CH01 Bearings

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B E A R I N G S I N T R O D U C T I O N

BEARINGS1. INTRODUCTION1.1 What is a Bearing?A bearing is a component which supports and locates a rotatingshaft, or which guides one component as it slides over another.

Furthermore, bearings are generally designed to permit relativemotion with minimal friction.

1.2 Types of BearingBearings can easily be classified into either rolling element orplain types. A rolling element bearing contains a number of rollingelements (either balls or rollers) and these form the mechanicallink between the relative moving surfaces. A plain bearing has noadditional moving parts and relies upon large area surface

contact to permit sliding of one surface over another. Specialmaterials are used for the bearing surfaces to reduce friction andwear. Within these two categories exist many further sub-categories based upon construction, load direction and operatingconditions.

Bearings can also be classified according to the direction of theload that they carry. A journal bearing supports a rotating shaftand constrains radial motion; primarily it carries radial load. Athrust bearing constrains axial shaft motion, and carries primarily

axial load. A linear bearing permits relative motion in a straightline, and carries primarily perpendicular load.

2 THE BASICS OF ROLLING ELEMENT BEARINGS2.1 The Operation of Rolling Element BearingsA rolling element bearing is comprised of two races, betweenwhich a number of rolling elements (balls or rollers) is held. Therolling elements are themselves separated by a soft metal or

polymer cage, which ensures that they are evenly spaced aroundthe races. The area of contact between the rolling elements andthe races is very small, and hence the frictional resistance to

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motion of this type of bearing is also small, which is why they aresometimes called anti-friction bearings.

In contrast to a plain bearing, where the relative motion at thebearing faces is sliding, the relative motion in the case of a rolling

element bearing is rolling. This underlies the need for verydifferent construction materials. Rolling element bearings aremade of exceptionally hard bearing steels. This is because theharder the material, the less the deformation caused by thecontact forces, hence the less the area of contact, and hence theless the friction. Even though the principle of operation is one ofrolling, some sliding does still occur, and for this reasonlubrication of rolling element bearings is an important as it is forplain bearings.

2.2 Types of Rolling Element BearingA fundamental distinction in types of rolling element bearing iswhether or not the bearing uses balls or rollers. Ball bearings tendto be used for lower load, higher speed, unidirectional loadapplications, whereas roller bearings tend to be used for higherload, lower speed, multi-directional load applications. In general,ball bearings are cheaper than rolling element bearings, theyoffer lower frictional resistance, and they are more widelyavailable.

The operational and performance distinctions between ball androller bearings all stem from the differences in contact area thatthey have between rolling elements and races. In theory, aspherical ball on a circular raceway has a single point of contact,whereas a circular section roller on a circular raceway has a lineof contact. This means that the load carried by a roller bearing isspread over a greater area than that of a ball bearing, giving it agreater load carrying and impact resistant capability, but also agreater frictional resistance than an equivalent ball bearing.Many type of ball and roller bearings exist. Simplest distinctioncomes from whether they are intended for axial, radial or multi-directional load transmission. Bearings intended for transmittingprimarily radial load are referred to as journal bearings, whereasthose for primarily axial load are termed thrust bearings.Over a dozen different types of journal bearings exist, rangingfrom the general purpose single row deep groove ball bearing, tothe specialist self-aligning spherical roller bearing. Similarly,thrust bearings range from single row ball bearings to spherical

roller bearings.The selection of an appropriate bearing is itself awhole field of study, but primary factors in the decision-making


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process include the relative ability to support radial and/or axialload, the tolerance to misalignment, the speed range ofoperation, and the available methods of location. There exists afurther specialist category of rolling element bearing called the

linear bearing, and again this represents a whole field in itself.2.3 Constraint of Rolling Element BearingsIt is normal practice that a rolling element bearing is mounted sothat its rotating race is a press fit into housing or onto a shaft,and its stationary race is a sliding fit. The specification of the fitsto use can be found in bearing handbooks and internationalstandards.

To permit thermal expansion during operation, without imposingundesirable axial load on a bearing, it is normal practice on a two-

bearing shaft to have one bearing fixed axially and the other freeto move.

Bearings are normally fixed to a shaft with shaft nuts and tabwashers or with slotted nuts and split pins. Larger bearings maybe fixed to a shaft with an end plate. Other possibilities includethe use of an end cap, a circlip or a tapered adaptor / withdrawalsleeve.

3 ROUTINE MAINTENANCE OF ROLLING ELEMENTBEARINGS3.1 Best Practice

The majority of best practice measures amount to little more thangood common engineering sense. They include appropriatestorage and handling, maintenance of cleanliness, and pre-installation lubrication.

It is important when mounting and dismounting bearings that thecorrect tools are used and that they are in good condition.Makeshift arrangements or badly worn tools are likely to lead todamage to the bearing or bearing assembly. Bearings should bewrapped in oil-proof paper when not in use. Only clean solventsand flushing oils should be used for bearing cleaning. Beforeinstalling a rolling element bearing both the housing and shaftshould be carefully inspected for burrs, nicks and scratches thatmay interfere with the fitting of the bearing. Bearings that are dryand unlubricated or have not been cleaned should not be spun. Ifcompressed air is used to clean a bearing care should be taken to

avoid spinning the bearing. Cotton waste or dirty rags should notbe used to clean bearings, only clean, lint free rags.

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The slushing compound used to protect a bearing in storage neednot be removed if it is petroleum based unless it has gone hard orbecome contaminated. The force applied when mounting ordismounting a bearing should always be applied to the ring with

the interference fit - it should never be applied in such a way thatthe force is transmitted through the rolling elements! Never strikea bearing directly with a hammer or mallet or with a soft metaldrift. Remember that all sealed and shielded bearings must bemounted cold. Bearings should never be heated with a nakedflame.

3.2 Tools and equipmentThe maintenance of rolling element bearings requires specialisedtools and equipment without these many tasks are extremelydifficult. Mounting dollies and sleeves - When mounting bearingswhich are a press fit it is important that pressure is appliedevenly. If the bearing is cocked due to uneven force then surfacescan be damaged and the bearing ring distorted. A set of mountingdollies and sleeves will ensure that bearings are driven homeevenly. Dollies and sleeves can be purchased directly from thebearing manufacturers, or can be made up to suit a particularapplication.

Hammers should be made from steel or soft material and shouldbe free from burrs. Copper or synthetic resin is suitable materialsfor soft hammers but lead and tin should not be used. Woodenmallets should not be used because of splinters. Only steel driftsshould be used and they should only be used for bending orstraightening tabs on locking washers or for driving shaft nuts. Anarbor press ensures that bearings are driven evenly especiallywhen the fit is tight. It should be used in preference to othermethods where practicable. Bearing pullers are essential for

dismounting. There are several types available. Pullers should bemaintained in good condition, so that they operate freely and theclaws are free of burrs. When handling large, heavy bearings it isimportant to have suitable lifting gear available such as tongs orslings.

Hook and impact spanners are needed for use with adaptersleeve nuts, withdrawal nuts and shaft nuts. Modern inductionheaters are much cleaner and easier to use than an oil bath whenbearings have to be heated for a shrink fit. When induction

heaters are not available, the traditional oil bath should be used.Various types of gauges are needed for measuring diametricalclearances, housing bores and shaft diameters. Ordinary feeler


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gauges are sufficient for clearances but a special set of boregauges may be required for measuring the bore of housings.Alternatively, standard inside and outside micrometers can beused for measuring diameters. The listed tools are the minimum

required for bearing maintenance. Where bearings are mountedon tapered shafts or on sleeves, hydraulic equipment may also berequired.

Where shafts have been provided with oil ducts, hydraulicpressure can be used for mounting and dismounting both straightbore and tapered bore bearings. Hydraulic force is supplied by anoil injection pump. Hydraulic nuts consist of a nut thatincorporates a groove in which an annular piston moves when oilpressure is built up behind it. Hydraulic nuts can be used for both

mounting and dismounting.As well as the equipment already mentioned, there are a numberof standard items which may be required from time to time. Thefollowing should be readily available: straight edge, marking blue,dial gauge, plumb line solvent, clean rags, etc. Tools should bekept clean, and in good condition. Bearings are precision itemsand tools and equipment should therefore be maintained, handledand stored accordingly.

3.3 Mounting proceduresBefore starting the mounting procedure for any bearing, thefollowing points should be considered: Check the manufacturer'sdrawing of the bearing arrangement. Ensure that it is clearlyunderstood. Establish which fit is the interference fit. Check thatthe necessary tools and equipment are available afterdetermining the mounting procedure to be used. Select a suitableworking environment - clean and adequately lit. If an old bearingis to be remounted make sure that it has been properly cleaned

and lightly lubricated. Protect the bearing in greaseproof paperuntil ready for fitting. New bearings should be kept wrapped untilready for fitting. Check that the bearing is the correct type andsize according to the manufacturer's recommendations. Checkthat the shaft and bearing housing are clean and free from burrsand other surface damage. Check the shaft and housingdimensions to ensure that they are correct according to drawings.Check shoulders and abutments for run-out - especially in thecase of thrust bearings. Run-out in the thrust face mounting will

cause rapid wear. It should be checked with a dial gauge.Pre-lubrication

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Normally, rolling element bearings are not lubricated until afterthey are mounted, although there may be exceptions, especiallywhen the bearings are inaccessible. For cold mounted bearings,however, a light lubrication of the bearing seat and the shaft

journal and housing will assist in assembly. It is also good practicewhere shaft nuts and adapter sleeve nuts are used for drive-up,that the threads be lightly lubricated; so that they createminimum resistance.

Selection of mounting methodThe method to be used for mounting a bearing will depend on thetype and size of bearing and the mounting arrangement. Themanufacturer's instructions or service manual should be followed

where available. Bearings with a bore of 100mm (4") or less canusually be cold mounted; larger bearings need to be heated. Forbearings with tapered bores, only very large bearings need to beheated. Cylindrical bore bearings need to be mountedmechanically whereas tapered bore bearings can be mountedusing hydraulic mounting tools. The following methods can beconsidered as standard for mounting rolling element bearings. Ifdoubt exists as to which method should be used then equipmentand bearing manufacturers should be consulted.

A press is fine for small bearings with cylindrical bore. A sleeveshould be used between bearing and press which has flat, paralleland burr-free end faces. Sleeve should bear on the interference fitbearing ring. For cold mounting of all cylindrical bore bearings adolly or sleeve can be used. The sleeve must bear on the racewith interference fit and care should be taken to ensure that thebearing is driven on smoothly and does not cock over. For smalland medium sized bearings up to 200mm (8") bore with taperedbore, either a hook or impact spanner can be used with a drive

nut to drive bearing onto tapered shaft or an adapter sleeve.When a drive nut is used in this manner the face of the nut facingthe bearing should be coated with a dry lubricant, such asmolybdenum disulphide, and the surface of the shaft or sleevecoated with light oil.

A hydraulic nut can be used for bearings with tapered bores.When oil is pumped into the nut the annular piston forces thebearing on to the tapered seat until it reaches a shoulder or therequired amount of axial drive-up. The hydraulic nut should be

used in conjunction with a suitable oil pump and oil of viscosityrecommended by the manufacturer. Oil can be injected betweeninner race and shaft to expand the bearing and reduce the


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friction, hence to make it easier to drive bearing onto seat. To beable to use this method the shaft must have been speciallymachined with an oilway and oil grooves to distribute the oilbetween the bearing and the shaft. The viscosity of the oil and

the pressure developed by the pump should be determined withthe supplier of the equipment. Sometimes withdrawal sleeves arealso specially machined with oil ducts and can be used with oilinjection. For large bearings on tapered bore shafts or withsuitably machined withdrawal sleeves, oil injection can be used inconjunction with a hydraulic nut.

For bearings with cylindrical bores and for large bearings withtapered bores it may be necessary to heat the bearing. Thebearing should be heated to around 80C to 90C above the shaft

temperature but never to more than 120C (250F). The heatingapparatus should be close to the equipment and the hot bearingshould be pushed home quickly and smoothly before it coolsdown or jams. An induction heater should be used where possible.If an oil bath is used, the bearing should be heated with the oiland not dropped into hot oil also it should be kept off the bottomof the bath by a grate to prevent distortion. If these two heatingmethods are not available then a hot plate or an oven may beused. Whatever method is used a careful check must be kept on

the temperature of the bearing by using a surface thermometer.When a bearing is an interference fit in the housing and it isimpossible to heat up the housing it may be necessary to cool thebearing by submerging it in a bath of alcohol cooled by dry ice orin a cryogenic liquid.

Special mounting considerationsAdjustment of tapered roller bearings - Tapered roller bearingsmust be set up either with clearance or with a certain amount of

preload, depending on the manufacturer's instructions. Thesimplest way to do this is to draw up the shaft nut or end plateuntil there is no play in the bearings and drag becomesnoticeable. If end play is required then the shaft nut can bebacked off. If preload is required then it can be pulled up harder.

The adjustment can be measured by mounting a dial gaugeagainst a shaft shoulder or gear face. In an arrangement withshims, once the play has been taken up the gap between the endplate and shaft end can be measured with a feeler gauge and

either the end play added or the preload subtracted to establishthe correct thickness of shim pack.

Where clearance is required it is always wise to take a final check

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of end play with a dial gauge against a shaft shoulder. If a shaftlocking nut arrangement is involved then a final check ofclearance will be needed after the locking nut has been tightenedbecause this will force the shaft nut towards the bearing by an

amount equal to the clearance in the threads. For a bearingmounted on a tapered bore, the degree of interference betweenthe inner ring and the shaft depends on how far the bearing isdriven up the tapered shaft. This dimension is known as axialdrive-up. In small spherical roller bearings, where clearancescannot be measured, the axial drive-up is used as a measure ofthe interference fit. Manufacturers' information should beconsulted for correct values of axial drive-up for differentbearings.

Measurement of clearances in spherical roller bearingsThe internal clearance in a spherical roller bearing is greater thanin a ball bearing and can be measured with feeler gauges.Spherical roller bearings are mounted on tapered seats, so as thebearing is pushed on to the taper the inner ring expands andreduces the internal clearance in the bearing. Hence the finalclearance is a direct function of the interference fit on the shaft.Measurement of the clearance is useful in giving an indication ofthe shaft fit. This can be accomplished as follows: Measure theun-mounted radial clearance by standing the bearing on a cleansurface and rotating the inner ring to seat the rollers in the outerring. Then use a feeler gauge to establish the clearance betweenthe uppermost roller and the outer ring. Do this by inserting thegauge between two top rollers and then rolling the roller underthe blade. Increase the feeler gauge thickness until the rollertraps the blade and cannot be withdrawn. When the bearing ismounted on a tapered shaft or sleeve, the clearance must bemeasured at the bottom instead of the top. As before, the outer

ring should be rotated a few times to ensure that the rollers areproperly seated and a feeler gauge inserted between the rollersand the bottom of the outer ring.

With self-aligning ball bearings the clearances are too small tomeasure with a feeler gauge. Normal practice is to tighten up theshaft nut and to check the clearance by swivelling and rotatingthe outer ring. When the clearances are correct the ring shouldrotate freely but there should be some resistance to swivelling.

3.4 Dismounting proceduresThe procedure used for dismounting a bearing will depend on the


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way in which the bearing is mounted and whether theinterference fit is on the shaft or in the housing. If the bearing isto be re-used then its relative position should be marked before itis dismounted i.e. which side is 'up' and which way does it face.

Whether the bearing is to be re-used or not care should be takennot to damage the bearing during the dismounting process sothat the evidence of failure is not disturbed and it can be used toestablish cause of failure. The dismounting force, like themounting force, should always be applied to the ring with theinterference fit. Care should also be taken not to damage thesurface of the shaft or housing. The following methods arecommonly used.

Interference fit on the shaft - The most common method for

dismounting bearings with either a cylindrical bore or a taperedbore is by the use of an outside puller. Bearings mounted onshafts that have been machined with oil ducts can be dismountedusing the oil injection method. Interference fit in the housing -When the bearing is an interference fit in the housing then thebearing may have to be hammered off using a dolly or a softmetal drift. Because of the tendency of soft metal drifts to chipthey should never be used for mounting. When dismounted in thisway bearings should be washed carefully before being re-used.

For bearings where the inner race can be swiveled, an insidepuller can be used.

Bearings on adapter sleeves - A hammer and dolly can be usedeither to drive the bearing off the sleeve or to drive the sleevefrom under the bearing depending on which way round the sleeveis mounted. The alternative is to use a hydraulic nut. Bearings onwithdrawal sleeves For small and medium sized bearings a hookor impact spanner can be used to drive up a withdrawal nut toforce out the sleeve. To make the process easier, the threads and

faces of the withdrawal nut should be lubricated withmolybdenum disulphide. For large bearings, a hydraulic nut isrecommended. If the withdrawal sleeve is machined with oil ductsthen oil injection can be used in conjunction with the withdrawal

nut and an impact spanner. 4. Failure patternsAs with plain bearings, it is important to determine accurately thecause of failure before a rolling element bearing is replaced. thisrequires careful analysis of the symptoms of malfunction andcareful inspection of the failed components. Generally speaking,malfunction and failure of rolling element bearings are caused bymuch the same problems as affect plain bearings. The symptoms

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of failure can be classified into those noticed during operation andthose found on inspection. The evidence of failure differs to someextent from that shown by plain bearings because of the physicaldifference in construction.

4. OPERATING SYMPTOMSOverheatingAs the condition of a rolling element bearing deteriorates, thefrictional resistance to motion increases and the operatingtemperature begins to rise. Unlike plain bearings, where almostany change in condition tends to cause an increase intemperature, only certain causes lead to overheating in rollingelement bearings. The two main causes of temperature rise are

inadequate lubrication and loss of internal clearance. Because ofthe sliding contact between the surfaces of a plain bearing almostany change in surface condition will increase the frictionalresistance to motion. But the rolling contact in a rolling elementbearing reacts rather differently. The temperature of a rollingelement bearing can be measured in the same way as a plainbearing and again it is deviation from the normal runningcondition that provides a warning of malfunction. A rise of 10Cabove normal running temperature is generally considered as

cause for alarm with a maximum allowable temperature for astandard bearing being around 125C (260F).

VibrationUnlike plain bearings, some vibration of rolling element bearingsis normal due to the action of the rolling elements themselves.With rolling element bearings it is particularly important that avibration signature pattern is established. Deviation from thehealthy signature pattern can then be monitored to detect change

in condition of the bearing. The acceptable limits for measuredvibration vary with the size and type of machine. Really they aremachine specific but recommended values can be found. Ifvibration levels are assessed by direct observation thenexperience is required to determine the severity of the problem.But an understanding of the principles of operation of the bearingand the material characteristics should make it fairly obviousvibration levels are becoming unacceptable. Vibration maydevelop in a bearing because of defects in the bearing itself or itmay be transmitted to the bearing from other parts of themachine or other adjacent equipment. Vibration produces rapidand repeated blows on bearing surfaces and contributes, sooner


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Rolling element bearings are not as susceptible to general wearfrom adhesion and abrasion as plain bearings, because of therolling rather than sliding motion of the surfaces, but thepresence of foreign material will cause surface damage in the

form of scratching and scoring.FatigueThe rolling action of balls and rollers sets up an intermittenttransfer of load from one raceway to another and generatescyclical stresses in the loaded area of the raceways. Thecontinuous deflection of the metal causes cracks to develop whichlead to flaking of the surfaces. As this condition worsens, themetal breaks down further and grain displacement takes place.

This condition is known as spalling.

FrettingFretting corrosion or false brinelling is most likely to occur when arolling element bearing is dry and stationary. It is due to slight,almost imperceptible motion between the contacting surfaces ofthe rolling elements and raceways. Such motion may betransmitted to the bearing from an external source or may comefrom the machine itself. Fretting produces a characteristic red-brown dust at the interface. Fretting corrosion is most lively to

occur when a machine is in transit or on stand-by.

BrinellingTrue brinelling, which false brinelling or fretting is said toresemble, consists of indentations in the raceways made by therolling elements when the bearing is subjected to shock loading

Pitting and flutingA pitted or a fluted effect may appear as a result of stray electric

currents arcing across the contact surfaces. It is usual for thepitting to develop first and if no action is taken to prevent furtherdamage this deteriorates further into a fluted or washboardeffect.

Smearing and gallingIf the rolling elements fail to rotate properly and rub or slide overthe raceways then smearing may occur due to the surface of theraceway 'picking up' on the rolling element.

CorrosionCorrosion of bearing surfaces may be evident as pitting or


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spelling of bearing raceways. Rust and other corrosion productsmay be present.

Wear patternsThe pattern or load zone produced on the internal surfaces of aball bearing can be an important clue to the cause of failure.Although other types of bearings also generate specific loadingpatterns they are much more difficult to analyze than ballbearings. In order to benefit from the study of load zones it isnecessary to differentiate between normal and abnormalpatterns. The normal pattern to be expected from a bearing willdepend on the way the load is carried and whether it is the inneror the outer ring that rotates.

4.3 Causes of failureThe same problems that cause malfunction and failure of plainbearings also cause failure of rolling element bearings. Rollingelement bearings can also be affected by factors that do notusually affect plain bearings due to their different configuration,

Thus the symptoms described above can result from one or moreof the following:

DirtAs with plain bearings, the presence of dirt or other foreignmaterial in the bearing will cause excessive wear and prematurefailure of a rolling element bearing. Cleanliness during assembly,constant filtering of lubricants and well maintained seals will helpto avoid this problem.

Inadequate lubricationThe oil supply system is as critical for rolling element bearings asfor plain bearings and all the recommendations for maintaining an

adequate supply of the correct lubricant apply. Failure of thesystem will lead to rapid deterioration in bearing condition due tosmearing and galling, and the likely seizure of the bearing. It isgenerally recognised that inadequate lubrication is the singlemost common cause of rolling element bearing failure.

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Improper assemblyIf a rolling element bearing is not correctly assembled then rapidwear will result from excessive or insufficient pre-loading,misalignment or failure to secure the inner and outer rings

correctly. Abnormal wear patterns such as those described in theprevious section will result and also the likelihood of fatiguefailure is increased.

MisalignmentFor rolling element bearings that do not have inherent self-aligning features the alignment of shaft and housing can becritical to the operation of the bearing. Cylindrical roller bearingsare the most susceptible to run-out and there are limits to the

tolerable run-out for all other types except self-aligning ballbearings and spherical roller bearings. Wear patterns should bestudied carefully in order to establish evidence of misalignmentand squareness of the housing and shaft alignment should beclosely checked if doubt exists.

OverloadIf a rolling element bearing is subjected to loads in excess ofthose for which it was designed, then its life will be shortened and

early fatigue failure will occur. The reduction in life span cangenerally be expected to be proportional to the cube of the loadincrease. This means if the load were doubled then bearing lifewould be reduced to approximately one-eighth of normal.Overloading can also occur due to excessive speed which leads tooverheating of the bearing. High temperatures may affect theproperties of the lubricant and cause centrifugal throw-out of oiland grease. Excessive wear will occur as the effectiveness of thelubricant deteriorates, and at very high speeds fatigue life may be

reduced due to the increased loading which results from highcentrifugal forces acting on the rolling elements.

MoistureThe presence of water in a bearing, due to leakage into thehousing or condensation of moisture due to temperature changes,can cause considerable damage from corrosion. Water may alsofind its way into the bearing as a result of improper washing anddrying techniques used during inspection. In an oil lubricatedbearing, the presence of significant quantities of water will causeemulsification of the lubricant.

Lubricant breakdown


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Oxidation of the lubricant may lead to the formation of acidiccompounds that may cause corrosion of bearing surfaces.Sometimes lubricant additives may be incompatible with bearingmaterials and also lead to corrosive attack.

Shock loadingRolling element bearings are more susceptible to shock loadingthan are plain bearings because of the point or line contact of therolling elements. Shock loads cause indentation of the racewaysand result in the brinelling effect.

Electric currentsStray electric currents, even at very low voltages, can arc acrossthe lubricant film and lead to pitting and fluting. This is morelikely to occur with electrical machinery but can also occur inother machines due to currents that build up statically and shortcircuit to ground through the bearing. This type of problem mayalso occur due to the incorrect positioning of earthing leads whenwelding is being carried out on a machine.

External vibrationsVibrations transmitted from other parts of a machine, or fromanother machine altogether, can cause damage to a rolling

element bearing whether it is running or stationary. If a bearing isin operation and is subjected to vibrations from an externalsource then its fatigue life is likely to be reduced depending onthe severity of the vibrations. External vibrations transmitted to astationary bearing may produce fretting corrosion.

5 THE BASICS OF PLAIN BEARINGS5.1 The Operation of Plain Bearings

The operation of a plain bearing relies on relative motion betweenplain surfaces which are either cylindrical, in the case of journalbearings, or flat, in the case of thrust or slider bearings. In orderthat motion may occur with minimum resistance the effect offriction between the two surfaces must be reduced as much aspossible. This is achieved by ensuring that the two surfaces aresmooth and by introducing a film of lubricant between them. Insome cases a bearing material which has self-lubricatingproperties may be used, otherwise a separate supply of oil orgrease must be provided. For the best operation it is preferable

that full-film lubrication is achieved so that metal-to-metal contactis eliminated. In some cases however, this may not be possible.

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When loads are very high it is impossible for a lubricant tomaintain separation of the surfaces and even with extremepressure lubricants, only boundary lubrication can be achieved. Inthe case of slider bearings, where positional accuracy of the

sliding element is required such as for machine tool carriages,full-film lubrication is undesirable because the lubricant film tendsto vary in thickness. Hence boundary lubrication, which allowssome metal-tometal contact and thus more accurately locates thesliding element, is preferable.

The material of a plain bearing is always softer than the materialof the shaft or slider it supports. This allows the bearing to wearout rather than the shaft or slider. The design of plain bearingassemblies is usually such that the bearing material itself can be

relatively easily replaced.The key to satisfactory operation of a plain bearing is themaintenance of the correct lubricant film. This is often assisted bymachined grooves in the bearing surface which act as reservoirsand ensure even distribution of lubricant across the bearingsurface. The grooves are usually connected to the lubricant inletwhich is often located just ahead of the most highly loaded areaof the bearing.

One of the most important aspects of bearing operation is thecontrol of temperature. This may be achieved either by ensuringan adequate flow of lubricant which acts as a coolant or bysurrounding the bearing with chambers in the housing throughwhich cooling water can circulate.

5.2 Types of Plain BearingJournal BearingsA plain journal bearing is essentially a tube which encircles, either

totally or partially, a rotating shaft. The tube is normally held in afixed housing and provides a support for the shaft and allows it torotate with minimum resistance. There are two principal types ofplain journal bearings - solid and split. A solid bearing is usuallycalled a bushing when it is of the thin-walled type i.e. up to 2mm,and a sleeve bearing when it is of the thick-walled type.

There are various types of split bearing but the most commonconsists of two halves which together form a full circle. They areused particularly for ease of shaft removal which can be

accomplished by removing the top half of the bearing only.

Similar to a split half bearing is a multipart bearing which is


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composed of more than two segments which fit together aroundthe journal to form a full circle.

A partial bearing is one which supports the shaft from the bottomonly and may comprise a half-circle or less. This type of bearing

can only be used when loads are relatively heavy and areconstant in direction.

Thrust BearingsA plain thrust bearing supports the end thrust of a shaft andrestrains axial movement. The bearing surface itself consists of astationary surface in the form of a pad against which the rotatingelement bears. The most common form of plain thrust bearing isthe thrust collar arrangement with the bearing face machined

with radial grooves as shown in Fig. 6-7.Lubricant must be fed into the bearing at the inner diameter sothat lubricant flows outwards across the bearing face. Runningspeeds are limited by lubricant viscosity.

A slight modification to the above is a profiled pad thrust bearingwhere the pads are shaped to provide a convergent lubricant filmand help to form a hydrodynamic wedge. The pad may be profiledto be unidirectional or bidirectional as shown below inFig. 6-8. Inorder to meet these complex requirements, a range of specialmaterials that are used purely as bearing materials has beendeveloped.

Tilting pad thrust bearings are able to accomodate a large rangeof speeds, loads, and lubricant. This is achieved by providingpivoted protect the journal from wear and scoring. pads which canassume a small angle with respect to the rotating collar surface,and this makes sure that a full hydrodynamic film is maintainedbetween the surfaces.

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Linear BearingsLinear bearings, which are often called ways or guides, supporteither linear or curvilinear motion and are often associated withreciprocating mechanisms. There are various forms that linear

bearings can take, some of which are shown in Fig. 6- 1 0.

5.3 Plain Bearing MaterialsThe requirements of a plain bearing material are complex and, insome senses, conflicting. For example, a bearing material mustbe hard enough to resist wear and yet soft enough to conform tothe contours of the journal during the running-in process. A plainbearing material must exhibit the following importantcharacteristics: Compressive strength to support unidirectional

loading without deformation. Fatigue strength to resist fluctuatingdynamic loadings. Embed ability To embed foreign matter andviscosities. Conformability To tolerate misalignment anddeflection under load. Compatibility To tolerate momentarymetal-to-metal contact without seizure. corrosion resistance Toresist attack due to water or oxidized lubricant products.

A wide range of specialist materials is used in modern plainbearings. White metal or babbitt alloys are tin or lead basedalloys. In general they are used in the form of a coating on a steel

bearing shell. Other materials include aluminium alloys, bronzealloys (which can be more cost effective than other types becausethey do not necessarily require a backing shell), and copper-leadalloys.

6. ROUTINE MAINTENANCE OF PLAIN BEARINGS6.1 Best Practice

There are a number of general considerations that relate to the

maintenance of plain bearings. Cleanliness is a keynote that mustbe observed when handling bearings of any kind. Dust and dirtparticles should be kept away from bearing surfaces and allbearing components should be kept dry and protected at alltimes. Good alignment is essential to plain bearings without self-aligning properties. When plain bearings are disassembled allparts should be marked so that they can be reinstalled in theiroriginal positions. The matching of parts during running-in makesthem unsuitable for operation in other positions. When fittingplain journal bearings careful attention should be paid to thecorrect tensioning of bolts. Bolting operations are normallyconducted by manufacturers with bolts tensioned to the same


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specification recommended in their maintenance instructions.Incorrect tensioning may affect clearances and bearing crush. Thekey to good bearing performance is having the correct clearancesand the correct lubricant. Bearing surfaces should be given

preliminary lubrication before the shaft is turned. The journalsurface or thrust collar should always be checked to ensure that itis perfectly smooth. In the case of a journal, a micrometer shouldbe used to check the size so that replacement bearings will havethe correct i.d. (inside diameter).

6.2 Assembly of Plain BearingsBushes and sleeves may be either a press-fit or a shrink-fit in thehousing. In the case of a press-fit the following procedure shouldbe adopted. Check the dimensions of the bush and housing andcompare with manufacturer's recommendations to ensure theyare correct. Ensure that the mating surfaces are clean, smoothand free of burrs. Press the bearing into the housing using anarbor press if possible. Hammering should be avoided as this mayruin the bearing. The bearing should be pressed in in onecontinuous motion and a lubricant should be used. Check the

inside diameter of the bush to see if it has been reduced due to'close-in' as a result of the press fit. Hone or ream the bush to therecommended size.

In the case of a shrink-fit, the bearing can be assembled either byheating the housing or by cooling the bearing. If the housing isheated, care should be taken to ensure that heating is uniform sothat no warping or distortion occurs. Heat up should not be toorapid and normally a temperature of around 100'C should providesufficient expansion. Avoid heating the housing any more than isabsolutely essential. If it is not convenient to heat the housing,the bearing may be packed in dry-ice or chilled in a deep-freezeunit before assembly. As in the case of a press-fit, it is advisableto check the inside diameter of the bearing when temperatureshave returned to normal to ensure that it has not been reduceddue to close-in. When assembling split bearings it is vital that thesurfaces of the bearing and housing are clean. Especially checkbetween the bearing and housing and the joint faces of both thebearing and housing. Also check the oil-ways and oil grooves. A

split bearing is provided with 'free spread' to ensure that bothhalves assemble correctly and do not foul the shaft along the joint

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line when bolted up. Free spread can be adjusted by tapping thebearing gently with a wooden mallet.

Manufacturer's recommendations should be checked to determinethe amount of free spread allowed. To ensure that the bearing

inserts have good contact with the housing they are made so thatthe inside diameter at right angles to the parting line is slightlylarger than the diameter across the parting line. This excess iscalled crush and ensures that when the bearing is bolted up it willbe held securely and in full contact with the housing. The bearinghalves should be installed in the housing so that the crush is evenon both sides of the housing. Bolts should be torqued upprogressively and evenly on both sides.

6.3 Clearances in Plain BearingsThe correct adjustment of clearances is vital to the operation ofplain bearings. If clearances are insufficient then metal-to-metalcontact will occur causing excessive wear. If clearances are toogreat then the hydrodynamic wedge will break down and alsocause metal-to-metal contact. In the case of journal bearingsthere are several ways in which clearances can be checked.

Measurement of the inside diameter of the bearing and outsidediameter of the journal can be made with either a micrometer or

gauges. Care should be taken if using a micrometer, to takereadings in several positions.

A soft material that will deform easily, such as plastic thread, canbe inserted between the bearing and the journal before the boltsare torqued up. After the bolts have been torqued and releasedagain, the flattened material can be compared with a suitablyprepared chart and the clearance read directly.

A third alternative is to insert lead or brass shims in between the

bearing and the housing. The shims are adjusted to the pointwhere the shaft turns easily but where an extra 0.025mm(0.001") added to the thickness of the shim would cause it to jam.Great care has to be taken not to damage the bearing surfaces ifthis method is used. If, whatever method is used, some out-of-roundness is found to exist, then it is the minimum clearancewhich is critical. In the case of thrust bearings it is also importantto measure the end float to ensure that sufficient clearance existsbetween the thrust collar and the bearing. This can be done by

forcing the shaft hard up in each direction and either measuringthe clearance at the thrust faces with gauges or shims or settingup a dial gauge and reading against a suitable shoulder on the


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shaft.

6.4 Start-up of PlantThe following procedures should be adopted during the start-up of

new or reconditioned plain bearings. Before the shaft is run,check the alignment with bearings in position. Bluing of thejournal will indicate whether the shaft is running true in thebearing. If the wear spots are at one end of one of the bearinghalves and at the opposite end of the other, then misalignment isindicated. Alignment should be corrected by shimming thebearing housing. Once the alignment is true, pre-lubricate thebearing surfaces before final assembly to ensure they do not startup dry. Run the shaft and check for noise and vibration. Checkbearing temperatures every half hour for the first two or threehours of operation. If temperatures are excessive, shut down themachine and inspect the bearing surfaces for wear-incharacteristics. Further alignment or scraping may be requiredbefore re-starting. Once the operation of the bearing issatisfactory, ensure that it is regularly supplied with the correctlubricant.

7. FAILURE OF PLAIN BEARINGS7.1 Symptoms of Rolling Element Bearing FailureBefore replacing a damaged bearing it is important to know theprecise cause of failure so that the problem can be properlycorrected, otherwise similar failures may keep occurring due tothe same cause.

Bearing problems and failures can be analysed in terms of effectsor symptoms or in terms of causes. From the point of view of faultdetection it is useful to look first of all at symptoms and then totrace the possible causes. Every failure is different in some

respects so it is extremely difficult to establish definite rulesabout causes. The same symptom may result from many differentcauses and systematic troubleshooting techniques are necessaryto eliminate all but the real cause. However, it is possible toestablish some general rules about the relationship betweensymptoms and causes that can serve as a guide in thetroubleshooting process. There are two levels at which thesymptoms of bearing failure can be detected. The first is whatmight be called the level of external symptoms which are

exhibited while the bearing is in operation. The second can becalled the level of internal symptoms which can only be observedwhen the bearing is shut down, dismantled and inspected. When

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the condition of a bearing begins to deteriorate, whatever thecause, then it may exhibit one or all of the following symptoms:

OverheatingThe temperature of a plain bearing during operation is a vitalindication of its condition. A properly lubricated bearing in goodcondition should not generate excessive temperatures and shouldrun cool enough to touch. The most efficient way to check bearingtemperature is to use a portable sensing device and to measurethe normal running temperature of the bearing once it has beenrun in. Routine checks at regular intervals will then show up anysudden or dramatic increases from the base level. When using acontact pyrometer or thermocouple to measure temperature it isimportant to ensure that good contact is achieved at a positionthat truly reflects the temperature of the bearing.

VibrationA plain bearing in good condition should run very smoothly, evenmore so than a rolling element bearing. As clearances increase,some vibration may develop which will tend to accelerate thedeterioration of the bearing. As with temperature, hand contact isusually sufficient to establish whether unnecessary vibration ispresent. It is advisable where possible, and especially for critical

equipment items, to establish a normal operating level ofvibration using a vibration monitor (see Chapter 10) and to makeroutine checks to establish when a deviation occurs. As withtemperature, a dramatic change in operating characteristicsindicates a need for further investigation.

NoiseA plain bearing should not produce high noise levels if it is in goodcondition. If the noise level increases it may be due either to

misalignment or excessive wear. Foreign particles trapped in thebearing may also cause noisy operation. In a reciprocatingmechanism the big and little end bearings produce a 'knocking'sound when wear becomes excessive. Noise levels can bemeasured independently although vibration monitoring is morecommonly used. Technicians should develop an 'ear' for thesound of machinery so that changes in both level andcharacteristics can be detected.

SeizureTotal seizure of a plain bearing is, of course, a symptom whichcannot be ignored. If the three characteristics referred to above


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are carefully monitored then seizure should not occur. Seizure isusually a result of excessive temperature build-up due to lack oflubrication and if temperatures are routinely checked then seizureshould be avoided. The danger in allowing undetected

deterioration to lead to ultimate seizure of a bearing is that sucha sudden catastrophic failure is likely to also cause severedamage to the housing and other machine components. Theexcessive temperature build-up can result in fire and the suddenshutdown of the machine may cause severe or even disastrousprocess problems.

Symptoms often occur simultaneously and the presence of oneoften leads to the others appearing. This means that cross-checkscan be made to establish whether a problem exists. For example,

if it is suspected that the noise level is increasing, then a check onoperating temperature may confirm that the condition of thebearing is deteriorating.

The operating symptoms alone rarely, if ever, provide sufficientevidence for the exact cause of failure to be determined. It maybe possible, once the external symptoms of failure are detected,to attribute them to the general cause of 'wear'and then merelyto proceed to replace the bearing and re-start the equipment.

This is a short-sighted approach however, and runs a strong risk

of a premature repetition of the same problem. In order to bemore precise about the cause of failure, it is necessary todismantle the bearing and inspect the bearing surfaces toestablish what internal symptoms exist. These can generally beidentified according to one of the following categories.

WearAny two surfaces that are in constant sliding contact, even thoughproperly lubricated, will eventually wear out. This process will, in

most cases, be apparent due to a general loss of metal across thebearing surface. The bearing surface may still be reasonably goodand the wear only clearly evident by the increased clearancesestablished by a dimension check. Wear occurs due to acombination of the processes of adhesion and abrasion.

FatigueFatigue is caused by cyclical stressing due to fluctuating loadsand although not as common in plain bearings as in rolling

element bearings it can often occur when load or speed becomesexcessive.

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GallingSometimes the adhesion due to cold welding of two surfaces incontact becomes so great that grain displacement of the bearingmaterial takes place leaving pits or craters. This is usually

associated with high loads which cause excessive pressurebetween points of contact.

ScoringWhen excessive amounts of dirt or large dirt particles are presentdue to contamination of the lubricant or because of abrasive wearthen scoring and erosion of the bearing surface will result.


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WipingWhen there is insufficient clearance in the bearing or overheatingoccurs due to inadequate lubrication then a superficial melting ofthe bearing surface and a flow of material occurs.

FrettingFretting is most likely to occur in plain bearing between the linerand the housing when the interference fit is inadequate and slightvibratory movement occurs. A characteristic fine brown debris ofoxidation material is the usual evidence of fretting or falsebrinelling as it is sometimes called. Fretting may also occurbetween the bearing surfaces while the shaft is stationary due tovibrations transmitted from external sources.

PittingLocal loss of metal in the form of pits or deep craters can resultfrom a number of causes. This can sometimes be due to strayelectric currents that arc across the bearing and cause pitting ofboth the surface of the bearing and the journal.

CorrosionEtching and surface discoloration are all evidence of corrosion.

7.2 Factors Causing Deterioration of Plain BearingsThere are several factors which experience has shown to becommon causes of plain bearing deterioration and failure.

Dirt ContaminationIngress of dirt is probably the most common cause of plainbearing failure. Dirt particles can become embedded in thebearing surface and can then act an abrasive causing excessivewear. It is also possible for dirt particles to be trapped between a

bearing shell and its housing, and this can distort the bearing withconsequent reduction in clearance and heat transfer. Dirt caningress into a bearing during assembly or via contaminatedlubricant, hence good maintenance practice and properly filteredlubrication systems are very important.

Lubrication ProblemsOil starvation is a common cause of bearing failure and may occurfor various reasons. The period immediately after overhaul is

always critical and it is recommended that the lubrication systembe primed to ensure lubricant is present on initial start-up. Thelubrication system should be kept under close observation for the

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period after start-up to ensure that pressures and temperaturesare established according to design. During normal operation,several factors may affect the supply of oil. Leaks, oil pumpfailure, blocked filters and blocked oil passages are all potential

problems. Care must be taken when installing bearing liners toensure that the oil hole is in line with the oil passage in thehousing. It should also be recognised that once a bearingbecomes badly worn the hydrodynamic wedge cannot beeffectively established and the wear rate will further increase dueto inadequate lubrication. Bearing failure may also occur due touse of an unsuitable lubricant due either to error or poorselection. Lubricant suppliers and equipment manufacturersshould be consulted if there is reason for concern about the

suitability of the lubricant used. Lubricants should be storedproperly and clearly marked so that the likelihood of error isreduced.

Breakdown of the lubricant, because of the formation of organicacids due to oxidation or the presence of specific elements in theoil such as sulphur compounds, may lead to corrosion of thebearing surfaces. An analysis of the lubricant after it has been inservice may be necessary to establish whether deterioration hastaken place. A review of the oil replacement period may be

sufficient to avoid recurring problems. This should be done inconsultation with the lubricant supplier.

Improper assemblyAs well as ensuring that cleanliness is observed during theassembly procedure, it is also important to ensure that thebearing is assembled correctly in all other respects. The mainconsiderations are that the bearing should be fitted square andsecure in its housing and that it does not get physically damaged

during assembly. Failure to ensure proper assembly may result inuneven or rapid wear and overheating.

MisalignmentIf the journal is not properly aligned in the bearing unevenbearing wear and possible damage to the journal will result. Thealignment procedures described earlier in this chapter should befollowed carefully. The alignment of shaft couplings should alsobe checked to ensure that misalignment there does not affect thetrue running of the journal in the bearing.

Overload


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Equipment overloads can occur for many different reasons andare particularly likely to occur when extra output is required. Themost common symptoms of overload are overheating and fatigue.When these are observed a review of operating conditions is

advisable in order to establish whether any overloading hasoccurred. Increased speeds, feed rates, levels, operatingtemperatures and pressures, etc. can all create overloadconditions which lead to machine elements such as bearingshaving to perform above design limits. Moisture The presence ofwater due to condensation or lack of effective sealing may causecorrosion of the bearing surfaces and the formation of rust.

Documents

CH01 Bearings