How Oil Works?

8/12/2019 How Oil Works?

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Friction is a ubiquitous part of our daily lives. Walking,writing, opening doors and

drawers and driving to work allinvolve energy being expended inorder to overcome friction.Although friction is generallythought of as a negative mechani-cal characteristic (approximately20% of a car's energy is spent inovercoming friction) it must beremembered that without it wewould not be able to walk and thebrakes on our cars would notwork.

Friction is usually seen as amechanical problem and the forcenecessary to overcome it has longinterested engineers. When maninvented the wheel it enabled himto move weights around with farless expenditure of energy than if they were dragged from A to B.What he had in fact done was toreplace sliding friction with

rolling friction which has a muchlower value.

This bulletin looks at the role of oil in reducing the harmful forcesof friction.

FRICTION

An understanding of friction willbe helpful as a starting point.

Friction can be defined as theresistance encountered when onebody moves relative to anotherbody with which it is in contact.

The laws andcoefficient of friction

The basic laws of friction were'sensed' by Leonardo da Vinci butwere not studied scientificallyuntil the 18th century by Coulomb

and Amontons, which led to thedefinition of three laws:

The first law states that the forceof friction that exists between twosurfaces is directly proportional tothe perpendicular pressurebetween them. In order to slide ametal cube of mass M across atable, a tangential force, F, must beapplied. If M is doubled then F

must also be doubled to get M tomove.

Oil

reduces

the harmful

forces of

friction.

How does oil work?

I S S U E 2

3

®

By John S. Evans, B.Sc.

A Division of the Set Point Technology Group

B U R E

A U V E R

I T

A S

1 8 2 8I N

T E

R N A T I O N

A L

I

S

O

9

0

0

2



2

Frictional

drag is

lower

once a

body is

in motion.

The second law states that theforce of friction is independent of the surface area of M in contactwith the surface over which it isbeing made to slide. If M were tohave an oblong cross section, it

would not matter whether the larg-er or smaller surface area wasplaced on the table; the force of friction, or the force required tomove both blocks, would remainthe same. Diagram 1 illustratesthis point.

The third law says that the forceof friction is dependent on thenature and state of the surfaces.

Smooth surfaces generate lessfriction than rough surfaces.

The first law can be restated as theratio between F and M for a par-ticular system is constant and F/Mis known as the coefficient of friction. This coefficient can beeasily measured by using gravityto make a metal block slide acrossa surface. Although the mass canbe weighed and the force to get itto move easily determined, inertiacauses problems.

Newton's first law of motion can

be interpreted as saying that bod-ies at rest tend to stay that wayunless acted upon. This means thatthe initial energy required to getthe metal block to start moving

(overcome static friction) is higherthan the energy required to keep itmoving (overcome dynamic fric-tion). Once the initial energy hasbeen expended to overcome thestatic friction, the coefficient of friction settles down to a constantvalue equal to the dynamic fric-tion. Thus frictional drag is loweronce a body is in motion.

Causes of friction

Surfaces that appear smooth andshiny to the naked eye will showpeaks and furrows when examinedwith a microscope or even a mag-nifying glass. This does not meanthat the component has been poor-ly machined but that componentsmanufactured to prescribed toler-ances may still have fairly roughsurfaces at microscopic level (see

Diagram 2).

When two surfaces are broughtinto contact, it is the tips of thepeaks that actually touch, meaningthat large areas of the surfacesnever come into contact. Forexample, the contact area of twoflat steel surfaces subjected to apressure of 0.5 kgcm-2 is about1/40 000th of the apparent surfacearea. The size of the contact areanot only depends on the surfacearea and roughness but also on theload acting on the two surfaces.The peaks where contact actuallytakes place are called asperities.

Because it is the asperities thattouch and because they make up avery small surface area, very highpressures and temperatures areachieved which can cause metal

deformation and cold welding totake place. When the surfaces slideover each other, microscopic

Diagram 1

Diagram 2

SURFACE MAGNIFIED

APPEARS SMOOTH



3

welds are produced between peakswhich, due to the relative motion,are then torn apart.

The laws of friction apply to oneclean metal surface sliding overanother. The laws break down

when thin metal films or oxidisedsurfaces are considered. They alsodo not apply to very hard surfaces(eg. a diamond) or very elasticsurfaces (eg. rubber) where thecontact area no longer depends onpressure.

Friction and wear

Whenever friction is overcome,

the dislocation of surface materialgenerates heat and this frictionalheat can be highly destructive tometal surfaces and cause wear totake place.

Additionally, when there is solidfriction (as opposed to fluid fric-tion) wear will also take place.Material is lost due to the cuttingaction of opposing asperities andto the shearing of microscopicwelds. In extreme cases the com-bination of high frictional temper-atures, welding and shearing cancause complete seizure of movingparts.

The harmful effects of frictioncannot be overemphasised. The

job of the engineer and particular-ly the lubrication engineer is tocontrol friction: to increase it

where it is needed and to decreaseit where it will cause damage.Lubrication reduces friction byreplacing solid friction with fluidfriction.

Sliding and rolling friction

When one body slides over anoth-er, the force of resistance encoun-tered at the points of contact is

known as sliding friction. If aball or cylinder were to roll over ametal surface, the relative velocity

of the points of contact are actual-ly zero and this results in rollingfriction. It should, however, benoted that rolling friction isalways accompanied by some slid-ing friction.

Two main types of rolling frictionexist. The first is where large tan-gential forces are experienced asin the case of a car wheel in con-tact with the road which generatesconsiderable sliding forces. Thesecond is where minimal tangen-tial forces are present as is thecase with a ball or roller bearing.This is sometimes known as freerolling.

Studies show that elastic deforma-tion of the roller and the surfaceoccurs and this gives rise to theresistance to motion (friction) thatis encountered. The energyreturned to the system when thedeformation returns to normal isless than the energy required tocause the deformation. This excessenergy is lost as frictional heat.

Tests show that rolling friction isnot influenced by the presence orabsence of a lubricant. However,lubrication is still important in thissituation because the elastic defor-mation of the surfaces introducessliding friction which can bereduced by the introduction of alubricant to convert solid frictioninto fluid friction.

LUBRICATION

The friction that exists betweentwo bodies in relative motioninvolves an appreciable energyloss which needs to be minimised.This is achieved by feeding lubri-cants between these sliding sur-faces which replace solid frictionwith fluid friction which can be farsmaller.

A lubricant must possess twobasic properties: some degree of

The

harmful

effects

of friction

cannot be

over-

emphasised.



4

fluidity to spread over the surfacesand adhesive power to allow thefluid to remain in place duringmotion. Oils are particularly wellsuited to this job.

When an oil is fed in between twomoving surfaces it causes them toseparate and in doing so eliminatesthe solid friction that existsbetween them. Unfortunately vis-cous drag (fluid friction) exists inthe oil so that friction can never betotally eliminated but it can begreatly reduced. The oil can causethe surfaces to separate in a num-ber of ways and, when this is notcompletely possible, friction is still

kept to a manageable level.

The lubrication regimes that willbe considered here are: boundarylubrication, mixed film lubricationand hydrodynamic lubricationwhich includes elastohydrodynam-ic lubrication (EHD).

Boundary lubrication

Three parameters need to be con-sidered when looking at the vari-ous lubrication regimes. They are:the load placed on the surfaces, thespeed at which they are movingrelative to each other, and the vis-cosity of the lubricant.

Viscosity is a very important prop-

erty of a lubricant and will be

Tribology

is the study

of the

interaction

of surfaces

in relative

motion to

each other.

A USEFUL analogy when look-ing at the different types of lubri-cation is to consider a car drivingalong a road in different condi-tions.

When driving on a dry road,there will be maximum frictionbetween the tyre and road surfaceand, if the brakes are applied,then there will be the shortestpossible stopping distance withthe maximum wear to the tyres(and road surface). This equatesto no lubrication at all.

If there has been a light show-

er and there are puddles on theroad and the brakes are applied,there is insufficient water on theroad to lift the tyres off the sur-face but there is enough water topool into low spots, preventingmaximum contact between thesurfaces. This is like boundarylubrication where braking andtyre wear will occur, but less thanwould occur on a dry road andwith longer braking distances.

If there has been a downpour

and a car is driving at high speedon a flat stretch of freeway andthe brakes are suddenly applied,then the wheels will lock and thecar will aquaplane. This is likehydrodynamic lubrication - the

tyres skim along the surface of the water without touching theroad, with minimal wear to thetyres. All braking is done by thebodywork and braking distance isdependent on how quickly thebodywork comes into contactwith something immovable(crashes).

If, under the same conditions,

the car has ABS brakes thensomething akin to mixed filmlubrication exists. The brakes andwheels are pulsed, allowing themto brake through the water sur-face and touch the road in shortbursts. Sometimes boundarylubrication takes place, some-times hydrodynamic. Brakingdistance is longer than in the caseof boundary lubrication and somewear to the tyres occurs, but

hopefully the car's bodywork does not come into play.

A PRACTICAL ANALOGY - DRIVING A CAR



5

defined at this point as a fluid'sresistance to flow at a particulartemperature. Fluids that have highviscosities do not flow easily(such as molasses) - high viscosityoils are sometimes referred to as'thick' oils. Fluids with low vis-cosities flow easily (such aswater) - low viscosity oils aresometimes referred to as 'thin' oils.

It is important to remember thatwhen an oil is heated up, its vis-cosity will decrease. In otherwords, it will 'thin out'. Likewisethe viscosity will increase as thetemperature decreases and the oilwill 'thicken up'. Because of this

an oil's viscosity must always beaccompanied by the temperatureat which the measurement wasmade for it to mean anything.

In a situation for ideal lubricationthe viscosity and the speed shouldbe high and the load low. If thiswere always the case then lubri-cated surfaces would always beseparated and only fluid frictionwould come into play.Unfortunately we do not live in anideal world and this is not alwayspossible.

In the case of a plain bearing and a journal there will be a smallamount of lubricant between themating surfaces. However,because of the load and zero speedon start up, the lubricant film willnot completely separate the bear-

ing from the journal. In this situa-tion the asperities will break through the lubricant film andmake contact; this is known as boundary lubrication. All is notlost, however, and the situation isfar better than if no oil were pre-sent at all. Typically, the coeffi-cient of friction of unlubricatedmetal surfaces ranges between 1.5and 0.5. With boundary lubrica-

tion it is between 0.1 and 0.02.

Viscosity

is a fluid’s

resistance

to flow at a

particular

temperature.

Boundary lubrication can be sub-divided into: oily lubrication, anti-wear lubrication and EP (extremepressure) lubrication.

1. Oily lubrication takes placethrough two distinct mechanisms:

Physical adsorption, wherebyoil molecules and compoundsblended into the oil are attractedelectrostatically to metal surfacesand form bimolecular layers dueto the polar nature of these com-pounds.

A chemical reaction also takesplace causing certain compounds

in the oil to attach themselveschemically to the metal surfaces.This allows very thin films, usual-ly only one or two moleculesthick, to form and help reducefriction whilst not totally separat-ing the metal surfaces (seeDiagram 3).

2. Antiwear lubrication occursbecause of the presence of chemi-cal compounds in the oil that arespecifically designed to react withmetal surfaces and form a protec-tive layer. The melting point of these compounds is fairly low andis below the temperature at whichEP compounds are effective.Antiwear additives are sometimesreferred to as mild EP additives.

3. EP lubrication is requiredwhere high pressures and hence

high temperatures are encoun-tered. This is achieved by theaddition of compounds that con-tain phosphorus and sulphur thatreact with the metal surfaces toform compounds such as iron sul-phide which has a high meltingpoint and low shear strength. Inother words, they make the metalsurfaces slippery.



N

P

Boundary

lubrication

Semifluid

lubrication

Hydrodynamic lubrication

6

solid friction is very high but isgradually replaced with fluid fric-tion as the journal begins to turn.Mixed film lubrication occurswhen boundary lubrication isreduced but the mating surfaces

are not quite separated and aresupported by a full film of lubri-cant (see Diagram 4).

Hydrodynamic lubrication

In theory the journal should belocated coaxially with the bearing.However, in practice, this does nothappen unless the load is extreme-ly small or the shaft is mountedvertically. In practical cases there

is an appreciable load on the oilfilm and the journal cannot rotatecoaxially to the bearing.

As speed increases a wedge isformed between the convergingsurfaces through which the oil isforced as it is entrained by therotating shaft. At this point thefluid has a greater velocity, andhydrodynamic pressure is created

which counterbalances the loadand keeps the mating surfacesapart.

Under this type of lubricationregime the coefficient of friction isreduced to the order of 0.005. Itwill be noted from the graph thatthe minimum friction is achieved

just before the onset of hydrody-namic lubrication. It is at thispoint that solid or static friction isreplaced by fluid or kinematicfriction. Unlike solid friction, fluidfriction is dependant on speed andgradually increases as the speedincreases.

Elastohydrodynamiclubrication (EHD)

The surfaces in relative motionthat require lubrication that have

been considered so far have hadthe same geometry, that is they fittogether like two spoons. In the

Mixed film lubrication

The coefficient of friction in aplain bearing is related to the loadon the bearing, the speed at whichit is turning and the viscosity of the oil that is lubricating the sys-tem.

Assuming the viscosity remainsconstant throughout (as will the

load) then the coefficient of fric-tion will vary with the change inspeed. On start-up the speed is low(zero) so that the coefficient of friction will be high but willdecrease as the speed increases.The graph below illustrates therelationship where initially the

Fluid

friction

increases

as speed

increases.

Diagram 4

Diagram 3

Metal repelling

Metal attracting

Metal surface

SPEED

F R I C T I O N



7

Pressure

is defined

as force

over area.

Hertzian pressures and the point orline of contact of the roller isknown as the Hertzian region.These high pressures cause boththe roller and the race to elastical-ly deform to a small degree whichspreads the pressure over a smallbut finite area. Oil is drawn intothe Hertzian region by hydrody-namic forces and when subjectedto these Hertzian pressures the vis-cosity increases (reversibly) dra-matically and has the consistencyof window putty. It is this increasein viscosity that enables the oil tolubricate under these extreme con-ditions and this lubrication regimeis known as elastohydrodynamic

lubrication.

case of a ball or roller bearing, thecontact area between the matingsurfaces is vanishingly small. Witha ball bearing in a race, the area of contact is a tangential point and,with a roller bearing, a tangentialline. Remember that pressure isdefined as force over area and inthis situation the area is verysmall. With normal loads in rollerelements, pressures of 1.5 Gpa(1,5 million kilopascals) can bereached. Under these conditionsboth the roller elements and the oilbehave in a non-Newtonian fash-ion (see Newtonian fluids sectionon back page).

These high pressures are known as

Diagram 5

HERTZIAN

REGION

HERTZIAN

PRESSURE

ELASTOHYDRODYNAMIC LUBRICATION

Pressure distribution over contact.



Produced by the

Wearcheck Division of

Set Point Technology

KWAZULU-NATAL

9 Le Mans Place, Westmead

P.O. Box 15108, Westmead, 3608.

Tel: (031) 700-5460

Fax: (031) 700-5471

E-mail: [email protected]

GAUTENG

25 San Croy Office Park,

Die Agora Road

(off Brabazon Road), Croydon.

P.O. Box 284, Isando, 1600

Tel: (011) 392-6322

Fax: (011) 392-6340

E-mail:

[email protected]

Pro-PrintFelicity Howden Public Relations 1/2002 Partners in Publishing

INTERNET SITE :

http://www.wearcheck.co.za

Publications are welcome to

reproduce this article or extracts

from it, providing the Wearcheck

Division of Set Point Technology

is acknowledged.

8

Lubricants

convert

solid friction

into

fluid friction.John Evans is the diagnosticmanager: mobile equipment for

the Wearcheck Division of Set

Point Technology.

Copies of previous Technical Bulletins can be accessed onWearcheck's web site:www.wearcheck.co.za.

Readers who would prefer toreceive future issues of Wearcheck Technical Bulletin and Monitorvia e-mail instead of in printedform, please e-mail a request to:[email protected].

NEWTONIAN fluids (includinglubricating oils) behave in thefollowing way: Consider anyfluid between two parallelplates of area A and separatedby a distance H. One plate isheld fixed whilst the othermoves with velocity V. As itmoves it carries the layer of fluid touching it, which in turncarries along the layer of fluidclosest to it and so on. The layerof fluid adjacent to the station-ary plate will also be stationary

so that the velocity of the differ-ent layers of fluid will varybetween 0 and V if there is lam-inar flow. The force required tocause this flow is given by thefollowing equation:

F =AV/H

The constant of proportionality() is the viscosity of the fluidand represents the resistance toflow at a particular temperature.It is dependant on the nature of the fluid and the temperatureonly.

NEWTONIAN FLUIDS

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