Đang tải... (xem toàn văn)
bearing failures and their causes
Bearing failures and their causesProduct information 401 ContentsIntroduction . 3Bearing failures and their causes 3How is bearing life defined? 3Path patterns and their interpretation 4Different types of bearing damage 9Wear 10Wear caused by abrasive particles 10Wear caused by inadequate lubrication . 11Wear caused by vibration 12Indentations . 14Indentations caused by faulty mounting or overloading . 14Indentations caused by foreign particles 16Smearing . 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 22Surface distress 23Corrosion . 24Deep seated rust 24Fretting corrosion . 25Damage caused by the passage of electric current 26Flaking (spalling) . 28Flaking 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 . 37Cracks . 38Cracks caused by rough treatment 39Cracks caused by excessive drive-up 40Cracks caused by smearing . 41Cracks caused by fretting corrison . 42Cage damage 43Vibration . 43Excessive speed 43Wear 43Blockage 43Other causes of cage damage . 43 3The 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 fieldAmong 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 the213 4IntroductionFigs 1–4 Progressive stages of flaking 456the 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 underFig 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-wayFig 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-wayPath patterns andtheir interpretation@@@@@@@@@@@@@@@@@@@@@@@@ 5879Fig 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-wayFig 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-wayFig 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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 6Path patterns and their interpretation1112Fig 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 washersFig 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 loadingFig 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 rings10@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 7141315Fig 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 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 pinching 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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 8Path patterns and their interpretation1718Fig 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 racewayFig 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 reductionFig 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 itscircumference16@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 9Each 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 damageSecondary damageFlakingCracksDifferent 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.10Different types of bearing damageFig 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 sections19WearIn 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 endthe 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. [...]... concerned, it is essential that the risk of damage to the bearings be taken into consideration at the design stage Consequently, where possible, ball bearings should be selected instead of roller bearings The ability of ball bearings to withstand vibrations without being damaged can also be considerably improved by applying axial preloading with the aid of springs, see fig 25 An oil bath, in which all rolling... become dented Cause Mounting pressure applied to the wrong ring Action Apply the mounting pressure to the ring with the interference fit Excessively hard drive-up on tapered seating Follow carefully the SKF instructions concerning mounting bearings on tapered seating Overloading while not running Raceways and rolling elements may become dented if the mounting pressure is applied to the wrong ring, so... bearings to extra loading, for instance by applying springs, see fig Appearance Diagonal smear streaks in the raceways 43 Details of how to calculate the minimum required axial loads are given in the SKF General Catalogue Cause Loading too light in relation to speed of rotation Action Preload the bearing by using springs Fig 42 Thrust ball bearing raceway with smear streaks on account of the rotational... water, is therefore highly dangerous to bearings Cause Presence of water, moisture or corrosive substances in the bearing over a long period of time Action Improve sealing Use lubricant with better rustinhibiting properties Fig 46 Deep seated rust in the outer ring of a cylindrical roller bearing 46 24 47 Fig 47 Extensive water etching on the inner ring of a spherical roller bearing Fretting corrosion If... internal clearance Flaking usually in the most heavily loaded zone Excessive drive-up on a tapered seating Do not drive the bearing so far up its tapered seating Follow carefully the instructions given by SKF Single row angular contact ball bearings or taper roller bearings adjusted to give excessive preload Re-adjust the bearings to obtain lighter preload Temperature differential between inner and outer . science. SKF has been well to the fore-front right from the start and has longled this fieldAmong the benefits resulting fromthis research has been the ability. ofmachines and exacting demands aremade upon their carrying capacity andreliability. Therefore it is quite naturalthat rolling bearings should have cometo