TI 1002a SRB Failures

8/12/2019 TI 1002a SRB Failures

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Contents Page

Different types of bearing damage. . . . . . . . . . . . . . . . . . . . . . . 3

Wear. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Wear caused by abrasive particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Wear caused by inadequate lubrication . . . . . . . . . . . . . . . . . . . . . . . . . 5

Wear caused by vibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Indentations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Indentations caused by faulty mounting or overloading . . . . . 8

Indentations caused by foreign particles . . . . . . . . . . . . . . . . . . . . . . . . 9

Smearing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Smearing of roller ends and guide flange . . . . . . . . . . . . . . . . . . . . . . . . 11Skid smearings of rollers and raceways . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Raceway smearing at intervals corresponding to theroller spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Smearing of external surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Surface distress. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Deep-seated rust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Fretting corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Electric current damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Flaking (spalling). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Cracks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Cracks caused by rough treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Cracks caused by excessive drive-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Cracks caused by smearing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

25Cracks caused by fretting corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Cage damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Ref: TI 7604; SRB/SRTB Handbook 1993-04

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Different types of bearing damage

Each of the different causes of bearing failure inadequate or unsuitablelubrication, careless handling, ineffective sealing, incorrect fits, etc. produces its own characteristic damage.

Such damage, known as primary damage, gives rise to secondary,

failure-inducing damage flaking and cracks. Even the primary damagemay necessitate scrapping the bearings because of, for instance, excessiveinternal clearance, vibrations, noise, and so on. A failed bearing frequentlydisplays a combination of primary and secondary damage.

The types of damage may be classified as follows:

Primary damage Secondary damage

Flaking Flaking

Wear Cracks

Indentations

Smearing

Surface distress

Corrosion

Electric current damage

Wear

In normal cases there is no appreciable wear in rolling bearings. Wear may,

however, occur as a result of the ingress of foreign particles into the bearingor when the lubrication is unsatisfactory. Vibrations in bearings which are notrunning can also cause wear.

Wear caused by abrasive particles

Appearance Cause Action

Small indentationsin the raceways androllers.Dull, worn surfaces.

Lubricant darkened.

Lack of cleanlinessbefore and duringmounting.

Lubricant contamina-ted by wear particles.

Ineffective seals.

Do not unpack bear-ing before it is to bemounted.Keep workshop clean

and use clean tools.Always use fresh cleanlubricant. Wipe thegrease nipples. Filterthe oil.

Check and if possible,improve the sealing.

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Wear caused by inadequate lubrication


Worn, frequentlymirror-like surfaces;at a later stage blueto brown discolortionbands.

Lubricant has graduallybeen used up or haslost its lubricatatingproperties.

The value is too low.

Too much water in thelubricant.

Check that the lubricantreaches the bearing.More frequent relubri-cation.

Choose higher viscosity.

Reduce lubricant watercontent or replacelubricant.

If there is not sufficient lubricant, or if the lubricant has lost its lubricatingproperties, it is not possible for an oil film with sufficient carrying capacity toform. Metal-to-metal contact occurs between rolling elements and raceways.In its initial phase, the wear that then results has roughly the same effect as

lapping. The peaks of the microscopic asperities, that remain after the pro-duction processes, are torn off and, at the same time, a certain rolling-out ef-fect is obtained. This gives the surfaces concerned a varying degree of mirror-like finish. At this stage surface distress can also arise, see page 15. If thelubricant is completely used up the temperature will rise rapidly. The surfaces

Fig. 3

aOuter ring of a spherical roller bearing that has not been adequately lubricated.The raceways have a mirror finish. The indentations are caused by foreign particles.

bSpherical roller with mirror-like surface due to lubricant starvation.

a b

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take on blue to brown discoloration bands. These discolorations appear mainlywhere grease lubrication is used. The temperature may even become so highas to cause the hardened material soften or even the bearing to seize up.

Fig. 4a

Roller of a spherical roller bear-ing with brown bands due tolubricant starvation.

Fig. 4b

Inner ring of spherical rollerwith brown bands due tolubricant starvation.

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Wear caused by vibration


Raceways withdepressions that areshiny or rusty at thebottom.

The bearing has beenexposed to vibrationswhile not running.

Secure the bearingduring transportation,by radial preloading.Provide a vibration-damping base. Employoil bath lubricationwhere possible.

When the bearing is not running, there is no lubricant film between the rollingelements and the raceways. The absence of lubricant film gives metal-to-metal contact and the vibrations produce small relative movements of rollersand rings. As a result of these movements, small particles break away fromthe surfaces and this leads to the formation of depressions in the raceways.

This damage is known as false brinelling. In many cases it is possible to seered rust at the bottom of the depressions. This is caused by oxidation of the

detached particles (which have a large area in relation to their volume) as aresult of their exposure to air. There is never any visible damage to the rollingelements.

Fig. 5

Vibration damage to the

inner ring of a cylindricalroller bearing. The damagehas arisen while the bear-ing was not running. It isevident, from the fainterfluting discernible betweenthe pronounced depressionswith corroded bottoms, thatthe ring has changed posi-tion for short periods.

In principle, the samepattern would appear on

the raceways of a sphericalroller bearing inner ring.

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The fluting resulting from vibrations sometimes closely resembles the flut-ing produced by the passage of electric current. However, in the latter casethe bottom of the depressions is dark in colour, not shiny or corroded. Thedamage caused by electric current is also distinguishable by the fact that therollers are marked as well.

Indentations

Raceways and rolling elements may become dented if mounting pressure isapplied incorrectly and if the bearing is subjected to abnormal loading whilenot running. Foreign particles in the bearing also cause indentations.

Indentations caused by faulty mounting or overloading


Indentations in theraceways of bothrings with spacingequal to the distancebetween the rollers.

Mounting pressure ap-plied to the wrong ring.

Excessively hard drive-up on tapered seating.

Overloading while notrunning.

Apply the mountingpressure to the ringwith the interference fit.

Follow carefully the SKFinstructions concerningmounting bearings ontapered seating.

Avoid overload or usebearings with higher

basic static load ratings.

Indentations in raceways and rollers occur when the bearing, while notrunning, is subjected to abnormally heavy loading in the form of impactsor pressure. The distance between the dents is the same as the rollerspacing.

If mounting pressure is applied to the wrong ring the damage originatesas smearing, and subsequently, if the pressure increases, develops intoa dent (Fig. 10).

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Indentations caused by foreign particles


Small indentationsdistributed aroundthe raceways of bothrings and on therollers.

Ingress of foreignparticles into thebearing.

Cleanliness to beobserved during themounting operation.Filter the lubricant.Improve seals.

Foreign particles, such as swarf and burrs, which have entered the bearingcause indentations when rolled into the raceways by the rolling elements.The particles need not even be hard; thin pieces of paper and thread fromcotton waste and cloth used for drying can produce indentations. Indentationscaused by these particles are in most cases small and distributed allover the raceways.

Fig. 6Indentations caused by dirt in one of theraceways of a spherical roller bearing.

50magnification.

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Smearing

When two inadequately lubricated surfaces slide against each other un-der load, material may be transferred from one surface to the other. Thisis known as smearing and the surfaces concerned become ripped up andlook torn. When smearing occurs, the material is sometimes heated (locally)

to such temperatures that rehardening takes place. This produces localisedstress concentrations that may cause cracking or flaking.

In rolling bearings, sliding primarily occurs at the roller end/ guide flangeinterfaces. Smearing may also arise when the rollers are subject to severeacceleration on their entry into the loaded zone. If the bearing rings rotaterelative to the shaft or housing, this may cause smearing in the bore and onthe outside surface and ring faces too.

Fig. 7Smearing of the surface of a roller from a spherical roller bearing.

100magnification.

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Fig. 8

Skid smearing in both racewaysof a spherical roller bearingouter ring.

Fig. 9

Skid smearing on the racewaysof a spherical roller bearinginner ring.

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Fig. 11One of the smearstreaks shown inFig. 10.

50magnification.

Fig. 10

Outer ring raceway of a spherical roller bearing withsmear streaks caused by blows against the inner ring.

Raceway smearing at intervals corresponding to the roller spacing


Transverse smear-ing streaks in theraceways spaced atintervals equal to thedistance between therollers.

Blows applied to thewrong ring or heavypreloading withoutrotating the bearing.

The inner ring assem-bly has been swivelledafter washing, whilethere was no lubricantbetween rollers andrings.

Rotate the bearingwhen it is being adjust-ed. Apply the mountingforce against the ringwith the tightest fit.Never allow the forceto pass through therollers.

Lubricate the bear-ing immediatelyafterwashing, especially ifthe bearing clearance issmall.

Smearing streaks may be found in the raceways of spherical roller bearings.These streaks are the result of careless handling or incorrect mountingpractice. Impacts or heavy pressure applied to the wrong ring, withoutrotating the bearing, can cause the rollers to produce narrow, transversestreaks of smearing in the raceways.

Bearings have to be lubricated immediately after washing because the cleanmetallic surfaces are susceptible to smearing when sliding against each other.

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Smearing of external surfaces


Torn and discolouredring bore or outsidesurface or faces.

Ring rotation relative toshaft or housing.

Select heavier interfer-ence fits.

Smearing may occur on the external surfaces of heavily loaded bearings.The smearing has then resulted from movement of the bearing ring relativeto its shaft or housing. Smearing of the inner ring bore, outer ring outsidesurface and ring faces can only be avoided if the fits are tight enough toprevent movement of the ring concerned in relation to its seating.

Heavier axial compression does not help in the case of inner rings.

Fig. 12

Smeared outsidesurface of aspherical roller

bearing outer ring.Material transferhas occurred fromhousing bore tobearing ring.

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Surface distress


Initially the damage isnot visible to the nakedeye. A more advancedstage is marked bysmall, shallow craterswith crystalline fracturesurfaces.

Inadequate or improperlubrication.

Improvelubrication.

If the lubricant film between raceways and rolling elements becomes too thin,the peaks of the surface asperities will momentarily come in contact witheach other. Small cracks then form in the surfaces and this is known assurface distress. These cracks must not be confused with the fatigue cracksthat originate beneath the surface and lead to flaking. The surface distress

cracks are microscopically small and increase very gradually to such a sizethat they interfere with the smooth running of the bearing.

If the lubrication remains satisfactory throughout, i.e. the lubricant film doesnot become too thin because of lubricant starvation or viscosity changesinduced by the rising temperature or by excessive loading, there is no risk ofsurface distress.

Fig. 13

The surface distress shown in Fig. 14.

Fig. 14

Surface distress in the form ofa band encircling a roller from aspherical roller bearing.

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Corrosion

Rust will form if water or corrosive agents get into the bearing in suchquantities that the lubricant cannot provide protection for the steel surfaces.This process will soon lead to deep-seated rust. Another type of corrosion isfretting corrosion.

Deep-seated rust


Greyish-black streaksacross the raceways,mostly coinciding withthe roller spacing.

Presence of water,moisture or corrosivesubstances in thebearing over a longperiod of time.

Improve sealing. Uselubricant with betterinhibiting properties.

A thin protective oxide film forms on clean steel surfaces exposed to air.However, this film is not impenetrable and if water or corrosive elementsmake contact with the steel surfaces, patches of etching will form. Thisdevelopment soon leads to deep-seated rust.

Deep seated rust is a great danger to bearings since it can initiate flakingor cracks. Acid liquids corrode the steel quickly, while alkaline solutions areless dangerous. The salts that are present in fresh water constitute, togetherwith the water, an electrolyte which causes galvanic corrosion, known aswater etching. Salt water, such as sea water, is therefore highly dangerousto bearings.

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Fig. 15

Deep-seated rust on a roller of aspherical roller bearing, caused bywater during standstill.

Fig. 17

Etched and rusty rollers of aspherical roller bearing, causedby moisture during standstill.

Fig. 16

Deep-seated rust in outer ring racewayof a spherical roller bearing, caused bywater in the bearing during standstill.

Fig. 18

Etched and rusty outer ring racewayof a spherical roller bearing, causedby moisture during standstill.

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Fretting corrosion


Areas of rust on the out-side surface of the outerring or in the bore of theinner ring. Raceway pathpattern heavily marked atcorresponding positions.

Fit too loose.Shaft or housing seatingwith errors of form.

Adjust seatings.

If the thin oxide film is penetrated, oxidation will proceed deeper into thematerial. An instance of this is the corrosion that occurs when there is a smallrelative movement between bearing ring and shaft or housing, on account ofthe fit being too loose. This type of rusting is called fretting corrosion and maybe relatively deep. The relative movement may also cause small particles ofmaterial to become detached from the surface. These particles oxidize quicklywhen exposed to the oxygen in the atmosphere.

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

supported and this has a detrimental effect on the load distribution in thebearings. Rusted areas also act as fracture notches.

Fig. 19

Fretting corrosion on the outerring of a spherical roller bearing.

Fig. 20

Fretting corrosion in the bore of aspherical roller bearing inner ring.

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Electric current damage


Dark brown or greyish-black fluting (corrugation)

or craters in racewaysand rollers.

Localised burns in race-ways.

Passage of electriccurrent.

Re-route the currentto bypass the bearing.

When welding, arrangeearthing to preventcurrent passingthrough the bearing.

When an electric current passes through a bearing, i.e. proceeds from onering to the other via the rollers, raceways may be damaged. At the contactsurfaces the process is similar to electric arc welding.

The material is heated to temperatures ranging from tempering to meltinglevels. This leads to the appearance of discoloured areas, varying insize, where the material has been tempered, re-hardened or melted.Small craters also form where the metal has melted.

The passage of electric current frequently leads to the formation of fluting(corrugation) in the bearing raceways and rollers.

Fig. 21

Fluting, caused by passage of electric current,in the outer ring of a spherical roller bearing.

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It can be difficult to distinguish between electric current damage and vibrationdamage. A feature of the fluting caused by electric current is the dark bottomof the corrugations, as opposed to the shiny or rusty bottom of the vibration-induced fluting. Another distinguishing feature is the lack of damage to therollers of bearings with raceway fluting caused by vibrations.

Both alternating and direct currents cause damage to bearings. Even lowamperage currents are dangerous. Non-rotating bearings are much moreresistant to electric current damage than bearings in rotation. The extentof the damage depends on a number of factors: current intensity, duration,bearing load, rotational speed and lubricant.

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

Fig. 22

Local long crater, caused bypassage of electric current,on the roller of a sphericalroller bearing.

Fig. 23

Local craters, caused bypassage of electric current,on the roller of a sphericalroller bearing.

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Flaking (spalling)

As already mentioned, flaking can occur as a result of normal fatigue, i.e. thebearing has reached the end of its nominal life span. However, this is not themost common cause of flaking since, with the aid of the simple SKF basicrating life equation, it is possible to select bearings with lives that are at least

as long as those of the other machine components. The flaking detected inbearings can generally be attributed to other factors. If the flaking isdiscovered at an early stage, when the damage is not too extensive, it isfrequently possible to diagnose its cause and take action to prevent arecurrence of the trouble.

When flaking has proceeded to a certain stage, it makes its presence knownin the form of noise and vibrations, which serve as a warning that it is timeto change the bearing.

The causes of premature flaking may be heavier external loading than hadbeen anticipated, preloading on account of incorrect fits or excessive drive-upa tapered seating, distortion due to shaft or housing seating form errors, axial

compression as a result of, for instance, thermal expansion, etc. Flaking mayalso be caused by other types of damage, such as indentations, deep-seatedrust, electric current damage or smearing.

Cracks

Cracks may form in bearing rings for various reasons. The most commoncause is rough treatment when the bearings are being mounted or dismount-ed. Hammer blows, applied direct against the ring or via a hardened chisel,

may cause fine cracks to form, with the result that pieces of the ring breakoff when the bearing is put into service. Excessive drive-up a tapered seatingor sleeve is another cause of ring cracking. The tensile stresses, arising in thering as a result of the excessive drive-up, produce cracks when the bearingis put into operation. The same result may be obtained when bearings areheated and then mounted on shafts manufactured to the wrong tolerances.

The smearing described in an earlier section may also produce cracks at rightangles to the direction of slide. Cracks of this kind may produce fractures rightacross the rings.

Flaking, that has occurred for some reason or other, acts as a fracture notchand may lead to cracking of the bearing ring. The same applies to fretting

corrosion.

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Cracks caused by rough treatment


Cracks or pieces brokenoff, generally in/from oneface of the bearing ring.

Blows, with hammer orhardened chisel, havebeen directed againstthe ring when the bear-ing was being mounted.

Always use a softdrift or mount-ing sleeve. Neversubject the bear-ing to direct hits.

Cracks caused by excessive drive-up


The bearing ring hascracked right through andhas lost its grip on theshaft.

Excessive drive-up atapered seating orsleeve against the ringwhen the bearing wasbeing mounted.

Interference fit on cylin-drical seating too heavy.

Follow carefullythe SKF instruc-tion concern-ing bearings ontapered seatings.

Change the fit.

Cracks caused by smearing

Appearance Cause

Crack(s) in conjunctionwith smearing of the

bearing ring. The ring mayhave cracked right across.

Smearing cracks generallyform across the smearing.

Smearing.

Cracks caused by fretting corrosion

Appearance Cause

Crack, transverse in inner

rings and generally longi-tudinal in outer rings, inconjunction with frettingcorrosion.

Fretting corrosion.

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Fig. 24

Spherical roller bearing inner ringthat has cracked right across fol-lowing smearing of one face. Thisring has been mounted to abut aspacer, but has not had a suffi-

ciently tight fit on the shaft.Consequently, the ring hasrotated relative to the shaft andthe spacer.

Fig. 26

Spherical roller bearing innerring with fretting corrosion and atransverse crack right through the

ring. The fretting corrosion hascaused the cracking.

Fig. 27

Longitudinal crack in outerring with fretting corrosion.

Fig. 25

Smearing damage on the face of a bearing

ring.Note the incipient transverse cracks.

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Cage damage

If, on examination of a failed bearing, a cage is found to be damaged, it mayin many cases prove to be difficult to ascertain the cause. Usually other com-ponents of the bearing are damaged too and this makes it even more difficultto discover the reason for the trouble. However, there are certain main causes

of cage failure, namely vibration, excessive rotational speeds, wear and block-age.

Vibrations

When a bearing is exposed to vibrations, the forces of inertia may be so greatthat fatigue cracks form in the cage material after a time. Sooner or laterthese cracks lead to cage failure.

Vibrations may be divided into two types:

aEccentric motion of the bearing generating vibrations which give heavyaccelerations. This type of vibration causes fatigue of the cage bars.A typical example is shaking screens.

Fig. 28a

Illustrates the type of vibrationwhich generates heavy accelera-tions by the eccentric motion ofthe bearing. This type of motiongenerates heavy mass forcesacting on the cages and theirlands.

Fig. 28b

Illustrates the type of vibrationfrom other parts of the machinery

or the surroundings. The latterinclude deck vibrations in shipsand impacts from rail joints. If theduration of this type of vibration isconsiderably less than the time ittakes for a roller to pass throughthe unloaded zone, the rollers maycause hammering damage to thecages.

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b Vibrations from other parts of the machinery or the surroundings thatgenerate peak accelerations. This type of vibration causes hammering ofthe cage pockets. By heavy hammering the shakedown limit for the cagematerial is exceeded and the cage bars are plastically deformed. Thisprocess leads to the cage pocket clearance increasing and consequently thehammering effect accelerates until the cage bars fracture. Typical examplesare impacts on bearings from rail joints and deck vibrations in ships.

Wear

Cage wear may be caused by inadequate lubrication, by excessive rollerskew (caused by inadequate lubrication), by high temperature or by abrasiveparticles.

The idea with rolling bearings is of course to avoid sliding friction. However,where the cages are concerned, sliding cannot be eliminated in the contactswith the other components of the bearing. This explains why the cages arethe first components to be affected when the lubrication becomes inadequate.The cages are always made of softer material than the other components ofthe bearing and consequently wear comparatively quickly. As the cage pockets

increase in size, due to wear, the roller guidance deteriorates, and this alsoapplies in cases where the cages are centred on the rollers. The resultantforces may lead to cage failure within a short space of time.

At operating temperatures around 200 C the strain-hardened zones ofunhardened steel cages start to soften and this leads to increased wear of thecage contact surfaces.

Blockage

Fragments of flaked material or other hard particles may get wedged betweena cage and a roller, preventing the latter from rotating around its own axis.

This leads to cage failure.Cages in bearings subject to severe acceleration and retardation, in con-junction with fluctuations in rotational speed, are affected by forces of inertia.These give rise to considerable pressures between the contacting surfaces,with consequent heavy wear.

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Fig. 29

A spherical roller bearing cage withbars deformed by roller hammering.The rollers have been exposed tovery high impact accelerations. Thecage pocket clearance has graduallyincreased and consequently also thehammering effect.

Fig. 30

A spherical roller bearing cagepocket with corner crack caused byroller hammering. The rollers havebeen exposed to excessive impactaccelerations.

Fig. 31

Wear of the outer part of bothraceways of a spherical rollerbearing inner ring. This damagehas been caused by the cagesbeing exposed to, for instance:

Inadequate lubrication

excessive cage accelerations onstarting-up

Excessive centrifugal accelerations(vibrations)

Excessive impact-inducedaccelerations (hammeringsituation)

TI 1002a SRB Failures

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