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Engineering Design Guide: GBHE-EDG-MAC-5701

Lubricants Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Engineering Design Guide: Lubricants CONTENTS SECTION 0 INTRODUCTION 1 SCOPE 1 2 LUBRICATION BASICS 2

2.1 Basic Functions of a Lubricant

2.2 Hydrostatic Fluid Film Lubrication

2.3 Hydrodynamic Fluid Film Lubrication

2.4 Boundary Lubrication

2.5 Mixed Lubrication 3 VISCOSITY 3

3.1 General

3.2 Dynamic Viscosity

3.3 Kinematic Viscosity 3.4 Measurement of Viscosity

3.5 Viscosity Classification of Lubricants

3.6 Viscosity Index

3.7 Viscosity Change with Pressure



4 MINERAL OILS 4

4.1 General Characteristics

4.2 British Standard 4475 Commentary

4.3 Oil Additives

4.4 Synthetic Oils 5 GREASES 5

5.1 Composition

5.2 Properties 6 SOLID LUBRICANTS 6 7 SELECTION OF LUBRICANTS 7 8 OPERATING FACTORS 8

8.1 Filtration

8.2 Operating Temperatures 8.3 Total Loss Lubrication Systems



9 LUBRICANT SUPPLY AND SCHEDULING 9

9.1 Selection of Supplier

9.2 Lubrication Schedules 10 HEATH AND SAFETY 10 11 MONITORING & MAINTENANCE OF OIL IN SERVICE 11

11.1 Analyze or Change?

11.2 Visual Analysis

1 I.3 Laboratory Analysis

11.4 Contamination Problems BIBLIOGRAPHY APPENDICES A VISCOSITY EQUIVALENTS B SYMBOLS AND PREFERRED UNITS



FIGURES I LUBRICANT CHANGE PERIODS AND TESTS 2 CHARACTERISTICS OF MINERAL LUBRICATING OILS

VG32 TO VG 460. 3 SERVICE MONITORING AND MAINTENANCE OF OIL IN

SERVICE ON LARGE SYSTEMS TABLES 1 ISO VISCOSITY CLASSIFICATION 2 OILS TO BS 4475 RECOMMENDED FOR USE BY GBHE 3 SUGGESTED OIL CHANGE PERIODS FOR SMALL INDUSTRIAL

SYSTEMS 4 VISUAL EXAMINATION OF USED LUBRICATING OILS 5 SUMMARY OF ROUTINE ANALYTICAL TESTS FOR INDUSTRIAL OILS DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE



0 INTRODUCTION Tribology means the study of friction, wear, lubrication and bearing design. It has been an internationally accepted title since 1966. The word is derived from the Greek:

'tribos' - rubbing

'ology' - study of Extensive research has given a considerable theoretical understanding of the lubrication aspects of tribilogy; most problems arise through the failure to apply this knowledge. Observations in industry indicate that: (a) Few engineers have an adequate working knowledge of the subject. (b) There is a need to acquire experience to influence trends in design. (c) Engineering developments lead inevitably to more onerous operating

conditions. 1 SCOPE Tribology is an interdisciplinary subject interacting surfaces in relative motion. This Engineering Design Guide is concerned only with the study of lubricants. 2 LUBRICATION BASICS 2.1 Basic Functions of a Lubricant These are: (a) Reduction or elimination of wear (b) Reduction of frictional forces (c) Dissipation of frictional heat



(d) Prevention of corrosion of bearing surfaces. (e) Hydraulic fluid for power transmission. 2.2 Hydrostatic Fluid Film Lubrication In this case the force between the contacting surfaces is balanced by pressure in a separating film of fluid. The pressure in the film is generated externally. 2.3 Hydrodynamic Fluid Film Lubrication In hydrodynamic lubrication, the pressure in the film is generated by the relative movement of the surfaces. This is usually a sliding motion with the surface having a slight convergent wedge in the direction of motion, but it can also be through the surfaces approaching normally. This latter form, known as 'squeeze-film lubrication', enables high, short duration loads to be sustained without film breakdown and is of great importance in reciprocating machines. For non-conformed contacts, sufficient local elastic deformation may occur for fluid film separation to be maintained at very small surface separation. This is known as “elastohydrodynamic lubrication” (EHL). With materials having a high elastic modulus sufficient pressure is generated between the surfaces to give a marked increase in the lubricant viscosity that helps to maintain a separating film; this occurs in rolling bearings, gears and cams. EHL also occurs with materials of low elastic modulus without viscosity increase; this mechanism occurs in rubber seals, human joints and is also responsible for the aquaplaning of tires. 2.4 Boundary Lubrication A thin solid or semi-solid film separates the contacting asperities on the surfaces.



2.5 Mixed Lubrication An intermediate condition between hydrodynamic and boundary lubrication where part of the load is supported by boundary lubricated solid contacts and part by hydrodynamic films. Optimum conditions of lubrication exist when the sliding surfaces are completely separated. At low speeds only boundary lubrication will occur and this is provided either by a lubricating solid or an adsorbed film of liquid lubricant. Even in mechanisms designed for full hydrodynamic lubrication, boundary lubrication will occur at starting and stopping or when the direction of motion is reversed. Under boundary conditions some wear of the surfaces is inevitable and in the case of heavy rotors hydrostatic jacking oil may be applied to the bearings at starting to replace the friction and the possibility of damage until there is sufficient velocity to generate a full hydrodynamic film. 3 VISCOSITY 3.1 General Viscosity is the most important single property of lubricating oil. Fundamentally it is resistance of the fluid to shear. It is dependent on temperature and, to a lesser extent, pressure and is usually measured at 40oC (previously 37.8oC or 100oF) and atmospheric pressure. This value is known as the Viscosity Grade. In lubrication duties the flow of oil is usually laminar. Transition from viscous to turbulent flow occurs as the velocity is increased above a critical value; this only occurs with thick lubricant films and high surface velocities, e.g. in large power generating steam turbines with bearing diameters above 500 mm. 3.2 Dynamic Viscosity For most lubricants the shear stress is proportional to the rate of shear, the constant 1\ being known as the dynamic viscosity

ᶯ = shear stress Rate of shear



The units of ᶯ are: (a) CGS System: dyn s = Poise (P)

m2 In practice, the centipoise cP (i.e. 0.01 P) is used. (b) SI System: Ns = 103 cP m2 3.3 Kinematic Viscosity Kinematic Viscosity is defined as:

Ѵ = Dynamic viscosity = ᶯ Density ρ The units of Ѵ are:

(a) CGS System:

cm2 = Stokes (St) s

The practical unit is the centistoke (cSt) which equals 0.01 St. Note that cSt = mm2/s. (b) SI System: m2 = 106 cSt s



3.4 Measurement of Viscosity The suspended level Capillary Viscometer is a commonly used measuring device. It gives results directly in kinematic viscosity and is based on the time it takes for a definite quantity of liquid to flow by gravity through a capillary tube. Efflux Viscometers use an orifice in place of the capillary tube, the chief forms being: (a) Saybolt Viscometer

This is used in the USA. There are two standard sizes, Universal and Furol. The viscosity is the time in seconds for efflux, e.g. Saybolt Universal Seconds, SSU. Normal reference temperatures are 100°F and 210°F.

(b) Redwood Viscometer

This is used in the UK. There are two sizes, Redwood No 1 and Redwood No 2. The viscosity is the time in seconds for efflux, e.g. Redwood 1 seconds. Normal reference temperatures are 90°F, 140°F, 200°F.

(c) Engler Viscometer

This is used in Europe. The viscosity is the ratio of the time of efflux of the oil to that of water at this same condition. Viscosity is quoted as degrees Engler, e.g. °E. Normal reference temperature is 50°C.

These empirical units are slowly going out of use. An equivalent table is given in Appendix A. 3.5 Viscosity Classification of Lubricants A system for this classification of industrial liquid lubricants was agreed in 1975 (ISO 3448, BS 4231). This defines 18 Viscosity Grades in the range 2 to 1500 cSt at 40°C; each grade is designated by the nearest whole number to its mid-point viscosity in centistokes and a range of + 10 of the midpoint value is permitted. The viscosity of each grade is approximately 50% greater than that of the preceding grade. Table I shows this classification.



TABLE 1 ISO VISCOSITY CLASSIFICATION

A quite separate classification is used for engine oils (Society of Automotive Engineers, SAE J300). This uses 100oC as a reference temperature and also gives limits at low temperatures where mineral oils show non-Newtonian viscosities (this is to cover cold start conditions). Arbitrary Grade Numbers are used with the suffix W for the low temperature limits. A normal mineral oil will have the same Grade Number as both temperature limits (e.g. 20/20W) but by the incorporation of certain additives it is possible to produce oils with different



low and high temperature grade numbers (e.g. 10W/40). Such oils are called multigrade oils. SAE Viscosity Grades are sometimes used in recommendations for industrial machines. Because of the different method of classification ISO Viscosity Grades and SAE Viscosity Grades are not strictly comparable; however for practical purposes the following relationships apply: SAE GRADE ISO GRADE Transmission

10 32 SAE 75 32 20 68 SAE 80 150 30 100 SAE 90 460 40 150 SAE 140 1000 50 220 60* 320 70* 460

* These Grades are obsolescent. 3.6 Viscosity Index The Viscosity Index (VI) concept was devised in 1929 as a way of expressing the viscosity/temperature relationship, for a particular oil, by a single empirical number. Reference oils were chosen having, what were considered at the time, maximum and minimum limits of viscosity/temperature sensitivity and they were assigned the end points of an 0 to 100 scale of VI. The VI of oil is calculated from determined viscosities at 40°C and 100oC and the use of tables which give the viscosities at 40oC of the 0, VI and 100 VI reference oils which have the same viscosity at 100oC as the oil under test. Oils with a low VI have a large change of viscosity with temperature. Oils with a high VI have a small change of viscosity with temperature.



Many oils are now produced with characteristics outside the upper and lower limits of the above scale. For example all "winter" oils have a VI of about 120. A new method has been developed to deal with oils over 100 VI (Viscosity Index Extension VIE) which gives similar values at 100 but greater differentiation at larger VI levels. Other more suitable methods, such as the Viscosity Temperature Coefficient (VTC) has been proposed but are rarely met in practice. A law suggested by Walther gives a theoretical relationship and is used in the ASTM chart. log E log (Ѵ + 0.6) = a - b log T where Ѵ is the oil viscosity centistokes

T is the temperature (absolute) a+b are constants for the particular oil.

For straight mineral oils Fig. 2 gives a guide to this variation. 3.7 Viscosity Change with Pressure Viscosity increases with pressure and this important in the elastohydrodynamic regime of lubrication and in hydraulics. Viscosity change follows a quasi-exponential law. For a typical mineral oil at 40oC, its viscosity will have doubled by 350 bar and will have increased twenty fold by 1500 bar. Oils which show a large change of viscosity with temperature (Low VI) show a large change with pressure. A theoretical relation is given below: ᶯ = ᶯo eαP

This equation seriously over-estimates the viscosity above 3000 bar. According to Wooster

α = (4.7 + 7.5 log to 10ᶯ0) 10-4



Where ᶯo is the viscosity at 1 bar in centipoise P is the pressure in bar 4 MINERAL OILS 4.1 General Characteristics Mineral oils are not pure chemicals, but consist of a very wide range of hydrocarbon of varying molecular weight and type. The three main types are paraffins, naphthenes and aromatics. The products of refining are a small range of base stocks of different viscosities, which are then blended to produce oils of the required viscosity. In addition to carbon and hydrogen, mineral oils contain small amounts of sulfur, usually less than 1%. This sulfur is not chemically active and normally of no consequence as far as lubrication goes, however if it gets into process streams it may lead to poisoning of catalysts etc. A summary of most of the important properties of mineral oil lubricants is given below: (a) Viscosity

Mineral lubricating oils can be manufactured in a continuous spectrum of viscosity (viz 2 to 1500 cSt at 400C). In practice only a restricted standardized range is produced.

(b) Oxidation Stability

Oxidation (the reaction between hydrocarbons in the oil and oxygen in the air) is a function of time and temperature. It is negligible with mineral oils below 400C. Straight mineral oils can normally be used up to 600C, for temperatures above this or where prolonged service is required oxidation stability can be increased by the incorporation of anti-oxidant additives. Oxidation is aggravated by the presence of water, metallic wear particles and other contaminants and also by churning and agitation. Fig 1 gives an indication of oil life as a function of temperature and access of air.



Oxidation is undesirable as it gives rise to: (1) Soluble products that increase the viscosity and may form deposits on

high temperature surfaces. (2) Insoluble products which can block oil holes, filters etc. (c) Acidity

Oils are neutral, but become acidic as a result of oxidation. The oxidation acids are not corrosive but provide a measure of deterioration.

(d) Demulsification

The ability of the oil to deal with water ingress (i.e. to separate out the water), is controlled in new oil to an acceptable level.

(e) Pour Point

Oils, when cooled sufficiently, form plastic waxy solids. Pour point is the temperature at which an oil just flows and this imposes a lower design limit on working temperature.

With the normal range of straight mineral oils, the pour point varies between +6°C and -18°C. Oils with values down to -60°C are obtainable and this is significant for the operation of refrigeration compressors.

(f) Density

The density of most oils lies between 850 and 960 kg/m3 (at 15°C).

An approximation of the thermal expansion is given below: ρ t = ρ0 ( 1 – βΔt ) Kg/L

where ρ0 is the density at 150C Kg/l

t = is the temperature rise °C β = 9.9 - 1.8 log10 ᶯ = dynamic viscosity at 15°C in centipoise



The bulk modules of oil can be expressed as:

c = (72.5 - 9.67 log10.) 10-6 Where

c = 1 x dρ or 1 x dV in bar -1 ρ dP V dP ᶯ = oil viscosity in centipoise at I bar. (g) Specific Heat

The specific heat of mineral oils between 00 and 200 0C within the specific gravity range 0.75 - 0.96 is obtained from:

s = 1.8 + 0.00366 t KJ/kg 0C

add 2% for paraffinic oils

subtract 2% for naphthenic oils.

(h) Thermal Conductivity

The thermal conductivity of mineral oils between -200 and 4000C within the specific gravity range 0.75 - 0.96 can be obtained from the following formula:

K = 0.13 - 7t x 10-3 W/m0C

accuracy + 10%

(j) Thermal Decomposition

Oils will break down above 3300C, even in the absence of oxygen, forming carbon deposits and with a decrease in viscosity and flash point. (See Fig. 1).



(k) Load Carrying

Boundary lubrication properties of straight mineral oils are adequate for most industrial applications, including spur and helical gears. There are exceptions, such as hypoid gears and high pressure ( > 100 bar) sliding-vane hydraulic pumps where additives are essential.

(1) Corrosion Protection

The small amounts of oxidation products in normal mineral oils give reasonable protection for ferrous alloys against corrosion; oils containing anti-oxidants require the incorporation of rust preventing additives.

(m) Aeration

Some mechanisms easily permit air entrainment in the bulk of the oil or foaming on the oil surface. Aeration is an undesirable situation which can be eased by system design and suitable selection of oil. Certain additives can be used to increase the rate of foam breakdown but they should be applied with caution as they tend to reduce the rate of air release.

The viscosity of a gas oil mixture is higher than that for the oil along by the factor (1 + 0.015 ) where is the volumetric percentage of the gas content.

The solubility of air in oil is sufficiently well given by

S = 1 ( 11 – 2 log10 Ѵ) Where Ѵ is the viscosity in centistokes

S is the volumetric percentage of air P is the absolute pressure in bar

This formula holds over a wide temperature range.



(m) Flash Point

This is the lowest temperature at which the vapor above an oil ignites when exposed to a flame, using a standard method for testing. Lubricating oils usually have flash points (closed) between 130°C and 250°C depending on viscosity. Flash Point is normally only of significance in lubricant manufacture as a control against the incorporation of low viscosity, volatile components.

4.2 British Standard 4475 Commentary

British Standard 4475, 'Specification for Straight Mineral Lubricating Oils' defines three quality levels of oil.

Table 2 gives a restricted selection of oils to BS 4475 and meets GBHE's requirements for general purpose lubrication.

TABLE 2 OILS TO BS 4475 RECOMMENDED FOR USE BY GBHE

Typical temperature-viscosity characteristics are given in Fig 2.



4.3 Oil Additives It is possible to modify the characteristics of a straight mineral oil by the incorporation of small amounts of other materials (usually < 1%). Additive oils should be used with discretion as the additives tend to be more reactive than the base oil and can react with system components or contaminants giving rise to harmful deposits. Additive oils are more expensive than straight oils. Important types of additive are as follows: (a) Anti-oxidant

These reduce the rate of degradation and are advantageous in the temperature range 60°C to 80°C. (See Fig 1). A side effect is that they create a need for a corrosion inhibitor.

(b) Extreme Pressure (EP)

Prevents welding and scuffing of sliding surfaces under severe operating conditions. Principally used in gear lubricants; particularly when associated with hypoid design and units subject to shock loading, and high pressure hydraulic pumps. Some EP additives react with yellow metals at ambient temperature and should be used only with steel systems. On gears other than hypoid there is no evidence of any advantage of EP oils over straight mineral oils.

(c) Detergent/Dispersant

Used in engine oils to prevent carbon deposits. Have no application in industrial machines other than engines.

The main application of additive oils is in large circulation systems (e.g. turbo-alternators and rotary compressors) where the use of an oil containing an anti-oxidant and rust inhibitor will increase the period between oil changes. In small systems without filters the need for oil change is determined by contamination rather than oxidation and straight oils provide the most economic choice. BS 489 covers a range of oxidation and rust-inhibited oils. With straight mineral oils there is no problem with equivalents or mixing products from different manufacturers. This is not the case with additive oils. It is thus necessary to study the description of the oil and its application before selecting an equivalent. Information on the composition of branded oils is not easy to obtain. Makers recommendations and guarantees should not be overlooked.



Experience within the company shows that in all but a very small number of applications, industrial machines can be satisfactorily lubricated by straight mineral oil. The lubrication requirements of reciprocating refrigerant compressors are met by straight mineral oils, but they have special requirements to ensure absence of solidification with inorganic refrigerants and adequate miscibility with halogenated refrigerants. Suitable materials are covered by BS 2626 which covers five viscosity grades. 4.4 Synthetic Oils These are only occasionally used in industry. They are of value in extending the temperature range of liquid film lubrication. In aircraft gas turbines, synthetic oils have now completely displaced mineral oils. They are more expensive in all cases. The chief types are: (a) Organic Acid Esters

High VI, but only available in low viscosity grades, ( < 10 cSt at 400C). Can be used up to 240 0C.

(b) Phosphate Esters

Can be used up to 120 0C. High resistance to combustion. Strips paint coatings. They have poor lubrication properties, particularly in high contact load applications.

(c) Polyglycols

High VI. Can be used up to 200 0C. Degradation products volatile. Outstandingly good boundary lubricants and for this reason very effective in worm gearboxes, allowing increased power transmission for the same temperature use.



(d) Synthetic Hydrocarbons (SHC)

Similar properties to mineral oil but sulfur-free and more resistant to oxidation. Built up from selected pure chemical compounds and therefore are free from "light" and "heavy" ends. This means the pour point is lower and the flash point higher than those of a straight mineral oil of equal nominal viscosity.

(e) Silicones

Extremely high VI. Poor boundary lubricating properties, but have extremely wide temperature range of application (-60 0C to 250 0C).

(f) Fluorocarbons

Low VI. Can be used up to 150 0C. Extremely good thermal and chemical stability; fire resistant and the only lubricant that can be used with liquid oxygen. Very expensive and have poor lubrication and corrosion preventive properties.

5.1 Composition Most lubricating greases are based on mineral oils of Viscosity Grade 68 to 220 mixed with a thickener to give a semi-solid product. Metallic soaps are the most common thickeners, but more recent developments include polyurea and Bentonite clay. The mechanical stability of a grease is a function of the base oil viscosity and the amount and type of thickener. Important types of grease, in ascending order of cost, are listed below: (a) Calcium (Lime)

Excellent water resistance and corrosion preventive properties within the temperature range. Maximum operating temperature 60°C (long term) 80°C (short term). Drop point 100°C (i.e. melt point under set conditions).



(b) Lithium

Good water resistance and incorporates additives to give corrosion preventive properties. Maximum operating temperature 80°C (long term), l40°C (short term). Drop point l80°C. Standard grease for rolling contact bearings.

(c) Polyurea

Similar to lithium greases, but with better temperature resistance, 100°C (long term); used particularly in the bearings of electric motors with Class F insulation.

(d) Bentonite Clay

Good water resistance and corrosion preventive properties. Maximum operating temperature 200°C (short/long term). High temperature duties. Poor lubricating properties and only suitable for rolling bearings at low speeds.

(e) Perfalkyl type. Very expensive, care should be taken in their use. 5.2 Properties The main advantage of grease over oil is that it does not flow under its own weight and hence allows simpler sealing arrangements. This is a major attraction in rolling bearings, lubricated flexible couplings and small sealed-for-life units. On the other hand because of this property grease does not circulate and cannot be used like oil to remove frictional heat. The most important physical property of grease is its consistency. This is determined by a penetrometer: a weighted metal core is allowed to sink into a sample of the grease at 24°C for a specific time and the distance it sinks, measured in tenths of a millimeter, is termed the penetration. Greases are classified in terms of their consistency using an arbitrary set of numbers originating from the National Grease Resource Institute of America (NLGI) •



The relationship between NLGI and penetration is as follows: NLGI Class 1 2 3 4 5 6 Penetration 310 265 220 175 130 85

-340 -295 -250 -205 -160 -115 In addition a range of partially thickened oils that flows in on their own weight are also described as greases; in order of increasing consistency they are given the NLGI Nos 00 and 0. The semi-fluid greases are used in gearboxes, Classes 1 and 2 greases in pumped systems, Classes 2 and 3 for rolling bearings with Class 3 grease preferred for vertical applications. Other properties of interest are: (a) Shear Stability

The shear stability is determined by the change in penetration as received and after working in a standard test apparatus. The worked penetration should be in the same NLGI class as the unworked values.

(b) Pumpability

Greases are non-Newtonian materials that is their viscosity varies with the rate of shear. The viscosity at any particular rate of shear is known as the Apparent Viscosity. Apparent Viscosities are required for estimating pressure drops in pipe runs. Plots of Apparent Viscosity against rate of shear can be obtained from the grease supplier.

(c) Extreme Pressure Properties

EP additives can be incorporated into greases, but with some loss in stability. EP greases are used in heavily loaded taper or spherical roller bearings to prevent scuffing.



6 SOLID LUBRICANTS These are used essentially for boundary lubrication and are no substitute for mineral oils. They provide lubrication when temperatures are too high for mineral oils, in situations where normal lubricants would be squeezed out, e.g. in assembly compounds, thread release agents. Sometimes used as an additive to conventional oils and greases. Graphite and Molybdenum Disulphide are the most well known. 7 SELECTION OF LUBRICANTS Machine manufacturers are conservative in their lubricant recommendations, frequently specifying precise Viscosity Grades and the use of additive-type oils when these are unnecessary. Most mechanisms are satisfactorily lubricated by straight mineral oils. This allows considerable reduction in lubricant stocks that not only reduces cost but reduces confusion and the risk of error. Table 5 gives a guide to lubricants for normal industrial machines based on experience in GBHE. Difficulties can arise during the warranty period of a machine if the lubricant recommended by the manufacturer is not used. The simplest way to overcome this is to specify a lubricant to BS 4475 at the tendering stage and place the onus on the Vendor to satisfy the Purchaser of cases in which this will not be acceptable. 8 OPERATING FACTORS 8.1 Filtration With a new machine, system flushing is essential, using a separate charge to remove the bulk of both liquid and solid contaminants and the temporary corrosive preventive with which parts are coated prior to supply.



8.2 Operating Temperatures The following checklist gives various temperatures associated with systems and bearings: (a) Oil Reservoir

60°C (optimum), 70°C (maximum). Values set to prevent degradation of mineral oils.

(b) Oil Inlet to Bearing - Circulating System.

40°C to 50°C (typical range).

Where bacterial contamination may cause problems 45°C to 55°C should be specified.

(c) Oil Outlet from Bearing - Circulating System

60°C to 70°C (typical), 80°C (maximum). Values set to prevent degradation of mineral oils.

(d) Plain White Metal Bearing

Temperatures measured by thermocouple embedded in the white metal with 1 mm of the bearing surface.

80°C (optimum), 100°C (normal maximum). Values set by the possibility of chemical reaction between bearing white metal and the lubricant, or contaminants in the lubricant, producing deposits and leading to failure.

130°C (accepted maximum for high speed bearings). Value set by the limit of physical properties of white metal, the metal starts to soften.

Oil degradation is little influenced by short term high temperature peaks in the bearing itself.

(e) Rolling Bearings

120°C (maximum for standard production bearings). Value set by bearing materials and lowering of oil viscosity.



Specially heat treated rolling bearings with some loss in load and carrying capacity are available to operate at temperatures up to 230°C but need synthetic oil.

8.3 Total Loss Lubrication Systems It is often preferable, e.g. in secheurs, conveyors, mills etc with a large number of widely separated moving parts to install a system for dispensing lubricant from a central position, either grease or oil, and either manually or automatically in a total loss system. Measuring valves are operated together or progressively, the pump either being in continuous or intermittent service. Should a blockage occur at anyone of the bearing points, it can be arranged that the resulting high pressure can initiate a visual or audible alarm. Pumpability is important in a centralized greasing system. Residence time of the grease or oil in the pipelines can be considerable and lead to problems. 9 LUBRICANT SUPPLY AND SCHEDULING 9.1 Selection of Supplier GBHE policy is to use straight mineral oils to BS4475 wherever possible; experience shows that this normally amounts to about 60-70% of the requirement on any particular plant or works. Contracts for oils to BS4475 should be awarded to suppliers on the basis of competitive tendering. The use of branded proprietary oils and greases and oils to particular specifications obtained from the major manufacturers. 9.2 Lubrication Schedules Lubricant schedules for new plants are drawn up by the Contractor or Machines Manufacturers to obtain the maximum rationalization. See Appendix B.



10 HEALTH AND SAFETY Oil can affect health by being swallowed or inhaled as droplets or vapor and by contact with skin or eyes. Prolonged contact with the skin can lead to dermatitis or, more rarely, skin cancer. Providing straightforward precautions are observed, there is not considered to be any risk. As a routine, hands and forearms should be washed before commencing a work period, barrier cream applied, and washed again at the end of the period. Cuts and scrapes should be attended to without delay. Hands should never be washed in petrol or similar solvent. Oil soaked rags should not be left in overall pockets. Data on Oral LD50 and Threshold Limit Values (TLV) are available from the oil suppliers covering their particular products. The following leaflets are also available: SHW 295 Effect on the Skin of Mineral Oil SHW 295A Cancer of the Skin Caused by Oil SHW 367 Dermatitis. A Cautionary Notice SHW 397 Cautionary Notes: Effects of Mineral

Oil on Skin 11 MONITORING & MAINTENANCE OF OIL IN SERVICE Lubricating oils deteriorate in service either because the lubricants degrade or they become contaminated. The majority of the lubricant tests covered by IP, ASTM, DIN etc., are aimed primarily at ensuring that the lubricant meets some specification level rather than monitoring the condition of the oil in service and are therefore not necessarily appropriate to the latter function. A monitoring scheme may be used not only to assess the fitness of the oil for further service but may indicate actions that can be taken to restore the condition of the oil.



11.1 Analyze or Change? Before adopting any monitoring system the cost of monitoring has to be considered. In the case of small systems it may be more economic to resort to changing the oil at some routine rather than attempt to assess its fitness for further service. The distinction between small and large systems is not precise but 100 - 200 liters may be taken as a guide. In fact few systems fall into this size: self-contained systems are usually less than 50 liters, circulation systems more than 1000 liters. Change periods for small systems have to be determined by practical experience. Suitable periods for typical industrial equipment are indicated in Table 3. TABLE 3 - SUGGESTED OIL CHANGE PERIODS FOR SMALL INDUSTRIAL SYSTEMS

For the larger systems a 2-tier system of monitoring is suggested consisting of a visual examination at say weekly intervals, backed up by oil analysis at some longer routine or as indicated necessary by the visual analysis. Fig 3.



11.2 Visual Analysis For visual analysis a sample of the circulating oil should be taken in a clean glass bottle (2-4 oz). It is helpful to retain a sample of the new oil and the previous week's sample so that a direct comparison can be made to see if any change has occurred, the samples being kept in a cupboard out of direct sunlight. If the oil is clean and has not noticeably darkened no action needs to be taken. If it is cloudy or opaque it should be stood for 30 – 60 minutes, preferably on a radiator where it can be heated to about 600C and re-examined. Table 4 indicates a scheme for visual analysis and the action to be taken. Although primarily of use in large systems, visual examination can also be of use of course in examining the oil from small systems where it is suspected that something may have happened to the oil before the routine change period has been reached. 11.3 Laboratory Analysis Laboratory analysis is expensive but it is justified with large systems. In the case of Works without their own laboratory facilities the oil supplier may be prepared to help or it may be necessary to use an external laboratory. The following indicates the most useful tests for oils in service. A 6-monthly routine is normally sufficient. (a) Viscosity

Most systems can tolerate a wide variation in viscosity.

Nevertheless a routine test should be carried out to check that the wrong oil has not been used for topping up and there has not been contamination with oil-soluble material. The viscosity at 40'C should not differ more than + 15% from the new oil value. (e.g. ISO VG 68. 57.8 to 78.2 cSt)



TABLE 4 VISUAL EXAMINATIONS OF USED LUBRICATING OILS

NOTES: 1 This includes paper and felt filters, not wire mesh strainers. 2 Both foams (mixtures of oil and air) and emulsions (mixtures of oil and

water) render the oil opaque. When an opaque sample is received it should be stood for 1 hour, preferably at 60°C (an office radiator provides a convenient source of heat): a foam breaks down, liberating the gas and leaving a clear oil; a stable emulsion persists after this time; a less stable emulsion shows a separated layer of water below the oil.

3 In a dark oil solids can be seen by inverting the bottle and looking at the

bottom. 4 Judgment and experience is necessary to decide how much

contamination' can be tolerated.



(b) Acidity

The oxidation process in mineral oils is auto-catalytic, that is once significant oxidation products have been formed they act as catalysts for further oxidation and deterioration proceeds rapidly.

Determination of acidity (neutralization number) provides a useful guide.

For straight oils the new oil value will be less than 0.1 mg KOH/gm. The end of the useful service life of an oil is indicated by an acid value of 2 - 3 mgKOH/gm; the value is not critical as sludge precipitation is not likely until a value of 40-50 mg KOH/gm but deterioration proceeds rapidly once a value of 2-3 mg KOH/gm has been reached and action at this level provides a useful margin of safety. Little can be done to restore the situation once this stage has been reached. Purging does not give much advantage because the oxidation products remaining act as catalysts.

The situation with additive-containing oils is different. With oils containing anti-oxidants, acidity development proceeds rapidly once the additive has been reduced below a certain level. The normal recommendation is to take 1 mg KOH/gm as an indication of the end of service life. It has been found however that in certain systems where rapid depletion of anti-oxidant can take place (e.g. when the oil becomes contaminated with oxidizing materials, as can happen on nitrous gas compressors or where there is a buildup of soluble copper in the oil) it is useful to check the antioxidant level and restore it to the new oil value by adding an anti-oxidant concentrate. In the case of systems prone to copper uptake, 8 ppm copper appears to be a critical level.

In certain additive-containing gear oils and high duty hydraulic oils, the acidity of the new oil is influenced by the additives and may have values ranging from 0.5 to 1.5 mg KOH/gm. In these oils the acidity first decreases as the additives are depleted and then increases again as deterioration occurs. The end of useful life is indicated when the acidity rises to a value of 1 mg KOH/gm, though in such cases it is best to consult the oil supplier for a more precise recommendation.

I



11.4 Contamination Problems 11.4.1 Contamination by Water Free water in suspension at concentrations as low as 75 ppm can turn a light colored, low viscosity oil at room temperature slightly hazy. Oil in good condition may contain up to 200 ppm of water and remain clear. Above this, water should quickly separate but if oils degrade or become contaminated the rate of separation is reduced and even stable emulsions can be formed. It is undesirable to operate with oils in this condition as corrosion may occur and the corrosion products - iron hydrates - act as emulsion stabilizers. If centrifuges or coalescers are fitted then these should be checked for satisfactory operation; the usual faults are too low oil temperature in centrifuges (the oil should be at 70 - 80°C) or binding in the case of coalescers through failure to drain. Even without purification equipment, failure to drain settled water from the oil tank may result in emulsion formation when the water is taken up by the pump. When stable emulsions are formed and cannot be broken down the oil should be changed as soon as possible. 11.4.2 Contamination with Air: Most lubricated mechanisms tend to entrain bubbles of air in the oil. It is important for the maintenance of satisfactory lubrication that the bubbles should migrate to the surface and the foam formed there quickly collapse. Lubrication systems are designed to allow these processes to occur. For example, oil drain lines should have a gentle slope (I in 40) and run less than half full; the oil should be returned to the reservoir surface and not dropped in from a height; the reservoir residence time should be sufficient to allow complete separation (4-10 minutes). Entrainment and foaming problems arise in lubrication systems either because excessive air is entrained or the rate of foam collapse is too low. Entrainment can arise in a number of ways; air leaks into the oil pump suction; rough edges to oil delivery nozzles; splashing of return oil on the reservoir surface; whipping of loose pieces on the surface of the oil. Slowing of bubble disengagement and foam collapse is the result of degradation or contamination of the oil; it is rarely possible to determine the cause by analysis as only very small amounts of the contaminant need to be present.



In small systems the best approach is to change the oil; if this does not improve the situation then it is necessary to look for the mechanical cause. When large quantities of oil are involved comparative tests on the new oil and oil from the system can be carried out to check whether the air release and foam collapse properties have deteriorated. Method IP313 can be used for the air release; Method IP146 for foam stability. A summary of the routine analytical tests and the action level applicable to normal industrial lubrication systems is given in Table 5. TABLE 5 - SUMMARY OF ROUTINE ANALYTICAL TESTS FOR INDUSTRIAL OILS

BIBL IOGRAPHY 1 M J Neale (Ed) Tribology Handbook, Butterworths 1973 2 Institute of Petroleum. Methods for Analysis and Testing for

Petroleum and its Products.



APPENDIX A

VISCOSITY EQUIVALENTS

This table may be used for approximate conversion from one viscosity scale to another, at the same temperature.



APPENDIX B SYMBOLS AND PREFERRED UNITS 1 Bulk Modules E Bar-1 2 Thermal Conductivity K W/M0C 3 Pressure P Bar 4 Solubility S - 5 Temperature t °c 6 Absolute Temperature T o K 7 Volume V M3 8 Constants a,b 9 Parameter 10 Density Kg/L 11 Kinematic Viscosity cSV 12 Dynamic Viscosity cP 13 Specific Heat kJ

Kg o C



FIGURE 1 LUBRICANT CHANGE PERIODS AND TESTS



FIGURE 2 CHARACTERISTICS OF MINERAL LUBRICATING OILS. VG 32 TO VG 460.



FIGURE 3 SERVICE MONITORING AND MAINTENANCE OF OIL IN

SERVICE ON LARGE SYSTEMS



DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE This Engineering Design Guide makes reference to the following documents: BRITISH STANDARDS BS 489 Steam Turbine Oils (referred to in Clause 4.3) BS 2626 Lubricating Oils for Refrigerant Compressors

(referred to in Clause 4.3) BS 4231 Classification for Viscosity Grades of Industrial Liquid (ISO 3448) Lubricants (referred to in Clause 3.5). BS 4475 Straight Mineral Lubricating Oils (referred to in

4.2, 7 and 9.1 and also Table 2). SHW 295 Effect on the Skin of Mineral Oil (referred to in

Clause 10) SHW 295A Cancer of the Skin Caused by Mineral Oil (referred to in

Clause 10) SHW 367 Dermatitis. A Cautionary Notice (referred to in Clause 10) SHW 397 Cautionary Notes: Effects of Mineral Oil on Skin (referred to

in Clause 10).

