d d i t i o n t o r o t a t i o n , c o u l d be t r a v -
ersed one r e l a t i v e t o the o t h e r so t h a t the c o n t a c t zone was made t o c o n t i n u a l l y
embody f r e s h m a t e r i a l (Fig.16).
A machine was a c c o r d i n g l y designed a t M.E.R.L. (now N.E.L.)
I n a l l t he aforement ioned machines, o n l y s l i d i n g takes p lace whereas i n many
mechanisms, i n v o l u t e gears f o r example, r o l l i n g as w e l l as s l i d i n g can take
p lace . Disc inachines such as t h e "Amsler" ( a l s o the M e r r i t t and S.A.E.
machines) have d i s c s which a r e loaded edgewise t o p rov ide va r ious combina t ions
o f r o l l i n g and s l i d i n g (F ig .17) . - P i t t i n g type f a i l u r e i s u s u a l l y assoc ia ted w i t h r o l l i n g con tac t and can be
s imu la ted i n the l abo ra to ry [ 6 ] us ing a s imp le m o d i f i c a t i o n o f t h e f o u r b a l l
machine (F ig .18) . Ins tead o f the th ree lower b a l l s be ing clamped i n t o p lace
they a r e a l lowed t o r o t a t e w i t h i n a s p e c i a l l y designed b a l l race [ 6 ] . Where i t
i s des i red t o i n v e s t i g a t e the p i t t i n g behaviour o f a spec ia l s t e e l , i t i s
p o s s i b l e t o s u b s t i t u t e a c o n i c a l l y ended t e s t p iece f o r t he c e n t r a l b a l l a l though
the th ree f r e e b a l l s must be re ta ined . A h i g h speed ve rs ion i s a l s o a v a i l a b l e .
20.4 EQUIPMENT FOR B A S I C RESEARCH
The i n v e s t i g a t i o n s pursued i n bas i c research l a b o r a t o r i e s may appear t o be
un re la ted t o p r a c t i c e by reason of the a r t i f i c i a l c o n d i t i o n s o f t e n imposed i n
o rde r t o e l u c i d a t e some fundamental r e l a t i o n s h i p . The c l o s i n g o f the communic-
a t i o n gap between the fundamental i n v e s t i g a t o r and the engineer whose p r a c t i c e
lies i n manufac tur ing i n d u s t r y f o r example, has been made e a s i e r by the deve l -
opment o f t he p r a c t i c e o f mode l l i ng complex systems us ing computers. Thus i n
Fig.19 the rec tang le marked A represents a real . machine-element sub jec ted t o the
environmental c o n d i t i o n s and load ing w i t h i n t h e i n d u s t r i a l environment. I t s
489
Lubricant sample
+ Load
F ig .15 Timken tes t
Fig.16 N.E.L. crossed c y l i n d e r machine
490
F
Fig .17 Amsler machine
l L
Fig.18 Rolling four ball machine
491
MATHEMATICAL LABORATORY EXPERIMENTS
E/
Fig.19 The r o l e o f the computer model i n r e l a t i n g bas i c research t o eng inee r ing p r a c t i c e
response t o these c o n d i t i o n s can be determined by a methodical s e r i e s o f t e s t s ,
r e s u l t s o f which a re represented by rec tang le 8. Assuming t h a t no t a l l i s we l l
o r t h a t some development i n p roduc t o r p roduc t i on c a p a c i t y i s requ i red , i t may
be necessary t o fo rmu la te some p r e d i c t i o n s o f behaviour l y i n g o u t s i d e the range
o f p rev ious exper ience. The system can then be represented by a s e r i e s o f
equat ions l i n k e d together t o form a mathematical model o f t he machine as ind-
i c a t e d by rec tang le C. S p e c i f i c a t i o n s f o r a p p l i e d c o n d i t i o n s and load ings would
be f e d i n and the computer would produce s o l u t i o n s t o the equat ions represented
by r e c t a n g l e D . These s o l u t i o n s would p rov ide p r e d i c t i o n s o f t he magnitude o f
ou tpu t q u a n t i t i e s which a r e represented by the arrows p o i n t i n g t o the l e f t from
D t o B. The arrows p o i n t i n g t o the r i g h t f rom €3 t o D represent the r e s u l t s o f
ac tua l t e s t . I f the re i s agreement between measured ou tpu ts and c a l c u l a t e d pre-
d i c t i o n s , t h e model can be sa id t o be complete. I f t he re i s divergence, the
model must be r e f i n e d and ad jus ted u n t i l acceptab le agreement r e s u l t s . Once
t h i s agreement has been a t t a i n e d , t he model may be used t o p r e d i c t the behaviour
o f an i n f i n i t e v a r i e t y o f machine elements and a p p l i e d c o n d i t i o n s prov ided they
l i e w i t h i n the range o f equat ions embodied i n the model.
At tempts t o c o n s t r u c t r e a l i s t i c mathematical models however, f r e q u e n t l y
revea l gaps i n understanding o f the phys i ca l system invo lved which can on ly be
f i l l e d by c a r e f u l l y c o n t r o l l e d l abo ra to ry exper iments such as those which were
necessary t o e l u c i d a t e the na tu re and opera t i ona l r e l a t i o n s h i p s o f e l a s t o -
hydrodynamic l u b r i c a t i o n (as i n d i c a t e d a t E ) . Such exper iments u s u a l l y form the
bas i s o f advanced t e s t methods made necessary by developing p r a c t i c e .
492
REFERENCES
Cree,J.C. (1953) C a t e r p i l l a r L1 and C h e v r o l e t L4 t e s t p r o c e s s , Symposium on Eng ine T e s t i n g o f L u b r i c a t i n g O i l . I n s t i t u t e o f P e t r o l e u m Roylance,B.J . (1977) The A p p l i c a t i o n o f E x i s t i n g Knowledge t h e S o l u t i o n o f I n d u s t r i a l T r i b o l o g y Problems. Proc. I . Mech. E. I n t h e Press. B a r w e l l ,F.T. (1979) ' B e a r i n g Systems - P r i n c i p l e s and P r a c t i c e ' C la rendon , O x f o r d . Meckleburg,K.R. (1975) ' F o r c e s i n t h e F a l e x c o n f i g u r a t i o n ' . T rans . ASLE., V01.18, pp. 97-104. Extreme Pressu re P r o p e r t i e s : F r i c t i o n and blear T e s t s : Four B a l l Machine. I P 239/77. The I n s t i t u t e o f Pe t ro leum. S tandards f o r P e t r o l e u m and i t s p r o d u c t s . Barwel1,F.T. and Scot t ,D. (1956) 'The e f f e c t o f l u b r i c a n t p i t t i n g f a i l u r e o f b a l l b e a r i n g s ' E n g i n e e r i n g , Vo1.182, pp. 9-12.
493
GLOSSARY
Terms and D e f i n i t i o n s
Abrasion - Wear by displacement o f m a t e r i a l caused by hard p a r t i c l e s o r hard
protuberances.
Abso lu te V i s c o s i t y - see v i s c o s i t y .
A d d i t i v e - A m a t e r i a l added t o a l u b r i c a n t f o r the purpose o f impar t ing new
p r o p e r t i e s o r o f enhancing e x i s t i n g p r o p e r t i e s .
Adhesive Wear - Wear by t rans fe rence o f m a t e r i a l f rom one sur face t o another
d u r i n g r e l a t i v e mot ion, due t o the process o f so l id -phase welding.
Anti-Wear A d d i t i v e - An a d d i t i v e used t o reduce wear.
Area o f Contact - The area o f con tac t between two s o l i d sur faces i s descr ibed
i n two ways.
( i ) Apparent Area: t he area o f con tac t de f i ned by the boundar ies
o f t h e macroscopic i n t e r f a c e o f t he bodies
( i i ) Real Area: t he sum o f t he l o c a l areas t r a n s m i t t i n g i n t e r f a c i a l
f o r c e d i r e c t l y between the bodies.
A s p e r i t i e s - The smal l sca le i r r e g u l a r i t i e s on a sur face .
B,O L i f e - see r a t i n g l i f e . ,
L
B a b b i t t Metal - A non- fe r rous bear ing a l l o y , e i t h e r t i n o r lead based con-
s i s t i n g o f va r ious amounts o f copper, antimony, t i n and lead.
Base Stock ( o i l ) - Ref ined pe t ro leum o i l used i n the p roduc t i on o f l u b r i c a n t s
and o t h e r products.
o t h e r base s tocks and/or a d d i t i v e s .
The base s tock may be used a lone or blended w i t h
Bear ing - A support o r guide by means o f which a moving p a r t i s pos i t i oned
w i t h respec t t o the o t h e r p a r t s o f a mechan’ism.
494
Bearing Area - The p r o j e c t e d bear ing load c a r r y i n g area when viewed i n the
d i r e c t i o n o f t he load.
Be i l by l aye r - An amorphous l a y e r o f deformed metal and ox ide p a r t i c l e s formed
by p o l i s h i n g .
Blending - The process o f m ix ing minera l o i l s t o o b t a i n des i red viscous
p r o p e r t i e s .
Boundary L u b r i c a t i o n - A c o n d i t i o n o f l u b r i c a t i o n i n which the f r i c t i o n and wear
between two sur faces i n r e l a t i v e mot ion a r e determined by the p r o p e r t i e s
o f the sur faces , and by the p r o p e r t i e s o f t he l u b r i c a n t o t h e r than b u l k
v i s c o s i t y .
B r i n e l l i n g - Inden ta t i on of t h e su r face o f a s o l i d body by repeated l o c a l
impact o r impacts, o r by s t a t i c over load.
C a v i t a t i o n Eros ion - Wear o f a s o l i d body moving r e l a t i v e l y t o a l i q u i d i n a
reg ion o f c o l l a p s i n g vapour bubbles which cause l o c a l h i g h impact
p ressure 0 r tempe r a t u res .
Centre L ine Average (CLA) - An Eng l i sh measure o f su r face topography repres-
e n t i n g the average depar tu re o f a l i n e p r o f i l e o f the sur face f rom the
c e n t r e l i n e .
Channe ing - The tendency o f a grease t o fo rm a channel by work ing down a
bear ing o r d i s t r i b u t i o n system, l eav ing shoulders t o a c t as a r e s e r v o i r
and sea l .
Clearance Ra t io - In a bear ing , t he r a t i o o f r a d i a l c learance t o s h a f t rad ius .
C o e f f i c i e n t of F r i c t i o n - The r a t i o ob ta ined by d i v i d i n g the t a n g e n t i a l f o r c e
r e s i s t i n g mot ion between two bod ies by the normal f o r c e p ress ing these
bodies together .
Composite Bear ing M a t e r i a l - A s o l i d m a t e r i a l composed o f a cont inuous o r
p a r t i c u l a t e s o l i d l u b r i c a n t phase dispensed th roughout a l oad bear ing
m a t r i x t o p rov ide cont inuous replenishment o f s o l i d l u b r i c a n t f i l m s as
wear occurs , and e f f e c t i v e heat t r a n s f e r f rom the f r i c t i o n sur face .
495
Corros ion I n h i b i t o r - A d d i t i v e s f o r p r o t e c t i n g l u b r i c a t e d surfaces aga ins t
chemical a t tack . They may be p o l a r compounds w e t t i n g the metal sur face
p r e f e r e n t i a l l y , or they may absorb the water t o form a w a t e r - i n - o i l
emulsion - o n l y the o i l touches the metal . Some co r ros ion i n h i b i t o r s
combine chemica l l y w i t h the metal t o g i v e a non-react ive surface.
Corros ive Wear - A process i n which chemical or e lect rochemical r e a c t i o n w i t h
the environment predominates.
C u t t i n g F l u i d - A f l u i d a p p l i e d t o a c u t t i n g t o o l t o a s s i s t i n the c u t t i n g
opera t i on by coo l i ng , l u b r i c a t i n g o r o the r means.
Detergent Add i t i ves - compounds which, when blended w i t h l u b r i c a t i n g o i l s ,
d isperse the d e t e r i o r a t i o n products f r o m the f u e l and l u b r i c a n t , es-
p e c i a l l y those formed under h i g h temperature cond i t i ons , and, thus,
minimise the format ion o f deposi ts l i a b l e t o cause p i s t o n - r i n g s t i c k i n g
or o t h e r t roub le .
Dispersant Add i t i ves - compounds which, when blended w i t h l u b r i c a t i n g o i l s ,
ma in ta in the products o f combustion f rom the fuel i n a f i n e l y dispersed
s t a t e , and thereby minimise sludge fo rma t ion and f i l t e r b lock ing ,
p a r t i c u l a r l y i n gaso l i ne engines opera t i ng under c o l d cond i t i ons .
Drop p o i n t - temperature a t which a drop o f grease o r o the r petroleum product
f i r s t detaches i t s e l f f rom the main b u l k o f ma te r ia l when a sample i s
s t e a d i l y heated under p resc r ibed cond i t i ons .
Duty Parameter - A dimensionless number which i s used t o evaluate the per-
formance o f bear ings.
Dynamic V i s c o s i t y - see: V i s c o s i t y .
E c c e n t r i c i t y Ra t io - In a bear ing, the r a t i o o f the e c c e n t r i c i t y t o the r a d i a l
c l earance.
Elasto-hydrodynamic L u b r i c a t i o n - A c o n d i t i o n o f l u b r i c a t i o n i n which the
f r i c t i o n and f i l m th ickness between two bodies i n r e l a t i v e mot ion a r e
determined by the e l a s t i c p r o p e r t i e s o f the bodies, i n combinat ion w i t h
the v iscous p r o p e r t i e s o f the l u b r i c a n t a t the p r e v a i l i n g pressure,
temperature and r a t e of shear.
496
Embeddabi l i ty - The a b i l i t y o f a bea r ing m a t e r i a l t o embed harmful f o r e i g n
p a r t i c l e s and reduce t h e i r tendency t o cause sco r ing o r abras ion .
Emulsion - A d i s p e r s i o n o f g lobu les o f one l i q u i d i n another i n which i t i s
i nso lub le .
EP (Extreme pressure) A d d i t i v e - A chemical substance c o n t a i n i n g one o r more
elements, e s p e c i a l l y su lphur , c h l o r i n e o r phosphorus, ab le t o reac t w i t h
metal sur faces t o g i v e i no rgan ic f i l m s o f h i g h m e l t i n g p o i n t . The
presence o f these f i l m s h inde rs we ld ing and se i zu re and thus prevents
s c u f f i n g and sco r ing , p a r t i c u l a r l y i n gears o p e r a t i n g under h i g h l oad
cond i t i ons .
Erosion - Eros ive wear i s loss o f m a t e r i a l f rom a s o l i d su r face due t o
r e l a t i v e mot ion i n c o n t a c t w i t h a f l u i d which con ta ins s o l i d p a r t i c l e s .
F a t t y Acids - Long cha in o rgan ic a c i d s which occur n a t u r a l l y as t h e i r g l y c e r i d e
e s t e r s i n animal and vegetab le o i l s and f a t s .
F i l l e r - A substance such as l ime , t a l c , mica and o t h e r powders, added t o
grease t o inc rease i t s cons i s tency or t o an o i l t o inc rease v i s c o s i t y .
Flash P o i n t - The lowest temperature a t which t h e vapour of a l u b r i c a n t can be
i g n i t e d under s p e c i f i e d cond i t i ons .
Flash Temperature - The maximum l o c a l temperature generated a t some p o i n t o f
c l o s e approach i n a s l i d i n g con tac t .
F lexure P i v o t - A type o f bea r ing f o r l i m i t e d movement i n which t h e moving
p a r t s a r e guided by f l e x u r e o f e l a s t i c members r a t h e r than by r o l l i n g
o r s l i d i n g surfaces.
F r e t t i n g - The removal o f ex t remely f i n e p a r t i c l e s f rom bear ing sur faces due t o
the inherent adhesive fo rces between the sur faces p a r t i c u l a r l y under the
c o n d i t i o n o f smal l ampl i tude v i b r a t i o n .
F r e t t i n g Cor ros ion - A fo rm o f f r e t t i n g i n which chemical r e a c t i o n predominates.
F r i c t i o n - The r e s i s t i n g f o r c e t a n g e n t i a l t o a common boundary between two
bodies when, under the a c t i o n o f an e x t e r n a l f o rce , one body moves o r
tends t o move r e l a t i v e t o the su r face o f t h e o the r .
491
F r i c t i o n Polymer - An amorphous o rgan ic depos i t which i s produced when c e r t a i n
meta ls a r e rubbed together i n the presence o f o rgan ic l i q u i d s o r gases.
G a l l i n g - A severe fo rm o f s c u f f i n g assoc ia ted w i t h gross damage t o the surfaces
o r f a i l u r e . The use o f t h i s fo rm should be avoided.
Grease - A l u b r i c a n t composed o f an o i l th ickened w i t h a soap o r o the r th ickener
t o a semi -so l i d o r s o l i d cons is tency . A l ime-based grease i s prepared
f rom a l u b r i c a t i n g o i l and Calcium soap. Sodium, Barium, L i t h i u m and
Aluminium based greases a r e a l s o used.
Hydrau l i c F l u i d - A f l u i d used f o r t ransmiss ion o f h y d r a u l i c p ressure o r ac t i on ,
n o t n e c e s s a r i l y i n v o l v i n g l u b r i c a n t p r o p e r t i e s . May be o i l , water o r
s y n t h e t i c ( f i r e r e s i s t a n t ) 1 i qu ids .
Hydrodynamic L u b r i c a t i o n - A system o f l u b r i c a t i o n i n which the shape and
r e l a t i v e mot ion o f t he s l i d i n g sur faces causes the fo rma t ion o f a f l u i d
f i l m hav ing s u f f i c i e n t p ressure t o separate the sur faces .
Hydros ta t i c L u b r i c a t i o n - A system o f l u b r i c a t i o n i n which the l u b r i c a n t i s
supp l i ed under s u f f i c i e n t e x t e r n a l p ressure t o separate the opposing
sur faces by a f l u i d f i l m .
I n i t i a l P i t t i n g - Surface f a t i g u e o c c u r r i n g du r ing the e a r l y stages o f opera t ion
o f gears, assoc ia ted w i t h removal o f h i g h l y s t ressed l o c a l areas and
runn ing - in .
Journa l - That p a r t o f a s h a f t o r a x l e which r o t a t e s o r o s c i l l a t e s r e l a t i v e l y
t o a r a d i a l bear ing .
Kinematic V i s c o s i t y - See: V i s c o s i t y .
L,o l i f e - See: Rat ing L i f e .
Lacquer - Hard, l u s t r o u s , v a r n i s h - l i k e , o i l i n s o l u b l e depos i t which tends t o
fo rm on the p i s t o n s and c y l i n d e r s o f i n t e r n a l combustion engines.
Load Car ry ing Capaci ty - The maximum load t h a t a s l i d i n g o r r o l l i n g system can
suppor t w i t h o u t f a i l u r e o r the wear exceeding the des ign l i m i t s f o r the
p a r t i c u l a r a p p l i c a t i o n .
498
Lubr icant - Any substance in terposed between two sur faces i n r e l a t i v e mot ion
f o r the purpose o f reducing the f r i c t i o n o r wear between them.
M i l d Wear - A form o f wear cha rac te r i sed by removal o f m a t e r i a l i n very small
fragments.
Non-Newtonian V i s c o s i t y - The apparent v i s c o s i t y o f a ma te r ia l i n which the
shear s t ress i s no t p ropor t i ona l t o the r a t e o f shear.
O i l - A l i q u i d o f vegetable, animal, minera l o r s y n t h e t i c o r i g i n f e e l i n g
sl ippery t o the touch.
O i l i n e s s - That p roper t y o f a l u b r i c a n t t h a t produces low f r i c t i o n under
cond i t i ons o f boundary l u b r i c a t i o n . The lower the f r i c t i o n , the g rea te r
the o i l i n e s s .
O i l Mis t (Fog) - An o i l atomised w i t h the a i d o f compressed a i r and then con-
veyed by the a i r i n a low-pressure d i s t r i b u t i o n system to m u l t i p l e
p o i n t s o f l u b r i c a n t a p p l i c a t i o n .
P i t t i n g - Any removal o r displacement o f m a t e r i a l r e s u l t i n g i n the format ion o f
sur face c a v i t i e s .
P l a i n Bear ing - Any simple s l i d i n g type o f bear ing as d i s t i n g u i s h e d from f i x e d -
pad, pivoted-pad o r r o l l i n g - t y p e bear ings.
Porous Bear ing - A bea r ing made from porous m a t e r i a l such as compressed metal
powders, the pores a c t i n g e i t h e r as r e s e r v o i r s f o r ho ld ing , o r passages
f o r supply ing l u b r i c a n t .
Pour Po in t - The lowest temperature a t which a l u b r i c a n t can be observed t o
f l o w under s p e c i f i e d cond i t i ons .
PTFE - P o l y t e t r a f l u o r e t h y l e n e , a polymer hav ing outs tanding l o w - f r i c t i o n
p r o p e r t i e s over a wide temperature range.
PV Factor - The product o f bear ing pressure and sur face v e l o c i t y .
499
Rat ing L i f e - The f a t i g u e l i f e i n m i l l i o n s o f r e v o l u t i o n s o r hours
ope ra t i ng speed which 90 per cen t o f a group of s u b s t a n t i a l
r o l l i n g element bear ings w i l l su rv i ve under a g iven load.
cent r a t i n g 1 i f e i s f r e q u e n t l y r e f e r r e d t o as l lL,o-l i f e " or
a t a g iven
y i d e n t i c a l
he 90 per
"B,o-l i f e " .
Redwood V i s c o s i t y - A comrnerical measure o f v i s c o s i t y expressed as the t ime i n
seconds requ i red f o r 50 cub ic cent imeters o f a f l u i d t o f l o w through a
tube o f 10 mm leng th and 1 . 5 mm diameter a t a g i ven temperature.
Root Mean Square Height (RMS) - An American measure o f sur face topography
represent ing the average depar ture o f a l i n e p r o f i l e of t he su r face
from a mean l i n e .
SAE - Society o f Automotive Engineers.
Saybol t V i s c o s i t y - A commerical measure o f v i s c o s i t y expressed as the t ime i n
seconds requ i red f o r 60 cub ic cent imeters o f a f l u i d t o f l o w through the
o r i f i c e o f t he Standard Saybol t Universa l Viscometer a t a g iven tem-
pe ra tu re under s p e c i f i e d cond i t i ons .
Scor ing - The fo rma t ion o f severe scratches i n the d i r e c t i o n o f s l i d i n g .
Scratch ing - The fo rma t ion o f f i n e scratches i n the d i r e c t i o n o f s l i d i n g
Scu f f i ng - Loca l i sed damage caused by the occurrence o f sol id-phase welding
between s l i d i n g surfaces, w i thou t l o c a l sur face me l t i ng .
Severe Wear - A form o f wear cha rac te r i sed by removal o f ma te r ia l i n r e l a t i v e l y
l a r g e fragments.
Soap - I n l u b r i c a t i o n , a compound formed by the r e a c t i o n o f a f a t t y ' a c i d w i t h
a metal o r metal compound.
S o l i d Lub r i can t - Any s o l i d used as a powder o r t h i n f i l m on a sur face t o
p rov ide p r o t e c t i o n f rom damage du r ing r e l a t i v e movement, and t o reduce
f r i c t i o n and wear.
S p a l l i n g - Separat ion o f p a r t i c l e s f rom a sur face i n the form o f f l akes .
S t i c k - S l i p - A r e l a x a t i o n o s c i l l a t i o n u s u a l l y assoc iated w i t h decrease i n the
c o e f f i c i e n t o f f r i c t i o n as the r e l a t i v e v e l o c i t y increases.
500
Synthet ic Lub r i can t - A l u b r i c a n t produced by synthes is r a t h e r than by e x t r a c t i o n
o r ref inement.
Thin F i l m L u b r i c a t i o n - A c o n d i t i o n o f l u b r i c a t i o n i n which the f i l m th ickness o f
the l u b r i c a n t i s such t h a t the f r i c t i o n between the sur faces i s de te r -
mined by the p r o p e r t i e s o f the sur faces as w e l l as by the v i s c o s i t y o f
the l u b r i c a n t .
Tota l Ac id Number (TAN) - The q u a n t i t y o f base, expressed i n terms o f the
equ iva len t number o f m i l l i g r a m s o f potassium hydrox ide t h a t i s requ i red
t o n e u t r a l i s e a l l a c i d i c c o n s t i t u e n t s present i n 1 gram o f sample.
Tota l Base Number (TBN) - The q u a n t i t y o f a c i d , expressed i n terms o f the
equ iva len t number o f m i l l i g r a m s o f potassium hydrox ide t h a t i s requ i red
t o n e u t r a l i s e a l l bas i c c o n s t i t u e n t s present i n 1 gram o f sample.
Varnish - A deposi t r e s u l t i n g f rom the o x i d a t i o n and/or po l ymer i sa t i on o f f u e l s ,
l u b r i c a t i n g o i l s , o r organic c o n s t i t u e n t s o f bea r ing m a t e r i a l s .
V i s c o s i t y - That b u l k p roper t y o f a f l u i d , s e m i - f l u i d o r semi -so l i d substance
which causes i t t o r e s i s t f l ow .
V i s c o s i t y i s de f i ned by the equat ion
dv q = -r/-
dx
T i s the shear s t ress , v the v e l o c i t y , ds the th ickness o f an element
measured perpendicu lar t o the d i r e c t i o n of f l o w ; dv/ds i s known as
the r a t e o f shear. V i s c o s i t y i n the normal, t h a t i s Newtonian sense
i s o f t e n c a l l e d dynamic o r abso lu te v i s c o s i t y .
v i s c o s i t y i s the r a t i o o f dynamic v i s c o s i t y t o dens i t y a t a s p e c i f i e d
temperature and pressure.
Kinematic o r s t a t i c
V i s c o s i t y Index (VI ) - A r b i t r a r y sca le used t o show the magnitude o f v i s c o s i t y
changes w i t h temperature i n l u b r i c a t i n g o i l s and o t h e r products.
Wear - The removal of m a t e r i a l from sur faces i n r e l a t i v e mot ion, normal ly by
abrasion, adhesion o r co r ros ion .
Wedge E f fec t - The establ ishment o f a pressure wedge i n a l u b r i c a n t .
W e t t a b i l i t y - A term used t o i n d i c a t e the ease w i t h which a l u b r i c a n t w i l l
spread o r f l o w over a bea r ing surface.
501
ZDDP - I n i t i a l s f o r z inc dialkyl-dithiophosphate, which i s w ide l y used a s an
extreme pressure agent. It i s a l s o an e f f e c t i v e o x i d a t i o n i n h i b i t o r
bu t should n o t be used i n mechanisms w i t h s i l v e r bear ings.
Acknowledgement. Permission t o quote terms and d e f i n i t i o n s received f rom
The I n s t i t u t e o f Petroleum and the OECD.
502
AUTHOR INDEX
Numbers under l i ned g i v e the page on which the complete re fe rence i s l i s t e d ,
o ther numbers r e f e r t o the page number on which the au thor ( o r h i s work) i s
mentioned i n the t e x t .
Amateau,M.F., 210 221 Archard,J.F., 13 2 Attwood,D.G., 3 lo Aubert,F., 220 222 -
Babichev, 17 2 Bailey,J.M., 18 2 Balmer,C., 329 Bartenev,G.M., 7 1 Barwell,F.T., 14 15 17 25 3s
471 482 488 492 - Bates,T.R., 13
Beilby,G., 4 lo 13 2 Bickerman,J.J., 14 2 Bingham,E., 278 290 Blackwel l ,J., 7 2 23 2 Block.H., 12 15 2 60 79 Booser,E.R., 47 49 78 131
Boothroyd,G., 461 2 Bosma,R., 60 79 Bowden,F.P., 12 15 - 28
Bowen,E.R., 9 11 13 25 280-
Brainard,W.A., 13 28 Brai thwaite,E.R., 3 10 130 221
Bridges,D.C., 428 445 Brown,T.W.F., 64 79 Bryce,J.B., 2 Buckley,D.H., 4 221 222 Bunshah,R.F., 220 222 Burke ,A. E. , 130
Cameron,A., 5 fi 52 78 130
Church i l l , J .R. , 130 Chynoweth,A., 3 lo Clausen,J., 284 - 290
Collacott,R.A., 24 30 427 428 433 445 Cree,J.C., 474 492 Crooks,C.S., 130 Czichos,H., 4 10
Davies,A.E., 428 - 445
Davis,T.A., 130 Oawson,P.H., 17 29 130
Decoufle,P., 277 289 Devine,M.J., 3
De Vos,H., 281 290 Dobychin,M.N., 4 lo Dombroski,R.M., 426 Dowson,O., 1 5 10 11 130
Duckworth,W.E., 206 221 Dumbleton,J.H., 5 fi 14 2 Orasche,H., 277 289
Eastham,D.R., 130 E l w e l l ,R.C., 47 78 Engel , P . A . , 18 3 Evans,F.C., 217 221 Eyres ,A. R., 275
Fein,R.S., 5 fi Fid le r ,F . , 17 29 130
503
Forrester,P.G., 130 131 208 ?21 Forsy th , I., 5 '0 Fowle,T. I., 184 193 196
Garner,D.A., 45 58 72 75 78 79 131
Gass,H., 220 222
Georges,J.M., 4 lo Glasser,W.A., 210 - 221
Godet,M., 130 Godfrey,D., 18 2 Gregory,J.C., 220 222 Grunberg,L., 20 2
-
Halligan,B.D., 330 353 370
Ha l l ing ,J . , 3 lo 221 222 Hancock,B.T., 5 11 Harvey,B.F., 428 445 Hastings,G.W., 5 fl Hibbs,L.E., 465 9 Higginson,G., 5 2 Hiley,R.W., 130 Hinterrnan,H.E., 7 11 220 222 Hirst,W., 14 2 Hjertzen,D.G., 132
H o l l igan,P.T., 9 Horton,A.W., 278 - 290
Hother-Lushington,S., 17 2 Hunter,M.S., 130
Jahanrnir,S., 13 a Jarnes,R.D.,
Jarnieson,D.T., 20 2 Jarvis,R.A., 132
Johnson,K.L., 5 2 Jones,G.J., 72 2 Jones,M.H., 471
Jones,M.P., 3 to Jost,H.P., 2 3 9 10 11 395 425 426
Juntz,R.S., 220 222 Justusson,W.M., 206 221
--
Kennedy-Srnith,R., 266 Kilbourn,D.F., 428 445 King,M., 4 lo Kirk,J.A., 13 3 Knight,R.E., 426 Kornbalov,V.S., 4 lo Krage lsky , I .V., 4 lo 14 9 Kruschov,M.M., 17 9
Lancaster,J.K., 6 11 217 221 Lansdown,A.R., 223 267
Laventev,V.V., 7 11 Leak,D.A., 206 221 Lee,C.S., 75 2 Lehrke,W.D., 2 lo Leitch,A., 278 290 Ling,F.F., 3 8
Linkinhoker,C.L., 49
Lloyd,K.A., 130
Love,P., 130 Loy,B., 21 2 Ludema,K.C., lo 13 27 28 Lund,J.W., 50 52 78 Lushbaugh,C.C., 277 2
Martin,F.A., 45 47 58 72 75 78 79 Mathieson,T.C., 429 445
--
McCoy,W.E., 425 McCullagh,P.J., 7 2 23 2 McDonald,D., 29 Mear,R.B., 130 Mear5,D.C.. 28 30 Meckleburg,K.R., 485 492 Merrett,J.G., 395 426
Mills,G.H., 21 25 27 29 30
Milne,A.A., 15 3 Ming Feng, I., 13 15 28
Moes,H., 60 2 Moore,M.A., 17 9 Morris,J.A., 130 Murray,P.W., 395 425
--
-
504
Neale,M.J., 2 lo 31 131 Newman,A.O., 64 79 Nicholson,O.W., 210 221
Nonnen,F.A., 2 lo -
Perkins,C.A., 130 Ph i l l i ps ,R . , 206 221 Pinkus,O., 52 78 Pocock,G., 443 - 445
Pratt,G.C., 130 217 221 Przbyszewski,J.S., 221 222
Quayle,J.P., 130
Raask,E., 2
Rabinowicz,E., 3 3 13 a Rafique,S.O., 130 Rapp,C., 281 2 Rastogi ,S.C., 284 290 Reda,A.A., 13 8 Reynolds,O., 5 fi Richardson,R.C.D., 3
Roehner,T.G., 130 Rollason,E.C., 454 - 470
Rose,A., 130 Roylance,B. J., 479 492 Ruff,W., 25 30 Ryman,F.O., 49 78
Saka,N., lo Sanderson,J., 278 - 289
Scala,R.A., 279 2 Schof ie ld,J. , 2 10 426
Scott,O., 1 3 4 5 7 10 11 13 14 15 17
18 20 21 23 24 25 27 3 205 211 212 216 218 221 488
492
Scott,H.H., 14 18 20 29
Sharp,W.F., 219 222 Shen,C., 14 3 Sherbiney,M.A., 221 222
--
- -
-
Shone,E.B., 80 130 Smal lheer,C.V., 266 Smart,E.F., 468 470 Smi th,A. I . , 216 221 Soul ,D.M., 242
Spalvins,T., 221 222 Spencer,J.B., 291 329 Sridharan,P., 13
S te rn l i ch t ,B . , 52 78 Suh,N.P., 1 4 to 13 15 8 Summers-Smith,O., 216 221 Swanson,T.D., 13 29
Tabor,D., 12 15 8 Tai t ,J . , 216 221 Taylor,C.M., 130 Teer,D.G., 221 222 Tevazwark,J.L., 5 1_1
Thomas,T.R., 4 5 lo Thomson ,K .K. , 52 78 Thony,C., 283 290 Tilley,G.P., 2 lo Tourret,R., 23 3 Tremain,G.R., 216 221 Trent,E.M., 215 221 446 452 464
468 470 -
Van Peteghem,T., 281 290 Venton,A.D.F., 428 445
Wagner,W.D., 277 289 Wahlberg,J.E., 279 290 Walsh,W., 3 lo Warriner,J.F., 131
Waterhouse,R.B., 18 19 2 Welsh,N.C., 15 2 Wentorf ,R.E., 465 470 West,C.H., 425 Westcott,V.C., 4 9 13 15 24
25 28 29 30
505
Wilkinson,H.C. , 428 - 445
Wilks,P.E. , 425 Wil l iams,D.F . , 5 11 W i 1 1 iams,G., 425 Wil l iams,J .E. , 454 470 Wilson,R.W., 7 fi 80 130 218 222 Woldman,N.E., 205 221 Wright ,E.P. , 23 9 Wright,K.H.R., 17 18 2
Zakay,V.F., 206 221 Zingmark,P.A., 281 290 Z l a t i n , L . , 3 lo Zorev,N.N., 448 - 470
506
SUBJECT INDEX
Abel method 244
Abrasion
d e f i n i t i o n 17 493 economics 3
Abrasive Wear 3 12 17
c u t t i n g t o o l s 214 461 467
damage 17
e f f e c t o f hardness 17
gears 194 202
p a r t i c l e s 296
p l a i n bear ings 81 97 207
res i s tance 206 213
r o l l i n g bear ings 173
seals 334 374 Accelerometers 434
A c i d i t y 244
Acid t reatment 275
Add i t i ves
a n t i
a n t i
a n t i
a n t i
a n t i
a n t i
c h a t t e r 265
co r ros ion 276
foam 247
ox idan ts 232 246 251 252 276
squawk 265
wear 231 246 260 276 281 493
b ioc ides 276 289
co r ros ion i n h i b i t o r 246 251 253 254
detergents 243 245 246 249 276 495
d ispersants 243 249 251 276 495 e m u l s i f i e r s 247 255 264 276
extreme pressure 231 247 258 259 276 467 474 496
495
e f f e c t on gears 191 193 200 201
e f f e c t on p l a i n bear ings 112
e f f e c t on s c u f f i n g 15
f r i c t i o n m o d i f i e r 246 264
hazards 275-290
head 280
metal c u t t i n g 262 467
metal deac t i va to r 247
pour p o i n t depressant 247 257
r u s t i n h i b i t o r 246 254
tack iness 247
v i s c o s i t y index improver 247 250 255
Adhesion wear theory 13 493
A i r entrainment 347
A1 ignment, gears 184-187
Almen Weiland t e s t 261
Amides 250
Analys is
Auger e l e c t r o n spectroscopy 4
ion spectroscopy 4
microprobe 4
scanning e l e c t r o n microscopy 4
spect rographic o i l 8 24 439
x ray photon 4
An t i c h a t t e r a d d i t i v e 265
An t i co r ros ion a d d i t i v e 276
An t i foam a d d i t i v e 247
A n t i ox idan t a d d i t i v e 232 246 251 252
An t i squawk a d d i t i v e 265
A n t i wear a d d i t i v e 231 246 260 276 281
Area o f con tac t
276
493
apparent 452 493
r e a l 452 493
A r t h r i t i s 28
Asbestos
packings 377 378 h e a l t h 393
A s p e r i t i e s 493
r o l e i n wear 13
A t t r i t i o n 465
Aus forming 206
507
Bac te r ia 282 289
Bad bonding 89
B a r r i e r cream 286
Bear ing area 494
m a t e r i a l s
aluminium a l l o y 83 87
aluminium lead 210
aluminium t i n 83 210
b a b b i t 82 493
copper l ead a l l o y s 83 86 208 474 lead bronze 83 86 208
ny lon 217
o v e r l a y 82 84 208
phosphor bronze 83 88
p l a s t i c 217
po lyace ta l 218
porous 88 498
p t f e 215-218
s i l i c o n bronze 88
s i l v e r 88
the rmop las t i c 217
thermoset 217
t r i m e t a l 82 88
wh i te metal 82 208
Bear ings
f l e x u r e 32 36 42
h y d r o s t a t i c 35 37
magnet ic 32
p l a i n j o u r n a l
bear ing load c a p a c i t y 54 61
c learance 65
des ign l i m i t s 46
e f f e c t o f bea r ing temperature 48 60
groov ing 56
misal ignment 67
o i l f i l m s t a b i l i t y 50 64
o i l f i l m t h i ckness 60
o i l o x i d a t i o n 50
sur face roughness 47
f a i l u r e s see f a i l u r e s
f l u i d f i l m 35
Bear ings
p l a i n j o u r n a l
heat balance 57
l u b r i c a t i o n requirements 402
m a t e r i a l s e l e c t i o n 43
maximum pressure 43
maximum temperature 71
minimum c learance 65
non laminar 72
performance 39 65
p l a s t i c 7 217
porous 32 88
power loss 67 70
p r o f i l e bore 72
pv f a c t o r 34 498
s p e c i f i c load r a t i n g 66
sur face f i n i s h 47
r o l l i ng element
a p p l i c a t i o n s 166
c lean ing 171
c 1 earances 144
c o r r e c t i o n f a c t o r 146
damage see f a i l u r e s
dimensions 144
d i sman t l i ng 168
f a t i g u e l i f e 35 139
f i t s 163
f r i c t i o n 150
grease 152
load c a r r y i n g c a p a c i t y 139
l u b r i c a t i o n 151 151
m a t e r i a l 211
mounting 167
no ise 159
o i l m i s t 158
o i l s e l e c t i o n 156
performance 39
sea ls 157
s e l e c t i o n 132
shock pu lse 163
speed l i m i t 145 148 149
508
t ype a x i a l p i s t o n pump 298
angu lar con tac t 134
b a l l t h r u s t 138 r a d i a l 45 67
c y l i n d r i c a l r o l l e r 135 r a t i o 494
double row angu lar c o n t a c t 134 spool va l ve 301
gear pump 298
double row r o l l e r 135
double row spher i ca l 137
needle 136
s e l f a l i g n i n g 133
s i n g l e row deep groove 133
spher i ca l r o l l e r t h r u s t 138
taper r o l l e r 136
rubb ing 34
l i f e 140
m a t e r i a l s e l e c t i o n 210
pv f a c t o r s 41
Be i l by l a y e r 454
Beta r a t i o 307
Bioc ides 282 289
Boroscope 437
Brake m a t e r i a l 7
B r i n e l l i n g 178 494
B r o o k f i e l d v iscometer 255
B u i l t up edge 454
Bypass see f i l t r a t i o n
Cams 216
Carbur i s ing 210 216 219
Carcinogenic compounds 279
Case hardening 210 219
C a v i t a t i o n
e ros ion 101 117-121 397 494
m a t e r i a l r e s i s t a n t 10 19
sur face damage 19
Centre l i n e average 494
Ceramics 6 213
Channeling 494
Che la t ing agents 254
Chemical vapour d e p o s i t i o n 7 220 464
Chromium p l a t i n g 216
Clearance c r i t i c a l 297 299
vane pump 298
Cleveland method 244
Cloud p o i n t 235
Cold c rank s imu la to r 255
Composites
bea r ing des ign da ta 6
bea r ing m a t e r i a l 6
bea r ing performance 6
m a t e r i a l a p p l i c a t i o n 207
Compression packing see Seals
Cond i t ion mon i to r i ng 427-445
Conradson method 244
Consumption o i l 267
Contact area 184 452 493
Contaminant a n a l y s i s 438
Contaminat ion
b u i l t i n 292 294
c o n t r o l 315
e f f e c t on d i r e c t i o n va lves 301
f l o w c o n t r o l s 302
gears 194
l u b r i c a n t s 234
motors 300
pressure c o n t r o l 302
pumps 297
va l ve spools 301
environmental 294
generated 296
l e v e l s 304
sea ls 347
s p e c i f i c a t i o n 305
sources 294 315
Coolant 467
Copolymers 250
509
Corros ion
ceramics 7 copper lead a l l o y s 107
i n h i b i t o r 251 253 254
p l a i n bear ings 102-117
r o l l i n g bear ings 181
res i s tance 102-117
see a l s o f a i l u r e s
C u t t i n g f l u i d s 6 262
C u t t i n g speeds 449 451
C u t t i n g tools 215
aluminium ox ide 450 464
boron n i t r i d e 450 465
ceramic 215 464
cemented carb ides 215 450 464
cos ts 451
h igh speed t o o l s t e e l 451 464
m a t e r i a l for 450 464
s i l i c o n ca rb ide 450
wear o f 7 215 459-467
Debr is
a n a l y s i s 24 419
c u t t i n g wear 25 26
f a t i g u e 27 440
laminar wear 25 440
rubbing wear 25 26
spher i ca l wear 25
synovia l f l u i d 28
Del ami na t ion theory 13
Delphi a n a l y s i s 428
Dermat i t i s 277
D e t e r i o r a t i o n l i m i t 444
Detergents 243 245 246 249
Dispersants 243 249 251
Di th iocarbonates 253
Dithiophosphates 252 253
Drawing 263
D r i l l i n g 263
Drop p o i n t 495
Duty parameter 495
Dye p e n e t r a t i o n 436
Dynamic load r a t i n g 139
E c c e n t r i c i t y r a t i o 46 51 495
Eddy c u r r e n t t e s t 437
E las to hydrodynamic l u b r i c a t i o n 5 495
polymers 217
t r a c t i o n 5
E l e c t r i c discharge 119 181
res i s tance method 437
E l e c t r o chemical machining 450
discharge machining 450
Embeddabi 1 i t y 81
Emulsions 237 255 282 318
Engine t e s t s 472
Eros ion
bear inq damage 117-121
ceramics 7 economics 2
pressure c o n t r o l 302
steam 38
wear 12 496
see a l s o f a i l u r e s
Extreme pressure a d d i t i v e 232 258 259 496
c u t t i n g o i l s 262
e f f e c t on s c u f f i n g 14
e f f e c t on p l a i n bear ings 112
e f f e c t on gears 191 193 200 201
t u r b i n e o i l s 262
see a l s o a d d i t i v e s
Eye i r r i t a t i o n 280
Fa i 1 ures
Gears
abras ive wear 194 202
al ignment 184-187
broken t e e t h 197
f a t i g u e p i t t i n g 189 197 198 474
foaming 204
f r e t t i n g 202
498
510
g a l l i n g 191 497
groov ing 188
no ise 203
overheat ing 203
r i d g i n g 188 474 475
r i p p l i n g 201 474 476
sco r ing 476
s c u f f i n g 191 199 474 476
v i b r a t i o n 203
p l a i n bear ings
a l l o y i n g 125
bad bonding 89
bad f i t t i n g 96
c a v i t a t i o n 117-121
c o r r o s i o n 102-117
e l e c t r i c a l d ischarge 119
e ros ion 117-121
excess ive lead 91
extraneous p a r t i c l e s 97-101
f a t i g u e 123
gas c a v i t i e s 91
ove rs i ze cuboids 91
thermal c y c l i n g 125
v i s c o s i t y 126
w i r e wool 122
r o l l ing bear ings
ab ras i ve 173
cage 175
co r ros ion 181
e l e c t r i c c u r r e n t 181
f a l s e b r i n e l l i n g 178
i n c o r r e c t mounting 174
smearing 177
v i b r a t i o n 178
wear 172
Falex t e s t 261
False b r i n e l l i n g 178
Fat igue
f r e t t i n g 18
p l a i n bear ings 123
r o l l i n g con tac t 20
see a l s o f a i l u r e s
Fau l t t r e e 429
Ferrography 9 25 439 442
F i l l e r 496
F i l t r a t i o n
e f f e c t o f f i r e r e s i s t a n t f l u i d s 318
emulsions 318
magnets 319
o f f l i n e 317 324
pressure l i n e 316 320
pump i n l e t 318 327
r e t u r n l i n e 317 322
sea ls 347
F i 1 t e r
abso lu te r a t i n g 306
a i r b rea the r 294 295
be ta r a t i o 307
bubble t e s t 306
bypass 308 309 320
c l a s s i f i c a t i o n 307
d i r t c a p a c i t y 313
e f f i c i e n c y 310 321
gears 194
l o c a t i o n 318
mean r a t i n g 306
mu l t i pass t e s t 307
nominal r a t i n g 305
o f f l i n e 317 324
pressure l i n e 316 320
p u l s a t i n g f l o w 308
r e t u r n l i n e 317 322 324
s e l e c t i o n 305-317
s i l t c o n t r o l 308 312
s i z i n g 312
F i r e r e s i s t a n t f l u i d 234 318
F lammab i l i t y 234
F lash p o i n t 235 244 496
F1 ash temperature 496
F lexure bear ing 32 36 496
performance 42
Flow zone 456
511
F lu id e ros ion 19
Flushing 292 294
Flux t e s t i n g 437
Four b a l l machine 232 261 485 487
Frequency spectrum 435
F re t t i ng
d e f i n i t i o n 18 496
gears 202
wear deb r i s 18
see f a i l u r e s
F r i c t i o n m o d i f i e r 264
F r i c t i o n we ld ing 452
FZG t e s t machine 261 478
Ga l l ing 191 497
Gears 184-204
a l ignment 184
contac t a rea 184
f a i l u r e s see f a i l u r e s
fa t i gue p a r t i c l e s 25
f i l t r a t i o n 194
l u b r i c a t i o n systems 195
ma te r ia l s e l e c t i o n 210
o i l v i s c o s i t y f o r 193 194
s c u f f i n g 191 199
t e s t s 261 478
Graphite 239 240 242 380
Grease
a p p l i c a t i o n 396
c l a s s i f i c a t i o n 399 497
r o l l i n g bear ing 152
se lec t i on 224 396
temperature l i m i t s 224 401
Grooving 188
Hardenab i 1 i t y
e f fec t o f chromium 211
e f f e c t o f vanadium 211
Hardness
too l s t e e l 214
e f f e c t on f a t i g u e s t reng th 207
Hazards
a d d i t i v e s 280
b a c t e r i a 282 289
b ioc ides 282 289
d e r m a t i t i s 277
eye i r r i t a t i o n 280
h e a l t h 545
o i l m i s t 277 287
o i l vapour 278
rec la imed o i l 283
r e - r e f ined o i 1 283
scro ta1 cancer 279
s k i n cancer 278
s y n t h e t i c l u b r i c a n t s 283
Hea l th and Safe ty a t Work Ac t 284
Holography 437
Hydros ta t i c bear ing 32 34 35
IAE t e s t 261
l n f r a red technique 437
Inspec t i on techniques 436
I r o n p r i n t i n g 99
Jos t r e p o r t 1
K u r t o s i s 429
Lacquer 472 474 497
Lubr i can t
a n t i f r i c t i o n p r o p e r t i e s 231
a n t i wear p r o p e r t i e s 231
e s t e r s 235
extreme pressure p r o p e r t i e s 231
l i m i t s o f o i l 224
l i m i t s of grease 224
l i m i t s o f s o l i d 224
minera l o i l 226
polyphenol e the rs 236
s e l e c t i o n 223-241 400 467
s i l i c o n e s 235
s o l i d 239
512
v i s c o s i t y 226 229
L u b r i c a t i o n systems
c o o l i n g 418
dua l l i n e 409
d i r e c t f e e d 403
d i r e c t p o s i t i v e 414
gas 239
gears 195
i n d i r e c t l i n e 405
m i c r o f o g 419
o i l 413
p o s i t i v e s p l i t 415
p r o g r e s s i v e 408
s e l e c t i o n 403 412
t o t a l loss 413
Machine e lements
h i g h e r p a i r 482 483
lower p a i r 482 483
Maintenance on c o n d i t i o n 427-445
Magnet ic b e a r i n g 32
Magnet ic p l u g 24 439 440
Mater i a 1
a b r a s i o n r e s i s t a n c e 206
compos i te 206
c o r r o s i o n r e s i s t a n c e 206
e l e v a t e d tempera ture 206 207
f i b r e r e i n f o r c e d 206 207
p l a s t i c b e a r i n g 7
s e l e c t i o n 205-222
Meta l c u t t i n g
a b r a s i o n 461
a t t r i t i o n 465
b u i l t up edge 454
c h i p 4 4 7 454
c l e a r a n c e a n g l e 447
c l e a r a n c e f a c e 447
c o o l a n t s 467
c r a t e r wear 460
c u t t i n g edge 447
c u t t i n g speed 447 451
c u t t i n q t o o l tempera ture 456
d i f f u s i o n 462
e f f e c t o f h e a t 451
e f f e c t o f s l i d i n g 458
e f f e c t o f speed 451
e f f i c i e n c y 446
f e e d 448
f l a n k wear 447
f l o w zone 455
h e a t g e n e r a t i o n 451
l u b r i c a n t s 467
q u i c k s t o p method 447
r a k e 447
s e i z u r e 452 458
s u r f a c e s h e a r i n g 461
tempera ture g r a d i e n t 457
t o o l f o r c e 448
t o o l wear 459
Molybdenum d i s u l p h i d e 239 240 242
M o n i t o r i n g t e c h n i q u e s
contaminant a n a l y s i s 8 429 433 438
spec t rum a n a l y s i s 434
t r e n d a n a l y s i s 433
v i b r a t i o n a n a l y s i s 8 429 432 433
waveform a n a l y s i s 436
Niemann FZG t e s t 261
N i t r i d i n g 210
No ise r o l l i n g b e a r i n g s 159
Non d e s t r u c t i v e t e s t i n g 436
dye p e n e t r a t i o n 436
eddy c u r r e n t 436
e l e c t r i c a l r e s i s t a n c e 436
f l u x 436
r a d i o g r a p h i c 436
u l t r a s o n i c 436
O i l d e g r a d a t i o n 270
f i l m i n s t a b i l i t y 50 64
f i l m w h i r l 72
m i s t 158 277 287 498
513
o x i d a t i o n l i m i t 50
standard t e s t s 244
vapours 278 287
O i l i n e s s 498
Over lay bear ings 208
Ox ida t i on res i s tance 6 207
Packed g land 370
P a r t i c l e s
type o f wear 24 440
e f f e c t on f l u i d power systems 296
P a r t i c l e coun t ing 439
Patch t e s t 439
Pensky Mar t i n Method 244
Phenates 248
Phenoles 252
Phosphonates 245
P is ton r i n g
m a t e r i a l s e l e c t i o n 215
wear 216
P i t t i n g 189 197 198 474 498
P l a i n bear ings see bear ings
P l a s t i c s 7 Porous bear ings 88 498
Pour p o i n t 235 244 257 498
P r o f i l e bore bear ings 72
PTFE 6 216 239 240 242
Pumps
PV f a c t o r 34 498
e f f e c t o f contaminant 297 298
Radial c learance 45 144
Radiographic t e s t 437
Ramsbottom method 244
Rat ing l i f e 499
Reaming 263
Reclaimed o i l safety 283
Reclamation 9 271
Redwood v i s c o s i t y 499
R e l i a b i l i t y a n a l y s i s 431
Re-ref i n i n g
a c i d c l a y 272
economics 273
Matthys process 273
Ridging gears 188 474 475
R ipp l i ng gears 201 474 496
R o l l ing bear ings see bear ings
R o l l i n g con tac t f a t i g u e
crack propagat ion 23
damage 20
d e f i n i t i o n 20
e f f e c t o f environment 20
e f f e c t o f l u b r i c a n t 23
e f f e c t o f m a t e r i a l 23
p a r t i c l e s 27
Root mean square he igh t 499
Rubbing bear ings see bear ings
Rust i n h i b i t o r 246 254
Ryder t e s t machine 261
Safety l u b r i c a n t 275-290
Sal i c y l a t e s 249
Saybol t v i s c o s i t y 499
Scor ing 476 499
Scu f f i ng 14
cams and tappets 216
e f f e c t o f extreme pressure a d d i t i v e 15
f a t i g u e p a r t i c l e s
gears 191 199 474 476
i n c i p i e n t 15
mechanism o f 14 499
p i s t o n r i n g s 215
sur face changes 15
Seal l u b r i c a t i o n 356 357
Sea 1 s
compression packings 370-394
abras ive wear 374
c o m p r e s s i b i l i t y 381
cos t 372 387
f a u l t s 390
514
f i t t i n g 388
fo r pumps 371
f o r va lves 371
gland design 374 375
env i ronmen t 384
h e a l t h aspects 391
i n s t a l l a t i o n 372
leakage 382
ma i ntenance 372
m a t e r i a l s 376-382
packed gland 370
r e l i a b i l i t y 372
r o l l i n g bear ings 157
s e l e c t i o n 383
sha f t wear 372
speed 385
s tandard i sa t i on 391
sur face f i n i s h 375
temperature 386
Seals l i p 338 339
assembly 351
contact area 338 345
e f f e c t o f deformat ion 351
humid i ty 351
1 i g h t 351
oxone 351
oxygen 351
temperature 350
energ ised 345
f r i c t i o n 344
leakage 339
ma te r ia l s e l e c t i o n 331-338
m u l t i l i p 343
performance 343
se lec t i ons 348
squeeze 338 339
storage 350
sur face roughness 343
Sea 1 s rec i procat i ng
double a c t i n g 340
energised sleeve 340 346
lobed 340
0 331 340 s i n g l e a c t i n g 340
Seals r o t a r y l i p 353-369
absorbed power 358
design 354
e c c e n t r i c i t y 359
f a u l t f i n d i n g 362
f i t t i n g 361
f r i c t i o n 358
hand1 ing 361
ma te r ia l s e l e c t i o n 367-369
maximum pressure 359
maximum speed 359
se rv i ce problems 362-365
s h a f t sur face 357
storage 360
Seals s t a t i c
compression packing 353 370-394
gasket 341
1 i p 341
r a d i a l face mechanical 353
squeeze 341
Shel l f o u r b a l l machine 232 262 485 387
Shock pu lse measurement 163
S i l t i n g 303
S i l t removal 308 312
Skin cancer 278 288
Skin p r o t e c t i o n 285
S o l i d l u b r i c a n t s 6 239 499
Solvent e x t r a c t i o n 276
Soap 499
Spa l l i ng 213 499
Spectroscopy
Auger e l e c t r o n 4
x ray photon 4
scanning i on 4
ion s c a t t e r i n g 4
Spectrographic o i l ana lys i s 8 24 439
Spectrum analysers 434
S t i c k s l i p 467 499
515
St r ibeck curve 227
Succinimides 250
Sulphonates 245
Surface f i n i s h
e f f e c t on o i l f i l m 48
Surface treatments
bo ron is ing 219
c a r b u r i s i n g 210 216 219
case hardening 210 219
chemical vapour depos i t i on 7 220 464
chromosing 219
chromium p l a t i n g 216
e l e c t r o depos i t i on 219
flame hardening 219
hard su r fac ing 219
ion imp lan ta t i on 7
ion n i t r i d i n g 221
ion p l a t i n g 7 221
molybdenum p l a t i n g 216
n i t r i d i n g 219 220
Noskuff 220
phosphating 219
plasma spray 219
shot peening 219
s i l i c o n i s i n g 219
s u i f BT 219
Sulphinuz 220
Synovial f l u i d 28
Synthet ic l u b r i c a n t s 276 500
Takiness 247
Tappet ma te r ia l 216
Test machines
Almen Weiland 261 485 486
h s l e r 488
cross c y l inder
Cygnus 485 486
Falex 261 486
I A E 261 478
Neimann FZG 26
488
478
Shel l f ou r b a l l 232 261 485 487
Timken 261 488
Thermography 437
Th iophosphonates 245
Threshold l i m i t value 277
Tools see c u t t i n g t o o l s
Tota l a c i d number 500
Tota l base number 500
T o x i c i t y 235 276 280 282
amines 281
c h l o r i n a t e d napthalenes 281
lead compounds 280
o r t h o phosphates 281
sod ium mercaptobenzothiazole 282
sodium n i t r i t e
t r i c h 1 o roe thy l ene 282
Trend a n a l y s i s 444
Tr i bol ogy
handbook 2 10
module 1
r e p o r t 1 2
u n i t s 1
Turbine o i l 262
U l t rason ic t e s t 437
V i b r a t i o n ana lys i s 8 434
V i s c o s i t y
absolute 226 600
index 229 500
index improver 229 255
k inemat ic 226 500
pressure r e l a t i o n s h i p 231
temperature r e l a t i o n s h i p 229 401
Waveform ana lys i s 436
Water based f l u i d s 247 255 264 467
Wea r
abras ives 194 202
adhesive 12 17
chemical 12
516
c u t t i n g t o o l s 215 459
debr i s ana lys i s 8 24 429
mechanical 12
p i s t o n r i n g 215
res i s tance t o abras ion 213
r e s i s t a n t m a t e r i a l s 213
W e t t a b i l i t y 500
White metals 82 208
Wire wool f a i l u r e 122
ZDDP 501
