Skf Bearing Failure and Causes

Bearing failures and their causes

Product information 401

Contents

Introduction ....................................................................................................... 3Bearing failures and their causes.................................................................. 3How is bearing life defined? .......................................................................... 3

Path patterns and their interpretation................................................................ 4Different types of bearing damage.................................................................... 9

Wear.............................................................................................................. 10Wear caused by abrasive particles ............................................................ 10Wear caused by inadequate lubrication..................................................... 11Wear caused by vibration .......................................................................... 12

Indentations................................................................................................... 14Indentations caused by faulty mounting or overloading............................. 14Indentations caused by foreign particles.................................................... 16

Smearing....................................................................................................... 17Smearing of roller ends and guide flanges ................................................ 17Smearing of rollers and raceways.............................................................. 18Raceway smearing at intervals corresponding to the roller spacing.......... 19Smearing of external surfaces ................................................................... 21Smearing in thrust ball bearings ................................................................ 22

Surface distress ............................................................................................ 23Corrosion....................................................................................................... 24

Deep seated rust........................................................................................ 24Fretting corrosion ....................................................................................... 25

Damage caused by the passage of electric current ...................................... 26Flaking (spalling) ........................................................................................... 28

Flaking caused by preloading .................................................................... 29Flaking caused by oval compression......................................................... 30Flaking caused by axial compression ........................................................ 31Flaking caused by misalignment................................................................ 32Flaking caused by indentations.................................................................. 33Flaking caused by smearing ...................................................................... 34Flaking caused by deep seated rust .......................................................... 35Flaking caused by fretting corrosion .......................................................... 36Flaking caused by fluting or craters ........................................................... 37

Cracks ........................................................................................................... 38Cracks caused by rough treatment ............................................................ 39Cracks caused by excessive drive-up........................................................ 40Cracks caused by smearing....................................................................... 41Cracks caused by fretting corrison............................................................. 42

Cage damage................................................................................................ 43Vibration..................................................................................................... 43Excessive speed........................................................................................ 43Wear .......................................................................................................... 43Blockage .................................................................................................... 43Other causes of cage damage................................................................... 43

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The life of a rolling bearing is de-fined as the number of revolutions thebearing can perform before incipientflaking occurs. This does not mean tosay that the bearing cannot be usedafter then. Flaking is a relatively long,drawn-out process and makes its pres-ence known by increasing noise andvibration levels in the bearing. There-fore, as a rule, there is plenty of time toprepare for a change of bearing.

magnitude of the load. Fatigue is theresult of shear stresses cyclicallyappearing immediately below the loadcarrying surface. After a time thesestresses cause cracks which graduallyextend up to the surface. As the rollingelements pass over the cracks frag-ments of material break away and thisis known as flaking or spalling. The flaking progressively increases in ex-tent (figs 1 to 4) and eventually makesthe bearing unserviceable.

Bearing failures andtheir causesBearings are among the most import-ant components in the vast majority ofmachines and exacting demands aremade upon their carrying capacity andreliability. Therefore it is quite naturalthat rolling bearings should have cometo play such a prominent part and thatover the years they have been thesubject of extensive research. Indeedrolling bearing technology has de-veloped into a particular branch of science. SKF has been well to the fore-front right from the start and has longled this field

Among the benefits resulting fromthis research has been the ability tocalculate the life of a bearing with con-siderable accuracy, thus making it poss-ible to match the bearing life with theservice life of the machine involved.

Unfortunately it sometimes happensthat a bearing does not attain its calcu-lated rating life. There may be manyreasons for this – heavier loading thanhas been anticipated, inadequate orunsuitable lubrication, careless hand-ling, ineffective sealing, or fits that aretoo tight, with resultant insufficientinternal bearing clearance. Each ofthese factors produces its own particu-lar type of damage and leaves its ownspecial imprint on the bearing. Con-sequently, by examining a damagedbearing, it is possible, in the majority of cases, to form an opinion on thecause of the damage and to take therequisite action to prevent a recurrence.

How is bearing lifedefined?Generally, a rolling bearing cannotrotate for ever. Unless operating condi-tions are ideal and the fatigue loadlimit is not reached, sooner or latermaterial fatigue will occur. The perioduntil the first sign of fatigue appears isa function of the number of revolutionsperformed by the bearing and the

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Introduction

Figs 1–4 Progressive stages of flaking

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the appearance and location of thepatterns prove to be useful aids in dia-gnosing the cause of the damage.

Deep groove ball bearings andthrust ball bearings have been used forillustrative purposes as they displaysuch characteristic path patterns.However, the figures are applicable,with some modifications, to other typesof bearing as well.

which the bearing has operated. Bylearning to distinguish between normaland abnormal path patterns there isevery prospect of being able to assesscorrectly whether the bearing has rununder the proper conditions.

The following series of figures illus-trates normal path patterns under diffe-rent rotational and loading conditions(figs 5 to 11) as well as typical patternsresulting from abnormal working condi-tions (figs 12 to 18).

In the majority of cases the damageto the bearing originates within theconfines of the path patterns and, oncetheir significance has been learned,

When a rolling bearing rotates underload the contacting surfaces of the roll-ing elements and the raceways norm-ally become somewhat dull in appear-ance. This is no indication of wear inthe usual sense of the word and is ofno significance to the bearing life. Thedull surface in an inner or outer ringraceway forms a pattern called, for thepurposes of this paper, the path pat-tern. This pattern varies in appearanceaccording to the rotational and loadingconditions. By examining the path pat-terns in a dismantled bearing that hasbeen in service, it is possible to gain agood idea of the conditions under

Fig 5 Uni-directional radial load. Rotatinginner ring – fixed outer ring.Inner ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-wayOuter ring: path pattern widest in the loaddirection and tapered off towards the ends.With normal fits and normal internal clear-ance, the pattern extends around slightlyless than half the circumference of the race-way

Fig 6 Uni-directional radial load. Fixedinner ring – rotating outer ring. Inner ring: path pattern widest in the loaddirection and tapered off towards the ends.With normal fits and normal internal clear-ance, the pattern extends around slightlyless than half the circumference of the race-wayOuter ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-way

Path patterns andtheir interpretation

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Fig 7 Radial load rotating in phase with theinner ring. Rotating inner ring – fixed outerring.Inner ring: path pattern widest in the loaddirection and tapered off towards the ends.With normal fits and normal internal clear-ance, the pattern extends around slightlyless than half the circumference of the race-wayOuter ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-way

Fig 8 Radial load rotating in phase with theouter ring. Fixed inner ring – rotating outerring.Inner ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-wayOuter ring: path pattern widest in the loaddirection and tapered off towards the ends.With normal fits and normal internal clear-ance, the pattern extends around slightlyless than half the circumference of the race-way

Fig 9 Uni-directional axial load. Rotatinginner or outer ring.Inner and outer rings: path pattern uniformin width, extended around the entire circum-ference of the raceways of both rings andlaterally displaced

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Path patterns and their interpretation

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Fig 11 Uni-directional axial load. Rotatingshaft washer – fixed housing washer.Shaft and housing washers: path patternuniform in width, extended around the entire circumference of the raceways ofboth washers

Fig 10 Combination of uni-directional radial and axial loads. Rotating inner ring –fixed outer ring.Inner ring: path pattern uniform in width,extended around the entire circumferenceof the raceway and laterally displacedOuter ring: path pattern extended aroundthe entire circumference of the raceway andlaterally displaced. The pattern is widest inthe direction of the radial loading

Fig 12 Uni-directional radial load + imbalance. Rotating inner ring – creepingouter ring.Inner and outer rings: path pattern uniformin width, extended around the entire circum-ference of the raceways of both rings

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Fig 13 Fits too tight – preloading. Uni-direc-tional radial load. Rotating inner ring – fixedouter ring.Inner ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-wayOuter ring: path pattern positioned in thecentre and extended around the entire cir-cumference of the raceway. The pattern iswidest in the direction of the radial loading

Fig 14 Oval compression of outer ring.Rotating inner ring – fixed outer ring.Inner ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-wayOuter ring: path pattern positioned in twodiametrically opposed sections of the race-way. The pattern is widest where the pinching has occurred

Fig 15 Outer ring misaligned. Rotatinginner ring – fixed outer ring.Inner ring: path pattern uniform in width,positioned in the centre and extendedaround the entire circumference of the race-wayOuter ring: path pattern in two diametricallyopposed sections, displaced diagonally inrelation to each other

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Path patterns and their interpretation

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Fig 17 Housing washer positioned eccent-rically relative to shaft washer. Rotatingshaft washer – fixed housing washer.Shaft washer: path pattern uniform in width,extended around the entire circumferenceof the racewayHousing washer: path pattern extendedaround the entire circumference of the race-way and off-centre relative to raceway

Fig 16 Inner ring misaligned. Rotating innerring – fixed outer ring. Inner ring: path pattern in two diametricallyopposed sections, displaced diagonally inrelation to each otherOuter ring: path pattern widest in the loaddirection and tapered off toward the ends.The internal clearance is reduced onaccount of the misalignment of the innerring; the length of the path pattern dependsupon the magnitude of the internal clear-ance reduction

Fig 18 Housing washer misaligned.Rotating shaft washer – fixed housing washer.Shaft washer: path pattern uniform in width,extended round the entire circumference ofthe racewayHousing washer: path pattern in the centreof the raceway but wider around part of itscircumference

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Each of the different causes of bearingfailure produces its own characteristicdamage. Such damage, known as pri-mary damage, gives rise to secondary,failure-inducing damage – flaking andcracks. Even the primary damage maynecessitate scrapping the bearings onaccount of excessive internal clear-ance, vibration, noise, and so on. A failed bearing frequently displays acombination of primary and secondarydamage.

The types of damage may be classi-fied as follows:

Primary damageWearIdentationsSmearingSurface distressCorrosionElectric current damage

Secondary damageFlakingCracks

Different types of bearingdamage

ActionDo not unpack bearing until justbefore it is to be mounted. Keepworkshop clean and use clean tools.

Check and possibly improve thesealing.

Always use fresh, clean lubricant.Wipe the grease nipples. Filter theoil.

CauseLack of cleanliness before and duringmounting operation.

Ineffective seals.

Lubricant contaminated by worn par-ticles from brass cage.

AppearanceSmall indentations around the race-ways and rolling elements. Dull,worn surfaces.

Grease discoloured green.

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Different types of bearing damage

Fig 19 Outer ring of a spherical roller bear-ing with raceways that have been worn byabrasive particles. It is easy to feel wherethe dividing line goes between worn andunworn sections

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WearIn normal cases there is no appre-ciable wear in rolling bearings. Wearmay, however, occur as a result of theingress of foreign particles into thebearing or when the lubrication isunsatisfactory. Vibration in bearingswhich are not running also gives rise towear.

Wear caused by abrasive particlesSmall, abrasive particles, such as gritor swarf that have entered the bearingby some means or other, cause wearof raceways, rolling elements andcage. The surfaces become dull to adegree that varies according to thecoarseness and nature of the abrasiveparticles. Sometimes worn particlesfrom brass cages become verdigrisedand then give light-coloured grease agreenish hue.

The quantity of abrasive particlesgradually increases as material is wornaway from the running surfaces andcage. Therefore the wear becomes anaccelerating process and in the end

the surfaces become worn to such anextent as to render the bearing unser-viceable. However, it is not necessaryto scrap bearings that are only slightlyworn. They can be used again aftercleaning.

The abrasive particles may haveentered the bearing because the seal-ing arrangement was not sufficientlyeffective for the operating conditionsinvolved. They may also have enteredwith contaminated lubricant or duringthe mounting operation.

ActionCheck that the lubricant reaches thebearing.More frequent relubrication.

CauseLubricant has gradually been used upor has lost its lubricating properties.

AppearanceWorn, frequently mirror-like, sur-faces; at a later stage blue to brown discolouration.

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Wear caused by inadequate lubricationIf there is not sufficient lubricant, or ifthe lubricant has lost its lubricatingproperties, it is not possible for an oilfilm with sufficent carrying capacity toform. Metal to metal contact occursbetween rolling elements and race-ways. In its initial phase, the resultantwear has roughly the same effect aslapping. The peaks of the microscopicasperities, that remain after the pro-duction processes, are torn off and, atthe same time, a certain rolling-outeffect is obtained. This gives the sur-faces concerned a varying degree of

mirror-like finish. At this stage surfacedistress can also arise, see page 23.

If the lubricant is completely usedup, the temperature will rise rapidly.The hardened material then softensand the surfaces take on blue to brownhues. The temperature may evenbecome so high as to cause the bear-ing to seize.

Fig 20 Cylindrical roller with mirror-like sur-face on account of lubricant starvation

Fig 21 Outer ring of a spherical roller bear-ing that has not been adequately lubricated.The raceways have a mirror finish

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CauseThe bearing has been exposed to vib-ration while stationary.

AppearanceDepressions in the raceways. Thesedepressions are rectangular in rollerbearings and circular in ball bear-ings. The bottom of these depres-sions may be bright or dull and oxi-dised.

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Fig 22 Outer ring of taper roller bearingdamaged by vibration during operation

Fig 23 Vibration damage to the ring ofcylinder roller bearing. The damage has arisen while the bearing was not running. Itis evident, from the fainter fluting discern-ible between the pronounced depressionswith corrosion at the bottom, that the ringhas changed position for short periods

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Wear caused by vibrationWhen a bearing is not running, there isno lubricant film between the rollingelements and the raceways. Theabsence of lubricant film gives metal tometal contact and the vibrations pro-duce small relative movements of roll-ing elements and rings. As a result ofthese movements, small particlesbreak away from the surfaces and thisleads to the formation of depressionsin the raceways. This damage isknown as false brinelling, sometimesalso referred to as washboarding. Ballsproduce sphered cavities while rollersproduce fluting.

In many cases, it is possible todiscern red rust at the bottom of thedepressions. This is caused by oxida-tion of the detached particles, whichhave a large area in relation to theirvolume, as a result of their exposure toair. There is never any visible damageto the rolling elements.

The greater the energy of vibration,the more severe the damage. Theperiod of time and the magnitude ofthe bearing internal clearance alsoinfluence developments, but the fre-quency of the vibrations does not ap-pear to have any significant effect.

Roller bearings have proved to bemore susceptible to this type of dam-

age than ball bearings. This is consid-ered to be because the balls can roll inevery direction. Rollers, on the otherhand, only roll in one direction; move-ment in the remaining directions takesthe form of sliding. Cylindrical rollerbearings are the most susceptible.

The fluting resulting from vibrationssometimes closely resembles the flut-ing produced by the passage ofelectric current. However, in the lattercase the bottom of the depression isdark in colour, not bright or corroded.The damage caused by electric currentis also distinguishable by the fact thatthe rolling elements are marked aswell as the raceways.


ActionSecure the bearing during transportby radial preloading.Provide a vibration-damping base.Where possible, use ball bearingsinstead of roller bearings.Employ oil bath lubrication, wherepossible.

Bearings with vibration damage areusually found in machines that are notin operation and are situated close tomachinery producing vibrations.Examples that can be cited are trans-former fans, stand-by generators andships’ auxiliary machinery. Bearings inmachines transported by rail, road orsea may be subject to vibration dam-age too.

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Fig 24 Inner and outer ring of a cylindricalroller bearing exposed to vibration. Theinner ring has changed position

Fig 25 Spring loading a deep groove ballbearing to prevent vibration damage

Fig 26 Outer ring of a self-aligning ball bearing damaged by vibration. The bearinghas not rotated at all

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Where machines subject to constantvibration are concerned, it is essentialthat the risk of damage to the bearingsbe taken into consideration at thedesign stage. Consequently, wherepossible, ball bearings should beselected instead of roller bearings. Theability of ball bearings to withstand vib-rations without being damaged canalso be considerably improved by app-lying axial preloading with the aid ofsprings, see fig 25. An oil bath, inwhich all rolling elements in the loadzone are immersed in the oil, has alsoproved to provide satisfactory protec-tion. A vibration-damping base helps toprevent damage too.

The bearings in machines that are tobe transported can be protected bylocking the shaft, thus preventing thesmall movements that have such adamaging effect on the bearings.

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ActionApply the mounting pressure to thering with the interference fit.

Follow carefully the SKF instructionsconcerning mounting bearings ontapered seating.

Avoid overloading or use bearingswith higher basic static load ratings.

CauseMounting pressure applied to thewrong ring.

Excessively hard drive-up on taperedseating.

Overloading while not running.

AppearanceIndentations in the raceways of bothrings with spacing equal to thedistance between the rolling ele-ments.

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IndentationsRaceways and rolling elements maybecome dented if the mounting pres-sure is applied to the wrong ring, sothat it passes through the rolling ele-ments, or if the bearing is subjected toabnormal loading while not running.Foreign particles in the bearing alsocause indentations.

Indentations caused by faultymounting or overloadingThe distance between the dents is thesame as the rolling element spacing.Ball bearings are prone to indentationsif the pressure is applied in such a waythat it passes through the balls duringthe mounting or dismounting opera-tions. Self-aligning ball bearings areparticularly susceptible to damage insuch circumstances. In spherical rollerbearings the damage originates assmearing (see page 17) and sub-sequently, if the pressure increases,develops into a dent. The same condi-tions apply in taper roller bearings that

Fig 27 Washer of a thrust ball bearing subjected to overloading while not running.The indentations are narrow and radiallyaligned, not sphered as in radial ball bear-ings

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are unduly preloaded without beingrotated.

Bearings that are mounted withexcessively heavy interference fits,and bearings with tapered bore thatare driven too far up the shaft seatingor sleeve, also become dented.

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Figs 28–30 An example of the results ofimproper handling. A roller in a double rowcylindrical roller bearing has sufferedimpact (fig 28). A periphery camera view ofthe roller shows two diametrically opposedindentations (fig 29). The roller has, in turn,dented the inner ring raceway (fig 30)

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ActionCleanliness to be observed duringthe mounting operation.Uncontaminated lubricant.Improved seals.

CauseIngress of foreign particles into thebearing.

AppearanceSmall indentations distributedaround the raceways of both ringsand in the rolling elements.

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Indentations caused by foreign particlesForeign particles, such as swarf andburrs, which have gained entry into thebearing cause indentations when rolled into the raceways by the rollingelements. The particles producing theindentations need not even be hard.Thin pieces of paper and thread fromcotton waste and cloth used for dryingmay be mentioned as instances of this.Indentations caused by these particlesare in most cases small and distributedall over the raceways.

Fig 31 Indentations, caused by dirt, in oneof the raceways of a roller bearing – 50 ×magnification

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AppearanceScored and discoloured roller endsand flange faces.

CauseSliding under heavy axial loading andwith inadequate lubrication.

ActionMore suitable lubricant.

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SmearingWhen two inadequately lubricated sur-faces slide against each other underload, material is transferred from onesurface to the other. This is known assmearing and the surfaces concernedbecome scored, with a “torn” appear-ance. When smearing occurs, thematerial is generally heated to suchtemperatures that rehardening takesplace. This produces localised stressconcentrations that may cause crack-ing or flaking.

In rolling bearings, sliding primarilyoccurs at the roller end-guide flangeinterfaces. Smearing may also arise

when the rollers are subjected to se-vere acceleration on their entry into the load zone. If the bearing rings ro-tate relative to the shaft or housing,this may also cause smearing in thebore and on the outside surface andring faces.

In thrust ball bearings, smearingmay occur if the load is too light inrelation to the speed of rotation.

Smearing of roller ends and guideflangesIn cylindrical and taper roller bearings,and in spherical roller bearings withguide flanges, smearing may occur onthe guiding faces of the flanges and

the ends of the rollers. This smearingis attributable to insufficient lubricantbetween flanges and rollers. It occurswhen a heavy axial load acts in onedirection over a long period, for instan-ce when taper roller bearings are sub-ject to excessive preloading. In caseswhere the axial load changes direction,smearing is much less common as theopportunity is provided for the ingressof lubricant when the roller end is tem-porarily relieved of load. Such smear-ing can be avoided to a considerableextent by selecting a suitable lubricant.

Fig 32 Smearing on the surface of a rollerfrom a spherical roller bearing – 100 × mag-nification

Fig 33 A cylindrical roller with end smear-ing caused by heavy axial loading andimproper lubrication

Fig 34 Guide flange smearing attributableto the same causes as the smearing shownin fig 33

32 33 34

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Smearing of rollers and racewaysIn certain circumstances, smearingmay occur on the surface of rollers andin raceways of spherical and cylindricalroller bearings. This is caused by rollerrotation being retarded in the unloadedzone, where the rollers are not drivenby the rings. Consequently their speedof rotation is lower than when they arein the loaded zone. The rollers are therefore subjected to rapid accelera-tion and the resultant sliding is so se-vere that in may produce smearing.

Fig 35 Skid smearing in both raceways ofa spherical roller bearing outer ring

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AppearanceScored and discoloured areas at thestart of the load zone in racewaysand on the surface of the rollers.

CauseRoller acceleration on entry into theloaded zone.

ActionMore suitable lubricant.Reduce bearing internal clearance.

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Raceway smearing at intervals corresponding to the roller spacingFar too often, when cylindrical rollerbearings are being mounted, the ringwith the roller and cage assembly isentered askew, without being rotated.The rollers then scratch the raceway ofthe other ring, causing smearing in theform of long, transverse streaks. Therollers may be smeared too. This typeof damage can be avoided if the bear-ing is well lubricated and one of therings is rotated. When large numbersof bearings are to be mounted it isexpedient to employ a mounting ring,see fig 36. Similar damage may arise ifthe bearing rings are mounted with fits

that are too tight in relation to the in-ternal clearance, so that preloadingoccurs.

Smear streaks may also be found inthe raceways of spherical and taperroller bearings. These streaks are theresult of careless handling or incorrectmounting practice. Blows or heavypressure applied to the wrong ring, without rotating the bearing, cause therollers to produce narrow, transversestreaks of smearing in the raceways,see fig 38.

Fig 37 A cylindrical roller bearing withsmear streaks in the inner ring raceway and on the rollers. The smearing has beencaused by the roller assembly being entered askew without being rotated

Fig 36 Mounting ring

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AppearanceTransverse smear streaks – spacedat intervals equal to the distancebetween the rollers – in the race-ways of cylindrical roller bearings.

Transverse smear streaks – spacedat intervals equal to the distancebetween the rollers – in the race-ways of spherical and taper rollerbearings.

CauseDuring the mounting operation, the ringwith the roller and cage assembly hasbeen entered askew on the other ring.

Blows applied to the wrong ring orheavy preloading without rotating thebearing.

ActionRotate the inner or outer ring duringentry. Lubricate the surfaces well.Use a mounting ring when fitting aseries of bearings.

Rotate the bearing when it is beingadjusted. Apply the mounting forceagainst the ring with the tightest fit;never allow the force to passthrough the rolling elements.

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Fig 38 Outer ring raceway of a sphericalroller bearing with smear streaks caused bya blow against the inner ring

Fig 39 One of the smear streaks shown infig 38 – 50 × magnification

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ActionSelect heavier interference fits.

CauseRing rotation relative to shaft or housing.

AppearanceScored and discoloured ring bore oroutside surface or faces.

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Smearing of external surfacesSmearing may occur on the externalsurfaces of heavily loaded bearings.Here, the smearing is the result ofmovement of the bearing ring relativeto its shaft or housing. Smearing of theinner ring bore, outer ring outside sur-face and ring faces can only be avoid-ed if the fits are tight enough to pre-vent movement of the ring concernedin relation to its seating. Increasing theaxial compression does not result inany improvement.

Fig 40 Smeared face of a cylindrical rollerbearing inner ring

Fig 41 Smeared outside surface of a spher-ical roller bearing outer ring. Material transferhas occurred from housing bore to bearingring

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AppearanceDiagonal smear streaks in the race-ways.

CauseLoading too light in relation to speed ofrotation.

ActionPreload the bearing by usingsprings.

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Smearing in thrust ball bearingsSmearing may occur in the racewaysof thrust ball bearings if the rotationalspeed is too high in relation to the loading. The centrifugal force then im-pels the balls to the outer part of theshallow raceways. There the balls donot roll satisfactorily and a great dealof sliding occurs at the ball-to-racewaycontacts. This leads to the formation of diagonal smear streaks in the outerpart of the raceway. In the case ofthrust ball bearings operating underlight loads and at high speeds, suchdamage can be prevented by subject-ing the bearings to extra loading, forinstance by applying springs, see fig

43. Details of how to calculate theminimum required axial loads aregiven in the SKF General Catalogue.

Fig 43 Preloading thrust ball bearings bymeans of springs

Fig 42 Thrust ball bearing raceway withsmear streaks on account of the rotationalspeed having been too high in relation tothe load

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ActionImprove lubrication.

CauseInadequate or improper lubrication.

AppearanceInitially the damage is not visible tothe naked eye. A more advancedstage is marked by small, shallowcraters with crystalline fracture sur-faces.

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Surface distressIf the lubricant film between racewaysand rolling elements becomes too thin,the peaks of the surface asperities willmomentarily come into contact witheach other. Small cracks then form inthe surfaces and this is known as sur-face distress. These cracks must notbe confused with the fatigue cracksthat originate beneath the surface andlead to flaking. The surface distresscracks are microscopically small andincrease very gradually to such a sizethat they interfere with the smooth run-ning of the bearing. These cracks may,however, hasten the formation of

sub-surface fatigue cracks and thusshorten the life of the bearing

If the lubrication remains satisfactorythroughout, i.e. the lubricant film doesnot become too thin because of lubric-ant starvation or viscosity changesinduced by the rising temperature oron account of excessive loading, thereis no risk of surface distress.

Fig 44 Surface distress in the form of aband encircling a roller from a spherical roller bearing

Fig 45 The surface distress depicted in fig45 – 100 × magnification

4544

AppearanceGreyish black streaks across theraceways, mostly coinciding with therolling element spacing. At a laterstage, pitting of raceways and othersurfaces of the bearing.

CausePresence of water, moisture or corros-ive substances in the bearing over along period of time.

ActionImprove sealing.Use lubricant with better rust-inhibiting properties.

24

Fig 46 Deep seated rust in the outer ring ofa cylindrical roller bearing

Fig 47 Extensive water etching on theinner ring of a spherical roller bearing

46 47

CorrosionRust will form if water or corrosiveagents reach the inside of the bearingin such quantities that the lubricantcannot provide protection for the steelsurfaces. This process will soon leadto deep seated rust. Another type ofcorrosion is fretting corrosion.

Deep seated rustA thin protective oxide film forms onclean steel surfaces exposed to air.However, this film is not impenetrableand if water or corrosive elementsmake contact with the steel surfaces,patches of etching will form. This de-velopment soon leads to deep seatedrust.

Deep seated rust is a great dangerto bearings since it can initiate flakingand cracks. Acid liquids corrode thesteel quickly, while alkaline solutionsare less dangerous. The salts that arepresent in fresh water constitute, to-gether with the water, an electrolytewhich causes galvanic corrosion,

known as water etching. Salt water,such as sea water, is therefore highlydangerous to bearings.


25

Fig 48 Fretting corrosion on the outer ringof a spherical roller bearing

Fig 49 Extensive fretting corrosion in thebore of a self-aligning ball bearing

48 49

Fretting corrosionIf the thin oxide film is penetrated, ox-idation will proceed deeper into thematerial. An instance of this is the cor-rosion that occurs when there is relat-ive movement between bearing ringand shaft or housing, on account of thefit being too loose. This type of dam-age is called fretting corrosion and maybe relatively deep in places. The relat-ive movement may also cause smallparticles of material to become de-tached from the surface. These par-tic-les oxidise quickly when exposed tothe oxygen in the atmosphere.

As a result of the fretting corrosion,the bearing rings may not be evenly

supported and this has a detrimentaleffect on the load distribution in thebearings. Rusted areas also act asfracture notches.

AppearanceAreas of rust on the outside surfaceof the outer ring or in the bore of theinner ring. Raceway path patternheavily marked at correspondingpositions.

CauseFit too loose.Shaft or housing seating with errors ofform.

ActionAdjust seatings.

ActionRe-route the current to by-pass thebearing.Use insulated bearings.

Re-route the current to by-pass thebearing. When welding, arrangeearthing to prevent current passingthrough the bearing.Use insulated bearings.

CausePassage of electric current throughrotating bearing.

Passage of electric current throughnon-rotating bearing.

AppearanceDark brown or greyish black fluting(corrugation) or craters in racewaysand rollers. Balls have dark disco-louration only. Sometimes zigzagburns in ball bearings raceways.

Localised burns in raceways and onrolling elements.

26

Fig 50 Fluting, caused by the passage ofelectric current, in the outer ring of a spher-ical roller bearing

Fig 51 The outer ring of a self-aligning ballbearing damaged by electric current

50 51

Damage caused bythe passage of electric currentWhen an electric current passesthrough a bearing, i.e. proceeds fromone ring to the other via the rolling ele-ments, damage will occur. At the con-tact surfaces the process is similar toelectric arc welding.

The material is heated to temperat-ures ranging from tempering to meltinglevels. This leads to the appearance ofdiscoloured areas, varying in size,where the material has been tem-pered, re-hardened or melted. Small

craters also form where the metal hasmelted.

The passage of electric current fre-quently leads to the formation of fluting(corrugation) in bearing raceways.Rollers are also subject to fluting, whilethere is only dark discolouration ofballs

It can be difficult to distinguish be-tween electric current damage and vib-ration damage. A feature of the flut-ingcaused by electric current is the darkbottom of the corrugations, as oppo-sed to the bright or rusty appear-anceat the bottom of the vibration-inducedfluting. Another distinguishing featureis the lack of damage to the rolling ele-

ments of bearings with raceway flutingcaused by vibrations.

Both alternating and direct currentscause damage to bearings. Even lowamperage currents are dangerous.Non-rotating bearings are much moreresistant to electric current damagethan bearings in rotation. The extent ofthe damage depends on a number offactors: current intensity, duration,bearing load, speed and lubricant.

The only way of avoiding damage ofthis nature is to prevent any electriccurrent from passing through the bearing.


27

53

52

54

Fig 52 Deep groove ball bearing withelectric current damage in zigzag pattern. Itis assumed that burns of this configurationarise when the momentary passage of highamperage current is accompanied by axialvibration

Fig 53 A railway axlebox bearing damagedby the passage of high amperage currentwhile the bearing was not running

Fig 54 Roller of a railway axlebox bearingdamaged by electric current (same bearingas in fig 53)

28

Flaking (spalling)Flaking occurs as a result of normalfatigue, i.e. the bearing has reachedthe end of its normal life span. How-ever, this is not the commonest causeof bearing failure. The flaking detectedin bearings can generally be attributedto other factors. If the flaking is dis-covered at an early stage, when thedamage is not too extensive, it is fre-quently possible to diagnose its causeand take the requisite action to preventa recurrence of the trouble. The pathpattern of the bearing may prove to beuseful, see page 4.

When flaking has proceeded to acertain stage, it makes its presenceknown in the form of noise and vibra-tions, which serve as a warning that itis time to change the bearing.

The causes of premature flakingmay be heavier external loading thanhad been anticipated, preloading onaccount of incorrect fits or excessivedrive-up on a tapered seating, oval dis-torsion owing to shaft or housing seat-ing out-of-roundness, axial compres-sion, for instance as a result of ther-mal expansion. Flaking may also becaused by other types of damage, suchas indentations, deep seated rust,electric current damage or smearing.

Fig 55 Flaked cone and rollers of taper roller bearing. Heavy loading and inad-equate lubrication are the causes of thisdamage

55


AppearanceHeavily marked path pattern inraceways of both rings.

Flaking usually in the most heavilyloaded zone.

CausePreloading on account of fits being tootight.

Excessive drive-up on a tapered seating.

Single row angular contact ball bear-ings or taper roller bearings adjustedto give excessive preload.

Temperature differential between innerand outer rings too great.

ActionAlter the fits or select bearing withlarger internal clearance.

Do not drive the bearing so far upits tapered seating. Follow carefullythe instructions given by SKF.

Re-adjust the bearings to obtainlighter preload.

Select bearing with larger internalclearance.

29

Flaking caused by preloading

Fig 56 Outer ring of a self-aligning ballbearing that has been driven too far up itstapered seating. The bearing had only per-formed a few revolutions when the damagewas detected. Flaking would soon haveoccurred if the bearing had been put intoservice

56

30

Flaking caused by oval compression

AppearanceHeavily marked path pattern at twodiametrically opposed sections ofeither bearing ring. Flaking in these sections.

CauseOval shaft or oval housing seating.The latter is a common defect in splithousings and machine frames.

The bore of plummer blocks mountedon an uneven base becomes ovalwhen the base bolts are tightened.

ActionUsually it is necessary to manufac-ture a new shaft or a new housingto remedy this defect. One expedi-ent is to spray metal on the compon-ents and then regrind. If it is a mat-ter of an oval shaft with the bearingmounted on a sleeve, it is possibleto adjust the shaft by grinding, incertain cases.

Adjust the base.


Fig 57 Flaking in the outer ring outer ringof spherical roller bearing that has beenmounted in a housing with oval bore

57

AppearanceDeep groove ball bearings: heavilymarked path pattern displaced toone side of both rings.Self-aligning ball bearings andspherical roller bearings: severelymarked raceway path patterns fromone row of rolling elements.Flaking in these areas.Single row angular contact ball bearings and taper roller bearings:same appearance as damage resulting from preloading, see page 29.

CauseIncorrect mounting, which results inaxial loading, e.g. excessive preload-ing of angular contact ball bearingsand taper roller bearings.

The non-locating bearing has jammed.

Axial freedom of movement has notbeen sufficient to accommodate thethermal expansion.

ActionCheck adjustment when mountingthe bearings.

Check the fit and lubricate the sur-faces.

If temperature differential betweenshaft and housing cannot be re-duced, provide greater freedom ofmovement.

31

Fig 58 Outer ring of a self-aligning ballbearing subjected to excessive axial loading. Flaking in the load zone

Fig 59 Flaked inner ring of a spherical roller bearing. The extent of the flakingaround one entire raceway indicates thatthe axial load has been very heavy in rela-tion to the radial load

58 59

Flaking caused by axial compression

32

Flaking caused by misalignment

Fig 61 Cylindrical roller bearing inner ringwith flaking at one side of the raceway, as aresult of overloading due to misalignment

Fig 60 Deep groove ball bearing outer ringthat has been out of alignment with theshaft. The ball path has an oval configura-tion on account of the misalignment. Theresult is the same as with oval compression,see page 31

60 61


AppearanceDeep groove ball bearings: diagonalpath pattern, severely marked attwo diametrically opposed sections.Cylindrical roller bearings: flaking atthe edge of the raceway.

CauseBearing seatings out of alignment.

Bearing mounted on the skew.

ActionRectify the seatings.

Use mounting sleeve with parallelfaces.

CauseIndentations resulting from faultymounting practice or overloading of thenon-rotating bearing, see page 28.

Indentations made by foreign particles,see page 16.

AppearanceFlaking in conjunction with indenta-tions coinciding with the rolling ele-ment spacing.

Flaking in conjunction with smallindentations.

33

Flaking caused by indentations

Fig 62 Various stages of flaking in theinner ring of a deep groove ball bearing.The ring has been mounted with interfer-ence fit on the shaft and blows have beendirected against the outer ring, causing themounting force to pass through the balls,which have produced the indentations

Fig 63 Flaking (the dark area) initiated bythe adjacent indentations – 100 × magnifi-cation

6362

34

Fig 64 Inner ring of a cylindrical roller bearing with extensive flaking caused bysmearing of the kind depicted in fig 65

Fig 65 Inner ring of a cylindrical roller bearing with smearing, at intervals corre-sponding to the roller spacing, caused byincorrect mounting

64 65

Flaking caused by smearing

AppearanceFlaking at the start of the load zonein raceways of roller bearings.

Flaking, coinciding with the rollerspacing, in raceways of roller bearings.

CauseSkid smearing, see page 18.

Transverse smearing resulting fromfaulty mounting practice, see page 19.


35

Flaking caused by deep seated rust

AppearanceFlaking originating from rust dama-ge.

CauseDeep seated rust, see page 24.

Fig 67 Cross section through the rollerdepicted in fig 66, showing the way in which the fatigue crack extends beneath thesurface. The fully developed flaking origin-ated in the same manner

Fig 68 Magnification of the damage shownin fig 66

6867

66Fig 66 Flaking originating from deep seated rust on the roller of a spherical roller bearing

36

Flaking caused by fretting corrosion

AppearanceFlaking in the raceway of either theinner or outer ring. Corroded area atcorresponding part of the inner boreof outside surface.

CauseFretting corrosion, see page 25.


Fig 69 Flaking in the raceways of the outerring of a spherical roller bearing. Correspond-ing area of advanced fretting corrosion onthe outside surface (for this photograph thering has been placed in front of a mirror).Development of the corrosion has beenaccompanied by an increase in volume thathas led to deformation of the bearing ringand localised overloading. The results havebeen premature fatigue and flaking

69

AppearanceFlaking in conjunction with bright orcorroded fluting or craters.

Flaking in conjunction with dark-coloured or burnt fluting or craters.

CauseWear resulting from vibrations whilethe bearing was not running, see page 12.

Electric current damage, see page 26.

37

Flaking caused by fluting or craters

Fig 70 The outer ring of a self-aligning ballbearing with flaking caused by craters thathave formed in conjunction with the pass-age of electric current. It is evident that theflaking has originated at the craters, fromwhere it has spread out in both directions.Detached fragments of flaked material havein turn caused indentations and further flaking

Fig 71 Flaking in both raceways of theinner ring of a spherical roller bearing. Theflaking has originated from the vibrationmarkings

71

70

38

CracksCracks may form in bearing rings forvarious reasons. The most commoncause is rough treatment when thebearings are being mounted or dis-mounted. Hammer blows, applieddirect against the ring or via a hard-ened chisel, may cause fine cracks toform, with the result that pieces of thering break off when the bearing is putinto service. Excessive drive up on atapered seating or sleeve is anothercause of ring cracking. The tensilestresses, arising in the rings as a resultof the excessive drive-up, producecracks when the bearing is put intooperation. The same result may beobtained when bearings are heatedand then mounted on shafts manufac-tured to the wrong tolerances.

The smearing described in an earliersection may also produce cracks atright angles to the direction of slide.Cracks of this kind produce fracturesright across the rings.

Flaking, that has occurred for somereason or other, acts as a fracturenotch and may lead to cracking of thebearing ring. The same applies to fret-ting corrosion.

Fig 72 Fractured outer ring of a self-align-ing ball bearing. The indentations visible atthe bottom edge of the ring were caused byrough treatment and the crack originated atone of these indentations

72


AppearanceCracks or pieces broken off, gener-ally at one face of the bearing ring.

ActionAlways use a soft drift or mountingsleeve. Never subject the bearing todirect hits.

CauseBlows, with hammer or hardened chisel, have been directed against thering when the bearing was beingmounted.

39

Fig 73 Cracked inner ring of a spherical roller bearing. One roller has been removedto allow the raceway of the left-hand of thephotograph to be examined. The roller hasthen been hammered back in place, causingpart of the centre flange to break away. Theimpacts have been transmitted via a roller inthe other row and part of the outer flangehas broken off too. At the same time the ringhas cracked right through

Fig 74 Inner ring of a spherical roller bear-ing with outer flange fracture produced bydirect hammering

73 74

Cracks caused by rough treatment

AppearanceThe bearing ring has cracked rightthrough and has lost its grip on theshaft.

CauseExcessive drive-up on a tapered seating or sleeve.

Interference fit on cylindrical seatingtoo heavy.

ActionFollow carefully the SKF mountinginstruction concerning bearings ontapered seatings.

Alter the fit.

40

Cracks caused by excessive drive-up

Fig 76 Fractured surface of the inner ring infig 75

Fig 75 Section of the inner ring of a spher-ical roller bearing – 3,5 × magnification. Thering has cracked because of excessive drive-up. The fracture originated at the dark areaby the bore chamfer

75 76


41

Cracks caused be smearing

AppearanceCrack or cracks in conjunction withsmearing of the bearing ring. Thering may have cracked right across.Smearing cracks generally formacross the smearing.

CauseSmearing, see page 17.

Fig 77 Spherical roller bearing inner ringthat has cracked right across following smearing of one face. The ring has beenmounted to abut a spacer that has not hada sufficiently tight fit on the shaft. Con-sequently the spacer has rotated relative tothe shaft and the bearing ring

Fig 78 Smearing damage on the face of abearing ring. Note the incipient transversecracks

7877

42

Fig 79 Spherical roller bearing inner ringwith fretting corrosion and a transversecrack right through the ring. The fretting cor-rosion has caused the cracking

Fig 80 Longitudinal crack in a deep grooveball bearing outer ring with fretting corrosion

79 80

Cracks caused by fretting corrosion

AppearanceCracks, transverse in inner ringsand generally longitudinal in outerrings, in conjunction with frettingcorrosion.

CauseFretting corrosion, see page 25.


43

Cage damageIf, on examination of a failed bearing,the cage is found to be damaged, itmay in many cases prove difficult toascertain the cause. Usually othercomponents of the bearing are dam-aged too and this makes it even moredifficult to discover the reason for thetrouble. However, there are certainmain causes of cage failure, viz. vibra-tion, excessive speed, wear and block-age.

VibrationWhen a bearing is exposed to vibra-tion, the forces of inertia may be sogreat as to cause fatigue cracks toform in the cage material after a time.Sooner or later these cracks lead tocage fracture.

Excessive speedIf the bearing is run at speeds inexcess of that for which the cage isdesigned, the cage is subjected toheavy forces of inertia that may lead tofractures. Frequently, where very highspeeds are involved, it is possible toselect bearings with cages of specialdesign.

WearCage wear may be caused by inad-equate lubrication or by abrasive par-

ticles. The idea with rolling bearings is of course to avoid sliding friction.However, where the cage is con-cerned, sliding cannot be eliminated inthe contacts with the other componentsof the bearing. This explains why thecage is the first component to be affec-ted when the lubrication becomes in-adequate. The cage is always made ofsofter material than the other compon-ents of the bearing and consequently itwears comparatively quickly. As thecage pockets increase in size, due towear, the rolling element guidancedeteriorates and this also applies tothe cage in cases where the cage iscentred on the rolling elements. Theresultant forces may lead to cage fail-ure within a short space of time.

BlockageFragments of flaked material or otherhard particles may become wedgedbetween the cage and a rolling ele-ment, preventing the latter from ro-tating round its own axis. This leads tocage failure.

Other causes of cage damageIf the rings of a deep groove ball bear-ing are fitted out of alignment witheach other, the path of the balls has anoval configuration. If the cage iscentred on the balls, it has to changeshape for every revolution it performs.

Fig 81 Fractured surface of the cageshown in fig 82. The fatigue cracks are clearly visible

Fig 82 Cage of a spherical roller bearing.Fatigue cracks have formed in the fillets

8281

Fatigue cracks then form in the mater-ial and sooner or later they lead tofractures.

There is a similar case when a thrustball bearing is fitted together with radialplain bearings. If clearance arises inthe plain bearings, the washers of thethrust bearing become displaced inrelation to each other. Then the ballsdo not follow their normal path andheavy stresses may arise in the cage.

Cages in bearings subject to severeacceleration and retardation, in conjun-ction with fluctuations in speed, areaffected by forces of inertia. Thesegive rise to considerable pressure be-tween the contacting surfaces, withconsequent heavy wear.

© Copyright SKF 1994Every care has been taken to ensure theaccuracy of the information contained inthis publication but no liability can beaccepted for any errors or omissions.

Printed in Sweden by Palmeblads Tryckeri AB

Publication PI 401 E

Reg. 770 · 15 000 · 1994-04

Documents

Skf Bearing Failure and Causes