12. Notes on Fundamentals of Tribology PAF CAE-1989

8/10/2019 12. Notes on Fundamentals of Tribology PAF CAE-1989

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FUNDAMENTALS OF TRIBOLOGY

MUHAMMAD ISMAIL

Squadron Leader

Department of Aerospace EngineeringCollege of Aeronautical Engineer ing

PAKISTAN AIRFORCE ACADEMY RISALPUR


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CONTENTS

Topic Page

Foreword ii

Preface iii

Tribology 1

Friction and its Measurement 1

Adhesion and Mechanism of Friction 7

Wear and Surface Damage 9

Surface Temperature of Sliding Solids 11

Rolling Friction 16

Viscosity 21

Lubrication 31

Boundary Lubrication 37

Types of Lubricants 40

Bibliography 41

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Foreword

This compilation of short notes on Tribology presents essential

knowledge about the subject in a crisp manner. Many other references on this

subject contain too exhaustive details which are not always immediately

required, and looking for the desired information becomes rather cumbersome.

The information in the present notes is based upon the current views on the

mechanism of friction and lubrication. It is extremely beneficial for the people,

especially for practicing maintenance engineers, who wish to become familiar

with the basic ideas current in this field.

Dr S K N ZAIDI

Group CaptainProfessor & Head of

Aerospace Engineering Department

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PREFACE

The absence of some study material in the context of tribology, in one

binding, has been felt for long at PAF College of Aeronautical Engineering.

The compilation of this handout is the first attempt in this direction.

The primary intention of this work is to provide the students and

practicing engineers with some appreciation and understanding of the

increasing important role which lubricants play in modern engineering and to

explain the physical principles on which bearing and sliding mechanisms

function. Hence, a brief account of the mechanism of friction, wear and surface

damage, surface temperature of sliding solids, and lubrication has been included

in this handout. The treatment is non-mathematical and has been kept very

simple.

MUHAMMAM ISMAIL

March, 1991 Squadron Leader

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TRIBOLOGY

Tribology is the science and technology of interacting surfaces in relative

motion (and the practices related thereto), including the subject of friction, wear

and lubrication.

FRICTION AND ITS MEASUREMENT

FRICTION

When one solid body is slid over another there is a resistance to the

motion which is called friction. Considering friction as a nuisance, attempts are

made to eliminate it or to diminish it to as small a value as possible. No doubt a

considerable loss of power is caused by friction (e.g. about 20% in motor cars,

9% in airplane piston engine and (1 -2)% in turbojet engines) but moreimportant aspect is the damage that is done by friction the WEAR or

SEIZURE of some vital parts of machines. This factor limits the design and

shortens the effective working life of the machines.

The Laws of Friction

There are two basic laws of friction;

First Law: The friction is independent of the area of contact

between the solids e.g. if one pulls a brick along a table, the friction is

same whether the brick is lying flat, or on its side, or standing on its end

[Fig 1(i), (ii), (iii)].

Fig-1. Figure illustrating the two basic laws of friction.

1

(i)

(ii)


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Measurement of Friction

To measure the friction, the basic requirements are simply a means of

applying a normal load W and a means of measuring a tangential force F. The

following section gives a brief account of some ways of doing this.

If the lower surface is flat, the simplest method is to use the gravity

loading and to tilt the lower surface until sliding begins (Fig 2)

Fig. 2. Two simple means of measuring friction

If is the angle at which sliding begins, then, normal force = w cos ,

and tangential force = w sin,

so that, = tan .

It is a convenient quick rough method to determine , but the vibration

during the tilting may produce error. Generally, once sliding is started at

an angle , the upper body accelerates down the slope. This is becausethe friction to start sliding (the static coefficient of friction s) is

generally greater than the friction which arises during sliding

(the coefficient of kinetic friction k).

The second method also uses the gravity loading but the lower

surface is kept horizontal and the tangential loading is applied by means

of dead load over pulley and = F/W.

Both the methods are however, defective because of the inertia of the

moving parts they cannot readily detect fluctuations which occur during sliding.

For this reason, it is often more fruitful to use a device of high natural

frequency. On the basis of this approach various sophisticated apparatus have

been developed. The schematic diagrams of two such devices are shown below

(Fig 3 & 4)

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Fig. a. Apparatus with high naturalFrequency for determining frictionbetween sliding solids. A, carriageholding lower surface (and heatingelement): B, Lower surface in form of

finely abraded flat surface: C. Uppersurface or slider, usually hemisphericalin shape: D, E. spring and screw forapplying normal load; F, Stiffeningspring supporting slider; G, Duraluminarm holding slider and loading spring;H, Bifilar suspension holding arm G; M,Mirror for recording deflexion of arm G.

Fig-4. Apparatus for measuring frictionbetween surfaces over a wide loadrange. A, Turntable carrying lower

specimen: B; C,Upper surface or slider;D, Cylindrical beam supporting slider;E, Lever arm holding beam D; G,Micrometer for raising or lowering endof lever arm and so flexing beam invertical plane; H, Galvanometerelements for recording flexing of beamin horizontal direction. By using wiresof various thicknesses for the beam Dapractical load range from about 5milligrams to 100 grams may beobtained.

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Effect of Speed

As was pointed out earlier, s> kbut the difference is not very great.

This is because the basic mechanism is essentially unchanged, the only

alteration being the effective time of contact at any instant. If the mechanism of

friction changes, however, a marked change in friction may occur. Forexample, with increasing speed of sliding, frictional heating may produce

softening or melting of surface layers and the friction may be largely due to the

rheological (concerning flow & deformation of metals) properties of a thin, soft

or molten surface film. This occurs at moderate speeds with ice and at

extremely high speeds with metals. Under these conditions the friction may be

very low ( < 0.1).

Frictional Characteristics

For two clean specimens of the same metal sliding together in air the

friction is high ( = 1 to 1.5) and some what irregular and the track shows very

heavy damage and tearing [Fig 5(a)].

Fig-5. Friction traces for:-(a) Copper sliding on copper (b) Load sliding on steel

Fig-5(c). Stick Slip motion

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When a soft metal slides on a harder metal the friction is of the same

order but generally shows a regular intermittent motion of a stick-slip nature

[Fig 5(b & c)]. During the stick the upper surface moves with the lower

surface until the restoring force is sufficient to initiate sliding. A rapid slip

then occurs and continues until the upper surface becomes again firmly attached

to the lower surface. Corresponding to the jerky motion the wear track on the

lower surface consists of intermittent smears of metal transferred from the

upper slider.

For hard metals sliding on soft metals the sliding is often smooth, and

corresponding to this the friction track consists of a well defined uniform

groove in the lower surface. In other cases the motion is intermittent and the

friction track is in the form of irregular groove.

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ADHESION AND MECHANISM OF FRICTION

Adhesion

When surfaces are placed together they make contact over the tips of

their asperities and the pressures here are extremely high. Over these regions

where intimate contact occurs, strong adhesion takes place and specimen

becomes, in effect, a continuous solid ( Fig 6 ). With metals this process may

be referred as Cold Welding.

Fig-6. Sketch showing plastic deformation at the points of real contact. Atthese regions junctions are formed. The surrounding regions are deformedelastically so that when the load is removed these elastic stresses are released,and the junctions are broken.

When the surfaces slide over one another the junctions so formed must be

sheared and the force to do this is nearly equal to the frictional resistance. If one

surface is much harder than the other, the asperities on the harder surface will

plough out grooves in the softer one and there will be an additional ploughing

term that must be considered in estimating the friction (generally neglected

being small in comparison).

Mechanism of Friction

If the material that has to be sheared has a mean shear strength S then

F=AxS, where A is the real area of contact and is proportional to the load and

independent of the size of the bodies, and

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Where, P is the yield pressure and is defined as the stress at which a substantial

amount of plastic deformation takes place under load (also known as yield

point). Experimentally, S is almost equal to the bulk shear strength of the softer

metal of the sliding pair.

In practice, the value of may be modified by small amounts of surface

contamination (impurity due to mixing of other materials).

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WEAR AND SURFACE DAMAGE

Wear is the actual removal of surface material due to the frictional force

between two mating surfaces. This can result in a change in component

dimension which can lead to looseness and subsequent improper operation. The

adhesion mechanism of friction enables us to understand the basic mechanismof metallic wear, when a junction shears during sliding it may shear in one or

other of four ways.

If the Junction is weaker than the materials on either side of it,

shear will occur along the interface itself. Consequently there will be

very little transfer of metal from one surface to the other and very little

wear. For example when a tin-base alloy slides on steel ( = 0.7),

practically no tin is transferred to steel.

If theJunction is stronger than one of the metals but weaker than

the other, shearing will take place, not at the interface itself, but a little

distance within the softer material, so that for quite a small increase in

friction, there may be anormous increase in metallic wear. Thus for a

lead-base alloy sliding on steel, the coefficient of friction is about unity,

whilst the wear may be fifty times higher than for corresponding tin-base

alloy.

This type of wear gradually builds up a film of softer metal on the

harder surface, so that, ultimately, the sliding is characteristic of similar

metals.

If the Junction is stronger than both metals, the shearing will not

occur at the interface. Most of the shearing will occur in the softer of the

two metals but occasionally small fragments of the harder metal will also

be ploughed out, e.g. when copper slides on steel, this type of 13ehavioris observed.

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Finally, we may consider the behavior of similar metals. Here the

Junctions are of the same material as both surfaces, but the process of

deformation and sliding will work harden them, and considerably

increase their shear strength. As a result, shearing will rarely occur at the

interface itself but within the bulk of the metals. Consequently, the

surface damage will be very large. It is for this reason that the sliding

together of similar metals may cause heavy wear.

Besides the adhesive wear, as discussed above, other possible types of

wears are described below:

Abrasive Wear. This type of wear occurs when hard particles or

a rough surface runs against a soft surface. One example is sand paper

smoothing wood, and the other is diamond powder embedded in lead lap

polishing a glass surface.

Fatigue Wear. As the surfaces run over each other, opposing

asperities are deformed and ultimately fatigued off.

Corrosive Wear. Oxygen, moisture or other active chemicals in

the lubricant form a layer which prevents the surfaces adhering together.

This layer is rubbed off during contact.

Delamination Wear. Delamination wear takes its name from

the fact that wear debris consists of flakes.

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SURFACE TEMPERATURE OF SLIDING SOLIDS

During sliding the real area of contact is generally very small and this has

a very important influence on the temperature developed during sliding. The

energy lost during sliding is dissipated mainly in the form of heat and this will

occur over a few small regions of contact.

Theoretical Calculation for Surface Temperature

In the following paragraphs two models for the theoretical calculation of

surface temperature of sliding objects are being discussed:

Model-1. In this case a cylinder is rubbing on a flat surface [Fig

7 (a) ] and it is assumed that heat is uniformly generated over circular

interface, and it is conducted up the body of cylinder and finally carried

away by radiation from the cylindrical surface.

If is fraction of the frictional heat which goes into the cylinder, kz

is the thermal conductivity of the cylinder, and is the radiation loss per

sq cm per unit degree temperature excess, then the temperature rise at the

interface is,

Where, J = Mechanical Equivalent of heat

The main defect of this model is that in fact contact occurs over a few

asperities, the area of which constitutes only a very small fraction of the

end surface of the cylinder. These are the regions at which the frictional

heat itself is generated. Consequently, the main loss of heat arises from

the conduction into the bulk of the metal from the minute frictional hot

spots.

Model 2. In this model, the actual contact region is treated as a

square of side 21, and k1, k2 are the thermal conductivities of the lower

moving surface and the upper stationary surface, respectively [Fig 7(b)].

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Fig. 8. Thermoelectric method of determining surface temperature. The

thermoelectric potential developed between dissimilar metals is measured by a high

frequency galvanometer or cathode ray oscilloscope.

Visual or Photographic Method. If bad thermal conductors

are slid together the frictional heat is conduced away much less

effectively than with metals. It is not possible to use the thermoelectric

method with non-conductors, however other methods can be employed

for noting temperature rise. For example, if polished surfaces of glass or

quartz are used,

Fig-9. Photographic record of hot spots formed between a steel pin rubbing ona glass plate load 1200 grams. The inner most track which just producesvisible blackening of the photographic plate corresponds to a sliding speed ofabout 70 cm/sec.

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and the apparatus is so arranged that a clear image of the rubbing surfaces

can be seen. It is found that if the sliding is carried out in dark, a number

of tiny stars of light appear at the interface between the rubbing surfaces.

The points of light are reddish in colour at low speeds and become whiter

and brighter as the speed or load is increased. These luminous points

correspond to small hot spots on the surface. Their varying positions and

distribution over the surface can be recorded by placing a photo-graphic

plate on a turn-table with the gloss side upward and allowed to rotate

with the metal slider resting on it (Fig 9).

Infra-Red Cell Method. Another method which is very

effective (if one of the surfaces is transparent) is to use a lead sulphide

cell. Lead sulphide is a semi-conductor and when electromagnetic

radiation falls on it, its electrical resistance falls. Modern lead sulphide

cells are available which are sensitive to infra-red radiations and whichhave a rapid response. Schematic diagram of experiential arrangement is

shown in Fig 10.

Fig-10. Apparatus for investigating frictional hot spots using a lead sulphideinfra-red cell. (A) Disc of glass or quartz; (B) upper surface slider; (C)Photocell; (D) Brass enclosure; (E) Chopper with alternate segments coveredwith a suitable filter.

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The Main Characteristics of Surface Temperature

The salient characteristics of the rise in surface temperature during the

process of friction are briefly described below:-

Local high temperatures are easily developed, although the load

may be light and the sliding speed may be only a few feet per second

(Fig 11).

Fig-11. Oscillograph record of a single hot spot developed on asteel sliderrubbing on glass, using apparatus shown in Fig. 10without the chopper.

The rise in temperature is normally limited by the melting point of

the solid.

Exception to sub para 20(b) can occur, if the metal can be readily

oxidized and if the oxidization is an exothermal process, the hot spottemperature may be very much higher because of the heat of oxidation

liberated at the oxidizing surface.

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ROLLING FRICTION

Other than sliding, an other way in which surfaces can move over one

another is rolling, and it is much easier to roll surfaces along than to slide them.

Types of Rolling

Rolling can take place in two different ways, as described below:-

The first type of rolling occurs when a car wheel is driven over a

road or a train wheel over a rail. Here considerable tangential forces are

involved in pulling the vehicle along, and the conventional frictional grip

between the wheel and the surface is of great importance.

The other type of rolling involves only a minute tangential tractioni.e. the rolling that occurs when a ball or cylinder rolls freely over

another surface called free rolling which is most commonly applied in

ball bearings and roller bearings. The resistance in these cases is

phenomenally low ( 0.001 ).

The rolling friction is accounted for by the elastic hysteresis losses within

the metals themselves and is scarcely affected by the presence or absence of

lubricant films. However, the lubricant films may play an important part inreducing surface attrition or wear but they have little effect on the rolling

resistance itself. In real ball and roller bearings, the behavior is complicated by

friction of the cage and other factors, but the hysteresis losses must play a very

important role.

Mechanism of Rolling

Reynolds, in his study or rolling friction, found that when a metalcylinder rolled over a rubber surface, it moved forward a distance less than its

circumference in each revolution of the cylinder. He assumed that a certain

amount of slip occurred between the roller and concluded that the occurrence of

this slip was responsible for the rolling resistance.

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Tabor, however, after more detailed investigation and repetition of

Reynolds experiments, showed that interfacial slip between a rolling element

and an elastic surface is in reality almost negligible and in any case quite

inadequate to account for the observed frictional losses. From the evidence of

his experiments Tabor concluded that rolling resistance arises primarily from

elastic-hysteresis losses in the materials of the rolling element and the surface.

This conclusion is supported by the experimental fact that rolling friction is

scarcely affected by the presence or absence of lubricant films. The typical

example of a ball rolling on a flat plate will give a clearer picture of the

physical mechanism now believed to be involved.

Fig (12) shows a steel ball resting on a flat steel plate; the actual arc of

contact is exaggerated in order to show the form of elastic deformation. As the

ball rolls in the direction of the arrow, the elastic deformation will assume a

shape similar to that as shown in Fig (12). A form of bulge will be pushed upfrom the plate surface in front of the ball and a cavity will follow a similar

bulge on the trailing edge of the ball. This type of deformation occurs when the

ball and plate are both made of material having similar elastic properties. The

plate surface over which the ball moves will undergo stretching and contraction

as the ball pushes a minute bow-wave of metal along in front of it. Similarly,

the ball surface itself will undergo cyclic stretching and contraction as it

continually modifies its original spherical shape.

Fig-12. Mechanism of Rolling Friction:-

(a) Elastic ball and plate at rest

(b) Elastic ball rolling on plate

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The continual deformation of the plate and distortion of the ball,

elastic-hysteresis, require energy and manifest themselves as the main sources

of resistance to rolling.

Friction of a Complete Rolling Contact Bearing

The conclusion that friction in rolling contact bearing is largely due to

elastic-hysteresis means that its magnitude will be influenced by all the factors

which normally cause variation in hysteresis effects. In the case of the rolling

elements alone, these would be the elastic properties of the materials, the speed

of rolling and the temperature. Additional factors which may influence the

frictional behavior of complete rolling contact bearing are the condition, shape

and relative positions of the surfaces, the magnitude and direction of the load,

the friction between the cage and the rolling elements, the friction between the

cage and the rings and the conditions of lubrication.

Friction of a Lubricant in a Rolling Contact Bearing

It is evident that lubrication is required to minimize sliding friction in

complete bearings. An additional function of the lubricant is to act as a

protection for the accurately-ground and highly-polished surfaces of the balls,

rollers and rings. If free moisture is allowed to contact the bearing elements,

corrosion and pitting will follow and the bearing life will be considerably

shortened. At the same time, a suitable lubricant should prevent the entry of

external contaminating matter in the form of dirt or abrasive dust.

In certain instances where ball or roller bearings operate at very high

speeds or under high ambient temperature conditions, e.g. in aircraft turbines, it

is necessary for the lubricant to serve as a heat-transfer medium by absorbing

heat generated at the contact areas of the rolling elements and carrying it away

from the bearing. In such cases, copious flow of lubricant serves to maintain an

even temperature throughout the bearing and prevents the development of a

high temperature differential between the inner and outer races.

Choice of Lubricant

The problem as to whether or not an oil or grease is the most suitable

lubricant for a particular rolling contact bearing is often determined solely by

the application itself.

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For the lubrication of ball and roller bearings mineral greases and mineral

oils are almost invariably used. Where circumstances permit, the aim of the

lubrication engineer should be first to choose the most satisfactory lubricant and

secondly to consider the design of a suitable bearing housing in which to

accommodate it. In any particular application, a decision must first of all be

made as to whether oil or grease is to be the choice. Some of the considerations

which influence this choice are discussed below.

Oil Lubrication

There is not doubt that oil is the most positive means of lubrication as its

fluidity enables it to penetrate readily to all parts of the bearing where sliding

friction is likely to occur. Oil is preferable under heavily-loaded high speed

conditions or high temperatures as it acts as a heat-transfer medium and serves

to prevent the bearing temperature from rising excessively. With greaselubrication no heat balance can be obtained; in fact, an excess of grease may

retain the heat. Operating speed is usually the main factor in determining the

adoption of oil as a lubrication medium since, for a given bearing bore,

manufacturers do not recommend the use of grease above a certain limiting

speed.

Other conditions in which oil is found to be the most suitable lubricant

are in light machines or precision instruments where resistance to rotation must

be kept to an absolute minimum and where bearings are completely enclosed incasings containing other parts for which oil lubrication is essential.

Grease Lubrication

Greases find much winder application than oils and are suitable in the

majority of bearing where there are no extremes of high temperature or high

speed. Some of the advantages of grease lubrication are:-

A grease is easier to retain in a bearing housing than oil and the

cleanliness in operation is of importance in textile or foodstuff machinery

where oil stains might be detrimental to the product.

A grease provides an excellent self-sealing device against the entry

of grit and harmful impurities such as might be present in the

environment of, say, cement kilns, quarries or chemical factories.

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In addition, grease provides an effective and almost permanent protective

coating on the rolling contact elements whereas during idle periods an oil might

tend to drain away and leave the surface dry and exposed to corrosion. From an

economic point of view, grease is a cheap and convenient lubricating medium

due to the infrequent attention and replenishment which grease-lubricated

bearings require.

Viscosity

. Viscosity, the single most important property of a hydraulic fluid, is a

measure of the sluggishness with which a fluid moves. It can be simply

described as the resistance of the lubricant to flow; more precisely it is the

property by virtue of which a fluid offers resistance to a shearing displacement.

In reality, the ideal viscosity for a given hydraulic system is a

compromise. Too high a viscosity results in:

(a) High resistance to flow, which causes a sluggish operation.

(b) Increases power consumption due to frictional losses.

(c) Increased pressure drop through valves and lines.

(d) High temperatures caused by friction.

On the other hand if the viscosity is too low, the result is:

(a) Increased leakage losses past seals.(b) Excessive wear due to break down of the oil film between moving

parts.

Viscous Flow

The concept of viscosity can be understood by examining two parallel

plates separated by an oil film of thickness y as shown in Fig 13.

Fig-13. Fluid velocity profile between parallel plates due to viscosity

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The lower plate is stationary, while the upper plate moves with a velocity

as it is being pushed by a forceFas shown. Due to viscosity, the oil adheres

to both surfaces. Thus, the velocity of the layer of fluid in contact with the

lower plate is zero, and the velocity of the layer in contact with the top plate is

. The consequence is a linearly varying velocity profile whose slope is / y.The absolute viscosity, z, of the oil can be represented mathematically:

z = Shear Stress in oil ()

Slope of the velocity profile (/ y)The shear stress in the fluid has units of force per unit area and is caused

by the adjacent sliding layers of oil film.

Calculations in hydraulic systems often involves the use of kinematic

viscosity rather that absolute viscosity. Kinematic viscosity (v) equals absolute

viscosity(z) divided by mass density( ):

v = z

Units of Viscosity

Two different units of absolute viscosity in the metric and British

systems are in current use.

Poise. When the shearing force on the moving surface is one

dyne, the wetted area one square centimeter, the relative velocity one

centimeter per second, the unit of viscosity is equal to one poise (named

after the French physicist Poiseuille).

Reyn. When the shearing force is one pound weight, the area

one square inch, the relative velocity one inch per second, the unit of

viscosity is equal to one reyen (name after the English scientist

Reynolds).

Given: 1 reyn = 6.895 x 104Poise

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Checking the units for the kinematic viscosity, we find the following two

units:

Stoke. In metric system, may be written in terms of

cm2/sec, and the kinematic viscosity of 1 cm2/sec is called stoke.

Newt. Using the English units, can be given by ft2 /sec, and

the kinematic viscosity of 1 ft2 /sec, is called Newt.

Given: 1 Newt = 6.45 x 104stokes

Measurement of Viscosity

Many types of viscosimeters (or viscometers), like Rotational, U-tube (or

capillary), Efflux, Saybolt and Redwood Falling Sphere are available butviscosity is usually measured by a Saybolt Viscosimeter which is schematically

shown in Fig (14). Basically, this device consists of an inner chamber

containing the sample oil to be tested. A separate outer compartment, which

completely surrounds the inner chamber, contains a quantity of oil whose

temperature is controlled by an electrical thermostat and heater. A standard

orifice is located at the bottom of the center oil chamber. When the oil sample

is at the desired temperature, the time it takes to fill a 60-cm3container through

the metering orifice is then recorded. The time(t), measured in seconds, is the

viscosity of the oil in official units called Saybolt Universal Seconds. Since a

thick liquid flows slowly, the SUS viscosity will be higher than that for a thin

liquid.

An empirical relationship between the viscosity in SUS and the

corresponding metric system units of centistokes(cS) is given as

(cS) = 0.226 t 195 t 100 SUS

t (cS) = 0.220 t 135 t 100 SUS

t

where represents the viscosity in centistokes and t is measured in SUS

or simply seconds.

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Fig-14 Saybolt Viscosimeter

Kinematic viscosity ( ) is related to the absolute viscosity (z) as,

(cS) = z(cP)/.

Since in the metric system, mass density equals specific graviy (generally 0.9

for most hydraulic fluids), therefore,

(cS) = z(cP)

0.9

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This test measures flow through a capillary instrument using gravity flow at

constant temperature. The time in seconds is then multiplied by the calibrationconstant for the viscosimeter to obtain the kinematic viscosity of the sample oil

in centistokes. The absolute viscosity can then be calculated by using aboveequation.

Absolute Viscosities of Typical Fluids at 68

o

F (20

o

C)

The viscosity values of some typical liquids is listed below for thegeneral awareness of the reader;

Fluid Abs. Viscosity

(Centi-poise)

Bitumen 105 - 109

Honey 2000

*SAE 50 oil 800

Glycerin 500

SAE 30 oil 300

Olive oil 100

SAE 10 oil 70

Ethylene-glycol 20

Mercury 1.6

Turpentine 1.5

Water 1.0

Petrol 0.6

Ether 0.2

*SAEAmerican Society of Automotive Engineers

Variation of Viscosity with Temperature

The viscosity of all liquids decrease with rise in temperature and this isparticularly apparent with hydrocarbon lubricating oils. The variation in

absolute and kinematic viscosities with temperature for a typical lightlubricating oil (SAE 10W) is shown in Figures (16) & (17) respectively.

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Fig. 16.- Variation of viscosity with temperature for SAE 10W Grade Oil

( Suffix W in SAE 10W sands for winter grade viscosity )

Fig. 17.- A. S. T. M. Standard Viscosity Temperature charts for liquid

petroleum products (D 34143), Kinematic viscosity, high range

This rapid variation in viscosity with temperature is of great importance inmany practical applications where oils are required to function over a wide

temperature range.

Variation of Viscosity with Pressure

Most lubricating oils undergo considerable increase in viscosity whensubjected to high pressure. Many oils become in substance plastic solids atpressures exceeding 20,000-30,000 psi. Typical examples are the high local

pressures developed when gear teeth mesh and the intense fluid-film pressuresin oil lubricated mechanisms, the components of which are subject to impact.

The manner in which viscosity varies with pressure for SAE 10W grade mineraloil is illustrated in Fig (18).

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Fig. 18.- Variation of viscosity with pressure, SAE 10W Oil

Viscosity Index

Oil becomes thicker as the temperature decreases and thins when heated.

Hence the viscosity of the given oil must be expressed at a specified

temperature. For most hydraulic applications, the viscosity normally equals

about 150 SUS at 100oF. It is a general rule of thumb that the viscosity should

never fall below 50 SUS or rise above 4000 SUS regardless of the temperature.Where extreme temperatures are encountered, the fluid should have a high

viscosity index (VI).

VI is a relative measure of oils viscosity change with respect to

temperature change. An oil having a low VI is one that exhibits a large change

in viscosity with temperature change. A high VI oil is one that has a relatively

stable viscosity which does not change appreciably with temperature change.

The original VI scale ranged from 0 to 100, respecting the poorest to bestcharacteristics known at that time. Today, with improved refining techniques

and chemical additives, oils exist with VI values well above 100. A high VI oil

is a good all-weather-type out for use with outdoor machines operating in

extreme temperature swings. For the calculation of VI of any hydraulic fluid

see Fig (19).

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Fig-19, Viscosity Index.

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VI Improvers

By adding small amounts of extremely viscous polymers to normal-grade

oils it is possible to modify the steepness of the viscosity-temperature curve.These polymers have relatively greater effects on the viscosity in the high range

than on the viscosity at low temperatures. Consequently, the VI of any type ofoil may be greatly improved. Because of these developments, VI rating is

becoming obsolescent as a means of assessing oils.

It is, however, still of value in expressing the relative change of viscositywith temperature and in comparing the merits of various oils in specialized

applications, as, for example, in hydraulic systems. For such systems syntheticoils having a VI of 160 and above are commercially available.

LUBRICATION

The behavior of surfaces is of course profoundly modified if a lubricantis added to them. We may consider two main types of lubrication:

Fluid or Hydrodynamic Lubrication.

Boundary lubrication which also includes the Extreme pressure(Extreme Temperature) Lubrication.

The general behavior of these groups can be represented schematically in the

following table.

Type of Lubrication Coefficient of

FrictionWear

FluidBoundary & Extreme Pressure

Unlubricated Surface

0.0010.0010.15

1.0

NoneSlight

Heavy

Fluid Lubrication

55. In fluid lubrication the moving surfaces are completely separated by acontinuous film of lubricant and the resistance to motion arises solely from theviscosity of the lubricant itself.

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Resistance in a Hydrodynamic Bearing

In case of a journal (the part of a shaft which is in contact with, and

supported by, a bearing) rotating in a bearing (support provided to hold arevolving shaft in its correct position) some idea of the resistance may be

obtained by assuming that the oil film is of uniform thickness all around thejournal (Fig 20).

Fig-20. A petroff hydrodynamic bearing in which the oil film is uniform.

If dimensions are in cm and N is RPM, then,

Surface velocity of shaft = (2 r N)/60 cm / sec.

It is generally accepted that there is no slip of the liquid at the solid surfaces so

that the whole of this velocity difference between the shaft and the bearing mustbe taken up by shear in the lubricant film. If film is thin, then,

Rate of shear in film = (2 r N)/(60 c),

Which is uniform in its value? According to Newtons law of viscous flow,

shear stress = Rate of shear stress x viscosity= ( 2 r Nz )/(60 c) dynes / cm2

Where z is the viscosity of oil in poises. If the length of the bearing is L, thearea over which this stress operates is 2rL. Since the resultant viscous force

operates at a distance from the center of the shaft; The Resisting Torque G is

given by:

G = ( 2 r L ) { ( 2 r N z ) / (60 c) }

= 4 x2y3L N z60 c

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For very lightly loaded bearings or for vertically running shafts this

relation gives values which are very close to the observed values of the resistingtorque. However, for loads across the oil film which are not negligible the

bearing cannot run in the central position. For the oil film to possess any load-bearing capacity the journal must rotate eccentrically relative to bearing so that

the lubricant is squeezed through the converging gap between the sufaces andits velocity increases under these conditions, a hydrodynamic pressure is built

up in the oil wedge and this is sufficient to keep the surfaces completelyseparated.

Fig. 21. Pressure distribution (shownshaded) in oil film for a journal running in ahalf bearing convergence in the oil filmloads to a pressure great enough tosupport the journal.

Fig. 22. Theoretical curve ofhydrodynamic lubrication betweenjournal and bearing.

For a journal running in a half bearing the pressure distribution is shown(Fig 21), and similar results are obtaining for a full bearing. It is seen that the

convergence in the oil film leads to an asymmetric pressure distribution givinga resultant upward thrust. This could not occur if the journal rotated centrally in

the bearing; the pressure would be zero throughout the film.

We have already seen that for a given journal and bearing the resistingtorque G is proportional to Nz, i.e. G (Nz). Thus if the nominal pressure on

the bearing is P (that is the load divided by the projected area of the bearing),the effective coefficient of friction is simply a linear function of NZ/P, i.e.

= f ( Nz / P )

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Another quantity of great importance is the distance of nearest approach

of the journal and bearing. This quantity is also dependent on the parameter

Nz/P. For a journal of diameter, D =1.25 inches, rotating in a bearing of length

L = 1.25 inches. With a diametrical clearance of 0.00125 in, and the distance

of nearest approach are plotted in Fig. 22.

The great advantage of hydrodynamic or fluid lubrication are that thereis, in the ideal case, no wear of the moving parts, and friction is extremely low.

The resistance to motion solely arises from the viscosity of the lubricant.

Clearly, lower the viscosity the lower the viscous resistance. However, there is

a limit to this, for as the viscosity is decreased, the distance of nearest approach

diminishes. If this becomes less than the height of the surface irregularities,

penetration of the hydrodynamic film occurs. For this reason it is customary to

use an oil of viscosity just sufficient to give a distance of nearest approach great

enough to ensure the maintenance of an unbroken hydrodynamic film.

Practical Complications in Fluid Lubrication

Although Reynolds theory (discussed above) remains the basic approach

to problem of hydrodynamic lubrication but practically, one complication arises

from side leakage of the oil; another from the effect of cavitation (the formation

of cavity between the surface of a moving body and a liquid normally in contact

with it) in oil which appears to break up the film into series of separate

filaments.

Apart from these we must take into account the following major factors:-

The effect of surface roughness.

The effect of pressure on the viscosity of oil: viscosity increase

with pressure.

The effect of temperature on the viscosity of oil: A rise intemperature of 10% may reduce the viscosity to half its value.

The effect of high rates of shear on viscosity of oil: this often leads

to a decrease in viscosity.

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These effects are all very difficult to deal with theoretically, but the most

important of these factors is the effect of temperature and much research has

gone into developing additives which reduce the temperature dependence of

viscosity. Development of synthetic lubricants is a result of these efforts.

Oxidation and Corrosion

Oxidation, which is caused by the chemical reaction of oxygen from the

air with particles of oil, can seriously reduce the life of a hydraulic fluid.

Petroleum oils are especially susceptible to oxidation because oxygen readily

unites with both carbon and hydrogen molecules. Most products of oxidation

are soluble in oil and are acidic in nature, which can cause corrosion of parts

throughout the system. The products of oxidation include insoluble gums,

sludge, and varnish which tend to increase the viscosity of the oil.

There are a number of parameters which hasten the rate of oxidation once

it begins. Included among these are heat, pressure, contaminants, water, and

metal surfaces. However, oxidation is most dramatically affected by

temperature. The rate of oxidation is very slow below 140oF but doubles for

every 20oF temperature rise. Additives are incorporated in many hydraulic oils

to inhibit oxidation. Since this increases the costs, they should be specified only

if necessary, depending on temperature and other environmental conditions.

Rust and corrosion are two different phenomena, although they both

contaminate the system and promote wear. Rust is the chemical reaction

between iron or steel and oxygen. The presence of moisture in the hydraulic

system provides the necessary oxygen. One primary source of moisture is from

atmospheric air which enters the reservoir through the breather cap.

Corrosion, on the other hand is the chemical reaction between a metal

and acid. The result of rusting or corrosion is the eating away of the metalsurfaces of hydraulic components. This can cause excessive leakage past the

sealing surface of the affected parts. Rust and corrosion can be resisted by

incorporating additives on the metal surfaces to prevent chemical reaction.

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Boundary Lubrication

When sliding surfaces are separated by lubricant films only few

molecules in thickness, the friction is influenced by the nature of the underlying

surface as well as by the chemical constitution of the lubricant. Viscosity plays

a little or no part in the frictional behavior. This type of lubrication is called

Boundary lubrication (or oiliness). The transition from the hydrodynamic to

boundary lubrication may be too gradual. As the sliding speed is decreased or

the load increased, the wedge of lubricant separating the surfaces becomes

thinner and thinner, and the number of surface asperities penetrating the film

becomes greater. The amount of boundary lubrication increases gradually,

while the fluid lubrication decreases. This type of mixed lubrication extends

over a very wide range of experimental conditions.

Cases of Boundary Lubrication

True boundary lubrication can, strictly speaking, only exist when sliding

speeds are so low and contact pressure so high that the existence of load-

supporting hydrodynamic wedge of lubricant are physically impossible. In

journal bearings, for example, boundary friction conditions may develop

whenever the shaft passes through zero speed during starting, stopping or

reversing. The rubbing surfaces of pistons, piston rings, crossheads, machine

tool guides, etc., all operate during part of their motion in the boundary region.

Boundary lubrication is evident in many metal-working processes such as deepdrawing, tube drawing, wire drawing, and metal cutting. Certain types of gears,

notably hypoid gears, operate under conditions of boundary friction.

Boundary Lubricants

All metals are covered with surface films of some kind. These can be

layers of water vapours, oxides, nitrides or sulphides or even dust, grease or

corrosion products. However, a boundary lubricant is defined as a film which is

deliberately introduced between the sliding surfaces and has the followingattributes:

A boundary lubricant must have a low shear strength.

It must adhere readily and firmly to the interacting surfaces.

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Many animal and vegetable fats and oils possess the properties of

effective boundary lubrication. From industrial point of view, the most

important are:

Stearic acid (lard oil and mutton tallow).

Palmitic acid (cotton seed oil).

Palm oilOleric acid (olive oil)

Extreme Pressure (EP) Lubricants

The efficacy of even the best boundary lubricants decreases as the

temperature increases and when they are desorbed (reverse process of

adsorption) the friction and wear are almost as high as in the absence of

lubrication. Furthermore, at very high temperatures ( 250oC and above) the

oxidation of the lubricant may occur and produce harmful products. Hence, for

the surfaces which operate under very severe conditions, more stable protective

films are required, which are provided by EP Lubricants. (This name is

misleading, it would be more appropriate to call them Extreme temperature

lubricants).

Extreme pressure lubricants are a class of lubricants to which certain

active chemicals or chemical groups have been added. These additions are

called EP additives and generally are:-

Chlorine (chlorinated esters etc).

Sulphur (Sulphurised fats and oils etc).

Phosphorus (tricresyl Phosphate etc).

Through chemical reactions with the metals surfaces these additives form

solid surface films of metallic chlorides, sulphides and probably phosphides.

These boundary films have a relatively low shear strength (e.g. Iron chloride

= 0.2; Iron Sulphide = 0.5) so that rubbing between the interacting surfacesoccurs in additive film and thus protects the underlying metal. Moreover, the

melting points of EP layers are high (e.g. iron chloride 1200oF; iron sulphide

2150oF) so they will remain attached to the base metal even under extreme

conditions.

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Gear Oils: These may be straight mineral oils of widely ranging

viscosity or compounded oil containing extreme-pressure (EP) additives

to improve the film strength and load-carrying ability. They are suitable

for enclosed gearsets of super, bevel, helical, spiral bevel, herringbone.

Such oils are best suited for average tooth loads and pinion speeds.

Machine or Engine Oils: They are straight mineral red oils

which are used for general lubrication of external operating parts of

engines, pumps, and compressors.

Hydraulic Oils: Although the prime object of a hydraulic fluid

is to transmit power, a further important requirement is that it should also

possess the qualities of an efficient hydrodynamic and boundary

lubricant. It must lubricate all moving parts particularly those in sliding

contact, to minimize wear. Mineral oils treated with suitable additivesand synthetic types of hydraulic fluids may be used in hydrokinetic

systems for this purpose.

Lubricating Greases

Lubricating greases commonly consist of mineral oils thickened, by the

addition of metallic soaps, to a solid or semi-solid consistency. Sometimes

fatty or synthetic oils are used instead of mineral oils. The metallic soap isnormally obtained from animal or vegetable fats mixed with a suitatable alkali,

a calcium (lime) or sodium (soda). Lithium, aluminum, lead or barium may

also be used for special purposes. Today precision manufacture is so fine and

seals are so perfect that life time lubrication in bearing is practicable.

Furthermore, greases are available which will function over very wide range of

temperature. They are also able to exclude moisture, dust, corrosive gases, etc.

particularly, in ball or roller bearing.

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Synthetic Lubricants

Synthetic lubricants are oily liquids which are not found naturally or not

produced directly during the normal manufacturing and refining processes of

the petroleum industry. However, most synthetics have some properties similar

to the petroleum-based lubricants and many are hydrocarbons which have

undergone a specific synthesis.

Industrial Synthetics

The most important specific chemical classes of compounds, which have

been found to have useful lubricating functions are:-

Esters Dibasic acid esters, Organo-phosphate esters.

Silicons - Silicate estersHydrocarbons - Chlorinated and fluorinated hydrocarbons

(Esters are derivatives of acids, obtained by exchange of replaceable

hydrogen for alkyl radicals, with fruity smell)

Characteristics of Synthetics

The most compelling reasons for using synthetics are listed below:

They work over a greatly extended range of temperature.

They exhibit thermal stability, oxidation resistance, hydrolysis

resistance, rust preventive qualities and fire resistance properties.

Many of them are excellent solvents for Viscosity Index (VI)

improvers, oxidation inhibitors and rust inhibitors.

Certain synthetics possess a high degree of natural detergency andact as scavenging agent for dirt and fuel deposits.

They are much costlier as compared to the petroleum products.

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Applications of Synthetics

The synthetics are finding increasing use in:-

Hot running bearings.

Heavily loaded high speed gears, and

Jet turbines.

Solid Lubricants

The modern engineering mechanisms, especially those applied to the

fields of nuclear power, aviation and ballistic missiles, are required to operate

over a very wide temperature range (from -50oF to 700oF ). It is very difficult

to meet these requirements from lubricants based on organic fluids, particularly

from the point of view of chemical stability at very high temperatures or incases where inaccessible bearing mechanisms are required to function

intermittently. To satisfy these exacting demands, solid lubricants have been

developed.

A solid lubricant may be broadly defined as a material which

permanently retains its solid sate when interposed between mutually sliding

surfaces and which facilitates sliding entirely within the confines of the

lubricant layer.

Types of Solid Lubricants

Some of the commonly used types of slid lubricants are:-

Boron Nitride.

Graphite.

Mica.Talc.

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Metal working Lubricants

The principal metal working processes are machining or grinding, deep

drawing, wire drawing, rolling, extrusion, stamping and blanking, moulding and

forging. All these processes require the use of lubricants or coolants in one

form or other to facilitate the required mechanical changes and to ensure an

acceptable product.

Constituents of Metal Working Lubricants

The principal constituents of metal-working lubricants are:

Water

Mineral oils

Fatty oils and fatty acidsWaxes

Soaps

Extreme Pressure additives

Mineral solids

Synthetics

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BIBLIOGRAPHY

Frication and Lubrication,

F.P. Bowden and D. Tabor (1960), Methuen & co Ltd London.

Lubrication and Friction,

Peter Freeman (1962), Pitman & Sons Ltd London.

Preventive Maintenance,

Petroman Course Manual.

Basic Lubrication Theory,

Alastair Cameron (1980), Ellis horwood Ltd. Chicester.

Fluid Power With Applicaton,Anthony Esposio (1980), Prentice-Hall, Inc., Englewood Cliffs,

New Jersey.

Documents

12. Notes on Fundamentals of Tribology PAF CAE-1989