Bearing failure

Product information 401Bearing failures and their causes

ContentsIntroduction ....................................................................................................... 3 Bearing failures and their causes .................................................................. 3 How is bearing life defined? .......................................................................... 3Path patterns and their interpretation................................................................ 4Different types of bearing damage .................................................................... 9 Wear.............................................................................................................. 10 Wear caused by abrasive particles ............................................................ 10 Wear caused by inadequate lubrication..................................................... 11 Wear caused by vibration .......................................................................... 12 Indentations................................................................................................... 14 Indentations caused by faulty mounting or overloading............................. 14 Indentations caused by foreign particles.................................................... 16 Smearing ....................................................................................................... 17 Smearing of roller ends and guide flanges ................................................ 17 Smearing of rollers and raceways.............................................................. 18 Raceway smearing at intervals corresponding to the roller spacing.......... 19 Smearing of external surfaces ................................................................... 21 Smearing in thrust ball bearings ................................................................ 22 Surface distress ............................................................................................ 23 Corrosion....................................................................................................... 24 Deep seated rust........................................................................................ 24 Fretting corrosion ....................................................................................... 25 Damage caused by the passage of electric current ...................................... 26 Flaking (spalling) ........................................................................................... 28 Flaking caused by preloading .................................................................... 29 Flaking caused by oval compression ......................................................... 30 Flaking caused by axial compression ........................................................ 31 Flaking caused by misalignment................................................................ 32 Flaking caused by indentations.................................................................. 33 Flaking caused by smearing ...................................................................... 34 Flaking caused by deep seated rust .......................................................... 35 Flaking caused by fretting corrosion .......................................................... 36 Flaking caused by fluting or craters ........................................................... 37 Cracks ........................................................................................................... 38 Cracks caused by rough treatment ............................................................ 39 Cracks caused by excessive drive-up........................................................ 40 Cracks caused by smearing....................................................................... 41 Cracks caused by fretting corrison............................................................. 42 Cage damage................................................................................................ 43 Vibration..................................................................................................... 43 Excessive speed ........................................................................................ 43 Wear .......................................................................................................... 43 Blockage .................................................................................................... 43 Other causes of cage damage................................................................... 43

IntroductionBearing failures and magnitude of the load. Fatigue is the The life of a rolling bearing is de- result of shear stresses cyclically fined as the number of revolutions thetheir causes appearing immediately below the load bearing can perform before incipientBearings are among the most import- carrying surface. After a time these flaking occurs. This does not mean toant components in the vast majority of stresses cause cracks which gradually say that the bearing cannot be usedmachines and exacting demands are extend up to the surface. As the rolling after then. Flaking is a relatively long,made upon their carrying capacity and elements pass over the cracks frag- drawn-out process and makes its pres-reliability. Therefore it is quite natural ments of material break away and this ence known by increasing noise andthat rolling bearings should have come is known as flaking or spalling. The vibration levels in the bearing. There-to play such a prominent part and that flaking progressively increases in ex- fore, as a rule, there is plenty of time toover the years they have been the tent (figs 1 to 4) and eventually makes prepare for a change of bearing.subject of extensive research. Indeed the bearing unserviceable.rolling bearing technology has de-veloped into a particular branch ofscience. SKF has been well to the fore-front right from the start and has longled this field Figs 1–4 Progressive stages of flaking 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 own 1 2special imprint on the bearing. Con-sequently, by examining a damagedbearing, it is possible, in the majorityof 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 revolutions 3 4performed by the bearing and the 3

Path patterns andtheir interpretationWhen a rolling bearing rotates under which the bearing has operated. By the appearance and location of theload the contacting surfaces of the roll- learning to distinguish between normal patterns prove to be useful aids in dia-ing elements and the raceways norm- and abnormal path patterns there is gnosing the cause of the damage.ally become somewhat dull in appear- every prospect of being able to assess Deep groove ball bearings andance. This is no indication of wear in correctly whether the bearing has run thrust ball bearings have been used forthe usual sense of the word and is of under the proper conditions. illustrative purposes as they displayno significance to the bearing life. The The following series of figures illus- such characteristic path patterns.dull surface in an inner or outer ring trates normal path patterns under diffe- However, the figures are applicable,raceway forms a pattern called, for the rent rotational and loading conditions with some modifications, to other typespurposes of this paper, the path pat- (figs 5 to 11) as well as typical patterns of bearing as well.tern. This pattern varies in appearance resulting from abnormal working condi-according to the rotational and loading tions (figs 12 to 18).conditions. By examining the path pat- In the majority of cases the damageterns in a dismantled bearing that has to the bearing originates within thebeen in service, it is possible to gain a confines of the path patterns and, once @@good idea of the conditions under their significance has been learned, @@ Fig 5 Uni-directional radial load. Rotating inner ring – fixed outer ring. Inner ring: path pattern uniform in width, @@ positioned in the centre and extended around the entire circumference of the raceway Outer ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly 5 less than half the circumference of the race- @@ @@ way @@ Fig 6 Uni-directional radial load. Fixed inner ring – rotating outer ring. Inner ring: path pattern widest in the load direction and tapered off towards the ends. With normal fits and normal internal clearance, the pattern extends around slightly less than half the circumference of the raceway Outer ring: path pattern uniform in width, positioned in the centre and extended 6 around the entire circumference of the race- way4

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-way @@ @ @@ @Outer ring: path pattern uniform in width, @@positioned in the centre and extendedaround the entire circumference of the race- 7way @@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- 8way @@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 9 @@ 5

Path patterns and their interpretation @@@@ @@@@ @@@@ 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 circumference of the raceway and laterally displaced Outer ring: path pattern extended around the entire circumference of the raceway and 10 laterally displaced. The pattern is widest in the direction of the radial loading@@@ @@@@@@ @@ @@@ 11 Fig 11 Uni-directional axial load. Rotating shaft washer – fixed housing washer. Shaft and housing washers: path pattern uniform in width, extended around the entire circumference of the raceways of both washers @@ @ 12 Fig 12 Uni-directional radial load + imbalance. Rotating inner ring – creeping outer ring. Inner and outer rings: path pattern uniform in width, extended around the entire circumference of the raceways of both rings6

@@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 extended @@ @ @@around 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 is 13widest in the direction of the radial loadingFig 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 14pinching has occurredFig 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 15 @@ @ 7

Path patterns and their interpretation @@ @@ @ Fig 16 Inner ring misaligned. Rotating inner ring – fixed outer ring. Inner ring: path pattern in two diametrically opposed sections, displaced diagonally in relation to each other@@@ @@ Outer ring: path pattern widest in the load direction and tapered off toward the ends. The internal clearance is reduced on account of the misalignment of the inner ring; the length of the path pattern depends 16 upon the magnitude of the internal clearance reduction@@@ @@@@@ @@@ @@ Fig 17 Housing washer positioned eccent- rically relative to shaft washer. Rotating@@@ @@@ shaft washer – fixed housing washer. Shaft washer: path pattern uniform in width, extended around the entire circumference of the raceway Housing washer: path pattern extended 17 around the entire circumference of the raceway and off-centre relative to raceway@@@ @@@@@@ 18 Fig 18 Housing washer misaligned. Rotating shaft washer – fixed housing washer. Shaft washer: path pattern uniform in width, extended round the entire circumference of the raceway Housing washer: path pattern in the centre of the raceway but wider around part of its circumference8

Different types of bearingdamageEach 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. Afailed bearing frequently displays acombination of primary and secondarydamage. The types of damage may be classi-fied as follows:Primary damageWearIdentationsSmearingSurface distressCorrosionElectric current damageSecondary damageFlakingCracks 9

Different types of bearing damageWear Wear caused by abrasive particles Small, abrasive particles, such as grit the surfaces become worn to such an extent as to render the bearing unser-In normal cases there is no appre- or swarf that have entered the bearing viceable. However, it is not necessaryciable wear in rolling bearings. Wear by some means or other, cause wear to scrap bearings that are only slightlymay, however, occur as a result of the of raceways, rolling elements and worn. They can be used again afteringress of foreign particles into the cage. The surfaces become dull to a cleaning.bearing or when the lubrication is degree that varies according to the The abrasive particles may haveunsatisfactory. Vibration in bearings coarseness and nature of the abrasive entered the bearing because the seal-which are not running also gives rise to particles. Sometimes worn particles ing arrangement was not sufficientlywear. from brass cages become verdigrised effective for the operating conditions and then give light-coloured grease a involved. They may also have entered greenish hue. with contaminated lubricant or during The quantity of abrasive particles the mounting operation. gradually increases as material is worn away from the running surfaces and cage. Therefore the wear becomes an accelerating process and in the end Appearance Cause Action Small indentations around the race- Lack of cleanliness before and during Do not unpack bearing until just ways and rolling elements. Dull, mounting operation. before it is to be mounted. Keep worn surfaces. workshop clean and use clean tools. Grease discoloured green. Ineffective seals. Check and possibly improve the sealing. Lubricant contaminated by worn par- Always use fresh, clean lubricant. ticles from brass cage. Wipe the grease nipples. Filter the oil. Fig 19 Outer ring of a spherical roller bearing with raceways that have been worn by abrasive particles. It is easy to feel where 19 the dividing line goes between worn and unworn sections10

Wear caused by inadequate mirror-like finish. At this stage surfacelubrication distress can also arise, see page 23.If there is not sufficient lubricant, or if If the lubricant is completely usedthe lubricant has lost its lubricating up, the temperature will rise rapidly.properties, it is not possible for an oil The hardened material then softensfilm with sufficent carrying capacity to and the surfaces take on blue to brownform. Metal to metal contact occurs hues. The temperature may evenbetween rolling elements and race- become so high as to cause the bear-ways. In its initial phase, the resultant ing to seize.wear 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 Appearance Cause Action Worn, frequently mirror-like, sur- Lubricant has gradually been used up Check that the lubricant reaches the faces; at a later stage blue to or has lost its lubricating properties. bearing. brown discolouration. More frequent relubrication.Fig 21 Outer ring of a spherical roller bear-ing that has not been adequately lubricated.The raceways have a mirror finishFig 20 Cylindrical roller with mirror-like sur-face on account of lubricant starvation 20 21 11

Different types of bearing damageWear caused by vibration In many cases, it is possible to age than ball bearings. This is consid-When a bearing is not running, there is discern red rust at the bottom of the ered to be because the balls can roll inno lubricant film between the rolling depressions. This is caused by oxida- every direction. Rollers, on the otherelements and the raceways. The tion of the detached particles, which hand, only roll in one direction; move-absence of lubricant film gives metal to have a large area in relation to their ment in the remaining directions takesmetal contact and the vibrations pro- volume, as a result of their exposure to the form of sliding. Cylindrical rollerduce small relative movements of roll- air. There is never any visible damage bearings are the most susceptible.ing elements and rings. As a result of to the rolling elements. The fluting resulting from vibrationsthese movements, small particles The greater the energy of vibration, sometimes closely resembles the flut-break away from the surfaces and this the more severe the damage. The ing produced by the passage ofleads to the formation of depressions period of time and the magnitude of electric current. However, in the latterin the raceways. This damage is the bearing internal clearance also case the bottom of the depression isknown as false brinelling, sometimes influence developments, but the fre- dark in colour, not bright or corroded.also referred to as washboarding. Balls quency of the vibrations does not ap- The damage caused by electric currentproduce sphered cavities while rollers pear to have any significant effect. is also distinguishable by the fact thatproduce fluting. Roller bearings have proved to be the rolling elements are marked as more susceptible to this type of dam- well as the raceways. Appearance Cause Action Depressions in the raceways. These The bearing has been exposed to vib- Secure the bearing during transport depressions are rectangular in roller ration while stationary. by radial preloading. bearings and circular in ball bear- Provide a vibration-damping base. ings. The bottom of these depres- Where possible, use ball bearings sions may be bright or dull and oxi- instead of roller bearings. dised. Employ oil bath lubrication, where possible. Bearings with vibration damage are usually found in machines that are not in operation and are situated close to machinery producing vibrations. Examples that can be cited are trans- former fans, stand-by generators and ships’ auxiliary machinery. Bearings in machines transported by rail, road or sea may be subject to vibration damage too. Fig 22 Outer ring of taper roller bearing damaged by vibration during operation Fig 23 Vibration damage to the ring of cylinder roller bearing. The damage has arisen while the bearing was not running. It is evident, from the fainter fluting discern- ible between the pronounced depressions 22 23 with corrosion at the bottom, that the ring has changed position for short periods12

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.Fig 24 Inner and outer ring of a cylindricalroller bearing exposed to vibration. Theinner ring has changed positionFig 25 Spring loading a deep groove ballbearing to prevent vibration damageFig 26 Outer ring of a self-aligning ballbearing damaged by vibration. The bearing 24has not rotated at all 25 26 13

Different types of bearing damageIndentations Indentations caused by faulty mounting or overloading are unduly preloaded without being rotated.Raceways and rolling elements may The distance between the dents is the Bearings that are mounted withbecome dented if the mounting pres- same as the rolling element spacing. excessively heavy interference fits,sure is applied to the wrong ring, so Ball bearings are prone to indentations and bearings with tapered bore thatthat it passes through the rolling ele- if the pressure is applied in such a way are driven too far up the shaft seatingments, or if the bearing is subjected to that it passes through the balls during or sleeve, also become dented.abnormal loading while not running. the mounting or dismounting opera-Foreign particles in the bearing also tions. Self-aligning ball bearings arecause indentations. particularly susceptible to damage in such circumstances. In spherical roller bearings the damage originates as smearing (see page 17) and sub- sequently, if the pressure increases, develops into a dent. The same conditions apply in taper roller bearings that Appearance Cause Action Indentations in the raceways of both Mounting pressure applied to the Apply the mounting pressure to the rings with spacing equal to the wrong ring. ring with the interference fit. distance between the rolling elements. Excessively hard drive-up on tapered Follow carefully the SKF instructions seating. concerning mounting bearings on tapered seating. Overloading while not running. Avoid overloading or use bearings with higher basic static load ratings. Fig 27 Washer of a thrust ball bearing subjected to overloading while not running. The indentations are narrow and radially 27 aligned, not sphered as in radial ball bear- ings14

28Figs 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, 29 30dented the inner ring raceway (fig 30) 15

Different types of bearing damageIndentations caused by foreignparticlesForeign particles, such as swarf andburrs, which have gained entry into thebearing cause indentations whenrolled 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. Appearance Cause Action Small indentations distributed Ingress of foreign particles into the Cleanliness to be observed during around the raceways of both rings bearing. the mounting operation. and in the rolling elements. Uncontaminated lubricant. Improved seals. Fig 31 Indentations, caused by dirt, in one 31 of the raceways of a roller bearing – 50 × magnification16

Smearing when the rollers are subjected to severe acceleration on their entry into the ends of the rollers. This smearing is attributable to insufficient lubricantWhen two inadequately lubricated sur- the load zone. If the bearing rings ro- between flanges and rollers. It occursfaces slide against each other under tate relative to the shaft or housing, when a heavy axial load acts in oneload, material is transferred from one this may also cause smearing in the direction over a long period, for instan-surface to the other. This is known as bore and on the outside surface and ce when taper roller bearings are sub-smearing and the surfaces concerned ring faces. ject to excessive preloading. In casesbecome scored, with a “torn” appear- In thrust ball bearings, smearing where the axial load changes direction,ance. When smearing occurs, the may occur if the load is too light in smearing is much less common as thematerial is generally heated to such relation to the speed of rotation. opportunity is provided for the ingresstemperatures that rehardening takes of lubricant when the roller end is tem-place. This produces localised stress Smearing of roller ends and guide porarily relieved of load. Such smear-concentrations that may cause crack- flanges ing can be avoided to a considerableing or flaking. In cylindrical and taper roller bearings, extent by selecting a suitable lubricant. In rolling bearings, sliding primarily and in spherical roller bearings withoccurs at the roller end-guide flange guide flanges, smearing may occur oninterfaces. Smearing may also arise the guiding faces of the flanges and Appearance Cause Action Scored and discoloured roller ends Sliding under heavy axial loading and More suitable lubricant. and flange faces. with inadequate lubrication.Fig 32 Smearing on the surface of a roller Fig 33 A cylindrical roller with end smear- Fig 34 Guide flange smearing attributablefrom a spherical roller bearing – 100 × mag- ing caused by heavy axial loading and to the same causes as the smearing shownnification improper lubrication in fig 33 32 33 34 17

Different types of bearing damageSmearing 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 aretherefore subjected to rapid accelera-tion and the resultant sliding is so se-vere that in may produce smearing. Appearance Cause Action Scored and discoloured areas at the Roller acceleration on entry into the More suitable lubricant. start of the load zone in raceways loaded zone. Reduce bearing internal clearance. and on the surface of the rollers. 35 Fig 35 Skid smearing in both raceways of a spherical roller bearing outer ring18

Raceway smearing at intervals that are too tight in relation to the in-corresponding to the roller spacing ternal clearance, so that preloadingFar too often, when cylindrical roller occurs.bearings are being mounted, the ring Smear streaks may also be found inwith the roller and cage assembly is the raceways of spherical and taperentered askew, without being rotated. roller bearings. These streaks are theThe rollers then scratch the raceway of result of careless handling or incorrectthe other ring, causing smearing in the mounting practice. Blows or heavyform of long, transverse streaks. The pressure applied to the wrong ring,rollers may be smeared too. This type without rotating the bearing, cause theof damage can be avoided if the bear- rollers to produce narrow, transverseing is well lubricated and one of the streaks of smearing in the raceways,rings is rotated. When large numbers see fig 38.of bearings are to be mounted it isexpedient to employ a mounting ring,see fig 36. Similar damage may arise if 36the bearing rings are mounted with fits Fig 36 Mounting ring Appearance Cause Action Transverse smear streaks – spaced During the mounting operation, the ring Rotate the inner or outer ring during at intervals equal to the distance with the roller and cage assembly has entry. Lubricate the surfaces well. between the rollers – in the race- been entered askew on the other ring. Use a mounting ring when fitting a ways of cylindrical roller bearings. series of bearings. Transverse smear streaks – spaced Blows applied to the wrong ring or Rotate the bearing when it is being at intervals equal to the distance heavy preloading without rotating the adjusted. Apply the mounting force between the rollers – in the race- bearing. against the ring with the tightest fit; ways of spherical and taper roller never allow the force to pass bearings. through the rolling elements.Fig 37 A cylindrical roller bearing withsmear streaks in the inner ring racewayand on the rollers. The smearing has beencaused by the roller assembly being 37entered askew without being rotated 19

Different types of bearing damage Fig 39 One of the smear streaks shown in fig 38 – 50 × magnificationFig 38 Outer ring raceway of a spherical 39roller bearing with smear streaks caused bya blow against the inner ring 3820

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. Appearance Cause Action Scored and discoloured ring bore or Ring rotation relative to shaft or Select heavier interference fits. outside surface or faces. housing.Fig 41 Smeared outside surface of a spher-ical roller bearing outer ring. Material transferhas occurred from housing bore to bearingringFig 40 Smeared face of a cylindrical rollerbearing inner ring 40 41 21

Different types of bearing damageSmearing in thrust ball bearings 43. Details of how to calculate theSmearing may occur in the raceways minimum required axial loads areof thrust ball bearings if the rotational given in the SKF General Catalogue.speed is too high in relation to theloading. 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 formationof 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 Appearance Cause Action Diagonal smear streaks in the race- Loading too light in relation to speed of Preload the bearing by using ways. rotation. springs. Fig 42 Thrust ball bearing raceway with smear streaks on account of the rotational speed having been too high in relation to the load Fig 43 Preloading thrust ball bearings by means of springs 42 4322

Surface distress sub-surface fatigue cracks and thus shorten the life of the bearingIf the lubricant film between raceways If the lubrication remains satisfactoryand rolling elements becomes too thin, throughout, i.e. the lubricant film doesthe peaks of the surface asperities will not become too thin because of lubric-momentarily come into contact with ant starvation or viscosity changeseach other. Small cracks then form in induced by the rising temperature orthe surfaces and this is known as sur- on account of excessive loading, thereface distress. These cracks must not is no risk of surface distress.be 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 Appearance Cause Action Initially the damage is not visible to Inadequate or improper lubrication. Improve lubrication. the naked eye. A more advanced stage is marked by small, shallow craters with crystalline fracture surfaces.Fig 45 The surface distress depicted in fig45 – 100 × magnificationFig 44 Surface distress in the form of aband encircling a roller from a sphericalroller bearing 44 45 23

Different types of bearing damageCorrosion Deep seated rust known as water etching. Salt water, A thin protective oxide film forms on such as sea water, is therefore highlyRust will form if water or corrosive clean steel surfaces exposed to air. dangerous to bearings.agents reach the inside of the bearing However, this film is not impenetrablein such quantities that the lubricant and if water or corrosive elementscannot provide protection for the steel make contact with the steel surfaces,surfaces. This process will soon lead patches of etching will form. This de-to deep seated rust. Another type of velopment soon leads to deep seatedcorrosion is fretting corrosion. rust. Deep seated rust is a great danger to bearings since it can initiate flaking and cracks. Acid liquids corrode the steel quickly, while alkaline solutions are less dangerous. The salts that are present in fresh water constitute, together with the water, an electrolyte which causes galvanic corrosion, Appearance Cause Action Greyish black streaks across the Presence of water, moisture or corros- Improve sealing. raceways, mostly coinciding with the ive substances in the bearing over a Use lubricant with better rust- rolling element spacing. At a later long period of time. inhibiting properties. stage, pitting of raceways and other surfaces of the bearing. Fig 46 Deep seated rust in the outer ring of a cylindrical roller bearing 46 47 Fig 47 Extensive water etching on the inner ring of a spherical roller bearing24

Fretting corrosion supported and this has a detrimentalIf the thin oxide film is penetrated, ox- effect on the load distribution in theidation will proceed deeper into the bearings. Rusted areas also act asmaterial. An instance of this is the cor- fracture notches.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 Appearance Cause Action Areas of rust on the outside surface Fit too loose. Adjust seatings. of the outer ring or in the bore of the Shaft or housing seating with errors of inner ring. Raceway path pattern form. heavily marked at corresponding positions.Fig 48 Fretting corrosion on the outer ringof a spherical roller bearingFig 49 Extensive fretting corrosion in the 48 49bore of a self-aligning ball bearing 25

Different types of bearing damageDamage caused by craters also form where the metal has ments of bearings with raceway fluting melted. caused by vibrations.the passage of The passage of electric current fre- Both alternating and direct currentselectric current quently leads to the formation of fluting cause damage to bearings. Even low (corrugation) in bearing raceways. amperage currents are dangerous.When an electric current passes Rollers are also subject to fluting, while Non-rotating bearings are much morethrough a bearing, i.e. proceeds from there is only dark discolouration of resistant to electric current damageone ring to the other via the rolling ele- balls than bearings in rotation. The extent ofments, damage will occur. At the con- It can be difficult to distinguish be- the damage depends on a number oftact surfaces the process is similar to tween electric current damage and vib- factors: current intensity, duration,electric arc welding. ration damage. A feature of the flut-ing bearing load, speed and lubricant. The material is heated to temperat- caused by electric current is the dark The only way of avoiding damage ofures ranging from tempering to melting bottom of the corrugations, as oppo- this nature is to prevent any electriclevels. This leads to the appearance of sed to the bright or rusty appear-ance current from passing through thediscoloured areas, varying in size, at the bottom of the vibration-induced bearing.where the material has been tem- fluting. Another distinguishing featurepered, re-hardened or melted. Small is the lack of damage to the rolling ele- Appearance Cause Action Dark brown or greyish black fluting Passage of electric current through Re-route the current to by-pass the (corrugation) or craters in raceways rotating bearing. bearing. and rollers. Balls have dark disco- Use insulated bearings. louration only. Sometimes zigzag burns in ball bearings raceways. Localised burns in raceways and on Passage of electric current through Re-route the current to by-pass the rolling elements. non-rotating bearing. bearing. When welding, arrange earthing to prevent current passing through the bearing. Use insulated bearings. Fig 50 Fluting, caused by the passage of electric current, in the outer ring of a spherical roller bearing 50 51 Fig 51 The outer ring of a self-aligning ball bearing damaged by electric current26

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 axial 52vibrationFig 53 A railway axlebox bearing damagedby the passage of high amperage current 53while the bearing was not runningFig 54 Roller of a railway axlebox bearingdamaged by electric current (same bearing 54as in fig 53) 27

Different types of bearing damageFlaking (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- 55 equate lubrication are the causes of this damage28

Flaking caused by preloading Appearance Cause Action Heavily marked path pattern in Preloading on account of fits being too Alter the fits or select bearing with raceways of both rings. tight. larger internal clearance. Flaking usually in the most heavily Excessive drive-up on a tapered Do not drive the bearing so far up loaded zone. seating. its tapered seating. Follow carefully the instructions given by SKF. Single row angular contact ball bear- Re-adjust the bearings to obtain ings or taper roller bearings adjusted lighter preload. to give excessive preload. Temperature differential between inner Select bearing with larger internal and outer rings too great. clearance.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 into 56service 29

Different types of bearing damageFlaking caused by oval compression Appearance Cause Action Heavily marked path pattern at two Oval shaft or oval housing seating. Usually it is necessary to manufac- diametrically opposed sections of The latter is a common defect in split ture a new shaft or a new housing either bearing ring. housings and machine frames. to remedy this defect. One expedi- Flaking in these sections. ent is to spray metal on the components and then regrind. If it is a mat- ter of an oval shaft with the bearing mounted on a sleeve, it is possible to adjust the shaft by grinding, in certain cases. The bore of plummer blocks mounted Adjust the base. on an uneven base becomes oval when the base bolts are tightened. Fig 57 Flaking in the outer ring outer ring 57 of spherical roller bearing that has been mounted in a housing with oval bore30

Flaking caused by axial compression Appearance Cause Action Deep groove ball bearings: heavily Incorrect mounting, which results in Check adjustment when mounting marked path pattern displaced to axial loading, e.g. excessive preload- the bearings. one side of both rings. ing of angular contact ball bearings Self-aligning ball bearings and and taper roller bearings. spherical roller bearings: severely marked raceway path patterns from The non-locating bearing has jammed. Check the fit and lubricate the sur- one row of rolling elements. faces. Flaking in these areas. Single row angular contact ball Axial freedom of movement has not If temperature differential between bearings and taper roller bearings: been sufficient to accommodate the shaft and housing cannot be re- same appearance as damage thermal expansion. duced, provide greater freedom of resulting from preloading, movement. see page 29.Fig 58 Outer ring of a self-aligning ballbearing subjected to excessive axialloading. Flaking in the load zoneFig 59 Flaked inner ring of a sphericalroller bearing. The extent of the flakingaround one entire raceway indicates thatthe axial load has been very heavy in rela- 58 59tion to the radial load 31

Different types of bearing damageFlaking caused by misalignment Appearance Cause Action Deep groove ball bearings: diagonal Bearing seatings out of alignment. Rectify the seatings. path pattern, severely marked at two diametrically opposed sections. Bearing mounted on the skew. Use mounting sleeve with parallel Cylindrical roller bearings: flaking at faces. the edge of the raceway. Fig 60 Deep groove ball bearing outer ring that has been out of alignment with the shaft. The ball path has an oval configuration on account of the misalignment. The result is the same as with oval compression, see page 31 Fig 61 Cylindrical roller bearing inner ring with flaking at one side of the raceway, as a result of overloading due to misalignment 60 6132

Flaking caused by indentations Appearance Cause Flaking in conjunction with indenta- Indentations resulting from faulty tions coinciding with the rolling ele- mounting practice or overloading of the ment spacing. non-rotating bearing, see page 28. Flaking in conjunction with small Indentations made by foreign particles, indentations. see page 16.Fig 63 Flaking (the dark area) initiated bythe adjacent indentations – 100 × magnifi-cationFig 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 62 63 33

Different types of bearing damageFlaking caused by smearing Appearance Cause Flaking at the start of the load zone Skid smearing, see page 18. in raceways of roller bearings. Flaking, coinciding with the roller Transverse smearing resulting from spacing, in raceways of roller faulty mounting practice, see page 19. bearings. Fig 64 Inner ring of a cylindrical roller bearing with extensive flaking caused by smearing of the kind depicted in fig 65 Fig 65 Inner ring of a cylindrical roller bearing with smearing, at intervals corre- 64 65 sponding to the roller spacing, caused by incorrect mounting34

Flaking caused by deep seated rust Fig 66 Flaking originating from deep seated rust on the roller of a spherical 66 roller bearing Appearance Cause Flaking originating from rust dama- Deep seated rust, see page 24. ge.Fig 68 Magnification of the damage shownin fig 66Fig 67 Cross section through the rollerdepicted in fig 66, showing the way inwhich the fatigue crack extends beneath thesurface. The fully developed flaking origin-ated in the same manner 67 68 35

Different types of bearing damageFlaking caused by fretting corrosion Appearance Cause Flaking in the raceway of either the Fretting corrosion, see page 25. inner or outer ring. Corroded area at corresponding part of the inner bore of outside surface. Fig 69 Flaking in the raceways of the outer ring of a spherical roller bearing. Correspond- ing area of advanced fretting corrosion on the outside surface (for this photograph the ring has been placed in front of a mirror). Development of the corrosion has been accompanied by an increase in volume that has led to deformation of the bearing ring 69 and localised overloading. The results have been premature fatigue and flaking36

Flaking caused by fluting or craters Fig 70 The outer ring of a self-aligning ball bearing with flaking caused by craters that have formed in conjunction with the passage of electric current. It is evident that the flaking has originated at the craters, from where it has spread out in both directions. Detached fragments of flaked material have in turn caused indentations and further 70 flaking Appearance Cause Flaking in conjunction with bright or Wear resulting from vibrations while corroded fluting or craters. the bearing was not running, see page 12. Flaking in conjunction with dark- Electric current damage, see page 26. coloured or burnt fluting or craters.Fig 71 Flaking in both raceways of theinner ring of a spherical roller bearing. Theflaking has originated from the vibration 71markings 37

Different types of bearing damageCracksCracks 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-aligning ball bearing. The indentations visible at the bottom edge of the ring were caused by 72 rough treatment and the crack originated at one of these indentations38

Cracks caused by rough treatment Appearance Cause Action Cracks or pieces broken off, gener- Blows, with hammer or hardened Always use a soft drift or mounting ally at one face of the bearing ring. chisel, have been directed against the sleeve. Never subject the bearing to ring when the bearing was being direct hits. mounted.Fig 73 Cracked inner ring of a sphericalroller 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 throughFig 74 Inner ring of a spherical roller bear-ing with outer flange fracture produced by 73 74direct hammering 39

Different types of bearing damageCracks caused by excessive drive-up Appearance Cause Action The bearing ring has cracked right Excessive drive-up on a tapered Follow carefully the SKF mounting through and has lost its grip on the seating or sleeve. instruction concerning bearings on shaft. tapered seatings. Interference fit on cylindrical seating Alter the fit. too heavy. Fig 75 Section of the inner ring of a spherical roller bearing – 3,5 × magnification. The ring has cracked because of excessive drive- up. The fracture originated at the dark area by the bore chamfer Fig 76 Fractured surface of the inner ring in fig 75 75 7640

Cracks caused be smearing Appearance Cause Crack or cracks in conjunction with Smearing, see page 17. smearing of the bearing ring. The ring may have cracked right across. Smearing cracks generally form across the smearing.Fig 78 Smearing damage on the face of abearing ring. Note the incipient transversecracksFig 77 Spherical roller bearing inner ringthat has cracked right across followingsmearing 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 77 78 41

Different types of bearing damageCracks caused by fretting corrosion Appearance Cause Cracks, transverse in inner rings Fretting corrosion, see page 25. and generally longitudinal in outer rings, in conjunction with fretting corrosion. Fig 79 Spherical roller bearing inner ring with fretting corrosion and a transverse crack right through the ring. The fretting corrosion has caused the cracking 79 80 Fig 80 Longitudinal crack in a deep groove ball bearing outer ring with fretting corrosion42

Cage damage ticles. The idea with rolling bearings is of course to avoid sliding friction. Fatigue cracks then form in the material and sooner or later they lead toIf, on examination of a failed bearing, However, where the cage is con- fractures.the cage is found to be damaged, it cerned, sliding cannot be eliminated in There is a similar case when a thrustmay in many cases prove difficult to the contacts with the other components ball bearing is fitted together with radialascertain the cause. Usually other of the bearing. This explains why the plain bearings. If clearance arises incomponents of the bearing are dam- cage is the first component to be affec- the plain bearings, the washers of theaged too and this makes it even more ted when the lubrication becomes in- thrust bearing become displaced indifficult to discover the reason for the adequate. The cage is always made of relation to each other. Then the ballstrouble. However, there are certain softer material than the other compon- do not follow their normal path andmain causes of cage failure, viz. vibra- ents of the bearing and consequently it heavy stresses may arise in the cage.tion, excessive speed, wear and block- wears comparatively quickly. As the Cages in bearings subject to severeage. cage pockets increase in size, due to acceleration and retardation, in conjun- wear, the rolling element guidance ction with fluctuations in speed, areVibration deteriorates and this also applies to affected by forces of inertia. TheseWhen a bearing is exposed to vibra- the cage in cases where the cage is give rise to considerable pressure be-tion, the forces of inertia may be so centred on the rolling elements. The tween the contacting surfaces, withgreat as to cause fatigue cracks to resultant forces may lead to cage fail- consequent heavy wear.form in the cage material after a time. ure within a short space of time.Sooner or later these cracks lead tocage fracture. Blockage Fragments of flaked material or otherExcessive speed hard particles may become wedgedIf the bearing is run at speeds in between the cage and a rolling ele-excess of that for which the cage is ment, preventing the latter from ro-designed, the cage is subjected to tating round its own axis. This leads toheavy forces of inertia that may lead to cage failure.fractures. Frequently, where very highspeeds are involved, it is possible to Other causes of cage damageselect bearings with cages of special If the rings of a deep groove ball bear-design. ing are fitted out of alignment with each other, the path of the balls has anWear oval configuration. If the cage isCage wear may be caused by inad- centred on the balls, it has to changeequate lubrication or by abrasive par- shape for every revolution it performs.Fig 82 Cage of a spherical roller bearing.Fatigue cracks have formed in the filletsFig 81 Fractured surface of the cageshown in fig 82. The fatigue cracks areclearly visible 81 82 43

© 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. Publication PI 401 EPrinted in Sweden by Palmeblads Tryckeri AB Reg. 770 · 15 000 · 1994-04

Bearing failure

by Quoc LeHoai

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Bearing failure Document Transcript