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9 Tribology

Edward H. Smith (Sections 9.1 and 9.3)

John Weston-Hays (Section 9.2)

Bert Middlebrook (Section 9.4)

Dennis R. Hatton (Section 9.5)

Tony G. Herraty (Section 9.6)

Philip Eliades (Section 9.7)

Keith T. Stevens and A. Davies (Section 9.8)

Michael W. J. Lewis (Section 9.9)

Ian Sherrington (Section 9.10)

Contents

9.1 Basic principles 9/39.1.1 Introduction 9/39.1.2 Lubrication regimes 9/39.1.3 Friction, wear and lubrication 9/4

9.2 Lubricants (oils and greases) 9/59.2.1 Physical characteristics 9/59.2.2 Additives 9/69.2.3 Lubricating-oil applications 9/79.2.4 General machinery oils 9/79.2.5 Engine lubricants 9/79.2.6 Hydraulic fluids 9/139.2.7 Machine tools 9/139.2.8 Compressors 9/189.2.9 Turbines 9/209.2.10 Transformers and switchgear 9/209.2.11 Greases 9/219.2.12 Corrosion prevention 9/229.2.13 Spray lubricants 9/239.2.14 Degreasants 9/239.2.15 Filtration 9/239.2.16 Centrifuging 9/249.2.17 Centralized lubrication 9/259.2.18 Storage of lubricants 9/259.2.19 Reconditioning of oil 9/269.2.20 Planned lubrication and maintenance

management 9/26

9.2.21 Condition monitoring 9/269.2.22 Health, safety and the environment 9/26

9.3 Bearing selection 9/279.3.1 Characteristics of bearings with

continuous motion 9/279.3.2 Bearing selection charts 9/28

9.4 Principles and design of hydrodynamic bearings 9/309.4.1 Introduction 9/309.4.2 Principles of hydrodynamic lubrication 9/309.4.3 Viscosity 9/319.4.4 Journal bearing design 9/329.4.5 Self-contained bearings 9/399.4.6 Thrust bearings 9/39

9.5 Lubrication of industrial gears 9/419.5.1 Methods of lubrication 9/429.5.2 Types of gear oils 9/449.5.3 Heat dissipation 9/459.5.4 Selection of gear lubricants 9/479.5.5 Service life of gear lubricants 9/49

9.6 Rolling element bearings 9/499.6.1 Introduction 9/499.6.2 Types 9/509.6.3 Selection 9/52

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9.6.4 Bearing life 9/529.6.5 Bearing friction and speed limits 9/549.6.6 Application 9/559.6.7 Lubrication 9/569.6.8 Bearing mounting and maintenance 9/589.6.9 Bearing failures 9/59

9.7 Materials for unlubricated sliding 9/609.7.1 Introduction 9/609.7.2 Performance rating 9/609.7.3 Counterface materials 9/629.7.4 Polymers and polymer composites 9/629.7.5 Carbon graphites 9/679.7.6 Solid lubricants 9/689.7.7 Metallic alloys and composites 9/699.7.8 Surface treatments and coatings 9/709.7.9 Conclusions 9/70

9.8 Wear and surface treatment 9/709.8.1 Introduction 9/709.8.2 Wear types 9/719.8.3 Surface treatments and coatings 9/739.8.4 Tribological data 9/839.8.5 Selection philosophy 9/859.8.6 Quality control 9/879.8.7 Closure 9/88

9.9 Fretting 9/889.9.1 Introduction 9/889.9.2 Source of relative movement 9/899.9.3 Characteristics 9/909.9.4 Parameters influencing fretting 9/919.9.5 Theoretical considerations 9/959.9.6 Fretting wear evaluation 9/989.9.7 Preventative measures - some

palliatives 9/989.9.8 Summary of palliatives 9/101

9.10 Surface topography 9/1029.10.1 Effects of surface topography 9/1029.10.2 Measurement 9/1099.10.3 Characterization of surface

measurements 9/1179.10.4 Summary, conclusions and future

developments 9/124

References 9/125

Further reading 9/131

Figure 9.2 A typical engineering surface

9.7.2.7 Rigid, full fluid-film (5 < M < 10)

In this situation the surfaces are kept apart by a pressurizedfluid such as oil or air. The clearance space is much larger thanthe average surface roughness, and therefore the surfaces canbe considered smooth. The pressurization of the fluid isachieved by external means in hydrostatic bearings, but isaccomplished in hydrodynamic contacts by the relative motionand geometry of the surfaces. Examples are crankshaft mainbearings in internal combustion engines (oil-lubricated hydro-dynamic journal bearings), shaft support bearings in power-generating turbines (often a combination of oil-lubricatedhydrostatic and hydrodynamic journal bearings), piston rings(oil-lubricated hydrodynamic slider bearings), dentists' drills(air-lubricated hydrostatic bearings for shaft support) andmagnetic disk heads (air-lubricated hydroynamic slider bear-ings). The friction coefficient in liquid-lubricated contacts fallstypically within a range of 0.004 to 0.01. An outline of theprinciples of rigid, hydrodynamic lubrication is presented inSection 9.4.

9.7.2.2 Elastic full fluid-film (1 < M < 10)

There are situations in liquid lubricated arrangements wherethe loads are so high that the pressure in the lubricant causeslocal elastic distortions of the surfaces. This form of hydrody-namic lubrication is called elastohydrodynamic lubrication.The nominal clearance can still be such that surface roughnesseffects can be ignored, although mixed elastohydrodynamiclubrication can occur.

Elastohydrodynamic lubrication is most commonly found ingears and rolling element bearings, which are discussed inSections 9.5 and 9.6, respectively.

9.1.2.3 Mixed (K M < 5)

Loads or speeds can be such that the opposing surfacesoccasionally come into contact. In these contacts, part of theload is carried by the liquid or gaseous lubricant, and part bythe interacting asperities. The rubbing together of asperitieswill increase friction, and this can be minimized by attention tothe compounds which can be made to adhere to the surfaces.Hypoid gears in automobile rear axles at times exhibit thisregime. Increased wear occurs, and friction coefficients inliquid-lubricated contacts of between 0.01 and 0.06 are found.

1 micrometre oxide layer

deformed layer

substrate material

Full Fluid Film

Mixed

Boundary

Figure 9.1 Lubrication regimes

9.1 Basic principles

9.1.1 Introduction

Machines transmit power between their moving componentswhich gives rise to friction and wear of the components.Tribology is, essentially, concerned with minimizing this fric-tion and wear, and is formally described as 'the science ofinteracting surfaces in relative motion'. The tribologist has tounderstand about the roughnesses of surfaces, the propertiesof liquid and solid lubricants, the principles of hydrodynamicand hydrostatic lubrication, the design of rolling elementbearings, the behaviour of surfaces under stress, the nature ofwear processes, and many other items which demand aninterdisciplinary approach to design and problem solving.

9.1.2 Lubrication regimes

The most usual way to reduce friction and wear is by lubrica-tion, employing gases, liquids or solids. Four basic lubricationregimes can be considered, characterized by the ratio, M, ofthe combined roughness, 7?, of both surfaces (in 7?a) to themean film thickness, h, i.e. M — RIh. Three of these regimesare illustrated in Figure 9.1, where the roughness of thesurfaces is greatly exaggerated for presentation reasons. Inreality, most surfaces more resemble gentle rollings hills onthe earth's surface than the craggy, mountainous featuresillustrated. Figure 9.2 shows the more realistic dimensions andstructure of a real surface, and a more detailed discussion ofsurface topography is presented in Section 9.10.

9.1.2.4 Boundary (M < 1)

In this regime, all the applied load is carried by the surfaceasperities, and the friction and wear which arises dependsupon the lubrication properties of the molecules on thesurfaces. Coefficients of friction between 0.06 and 0.1 aretypical when a low shear strength surface film is present. If nosuch film is present, then coefficients ranging between 0.2 and0.4 can be exhibited, even rising as high as 1.0 in some cases.The friction and wear of materials in dry sliding is discussed inSection 9.7.

9.1.3 Friction, wear and lubrication

9.1.3. 1 Friction

There are three general laws of friction:

1. Friction is independent of apparent contact area2. Limiting friction is proportional to the normal load3. Limiting friction is independent of sliding speed

Limiting, or breakout, friction is the friction force obtaining atthe point of slippage. The first two laws, proposed by Amon-ton in 1699, are generally applicable. The third law, proposedby Coulomb in 1785, has a reduced area of applicability.

The second law is more commonly written as:

fji = FIW

where

IUL = static, or breakout, coefficient of friction,F = limiting friction force,W - load applied normal to the surfaces.

In the light of law 3, a dynamic coefficient of friction can alsobe defined as the ratio of the sliding friction force to thenormal load. It can be significantly lower than its staticcounterpart.

The friction occurring at asperity contacts is usually attribut-able to one of two causes: abrasion, where the harder surface(or a harder particle) ploughs through the softer counterface;or adhesion, where the intimate contact between asperitiesgenerates local adhesive bonds which break when motion isinitiated. Friction can also arise due to hysteresis, as in thecase of rolling friction.

The simple adhesive theory of friction deduces that:

IJL = shear strength of surface/hardness of surface

This is a gross oversimplification1 but serves to indicate thatadhesive friction can be reduced by reducing the shearstrength of the bonds (by using dissimilar materials in opposi-tion), introducing low shear strength surface layers (often asboundary lubricants such as stearates, sulphides, etc.), and/orincreasing the hardness of the surface. There is evidence that asoft, thin layer of low shear strength material on a hardersubstrate can produce a component whose surface exhibitsboth low shear strength and high hardness - the latter beingacquired from the parent material if the surface film issufficiently thin.

The friction effects of abrasion can be minimized by suitablechoice of counterface materials so that ploughing is reduced orprevented, and/or the addition of boundary lubricants whichcan ease the ploughing process. Sometimes boundary lubri-cants occur naturally as, for example, oxide layers; sometimesa boundary lubricant can be added to a liquid lubricant, e.g.stearates which have polar properties permitting them to attachto a surface like bristles on a brush (see Section 9.5.2). Arnell etal.{ and Rabinowicz2 provide more detailed discussions.

9.7.3.2 Wear

Wear occurs through adhesion, abrasion, fretting, corrosionor fatigue. The first two modes are sometimes called slidingwear and are discussed below. Further detailed discussion ofwear is presented in Sections 9.7-9.9. Sun3 has suggested thatsliding wear can also be caused by delamination of surfaces. Inaddition to the discussion in this chapter, the reader is referredto the Wear Control Handbook4 for further details.

Adhesive wear arises from the breaking of the adhesivebonds formed when asperities contact and deform. The result-ing wear particles may be smeared on one of the surfaces,form work-hardened abrasive particles, or be ejected from thecontact. Abrasive wear occurs when harder asperities ploughthrough a softer counterface (two-body abrasion) or when aharder third particle gouges the surface of a softer material(three-body abrasion).

Wear equations Wear by sliding (adhesive, abrasive or dela-mination) is usually measured in terms of the specific wearrate, /c, defined as the volume of material worn away for a unitload and unit sliding distance. Thus:

Ic = vl(W. L) (9.1)

where

v = volume of worn material (m3),W = normal applied load (N),L = total sliding distance (m).

The standard units are m2 N"1. Values of 10"16 or lower areindicative of mild wear, and values greater then 10~14 repre-sent severe wear. Most material wear rates lie within the range10-17andlO~12.

PV factors in wear Rearrangement of equation (9.1) can beeffected to produce:

v = k.W.L (9.2)

Dividing both sides of this equation by the apparent area ofcontact, a, yields:

h = (k.W.L)/a (9.3)

where H is the wear depth.Since WIA is the bearing pressure, P, equation (9.3) can be

written as:

h = k.P.l (9.4)

Dividing both sides of the equation by time, t, and rearranginggives

k = hl(P.V.i) (9.5)

where the sliding speed, V, is equal to LIT.

Sometimes the constant, /c, which is numerically identical tothat in equation (9.1), is called the Wear Factor.

Equation (9.5) can be rearranged to yield:

k.t/h = 1/(F. V) (9.6)

If the parameter A; is a constant for a particular configuration,then an acceptable value for the term (k.t/h) can be deter-mined since the required life, t, and the acceptable maximumwear depth, h, can both be specified. Equation (9.6) thenshows that the parameter (k.t/h) is inversely proportional tothe product of P and V. This is often quoted by manufacturersof dry or partially lubricated bearings as the bearing's 'PVfactor'.

This same product, PV, also influences the temperature riseat the sliding contact. This can be deduced from the fact that:

• Power loss is proportional to the product of friction forceand speed.

• Heat loss by conduction is proportional to temperaturedifference, 7\ between the sliding surface and its surround-ings, the apparent area of the contact, A, and the conductiv-ity of the bearing and its surroundings.

Thus the temperature rise, 7\ at the sliding interface is givenby:

T= c. /LtP. V

where c is a constant which characterizes the resistance of thebearing configuration to heat dissipation. This is discussedfurther in Section 9.7.

9.1.3.3 Fluid film lubricationThis form of lubrication can be achieved using liquids or

gases. The aim is to keep the surfaces separated during normaloperation. The most common liquid lubricants are mineraloils, which are discussed in Section 9.2. Air is probably themost common gaseous lubricant.

Liquid film bearings usually exhibit laminar flow, and canbe designed using the Reynolds equation as discussed inSection 9.3. A modified form of the familiar Reynoldsnumber, Re, is used in fluid film lubrication studies, definedas:

Re = (p. u. /./T]). (M)

where

p = density of the lubricant,u = a representative velocity of the lubricant,/ = characteristic dimension of the bearing (e.g. length orwidth),17 = dynamic viscosity of the lubricant,h — characteristic film thickness.

This ratio assesses the relative importance of viscous andso-called inertia forces. If Re <l 1, the former dominate thelatter and laminar flow pertains. When Re ^> 1, the latterdominate the former and turbulent flow is present. Somehigher speed bearings operate in the turbulent regime. Furtherreading on turbulent flow can be found in reference 5.

9.2 Lubricants (oils and greases)

9.2.1 Physical characteristics

Reference will be made to the physical characteristics oflubricants as they affect their selection for various applica-tions. These terms are well known to the lubricant supplier butare not always fully understood by the user. Brief descriptionsof these characteristics are therefore given so that theirsignificance may be appreciated.

9.2.7.7 Viscosity

This is the most important physical property of a lubricatingoil; it is a measure of its internal friction or resistance to flow.In simple terms, it provides a measure of the thickness (notdensity) of a lubricating oil at a given temperature; the higherthe viscosity, the thicker the oil. Accurate determination ofviscosity involves measuring the rate of flow in capillary tubes,the unit of measurement being the centistoke (cSt). As oilsbecome thinner on heating and thicker on cooling a viscosityfigure must always be accompanied by the temperature atwhich it was determined.

The number of commercial viscosity systems can be confus-ing, and as kinematic viscometers are much more sensitive andconsistent, there is a growing tendency to quote kinematicviscosities. The International Standards Organization (ISO)uses kinematic viscosity in its viscosity grade classification(Table 9.1). These ISO grade numbers are used by most oilcompanies in their industrial lubricant nomenclature. Thisprovides the user with a simple verification of conformityregarding viscosity between plant manufacturer and oilsupplier recommendations and also in the monitoring ofcorrect oil usage on his plant.

9.2.7.2 Viscosity Index (VI)

This is a way of expressing the rate of change of viscosity withtemperature. All oils become less viscous as the temperatureincreases. The rate of change of viscosity varies with differentoils and is mainly dependent on the type of crude from whichthe oil is derived and the refining method. The higher the VIfigure, the lower is the variation in viscosity relative totemperature. The VI of an oil is an important property inapplications where the operating temperature is subject toconsiderable change.

9.2.1.3 Pour point

This is a rough measure of a limiting viscosity. It is thetemperature 2.50C above that at which the oil ceases to flowwhen the vessel in which it has been cooled is held horizontallyfor 5 s. The pour point is a guide to behaviour and care shouldalways be taken that operating temperatures are above thefigure specified by the oil manufacturer as the pour point of agiven oil.

9.2.7.4 Flash point

The flash point of an oil is the temperature at which it givesoff, under specified conditions, sufficient vapour to form aflammable mixture with air. This is very different from the

Table 9.1 ISO viscosity grade chart

ISOviscositygrade

2357

101522324668

100150220320460680

10001500

Mid-pointkinematicviscosity

2.23.24.66.8

101522324668

100150220320460680

10001500

Kinematic viscosity limitscSt at 4(TC (1040F)

min. max.

1.98 2.422.88 3.524.14 5.066.12 7.489.00 11.0

13.5 16.519.8 24.228.8 35.241.4 50.661.2 74.890.0 110

135 165198 242288 352414 506612 748900 1100

1350 1650

temperature of spontaneous combustion. The test is an empi-rical one and the result depends upon the instrument used andthe prescribed conditions. For example, the flash point may be'closed' or 'open', depending on whether the test apparatushas a lid or not. As far as lubricating oils are concerned, thetest is of limited significance, although it can be indicative ofcontamination (for example, the dilution of crankcase oil byfuel).

9.2.1.5 Penetration of grease

The most important physical property of a lubricating grease isits consistency, which is analogous to the viscosity of a liquid.This is determined by an indentation test in which a weightedmetal cone is allowed to sink into the grease for a specifiedtime. The depth to which the cone penetrates, in tenths of amillimetre, is a measure of the consistency. There is a widelyaccepted scale, that of the American National LubricatingGrease Institute (NLGI), that relates penetration to a con-sistency number.

The penetration test is used mainly to control manufactureand to classify greases and is, within limits, a guide to selection.Penetrations are often qualified by the terms 'worked' and'unworked'. As greases are thixotropic, that is, they soften asa result of shear but harden again after shearing has stopped,the worked penetration for a particular grease may be appreci-ably greater than the unworked penetration. The differencebetween these two figures may be a useful guide to theselection of greases for operating conditions that involve muchchurning - as small a difference as possible being desirable(see Table 9.2).

9.2.1.6 Drop point of grease

The drop point of a grease is an indication of change from asoft solid to a viscous fluid; its value depends completely onthe conditions of test, particularly the rate of heating. Thegrease sample, which is held in a small metal cup with anorifice, is heated at a predetermined rate. The drop point isthe temperature at which a drop of the sample falls from thecup.

The drop point is of limited significance as far as the user isconcerned, for it gives no indication of the condition of thegrease at lower temperatures, or of change in consistency orstructure with heat. It is a very rough indication of a grease'sresistance to heat and a guide to manufacture. The differencebetween the highest temperature at which a grease can be usedand the drop point varies very much between types. It is at itsmaximum with some soda greases and much smaller withmulti-purpose lithium products and modern complex greases.

Table 9.2 NLGI consistency classification for greases

9.2.2 AdditivesMuch highly stressed modern machinery runs under condi-tions in which a straight mineral oil is not adequate. Even thehighest quality mineral oil can be unsatisfactory in response ofits resistance to oxidation and its behaviour under pureboundary conditions, but it is possible to improve thesecharacteristics by the addition of relatively small amounts ofcomplex chemicals. This use of additives resembles in manyways the modification of the properties of steel by the additionof small amounts of other chemicals. It will be of value to havesome knowledge of the effect of each type of additive.

9.2.2.1 Anti-oxidants

When mixed with oxygen, lubricating oil undergoes chemicaldegradation resulting in the formulation of acidic products andsludge. This reaction, which is affected by temperature, thepresence of catalysts such as copper and the composition ofthe oil, can be delayed by the inclusion of suitable additives.

Anti-oxidants are the most extensively used additives andwill be found in oils and greases which are expected to operatefor considerable periods or under conditions which wouldpromote oxidation. Typical examples are crankcase oils andbearing greases.

9.2.2.2 Anti-foam

The entrainment of air in lubricating oil can be brought aboutby operating conditions (for example, churning) and by baddesign such as a return pipe which is not submerged. The airbubbles naturally rise to the surface, and if they do not burstquickly, a blanket of foam will form on the oil surface. Furtherair escape is thus prevented and the oil becomes aerated. Oilin this condition can have an adverse effect on the systemwhich, in extreme cases, could lead to machine failure. Thefunction of an anti-foam additive is to assist in the burst of airbubbles when they reach the surface of the oil.

9.2.2.3 Anti-corrosion

The products of oil oxidation will attack metals, and this canbe prevented by keeping the system free from pro-oxidativeimpurities and by the use of anti-oxidants. These additives willnot, however, prevent rusting of ferrous surfaces when air andwater are present in the mineral oil. The presence of absorbedair and moisture is inevitable in lubricating systems andtherefore the oil must be inhibited against rusting. Theseadditives, which are homogeneously mixed with the oil, havean affinity for metal, and a strongly absorbed oil film is formedon the metal surface which prevents the access of air andmoisture.

9.2.2.4 Anti-wear

The increasing demands being made on equipment by therequirement for increased output from smaller units createproblems of lubrication, even in systems where full-fluid filmconditions generally exist. For instance, at start-up, after aperiod of rest, boundary lubrication conditions can exist andthe mechanical wear that takes place could lead to equipmentfailure. Anti-wear additives, by their polar nature, help the oilto form a strongly absorbed layer on the metal surface whichresists displacement under pressure, thereby reducing frictionunder boundary conditions.

NLGI number

000OO

O123456

ASTM worked penetration at 7TF

455-475400-435355-385310-340265-295220-250175-205130-16085-115

9.2.2.5 Extreme pressure

Where high loading and severe sliding speeds exist betweentwo metal surfaces, any oil film present is likely to be squeezedout. Under these conditions very high instantaneous pressuresand temperatures are generated. Without the presence ofextreme pressure additives the asperities would be weldedtogether and then torn apart. Extreme pressure additives reactat these high temperatures with the metal or another oilcomponent to form compounds which are more easily de-formed and sheared than the metal itself, and so preventwelding. Oils containing extreme pressure additives are gen-erally used in heavily loaded gearboxes which may also besubjected to shock loading.

9.2.2.6 Detergent!dispersant

The products of combustion formed in internal combustionengines, combined with water and unburnt fuel, will formundesirable sludge which can be deposited in the engine andso reduce its operation life and efficiency. Detergent/dispersant additives prevent the agglomeration of these pro-ducts and their deposition in oilways by keeping the finelydivided particles in suspension in the oil. They are used inengine-lubricating oils where, when combined with anti-oxidants, they prevent piston-ring sticking. They are essentialfor high-speed diesels, and also desirable for petrol engines.

9.2.2.7 Viscosity Index improvers

When mineral oils are used over an extended temperaturerange it is frequently found that the natural viscosity/temperature relationship results in excessive thinning out inthe higher-temperature region if the desired fluidity is to bemaintained at the lower region. The addition of certainpolymers will, within limits, correct this situation. They are ofparticular value in the preparation of lubricating oils forsystems sensitive to changes in viscosity such as hydrauliccontrols. They are also used in multigrade engine oils.

9.2.3 Lubricating-oil applications

There is a constant effort by both the supplier and consumer oflubricants to reduce the number of grades in use. The variouslubricant requirements of plant not only limit the extent of thisrationalization but also create the continuing need for a largenumber of grades with different characteristics.

It is not possible to make lubricants directly from crude oilthat will meet all these demands. Instead, the refinery pro-

duces a few basic oils and these are then blended in varyingproportions, together with additives when necessary, to pro-duce an oil with the particular characteristics required. Insome instances the continued increase in plant performance iscreating demands on the lubricant which are at the limit of theinherent physical characterisics of mineral oil. Where theoperational benefit justifies the cost, the use of synthetic basestocks is being developed.

Where these are considered for existing plant, seal and paintcompatibility needs to be reviewed before such products areintroduced. The problems which face the lubricant suppliercan best be illustrated by looking at the requirements ofcertain important applications.

9.2.4 General machinery oils

These are lubricants for the bearings of most plant, wherecirculating systems are not involved. These are hand, ring,bottle or bath lubricated bearings of a very wide range ofequipment; line shafting, electric motors, many gear sets andgeneral oil-can duties. The viscosity of these oils will vary tosuit the variations in speed, load and temperature.

While extreme or arduous usage conditions are not metwithin this category, the straight mineral oils which areprescribed must possess certain properties. The viscosity levelshould be chosen to provide an adequate lubricant filmwithout undue fluid friction, though this may also beinfluenced by the method of application. For instance, aslightly higher viscosity might be advisable if intermittent handoiling has to be relied upon. Although anti-oxidants are notgenerally required, such oils must have a reasonable degree ofchemical stability (Figure 9.3).

9.2.5 Engine lubricants

The type of power or fuel supply available will influence thedecision on prime mover to be used. This is often electricpower, but many items of plant such as compressors, gene-rators or works locomotives, will be powered by dieselengines, as will most of the heavy goods vehicles used in andoutside the works.

The oils for these engines have several functions to performwhile in use. They must provide a lubricant film betweenmoving parts to reduce friction and wear, hold products ofcombustion in suspension, prevent the formation of sludgesand assist in cooling the engine. Unless the lubricant chosenfulfils these conditions successfully, deposits and sludge willform with a consequent undesirable increase in wear rate anddecrease in engine life.

Figure 9.3 Ring oiled bearings

9.2.5.1 Frictional wear

If the effects of friction are to be minimized, a lubricant filmmust be maintained continuously betwen the moving surfaces.Two types of motion are encountered in engines, rotary andlinear. A full fluid-film between moving parts is the ideal formof lubrication, but in practice, even with rotary motion, this isnot always achievable. At low engine speeds, for instance,bearing lubrication can be under boundary conditions.

The linear sliding motion between pistons, piston rings andcylinder walls creates lubrication problems which are some ofthe most difficult to overcome in an engine. The ring isexerting a force against the cylinder wall while at the sametime the ring and piston are moving in the cylinder with asliding action. Also, the direction of piston movement isreversed on each stroke. To maintain a full fluid oil film on thecylinder walls under these conditions is difficult and boundarylubrication can exist. Frictional wear will occur if a lubricantfilm is either absent or unable to withstand the pressures beingexerted. The lubricant will then be contaminated with metalwear particles which will cause wear in other engine parts asthey are carried round by the lubricant.

9.2.5.2 Chemical wear

Another major cause of wear is the chemical action associatedwith the inevitable acidic products of fuel combustion. Thischemical wear of cylinder bores can be prevented by having anoil film which is strongly adherent to the metal surfacesinvolved, and which will rapidly heal when a tiny ruptureoccurs. This is achieved by the use of a chemical additiveknown as a corrosion inhibitor.

9.2.5.3 Products of combustion and fuel dilution

As it is not possible to maintain perfect combustion conditionsat all times, contamination of the oil by the products ofcombustion is inevitable. These contaminants can be eithersolid or liquid.

When an engine idles or runs with an over-rich mixture thecombustion process is imperfect and soot will be formed. Aquantity of this soot will pass harmlessly out with the exhaustbut some will contaminate the oil film on the pistons andcylinders and drain down into the crankcase. If there is anywater present these solids will emulsify to form sludges whichcould then block the oilways. Filters are incorporated into theoil-circulation system to remove the solid contaminants to-gether with any atmospheric dust which bypasses the airfilters.

One of the liquid contaminants is water, the presence ofwhich is brought about by the fact that when fuel is burnt itproduces approximately its own weight in water. When theengine is warm this water is converted into steam, whichpasses harmlessly out of the exhaust. However, with coldrunning or start-up conditions this water is not converted anddrains into the sump. Having dissolved some of the combus-tion gases, it will be acidic in nature and will form sludges.

Another liquid contaminant is unburnt fuel. A poor-qualityfuel, for example, may contain high boiling point constituentswhich will not all burn off in the combustion process and willdrain into the sump. The practice of adding kerosene to fuel tofacilitate easy starting in very cold weather will eventuallycause severe dilution of the lubricating oil. Excessive use ofover-rich mixture in cold weather will mean that all the fuel isnot burnt because of the lack of oxygen and again, someremains to drain into the sump.

Poor vaporization of the fuel will also produce oil dilution.Generally, this fuel will be driven off when the engine

becomes warm and is running at optimum conditions.However, severe dilution of the oil by fuel could have seriousresults as the viscosity of the oil will be reduced to anunacceptable level.

9.2.5.4 Oxidation

The conditions of operation in an engine are conducive to oiloxidation, and this is another problem to be overcome by thelubricant. In the crankcase, the oil is sprayed from variouscomponents in the form of an oil mist which is in contact with alarge quantity of air and at a fairly high temperature. Oxida-tion produces complex carbonaceous products and acidicmaterial and these, combined with fuel contaminants, willform stable sludges. In the combustion chamber, where thetemperatures are very much higher, the oil is scraped up thecylinder walls by the piston ascending at very high speeds andis again present in the form of an oil mist. A form of carbondeposit is produced by a combination of heat decompositionand oxidation. Some of this deposit will remain, but some willpass into the sump. The effect of oxidation adds to theproblem of oil contamination by the products of combustion,resulting in the formation of a resin-like material on thepistons and hot metal parts known as 'lacquer' and acidicmaterial which will attack bearing metals such as copper-lead.

These problems of engine lubrication can be overcome byusing a highly refined oil. The resistance to oxidation is furtherenhanced by the use of anti-oxidants. The addition ofcorrosion-inhibitors counters acidic materials produced bycombustion at low engine temperatures.

Detergent-dispersant additives are incorporated so that thecarbonaceous matter produced by imperfect combustion isretained in suspension in the oil, preventing it from beingdeposited on the engine surfaces. Such an oil is known as afully detergent-type lubricant. All these additives are grad-ually consumed during operation and the rate of decline intheir usefulness will determine the oil-change period. This rateis, in turn, influenced by the conditions of operation.

9.2.5.5 The SAE viscosity system

This classification was devised by the Society of AutomotiveEngineers (SAE) in America by dividing the viscosity spaninto four and giving each of the divisions a number - SAE 20,30, 40 and 50. The thinnest (SAE 20), for example, coveredthe range 5.7-9.6 cSt specified at 21O0F, which was consideredto be a temperature typical of a hot engine. (The SAEoriginally specified temperatures in 0F, because that was theconvention. Today, temperatures are quoted in 0C.)

Later, the SAE series was extended to include muchlower-viscosity oils because of the growing demand for easierwinter starting. The viscosities of the three new grades werespecified at O0F (typical of cold morning temperatures) andeach was given the suffix W for Winter - SAE 5W, 1OW and2OW. Later still, grades of OW, 15W and 25W were added tosatisfy the more precise requirements of modern engines(Table 9.3).

9.2.5.6 Multigrades

All oils become less viscous when heated and more viscouswhen cooled, but some are less sensitive than others to theseviscosity/temperature effects. The degree of sensitivity isknown as Viscosity Index (VI). An oil is said to have high VI ifit displays a relatively small change of viscosity for a givenchange of temperature.

In the 1950s, developments in additive technology led to theproduction of engine oils with unusually high VIs, known as

a As measured in the Cold Cranking Simulator (CCS).h As measured in the Mini Rotary Viscometer (MRV).

multigrade oils. A multigrade oil's high resistance to tempera-ture change is sufficient to give it the combined virtues of alow-viscosity grade at low (starting) temperatures and a high-viscosity one at running temperatures. An SAE 20W-40multigrade, for example, is as thin at -2O0C as a 2OW oil, butas thick at 10O0C as an SAE 40 oil. Thus the multigradecombines full lubrication protection at working temperatureswith satisfactorily easy starting on frosty mornings. Figure 9.4is a viscosity-temperature graph for six monograde oils and a10W-40 multigrade, showing how the multigrade has thehigh-temperature properties of an SAE 40 oil and the low-temperature properties of an SAE 1OW. Thus the multigradeis suitable for all-year-round use.

9.2.5.7 Performance ratings

The SAE numbering system refers purely to the viscosity ofthe oil, and is not intended to reflect lubricating performance(there is no such thing as an 'SAE quality' oil, for example).

Engine oils are marketed in a range of performance levels, andneed to be classified according to the severity of serviceconditions in which they are designed to operate. Accordingly,the American Petroleum Institute (API) has drawn up acoding system in which oils are subjected to a series ofclassifying bench-tests known as the 'Sequence' tests.

9.2.5.8 The API service classifications

In the API system the least demanding classification for apetrol engine was originally designated SA. The mostdemanding is, at present, SG. (The S stands for ServiceStation.) Constant development of both engines and oilsmeans that from time to time the highest ratings are super-seded by even higher ratings. The API system also classifiesdiesel engine oils by their severity of service. Here thecategories have the prefix C, which stands for Commercial.

Petrol enginesSA Service typical of engines operated under mild condi-

tions. This classification has no performance require-ments.

SB Service typical of engines operating in conditions suchthat only minimum protection of the type afforded byadditives is desired. Oils designed for this service havebeen used since the 1930s; they provide only anti-scuffcapability and resistance to oil oxidation and bearingcorrosion.

SC Service typical of petrol engines in 1964-1967 cars andtrucks. Oils designed for this service provide control ofhigh- and low-temperature deposits, wear, rust andcorrosion.

SD Service typical of 1967-1970 petrol engines in cars andsome trucks; but it may apply to later models. Oilsdesigned for this service provide more protection thanSC against high- and low-temperature deposits, wear,rust and corrosion; and may be used where SC isrecommended.

SE Service typical of petrol engines in cars and sometrucks in 1972-1979. Oils designed for this serviceprovide more protection against oxidation, high-temperature deposits, rust and corrosion than SD orSC, and may be used where those classifications arerecommended.

SF Service typical of petrol engines in cars and sometrucks from 1980. Oils developed for this serviceprovide better oxidation stability and anti-wear perfor-mance than SE oils. They also provide protectionagainst engine deposits, rust and corrosion. Oils meet-ing SF may be used wherever SE, SD or SC isrecommended.

SG Service typical of petrol engines in present cars, vansand light trucks. Oils developed for this service pro-vide improved control of engine deposits, oil oxidationand engine wear relative to oils developed for previouscategories. Oils meeting SG may be used wherever SF,SE, SF/CC or SE/CC are recommended.

Diesel enginesCA Service typical of diesel engines operated in mild to

moderate duty with high-quality fuels. Occasionallythis category has included petrol engines in mildservice. Oils designed for this service were widely usedin the late 1940s and 1950s; they provided protectionfrom bearing corrosion and light-temperature depo-sits.

CB This category is basically the same as CA, but im-proved to cope with low-quality fuels. Oils designed

Table 9.3 Viscosity chart

OW5W

1OW15W2OW25W

20304050

Maximumviscosity cPat °Ca

3250 at -303500 at -253500 at -203500 at -154500 at -106000 at -5

Maximumborderline pumpingtemperature (°C)b

-35-30-25-20-15-10

Viscosity (cSt)at 10(PC

min. max.

3.8 —3.8 —4.1 —5.6 —5.6 —9.3 —

5.6 9.39.3 12.5

12.5 16.316.3 21.9

SAE 10W-40 Multigrade

Temperature 0C

Viscosity cSt

Figure 9.4 Multigrade chart

LIVE GRAPHClick here to view

for this service were introduced in 1949.CC Service typical of lightly supercharged diesel engines

operated in moderate to severe duty. Has includedcertain heavy-duty petrol engines. Oils designed forthis service are used in many trucks and in industrialand construction equipment and farm tractors. Theseoils provide protection from high-temperature depo-sits in lightly supercharged diesels and also from rust,corrosion and low-temperature deposits in petrolengines.

CD Service typical of supercharged diesel engines in high-speed high-output duty requiring highly effective con-trol of wear and deposits. Oils designed for this serviceprovide protection from bearing corrosion and high-temperature deposits in supercharged diesel enginesrunning on fuels of a wide quality range.

CDII Service typical of two-stroke cycle diesel enginesrequiring highly effective control over wear and depo-sits. Oils designed for this service also meet all therequirements of CD.

CE Service typical of certain turbocharged or super-charged heavy-duty diesel engines operating underboth low speed-high load and high speed-low loadconditions. Oils designed for this service must alsomeet the requirements specified for CC and CDclassifications.

Before an oil can be allocated any given API performancelevel it must satisfy requirements laid down for various enginetests. In the SG category, for example, the engine tests are asfollows:

Service UD measures the tendency of the oil to rust or corrodethe valve train and to influence the value lifter operation.Sequence HIE measures high-temperature oil oxidation,sludge and varnish deposits, cam-and-tappet wear, cam andlifter scuffing and valve lifter sticking.Sequence VE evaluates sludge deposits, varnish deposits,oil-ring clogging and sticking, oil-screen plugging and camwear.Caterpillar IH2 determines the lubricant effect on ring stick-ing, ring and cylinder wear, and accumulation of pistondeposits.CRC L-38: the characteristics assessed are resistance to oxida-tion, bearing corrosion, tendency to formation of sludge andvarnish, and change of viscosity.

In the CE category the tests are:

Caterpillar IG2: the lubricant characteristics determined arering sticking, ring and cylinder wear, and accumulation depo-sits under more severe test conditions than those for Caterpil-lar IH2.Cummins NTC-400 measures crownland and piston deposits,camshaft roller follower pin wear and oil consumption.Mack T6 assesses oil oxidation, piston deposits, oil consump-tion and ring wear.Mack 77 evaluates oil thickening.CRC L-38 (as above).

Other specifications Various authorities and military bodiesissue specifications relating to the service performance ofengine oils. In some instances the ratings are almost identicalwith those of the API, but most of them are not preciselyparallel because they cover performance factors encounteredin particular engines and particular categories of service.

The most common of the other specifications are those withthe prefix MIL, issued by the US military authorities. MIL-L-2104E approximates to the API CE rating for diesel lubri-cants, although it also relates to petrol engines that require

API SE performance. MIL-L-46152D covers oils for bothdiesel and petrol engines, and approximates to API SG/CC(Figure 9.5).

CCMC ratings Another important set of performance speci-fications is produced by the European Vehicle Manufacturers'Association, known by its initials CCMC*. The CCMC ratingG-I corresponds roughly to API SE, and G-2 to API SF. G-3(comparable to MIL-L-46152B, for petrol engines only)covers fuel-efficient and light-viscosity lubricants blendedfrom special high-quality base oils. CCMC also issues specifi-cations for diesel lubricants: D-I approximates to API CC,D-2 to API CD and MIL-L-2104D, and D-3 to API CE andMIL-L-2104E.

To qualify for the CCMC categories G and D, an oil mustmeet the requirements of the following tests in addition to therelevant API classification tests.

For the G categoryFord Kent, which evaluates cold ring sticking, piston skirtvarnish, oil thickening and consumption.Fiat 132, to evaluate the tendency of the oil to cause pre-ignition.Daimler Benz OM 616 to evaluate wear of cylinders and cams.Bosch Injector Rig, measuring the mechanical stability of theoil to assess its shear stability.Noack Test, to measure the weight loss due to evaporation ofthe oil.

High shear/high temperature viscosity test, to assess the oil'scapability for resisting shear, and so retaining its viscosity, athigh temperatures.Tests for oil/seal compatibility and oil consumption are still tobe established.

For the D categoryBosch Injector Rig, Noack, and D-B OM 616 tests as abovetogether with:

For Dl and D2 only, MWM-B evaluating varnish, carbondeposits, and ring-sticking;For D3 only, D-B OM 352A bore polishing and pistoncleanliness;For PDl only, VW L6L to evaluate ring sticking and pistoncleanliness.

9.2.6 Hydraulic fluids

The wide application of hydraulic systems has undoubtedlybeen stimulated by the increasing use of fully automaticcontrols for sequences of operations where the response tosignals must be rapid and the controls themselves light andeasily operated. These needs are met by hydraulic circuitswhich, in addition, provide infinitely variable speed control,reversal of high-speed parts without shock, full protectionagainst damage from overhead and automatic lubrication.

Over the years the performance standards of hydraulicequipment have risen. Whereas a pressure of about 7000 kPaused to be adequate for industrial hydraulic systems, nowa-days systems operating with pressures of 15 000-25 000 kPaare common. Pressures above 35 000 kPa are to be found inapplications such as large presses for which suitable high-

*Comite des Constructeurs d'Automobiles du Marche Commun represents jointindustry opinion on factors such as lubricant specifications, emissions, vehicledesign and safety standards. With regard to crankcase lubricants, CCMC definessequences of engine tests, and the tests themselves are defined by CEC (Coordinat-ing European Committee for the Development of Performance Tests for Lubri-cants and Engine Fuels: a joint body of the oil and motor industries).

pressure pumps have been developed. Additionally, systemshave to provide increased power densities, more accurateresponse, better reliability and increased safety. Their use innumerically controlled machine tools and other advancedcontrol systems creates the need for enhanced filtration. Fullflow filters as fine as 1-10 /urn retention capability are now tobe found in many hydraulic systems.

With the trend toward higher pressures in hydraulic systemsthe loads on unbalanced pump and motor components becomegreater and this, coupled with the need for closer fits tocontain the higher pressures, can introduce acute lubricationproblems. Pumps, one of the main centres of wear, can bemade smaller if they can run at higher speeds or higherpressures, but this is only possible with adequate lubrication.For this reason, a fluid with good lubrication properties is usedso that 'hydraulics' is now almost synonymous with 'oilhydraulics' in general industrial applications. Mineral oils areinexpensive and readily obtainable while their viscosity can bematched to a particular job.

The hydraulic oil must provide adequate lubrication in thediverse operating conditions associated with the componentsof the various systems. It must function over an extendedtemperature range and sometimes under boundary conditions.It will be expected to provide a long, trouble-free service life;its chemical stability must therefore be high. Its wear-resistingproperties must be capable of handling the high loads inhydraulic pumps. Additionally, the oil must protect metalsurfaces from corrosion and it must both resist emulsificationand rapidly release entrained air that, on circulation, wouldproduce foam.

Mineral oil alone, no matter how high its quality, cannotadequately carry out all the duties outlined above and hencethe majority of hydraulic oils have their natural propertiesenhanced by the incorporation of four different types ofadditives. These are: an anti-oxidant, an anti-wear agent, afoam-inhibitor and an anti-corrosion additive. For machines inwhich accurate control is paramount, or where the range ofoperating temperatures is wide - or both - oils will be formu-lated to include a VI improving additive as well.

9.2.6.1 Viscosity

Probably the most important single property of a hydraulic oilis its viscosity. The most suitable viscosity for a hydraulicsystem is determined by the needs of the pump and the circuit;too low a viscosity induces back-leakage and lowers thepumping efficiency, while too high a viscosity can causeoverheating, pump starvation and possibly cavitation.

9.2.6.2 Viscosity Index

It is desirable that a fluid's viscosity stays within the pumpmanufacturer's stipulated viscosity limits, in order to accom-modate the normal variations of operating temperature. Anoil's viscosity falls as temperature rises; certain oils, however,are less sensitive than others to changes of temperatures, andthese are said to have a higher VI. Hydraulic oils are formu-lated from base oils of inherently high VI, to minimize changesof viscosity in the period from start-up to steady running andwhile circulating between the cold and hot parts of a system.

Figure 9.5 Approximate relationship between classifications and test procedures

Engine typeOil qualityConditions

API

US Militarytest procedures

CCMCspecilications

Additivetreatment level

Super High Performance DieselPassenger (Car) Diesel

Withdrawn Jan 90

Withdrawn Jan 90Withdrawn Jan 90Delisled —1.1.90

Gasoline DieselLow LowMild Mild

HighSevere

HighSevere

HighLowHigh Low Nil

PRESSURE, ATMOSPHERES ABS

Figure 9.6

9.2.6.3 Effects of pressure

Pressure has the effect of increasing an oil's viscosity. While inmany industrial systems the working pressures are not highenough to cause problems in this respect, the trend towardshigher pressures in equipment is requiring the effect to beaccommodated at the design stage. Reactions to pressure aremuch the same as reactions to temperature, in that an oil ofhigh VI is less affected than one of low VI. A typical hydraulicoil's viscosity is doubled when its pressure is raised fromatmospheric to 35 000 kPa (Figure 9.6).

9.2.6.4 Air in the oil

In a system that is poorly designed or badly operated, air maybecome entrained in the oil and thus cause spongy and noisyoperation. The reservoir provides an opportunity for air to bereleased from the oil instead of accumulating within thehydraulic system. Air comes to the surface as bubbles, and ifthe resultant foam were to become excessive it could escapethrough vents and cause loss of oil. In hydraulic oils, foamingis minimized by the incorporation of foam-breaking additives.The type and dosage of such agents must be carefully selected,because although they promote the collapse of surface foamthey may tend to retard the rate of air release from the body ofthe oil.

9.2.6.5 Oxidation stability

Hydraulic oils need to be of the highest oxidation stability,particularly for high-temperature operations, because oxida-tion causes sludges and lacquer formation. In hydraylic oils, ahigh level of oxidation stability is ensured by the use of baseoils of excellent quality, augmented by a very effective combi-nation of oxidation inhibitors.

A very approximate guide to an oil's compatibility withrubbers commonly used for seals and hoses is given by theAniline Point, which indicates the degree of swelling likely to

arise; a high figure indicates a high level of compatibility. Thissystem has been superseded by the more accurate Seal Com-patibility Index (SCI), in which the percentage volume swell ofa 'standard' nitrile rubber is determined after an immersiontest in hot oil.

9.2.6.6 Fire-resistant fluids

Where fire is a hazard, or could be extremely damaging,fire-resistant hydraulic fluids are needed. They are referred toas 'fire resistant' (FR) so that users should be under noillusions about their properties. FR fluids do not extinguishfires: they resist combustion or prevent the spread of flame.They are not necessarily fireproof, since any fluid will even-tually decompose if its temperature rises high enough. Nor arethey high-temperature fluids, since in some instances theiroperating temperatures are lower than those of mineral oils.FR fluids are clearly essential in such applications as electricwelding plants, furnace-door actuators, mining machinery,diecasters, forging plant, plastics machinery and theatricalequipment. When leakage occurs in the pressurized parts of ahydraulic system the fluid usually escapes in the form of ahigh-pressure spray. In the case of mineral oils this spraywould catch fire if it were to reach a source of ignition, orwould set up a rapid spread of existing flame. FR fluids aretherefore formulated to resist the creation of flame from asource of ignition, and to prevent the spread of an existingfire.

Four main factors enter into the selection of a fire-resistantfluid:

1. The required degree of fire-resistance2. Operational behaviour in hydraulic systems (lubrication

performance, temperature range and seal compatibility, forexample)

3. Consideration of hygiene (toxicological, dermatologicaland respiratory effects)

4. Cost

9.2.6.7 Types of fluid

The fluids available cover a range of chemical constituents,physical characteristics and costs, so the user is able to choosethe medium that offers the best compromise for operationalsatisfaction, fire-resistance and cost effectiveness. Four basictypes of fluid are available and are shown in Table 9.4.

In a fully synthetic FR fluid the fire resistance is due to thechemical nature of the fluid; in the others it is afforded by the

Table 9.4 CETOP classifications of fire-resistant hydraulic fluids

CETOP: Comite European des Transmissions Oleohydrauliques etPneumatiques.

Class

HF-A

HF-B

HF-C

HF-D

Description

Oil-in-water emulsions containing a maximumof 20% combustible material. These usuallycontain 95% waterWater-in-oil emulsions containing a maximumof 60% combustible material. These usuallycontain 40-45% waterWater-glycol solutions. These usually contain atleast 35% waterWater-free fluids. These usually refer to fluidscontaining phosphate esters, other organicesters or synthesized hydrocarbon fluids

RATIO OF VISCOSITIES

LOW-VI OIL

HIGH-VI OIL

LIVE GRAPHClick here to view

presence of water. The other main distinction between the twogroups is that the fully synthetic fluids are generally betterlubricants and are available for use at operating temperaturesup to 15O0C, but are less likely to be compatible with theconventional sealing materials and paints than are water-basedproducts.

When a water-based fluid makes contact with a flame or aaahot surface its water component evaporates and forms a steamblanket which displaces oxygen from around the hot area, andthis obviates the risk of fire. Water-based products all containat least 35% water. Because water can be lost by evaporation,they should not be subjected to operating temperatures aboveabout 6O0C. Table 9.5 shows a comparison of oil and FRfluids.

9.2.6.8 High water-based hydraulic fluids

For a number of years HF-A oil-in-water emulsions have beenused as a fire-resistant hydraulic medium for pit props.Concern over maintenance costs and operational life hascreated interest in a better anti-wear type fluid. Micro-emulsions are known to give better wear protection than thenormal oil-in-water emulsions. At the same time the carindustry, in attempts to reduce costs especially from leakageson production machinery, has evaluated the potential forusing HWBHF in hydraulic systems. As a result, in many partsof industry, not only those where fire-resistant hydraulic fluidsare needed, there is a increasing interest in the use ofHWBHF.

Such fluids, often referred to as 5/95 fluid (that being theratio of oil to water), have essentially the same properties aswater with the exception of the corrosion characteristics andthe boundary lubrication properties which are improved bythe oil and other additives. The advantages of this type of fluidare fire resistance, lower fluid cost, no warm-up time, lowerpower consumption and operating temperatures, reducedspoilage of coolant, less dependence on oil together withreduced transport, storage, handling and disposal costs, andenvironmental benefits.

In considering these benefits the the user should not over-look the constraints in using such fluids. They can be summa-rized as limited wear and corrosion protection (especially withcertain metals), increased leakage due to its low viscosity,limited operating temperature range and the need for addi-tional mixing and in-service monitoring facilities.

Because systems are normally not designed for use with thistype of fluid, certain aspects should be reviewed with theequipment and fluid suppliers before a decision to use suchfluids can be taken. These are compatibility with filters, seals,gaskets, hoses, paints and any non-ferrous metals used in the

Table 9.5 Comparison of oil and FR fluids

equipment. Condensation corrosion effect on ferrous metals,fluid-mixing equipment needed, control of microbial infectiontogether with overall maintaining and control of fluid dilutionand the disposal of waste fluid must also be considered.Provided such attention is paid to these design and operatingfeatures, the cost reductions have proved very beneficial to theoverall plant cost effectiveness.

9.2.6.9 Care of hydraulic oils and systems

Modern additive-treated oils are so stable that deposits andsludge formation in normal conditions have been almosteliminated. Consequently, the service life of the oils which isaffected by oxidation, thermal degradation and moisture isextended.

Solid impurities must be continuously removed becausehydraulic systems are self-contaminating due to wear of hoses,seals and metal parts. Efforts should be made to exclude allsolid contaminants from the system altogether. Dirt is intro-duced with air, the amount of airborne impurities varying withthe environment. The air breather must filter to at least thesame degree as the oil filters.

It is impossible to generalize about types of filter to be used.Selection depends on the system, the rate of contaminationbuild-up and the space available. However, a common ar-rangement is to have a full-flow filter unit before the pumpwith a bypass filter at some other convenient part of thesystem. Many industrial systems working below 13 500 kPacan tolerate particles in the order of 25-50 /im with no seriouseffects on either valves or pumps.

Provided that the system is initially clean and fitted withefficient air filters, metal edge-strainers of 0.127 mm spacingappear to be adequate, although clearances of vane pumpsmay be below 0.025 mm. It should be remembered that anexcessive pressure drop, due to a clogged full-flow fine filter,can do more harm to pumps by cavitation than dirty oil.

If flushing is used to clean a new system or after overhaul itshould be done with the hydraulic oil itself or one of lighterviscosity and the same quality. As the flushing charge cir-culates it should pass through an edge-type paper filter of largecapacity. It is generally preferable to use a special pump ratherthan the hydraulic pump system, and the temperature of the oilshould be maintained at about 4O0C without local overheating.

9.2.7 Machine tools

Lubricants are the lifeblood of a machine tool. Withoutadequate lubrication, spindles would seize, slides could notslide and gears would rapidly distintegrate. However, thereduction of bearing friction, vital though it is, is by no meansthe only purpose of machine-tool lubrication. Many machinesare operated by hydraulic power, and one oil may be requiredto serve as both lubricant and hydraulic fluid. The lubricantmust be of correct viscosity for its application, must protectbearings, gears and other moving parts against corrosion, and,where appropriate, must remove heat to preserve workingaccuracies and aligments. It may additionally serve to seal thebearings against moisture and contaminating particles. Insome machine tools the lubricant also serves the function of acutting oil, or perhaps needs to be compatible with the cuttingoil. In other tools an important property of the lubricant is itsability to separate rapidly and completely from the cuttingfluid. Compatibility with the metals, plastics, sealing elementsand tube connections used in the machine construction is animportant consideration.

In machine-tool operations, as in all others, the wisestcourse for the user is to employ reputable lubricants in themanner recommended by the machine-tool manufacturer and

Fire resistanceRelativedensityViscosity IndexVapourpressureSpecial sealsSpecial paintsRustprotection

Mineraloil

Poor0.87

HighLow

NoNoVery good

Water-in-oilemulsion

Fair0.94

HighHigh

PartlyNoGood

Water-glycol

Excellent1.08

HighHigh

PartlyYesFair

Phosphateester

Good1.14

LowLow

YesYesFair

the oil company suppying the product. This policy simplifiesthe selection and application of machine-tool lubricants. Theuser can rest assured that all the considerations outlined abovehave been taken into account by both authorities.

The important factors from the point of view of lubricationare the type of component and the conditions under which itoperates, rather than the type of machine into which it isincorporated. This explains the essential similarity of lubricat-ing systems in widely differing machines.

9.2.7.1 Bearings

As in almost every type of machine, bearings play an impor-tant role in the efficient functioning of machine tools.

9.2.7.2 Roller bearings

There is friction even in the most highly finished ball or rollerbearing. This is due to the slight deformation under load ofboth the raceway and the rolling components, the presence ofthe restraining cage, and the 'slip' caused by trying to makeparts of different diameter rotate at the same speed. Inmachine tools the majority of rolling bearings are grease-packed for life, or for very long periods, but other means oflubrication are also used (the bearings may be connected to acentralized pressure-oil-feed system for instance). In othercases, oil-mist lubrication may be employed both for spindlebearings and for quill movement. In headstocks and gear-boxes, ball and roller bearings may be lubricated by splash oroil jets.

9.2.7.3 Plain journal bearings

Plain bearings are often preferred for relatively low-speedspindles operating under fairly constant loads, and for thespindles of high-speed grinding wheels. These bearings ride ona dynamic 'wedge' of lubricating oil. Precision plain bearingsare generally operated with very low clearances and thereforerequire low-viscosity oil to control the rise of temperature.Efficient lubrication is vital if the oil temperature is to be keptwithin reasonable limits, and some form of automatic circula-tion system is almost always employed.

9.2.7.4 Multi-wedge bearings

The main drawback of the traditional plain bearing is itsreliance on a single hydrodynamic wedge of oil, which undercertain conditions tends to be unstable. Multi-wedge bearingsmake use of a number of fixed or rocking pads, spaced atintervals around the journal to create a series of opposed oilwedges. These produce strong radial, stabilizing forces thathold the spindle centrally within the bearing. With the best ofthese, developed especially for machine tools, deviation of thespindle under maximum load can be held within a fewmillionths of a centimetre.

9.2.7.5 Hydrostatic bearings

To avoid the instabilities of wedge-shaped oils films, a lubri-cating film can be maintained by the application of pressurizedoil (or, occasionally, air) to the bearing. The hydrostaticbearing maintains a continuous film of oil even at zero speed,and induces a strong stabilizing force towards the centre whichcounteracts any displacement of the shaft or spindle. Disad-vantages include the power required to pressurize the oil andthe necessary increase in the size of the filter and circulatorysystem.

9.2.7.6 Slideways

Spindles may be the most difficult machine-tool componentsto design, but slideways are frequently the most troublesometo lubricate. In a slideway the wedge-type of film lubricationcannot form since, to achieve this, the slideway would need tobe tilted.

9.2.7.7 Plain slideways

Plain slideways are preferred in the majority of applications.Only a thin film of lubricant is present, so its properties -especially its viscosity, adhesion and extreme-pressure charac-teristics - are of vital importance. If lubrication breaks downintermittently, a condition is created known a 'stick-slip' whichaffects surface finish, causes vibration and chatter and makesclose limits difficult to hold. Special adhesive additives areincorporated into the lubricant to provide good bonding of theoil film to the sliding surfaces which helps to overcome theproblems of table and slideway lubrication. On long traverses,oil may be fed through grooves in the underside of theslideway.

9.2.7.8 Hydrostatic slideways

The use of hydrostatic slideways - in which pressurized oil orair is employed - completely eliminates stick-slip and reducesfriction to very low values; but there are disadvantages in theform of higher costs and greater complication.

9.2.7.9 Ball and roller slideways

These are expensive but, in precision applications, they offerthe low friction and lack of play that are characteristic of themore usual rolling journal bearings. Lubrication is usuallyeffected by grease or an adhesive oil.

9.2.7.10 Leadscrews and nuts

The lubrication of leadscrews is similar in essence to that ofslideways, but in some instances may be more critical. This isespecially so when pre-load is applied to eliminate play andimprove machining accuracy, since it also tends to squeeze outthe lubricant. Leadscrews and slideways often utilize the samelubricants. If the screw is to operate under high unit stresses -due to pre-load or actual working loads - an extreme-pressureoil should be used.

9.2.7.11 Recirculating-ball leadscrews

This type was developed to avoid stick-up in heavily loadedleadscrews. It employs a screw and nut of special form, withbearing balls running between them. When the balls run offone end of the nut they return through an external channel tothe other end. Such bearings are usually grease-packed forlife.

9.2.7.72 Gears

The meshing teeth of spur, bevel, helical and similar involutegears are separated by a relatively thick hydrodynamic wedgeof lubricating oil, provided that the rotational speed is highenough and the load light enough so as not to squeeze out thelubricant. With high loads or at low speeds, wear takes place ifthe oil is not able to maintain a lubricating film under extremeconditions.

Machine-tool gears can be lubricated by oil-spray, mist,splash or cascade. Sealed oil baths are commonly used, or thegears may be lubricated by part of a larger circulatory system.

9.2.7.73 Hydraulics

The use of hydraulic systems for the setting, operation andcontrol of machine tools has increased significantly. Hydraulicmechanisms being interlinked with electronic controls and/orfeedbacks control systems. In machine tools, hydraulicsystems have the advantage of providing stepless and vibra-tionless transfer of power. They are particularly suitable forthe linear movement of tables and slideways, to which ahydraulic piston may be directly coupled.

One of the most important features for hydraulic oil is aviscosity/temperature relationship that gives the best compro-mise of low viscosity (for easy cold starting) and minimum lossof viscosity at high temperatures (to avoid back-leakage andpumping losses). A high degree of oxidation stability isrequired to withstand high temperatures and aeration inhydraulic systems. An oil needs excellent anti-wear character-istics to combat the effects of high rubbing speeds and loadsthat occur in hydraulic pumps, especially in those of the vanetype. In the reservoir, the oil must release entrained air readilywithout causing excessive foaming, which can lead to oilstarvation.

9.2.7.14 Tramp oil

Tramp oil' is caused when neat slideway, gear, hydraulic andspindle lubricants leak into water-based cutting fluids and cancause problems such as:

• Machine deposits• Reduced bacterial resistance of cutting fluids and subse-

quent reduction in the fluid life• Reduced surface finish quality of work pieces• Corrosion of machine surfaces

All these problems directly affect production efficiency. Re-cent developments have led to the introduction of syntheticlubricants that are fully compatible with all types of water-based cutting fluids, so helping the user to achieve maximummachine output.

9.2.7.75 Lubrication and lubricants

The components of a hydraulic system are continuously lubri-cated by the hydraulic fluid, which must, of course, be suitablefor this purpose. Many ball and roller bearings are grease-packed for life, or need attention at lengthy intervals. Mostlubrication points, however, need regular replenishment if themachine is to function satisfactorily. This is particularly true ofparts subjected to high temperatures.

With the large machines, the number of lubricating pointsor the quantities of lubricants involved make any manuallubrication system impracticable or completely uneconomic.Consequently, automatic lubrication systems are oftenemployed.

Automatic lubrication systems may be divided broadly intotwo types: circulatory and 'one-shot' total-loss. These cover,respectively, those components using relatively large amountsof oil, which can be cooled, purified and recirculated, andthose in which oil or grease is used once only and then lost.Both arrangements may be used for different parts of the samemachine or installation.

9.2.7.76 Circulatory lubrication systems

The circulatory systems used in association with machine toolsare generally conventional in nature, although occasionallytheir exceptional size creates special problems. The normalinstallation comprises a storage tank or reservoir, a pump and

filter, suitable sprays, jets or other distribution devices, andreturn piping. The most recent designs tend to eliminate wickfeeds and siphon lubrication.

Although filtration is sometimes omitted with non-criticalball and roller bearings, it is essential for most gears and forprecision bearings of every kind. Magnetic and gauze filtersare often used together. To prevent wear of highly finishedbearings surfaces the lubricant must contain no particle aslarge as the bearing clearance.

Circulatory systems are generally interlocked electrically ormechanically with the machine drive, so that the machinecannot be started until oil is flowing to the gears and mainbearings. Interlocks also ensure that lubrication is maintainedas long as the machine is running. Oil sight-glasses at keypoints in the system permit visual observations of oil flow.

9.2.7.77 Loss-lubrication systems

There are many kinds of loss-lubrication systems. Most typesof linear bearings are necessarily lubricated by this means. Anincreasingly popular method of lubrication is by automatic ormanually operated one-shot lubricators. With these devices ametered quantity of oil or grease is delivered to any number ofpoints from a single reservoir. The operation may be carriedout manually, using a hand-pump, or automatically, by meansof an electric or hydraulic pump. Mechanical pumps areusually controlled by an electric timer, feeding lubricant atpreset intervals, or are linked to a constantly moving part ofthe machine.

On some machines both hand-operated and electricallytimed one-shot systems may be in use, the manual systembeing reserved for those components needing infrequent at-tention (once a day, for example) while the automatic systemsfeeds those parts that require lubrication at relatively briefintervals.

9.2.7.18 Manual lubrication

Many thousands of smaller or older machines are lubricatedby hand, and even the largest need regular refills or topping upto lubricant reservoirs. In some shops the operator may befully responsible for the lubrication of his own machine, but itis nearly always safer and more economical to make oneindividual responsible for all lubrication.

9.2.7.79 Rationalizing lubricants

To meet the requirements of each of the various componentsof a machine the manufacturer may need to recommend anumber of lubricating oils and greases. It follows that, wherethere are many machines of varying origins, a large number oflubricants may seem to be needed. However, the needs ofdifferent machines are rarely so different that slight modifica-tion cannot be made to the specified lubricant schedule. It isthis approach which forms the basis for BS 5063, from whichthe data in Table 9.6 have been extracted. This classificationimplies no quality evaluation of lubricants, but merely givesinformation as to the categories of lubricants likely to besuitable for particular applications.

A survey of the lubrication requirements, usually carriedout by the lubricant supplier, can often be the means ofsignificantly reducing the number of oils and greases in aworkshop or factory. The efficiency of lubrication may well beincreased, and the economies effected are likely to be substan-tial.

Remarks

May be replaced by CB 68

CB 32 and CB 68 may be usedfor flood-lubricated mechanicallycontrolled clutches; CB 32 and CB 68may be replaced by HM 32 and HM 68

May also be used for manualor centralized lubrication oflead and feed screws

May also be used for applicationsrequiring particularly low-viscosity oils,such as fine mechanisms, hydraulic orhydro-pneumatic mechanismselectro-magnetic clutches, air linelubricators and hydrostatic bearings

May also be used for the lubrication ofall sliding parts - lead and feed screws,cams, ratchets and lightly loaded wormgears with intermittent service; if alower viscosity is required HG 32 maybe used.

Detailed application

Total-loss lubrication

Pressure and bath lubricationof enclosed gears and alliedbearings of headstocks, feedboxes, carriages, etc. whenloads are moderate; gears canbe of any type, other thanworm and hypoid

Pressure and bath lubricationof enclosed gears of any type,other than hypoid gears, andallied bearings when loads arehigh, provided that operatingtemperature is not above 7O0C

Pressure and bath lubricationof plain or rolling bearingsrotating at high speed

Lubrication of all types ofmachine tool plain-bearingslideways; particularlyrequired at low traversespeeds to prevent adiscontinuous or intermittentsliding of the table (stick-slip)

Typical application

General lubrication

Enclosed gears -general lubrication

Heavily loaded gearsand worm gears

Spindles

Slideways

Viscositygrade no.(BS 4231)

68

3268

150320

1022

68220

Table 9.6 Classification of lubricants

Class Type of lubricant

AN Refined mineral oils

CB Highly refined mineral oils(straight or inhibited) withgood anti-oxidationperformance

CC Highly refined mineral oilswith improved loading-carryingability

FX Heavily refined mineral oilswith superior anti-corrosionanti-oxidation performance

G Mineral oils with improvedlubricity and tackinessperformance, and whichprevent stick-slip

May also be used for the lubrication ofplain or rolling bearings and all typesof gears, normally loaded worm andhypoid gears excepted, HM 3X andHM 68 may replace CB 32 and CB 68,respectively

May also be used for the lubrication ofslide ways, when an oil of this viscosityis required

Operation of general hydraulicsystems

Specific application formachines with combinedhydraulic and plain bearings,and lubrication systems wherediscontinuous or intermittentsliding (stick-slip) at low speedis to be prevented

Detailed application

Hydraulic systems

Combined hydraulic andslideways systems

Typical application

3268

32

Consistencynumber

Highly refined mineral oilswith superior anti-corrosion,anti-oxidation, and anti-wearperformance

Refined mineral oils of HMtype with anti-stick-slipproperties

Type of lubricant

HM

HG

Class

XM 1: Centralized systemsXM 2: Dispensed by cup or hand gun or in centralized systemsXM 3: Normally used in prepacked applications such as electric motor

bearings

Plain and rolling bearingsand general greasingof miscellaneous parts

123

Premium quality multi-purposegreases with superior anti-oxidationand anti-corrosion properties

XM

Note: It is essential that lubricants are compatible with the materials used in the construction of machine tools, and particularly with sealing devices.The grease X is sub-divided into consistency numbers, in accordance with the system proposed by the National Lubricating Grease Institute (NLGI) of the USA. These consistency numbers are related to the worked penetrationranges of the greases as follows:

Consistency number Worked penetration range1 310-3402 265-2953 220-250

Worked penetration is determined by the cone-penetration method described in BS 5296.

9.2.8 Compressors

Compressors fall into two basic categories: positive-displacement types, in which air is compressed by the'squashing' effect of moving components; and dynamic(turbo)-compressors, in which the high velocity of the movingair is converted into pressure. In some compressors the oillubricates only the bearings, and does not come into contactwith the air; in some it serves an important cooling function; insome it is in intimate contact with the oxidizing influence ofhot air and with moisture condensed from the air. Clearly,there is no such thing as a typical all-purpose compressor oil:each type subjects the lubricant to a particular set of condi-tions. In some cases a good engine oil or a turbine-quality oil issuitable, but in others the lubricant must be special com-pressor oil (Figure 9.7).

9.2.5.7 Quality and safety

Over the years the progressive improvements in compressorlubricants have kept pace with developments in compressortechnology, and modern oils make an impressive contributionto the performance and longevity of industrial compressors.More recently a high proportion of research has been directedtowards greater safety, most notably in respect of fires andexplosions within compressors. For a long time the causes ofsuch accidents were a matter of surmise, but it was noticedthat the trouble was almost invariably associated with highdelivery temperatures and heavy carbon deposits in deliverypipes. Ignition is now thought to be caused by an exothermic(heat-releasing) oxidation reaction with the carbon deposit,which creates temperatures higher than the spontaneous igni-tion temperature of the absorbed oil.

Experience indicates that such deposits are considerablyreduced by careful selection of base oils and antioxidationadditives. Nevertheless, the use of a top-class oil is no

guarantee against trouble if maintenance is neglected. Forcomplete safety, both the oil and the compressor system mustenjoy high standards of care.

9.2.8.2 Specifications

The recommendations of the International Standards Organi-zation (ISO) covering mineral-oil lubricants for reciprocatingcompressors are set out in ISO DP 6521, under the ISO-L-DAA and ISO-L-DAB classifications. These cover applica-tions wherever air-discharge temperatures are, respectively,below and above 16O0C For mineral-oil lubricants used inoil-flooded rotary-screw compressors the classifications ISO-L-DAG and DAH cover applications where temperatures are,respectively, below 10O0C and in the 100-11O0C range. Formore severe applications, where synthetic lubricants might beused, the ISO-L-DAC and DAJ specifications cover bothreciprocating and oil-flooded rotary-screw requirements.

For the general performance of compressor oils there isDIN 51506. This specification defines several levels of perfor-mance, of which the most severe - carrying the code lettersVD-L - relates to oils for use at air-discharge temperatures ofup to 22O0C.

The stringent requirements covering oxidation stability aredefined by the test method DIN 51352, Part 2, known as thePneurop Oxidation Test (POT). This test simulates the oxidiz-ing effects of high temperature, intimate exposure to air, andthe presence of iron oxide which acts as catalyst - all factorshighly conducive to the chemical breakdown of oil, and theconsequent formation of deposits that can lead to fire andexplosion.

Rotary-screw compressor mineral oils oxidation resistanceis assessed in a modified Pneurop oxidation test using ironnaphthenate catalyst at 12O0C for 1000 h. This is known as therotary-compressor oxidation test (ROCOT).

9.2.8.3 Oil characteristics

Reciprocating compressors In piston-type compressors theoil serves three functions in addition to the main one oflubricating the bearings and cylinders. It helps to seal the fineclearances around piston rings, piston rods and valves, andthus minimizes blow-by of air (which reduces efficiency andcan cause overheating). It contributes to cooling by dissipatingheat to the walls of the crankcase and it prevents corrosionthat would otherwise be caused by moisture condensing fromthe compressed air.

In small single-acting compressors the oil to bearings andcylinders is splash-fed by flingers, dippers or rings, but thelarger and more complex machines have force-feed lubricationsystems, some of them augmented by splash-feed. The cyl-inders of a double-acting compressor cannot be splash-lubricated, of course, because they are not open to thecrankcase. Two lubricating systems are therefore necessary -one for the bearings and cross-head slides and one feeding oildirectly into the cylinders. In some cases the same oil is usedfor both purposes, but the feed to the cylinders has to becarefully controlled, because under-lubrication leads to rapidwear and over-lubrication leads to a build-up of carbondeposits in cylinders and on valves. The number and positionof cylinder-lubrication points varies according to the size andtype of the compressor. Small cylinders may have a singlepoint in the cylinder head, near the inlet valve; larger onesmay have two or more. In each case the oil is spread by thesliding of the piston and the turbulence of the air.

In the piston-type compressor the very thin oil film has tolubricate the cylinder while it is exposed to the heat of theFigure 9.7 Compressor types

COMPRESSOR

DYNAMlC(TVRBO) POSITIVE DISPLACEMENT

AXJALFLOW CENTRIFUGAL ROTARY RECIPROCATING

CROSSHEAD TRUNK

ONEROTOR TWOROTORS

SLIDINGVANE

LIQUIDRING

SINGLESCREW

LOBE(ROOTS)

SCREW

compressed air. Such conditions are highly conducive tooxidation in poor-quality oils, and may result in the formationof gummy deposits that settle in and around the piston-ringgrooves and cause the rings to stick, thereby allowing blow-byto develop.

Rotary compressors - vane type The lubrication system ofvane-type compressors varies according to the size and outputof the unit. Compressors in the small and 'portable' grouphave neither external cooling nor intercooling, because toeffect all the necessary cooling the oil is injected copiously intothe incoming air stream or directly into the compressorchamber. This method is known as flood lubrication, and theoil is usually cooled before being recirculated. The oil iscarried out of the compression chamber by the air, so it has tobe separated from the air; the receiver contains baffles that'knock out' the droplets of oil, and they fall to the bottom ofthe receiver. Condensed water is subsequently separated fromthe oil in a strainer before the oil goes back into circulation.

Vane-type pumps of higher-output are water-jacketed andintercooled: the lubricant has virtually no cooling function soit is employed in far smaller quantities. In some units the oil isfed only to the bearings, and the normal leakage lubricates thevanes and the casing. In others, it is fed through drillings in therotor and perhaps directly into the casing. This, of course, is atotal-loss lubrication technique, because the oil passes outwith the discharged air.

As in reciprocating units, the oil has to lubricate while beingsubjected to the adverse influence of high temperature. Thevanes impose severe demands on the oil's lubricating powers.At their tips, for example, high rubbing speeds are combinedwith heavy end-pressure against the casing.

Each time a vane is in the extended position (once perrevolution) a severe bending load is being applied between itand the side of its slot. The oil must continue to lubricatebetween them, to allow the vane to slide freely. It must alsoresist formation of sticky deposits and varnish, which lead torestricted movement of the vanes and hence to blow-by and, insevere cases, to broken vanes.

Rotary compressors — screw type The lubrication require-ments for single-screw type compressors are not severe, but inoil-flooded rotary units the oxidizing conditions are extremelysevere because fine droplets of oil are mixed intimately withhot compressed air. In some screw-type air compressors therotors are gear driven and do not make contact. In others, onerotor drives the other. The heaviest contact loads occur wherepower is transmitted from the female to the male rotor: herethe lubricant encounters physical conditions similar to thosebetween mating gear teeth. This arduous combination ofcircumstances places a great demand on the chemical stability,and lubricating power, of the oil.

Other types Of the remaining designs, only the liquid-pistontype delivers pressures of the same order as those just men-tioned. The lobe, centrifugal and axial-flow types, are moreaccurately termed 'blowers', since they deliver air in largevolumes at lower pressures. In all four cases only the 'external'parts - bearings, gears or both - require lubrication. There-fore the oil is not called upon to withstand the severe serviceexperienced in reciprocating and vane-type compressors.Where the compressor is coupled to a steam or gas turbine acommon circulating oil system is employed. High standards ofsystem cleanliness are necessary to avoid deposit formation inthe compressor bearings.

Refrigeration compressors The functions of a refrigeratorcompressor lubricant are the same as those of compressor

lubricants in general. However, the close association betweenrefrigerant and lubricant does impose certain additional de-mands on the oil. Oil is unavoidably carried into the circuitwith refrigerant discharging from the compressor. In manyinstallations provision is made for removal of this oil.However, several refrigerants, including most of the halogenrefrigerants, are miscible with oil and it is difficult to separatethe oil which enters the system which therefore circulates withthe refrigerant. In either case the behaviour of the oil in coldparts of the systems is important, and suitable lubricants haveto have low pour point and low wax-forming characteristics.

Effects of contamination The conditions imposed on oils bycompressors - particularly by the piston type - are remark-ably similar to those imposed by internal combustion engines.One major difference is, of course, that in a compressor nofuel or products of combustion are present to find their wayinto the oil. Other contaminants are broadly similar. Amongthese are moisture, airborne dirt, carbon and the products ofthe oil's oxidation. Unless steps are taken to combat them, allthese pollutants have the effect of shortening the life of boththe oil and the compressor, and may even lead to fires andexplosions.

Oxidation High temperature and exposure to hot air are twoinfluences that favour the oxidation and carbonization ofmineral oil. In a compressor, the oil presents a large surfacearea to hot air because it is churned and sprayed in a fine mist,so the oxidizing influences are very strong - especially in thehigh temperatures of the compressor chamber. The degree ofoxidation is dependent mainly on temperature and the abilityof the oil to resist, so the problem can be minimized by thecorrect selection of lubricant and by controlling operatingfactors.

In oxidizing, an oil becomes thicker and it deposits carbonand gummy, resinous substances. These accumulate in thepiston-ring grooves of reciprocating compressors and in theslots of vane-type units, and as a result they restrict freemovement of components and allow air leakages to develop.The deposits also settle in and around the valves of piston-typecompressors, and prevent proper sealing.

When leakage develops, the output of compressed air isreduced, and overheating occurs due to the recompression ofhot air and the inefficient operation of the compressor. Thisleads to abnormally high discharge temperatures. Highertemperature leads to increased oxidation and hence increasedformation of deposits, so adequate cooling of compressors isvery important.

Airborne dirt In the context of industrial compressors, dust isa major consideration. Such compressors have a very highthroughput of air, and even in apparently 'clean' atmospheres,the quantity of airborne dirt is sufficient to cause trouble if thecompressor is not fitted with an air-intake filter. Many of theairborne particles in an industrial atmosphere are abrasive,and they cause accelerated rates of wear in any compressorwith sliding components in the compressor chamber. The dirtpasses into the oil, where it may accumulate and contributevery seriously to the carbon deposits in valves and outletpipes. Another consideration is that dirt in an oil is likely toact as a catalyst, thus encouraging oxidation.

Moisture Condensation occurs in all compressors, and theeffects are most prominent where cooling takes place - inintercoolers and air-receivers, which therefore have to bedrained at frequent intervals. Normally the amount of mois-ture present in a compression chamber is not sufficient toaffect lubrication, but relatively large quantities can have a

serious effect on the lubrication of a compressor. Very wetconditions are likely to occur when the atmosphere is excess-ively humid, or compression pressures are high, or the com-pressor is being overcooled.

During periods when the compressor is standing idle themoisture condenses on cylinders walls and casings, and if theoil does not provide adequate protection this leads to rusting.Rust may not be serious at first sight, and it is quickly removedby wiping action when the compressor is started, but the rustparticles act as abrasives, and if they enter the crankcase oilthey may have a catalytic effect and promote oxidation. Insingle-acting piston-type compressors, the crankcase oil iscontaminated by the moisture.

9.2.9 Turbines

9.2.9.7 Steam

Although the properties required of a steam-turbine lubricantare not extreme it is the very long periods of continuousoperation that creates the need for high-grade oils to be used.The lubricating oil has to provide adequate and reliablelubrication, act as a coolant, protect against corrosion, as ahydraulic medium when used in governor and control systems,and if used in a geared turbine provide satisfactory lubricationof the gearing. The lubricant will therefore need the followingcharacteristics.

Viscosity For a directly coupled turbine for power generationa typical viscosity would be in the range of 32-46 cSt at 4O0C.Geared units require a higher viscosity to withstand toothloadings typically within the range of 68-100 cSt at 4O0C.

Oxidation resistance The careful blending of turbine oils,using components which, by selective refining, have a reducedtendency to oxidize, produces the required long-term stability.The high temperatures and pressures of modern designs add tothese demands, which are combatted by the incorporation ofsuitable anti-oxidant additives.

Demulsibility The ability of the lubricant to separate readilyand completely from water, in either a centrifuge or a settlingtank, is important in a turbine lubricant. Otherwise theretained water will react with products of oxidation andparticle contaminants to form stable emulsions. These willincrease the viscosity of the oil and form sludges which canresult in a failure. Careful and selective refining ensures agood demulsibility characteristic. Inadequate storage andhandling can seriously reduce this property.

Corrosion resistance Although the equipment is designed tokeep the water content at a minimum level, it is virtuallyimpossible to eliminate it entirely. The problem of rusting istherefore overcome by using corrosion inhibitors in the lubri-cant formulation.

Foaming resistance Turbine oils must be resistant to foam-ing, since oil-foam reduces the rate of heat transfer from thebearings, promotes oxidation by greatly extending the area ofcontact between air and oil. It is also an unsatisfactorymedium for the hydraulic governor controls. Careful refiningis the primary means of achieving good resistance to foaming.Use of an anti-foam additive may seem desirable but thisshould be approached with caution. If it is used in quantitieshigher than the optimum it can in fact assist air entrainment inthe oil by retarding the release of air bubbles.

9.2.9.2 Gas

The lubricants generally specified for conventional gas tur-bines invariably fall within the same classification as thoseused for steam turbines and are often categorized as 'turbineoils'. In those cases where an aircraft type gas turbine has beenadapted for industrial use the lubricant is vitally important totheir correct operation. Specifications have been rigidly laiddown after the most exhaustive tests, and it would be unwise,even foolhardy, to depart from the manufacturers' recommen-dations. No economic gain would result from the use ofcheaper, but less efficient, lubricants.

9.2.9.3 Performance standards

In the UK there is BS 489:1983. In Europe there is DIN 51515together with manufacturers' standards such as those set byBrown Boverie and Alsthom Atlantique. In the USA thereare the ASTM standards and the well-known General Electricrequirements.

The total useful life of a turbine oil is its most importantcharacteristic. ASTM method D943 (IP 157) measures the lifeindirectly by assessing the useful life of the oxidation inhibitorcontained in the formulation and is often referred to as theTOST 'life' of the oil. Rust prevention is generally assessed bythe ASTM D665 (IP 135) method.

There are many other specifications designed by equipmentbuilders, military and professional societies, as well as users.Care always needs to be taken when purchasing turbine oil tospecification. The cheapest oil, albeit conforming to thespecification, may not necessarily be the best within thatspecification for the particular purpose. For instance, theadditive package is rarely (if ever) defined, so that unexpectedreactions can occur between oils which could affect overallperformance.

9.2.10 Transformers and switchgear

The main requirement for a power-transmission equipment oilis that it should have good dielectric properties. Oil used intransformers acts as a coolant for the windings; as an insulantto prevent arcing between parts of the transformer circuits;and prevents the ionization of minute bubbles of air and gas inthe wire insulation by absorbing them and filling the voidsbetween cable and wrapping. In switchgear and circuitbreakers it has the added function of quenching sparks fromany arc formed during equipment operation. Oils for use inpower transmission equipment should have the followingproperties; high electric strength, low viscosity, high chemicalstability and low carbon-forming characteristics under theconditions of electric arc.

9.2.10.1 Performance standards

The efficiency of transformer oils as dielectrics is measured by'electric strength' tests. These give an indication of the voltageat which, under the test conditions, the oil will break down.Various national standards exist that all measure the samebasic property of the oil. In the UK it is BS 148:1984. There isan international specification, IEC 296/1982, which may bequoted by equipment manufacturers in their oil recommenda-tions.

9.2.70.2 Testing

How frequently the oil condition should be tested depends onoperating and atmospheric conditions; after the commission-ing sample, further samples should be taken at three months

and one year after the unit is first energized. After this, undernormal conditions, testing should be carried out annually. Inunfavourable operating conditions (damp or dust-laden at-mospheres, or where space limitations reduce air circulationand heat transfer) testing should be carried out every sixmonths.

Testing should include a dielectric strength test to confirmthe oil's insulation capability and an acidity test, which indi-cates oil oxidation. While acid formation does not usuallydevelop until the oil has been in service for some time, when itdoes occur the process can be rapid. If acidity is below 0.5 mgKOH/g no action would seem necessary. Between 0.5 and1 mg KOH/g, increased care and testing is essential. Above 1the oil should be removed and either reconditioned or dis-carded. Before the unit is filled with a fresh charge of oil itshould be flushed. These suggestions are contained in a BritishStandards Code of Practice.

Sludge observations will show if arcing is causing carbondeposits which, if allowed to build up will affect heat transferand could influence the oil insulation. There is also a flashpoint test, in which any lowering of flash point is an indicationthat the oil has been subjected to excessive local heating orsubmerged arcing (due to overload or an internal electricalfault). A fall in flash point exceeding 160C implies a fault, andthe unit should be shut down for investigation of the cause.Lesser drops may be observed in the later stages of oil life, dueto oxidation effects, but are not usually serious. A 'crackle'test is a simple way of detecting moisture in the oil. Wherewater is present the oil should be centrifuged.

9.2.11 Greases

Grease is a very important and useful lubricant when usedcorrectly, its main advantage being that it tends to remainwhere it is applied. It is more likely to stay in contact withrubbing surfaces than oil, and is less affected by the forces ofgravity, pressure and centrifugal action. Economical andeffective lubrication is the natural result of this property and areduction in the overall cost of lubrication, particularly inall-loss systems, is made possible.

Apart from this, grease has other advantages. It acts both asa lubricant and as a seal and is thus able, at the same time as itlubricates, to prevent the entry of contaminants such as waterand abrasive dirt. Grease lubrication by eliminating the needfor elaborate oil seals can simplify plant design.

Because a film of grease remains where it is applied formuch longer than a film of oil, it provides better protection tobearing and other surfaces that are exposed to shock loads orsudden changes of direction. A film of grease also helps toprevent the corrosion of machine parts that are idle for lengthyperiods.

Bearings pre-packed with grease will function for extendedperiods without attention. Another advantage is the almostcomplete elimination of drip or splash, which can be aproblem in certain applications. Grease is also able to operateeffectively over a wider range of temperatures than any singleoil.

There are certain disadvantages as well as advantages inusing grease as a lubricant. Greases do not dissipate heat aswell as fluid lubricants, and for low-torque operation tend tooffer more resistance than oil.

9.2.1 Ll Types of grease

The general method of classifying greases is by reference tothe type of soap that is mixed with mineral oil to produce thegrease, although this has rather less practical significancenowadays than it had in the past. One example of this is the

multi-purpose grease that may replace two or three differenttypes previously thought necessary to cover a particular fieldof application. Nevertheless, there are unique differences inbehaviour between greases made with different metal soaps,and these differences are still important in many industrialuses, for technical and economic reasons.

Calcium-soap greases The line-soap (calcium) greases havebeen known for many years but are still probably the mostwidely used. They have a characteristic smooth texture,thermal stability, good water resistance and are relativelyinexpensive. The softer grades are easily applied, pump welland give low starting torque. Their application is limited bytheir relatively low drop points, which are around 10O0C. Thismeans that, in practice, the highest operating temperature isabout 5O0C.

Nevertheless, they are used widely for the lubrication ofmedium-duty rolling and plain bearings, centralized greasingsystems, wheel bearings and general duties. The stiffer varie-ties are used in the form of blocks on the older-type brasses.Modifications of lime-base grease include the graphited varie-ties and those containing an extreme pressure additive. Thelatter are suitable for heavily loaded roller bearings such as insteel-mill applications.

Sodium-soap greases The soda-soap (sodium) greases were,for some considerable time, the only high-melting pointgreases available to industry. They have drop points in theregion of 15O0C and their operating maximum is about 8O0C.These greases can be 'buttery', fibrous or spongy, are notparticularly resistant to moisture and are not suitable for use inwet conditions. Plain bearings are very frequently lubricatedwith soda-based greases.

For rolling-contact bearings, a much smoother texture isrequired, and this is obtained by suitable manufacturingtechniques. Modified grades may be used over the sametemperature range as that of the unmodified grade and, whenthey are correctly formulated, have a good shear resistanceand a slightly better resistance to water than the unmodifiedgrades.

Lithium-soap greases These products, unknown before theSecond World War, were developed first as aircraft lubricants.Since then the field in which they have been used has beengreatly extended and they are now used in industry as multi-purpose greases. They combine the smooth texture of thecalcium-based greases with higher melting points than soda-soap greases, and are almost wholly manufactured in themedium and soft ranges. Combined with suitable additives,they are the first choice for all rolling-contact bearings, as theyoperate satisfactorily up to a temperature of 12O0C and at evenhigher for intermittent use. Their water resistance is satisfac-tory and they may be applied by all conventional means,including centralized pressure systems.

Other metal-soap greases Greases are also made from soapsof strontium, barium and aluminium. Of these, aluminium-based grease is the most widely used. It is insoluble in waterand very adhesive to metal. Its widest application is in thelubrication of vehicle chassis. In industry it is used for rolling-mill applications and for the lubrication of cams and otherequipment subject to violent oscillation and vibration, whereits adhesiveness is an asset.

Non-soap thickened greases These are generally reserved forspecialist applications, and are in the main more costly thanconventional soap-based greases. The most commonsubstances used as non-soap thickeners are silicas and clays