Understandingbearing failures

Welcome to the SMB Bearings guideto understanding bearing failuresBearing failures present a paradox: theyare both relatively rare and relativelycommon. They are rare because,statistically speaking, only a smallpercentage of bearings in use fail. Themajority exceed their service life and ifyou are using high quality bearings andmaintaining them properly, the risks arefurther reduced.Yet although bearing failures may be rarein relative terms, the absolute numberof bearing failures is still relativelyhigh. That’s because there are so manybearings in use that even a relativelysmall percentage of failures is still a highnumber.Regardless of the rarity or otherwiseof bearing failure, the results can beextremely costly. A failed bearing canlead to expensive unplanned downtime,or secondary damage to equipmentand machinery. It’s no surprise thatmanufacturers and other users of bearingswant to know more about what causesthese failures and how they can bestdiagnose problems with their bearings.That’s where this whitepaper fits in.This guide should give you a basicunderstanding of what causes bearingfailures, what you can do to eliminate orreduce the risks and how you diagnose theproblem correctly when you encounterdamage or failure.The diagnosis element is key becausethe story isn’t over when a bearing fails.Different failures leave different imprintsor clues that suggest their causes. Insome cases, only expert analysis withexpensive equipment can uncover theroot of the problem, a luxury that not allmanufacturers possess.Although visual inspection has itslimitations, in many instances a goodbasic understanding of bearing failurescan equip you with the necessary tools todiagnose the problem. Even if you cannotdiagnose the exact problem, there willoften be clues that can help narrow downthe range of possible culprits.The value of diagnosis lies in thecorresponding remedial action. If youknow what has likely caused the bearing tofail, you can take the necessary action toprevent damage or failure from repeating.This will save you hassle and potentially, alot of money.The guidance contained here mostlyadheres to the ISO classification forbearing damage, and I have tried toavoid overly technical language. If youalready understand the basics, there aremore lengthy, technical guides availableelsewhere. I hope that what is lost insimplicity is gained in readability and easeof use. As always, if you have more detailedquestions then feel free to get in touch.Best regards,Chris JohnsonManaging director of SMB Bearings3

Understanding failuresMost bearings will outlive their service life. Only onein approximately 200 bearings will fail. However, asresearch by bearing manufacturer SKF has estimated,there are so many bearings in operation thatapproximately 50 million bearings are replaced due tofailure every year.Having a basic understanding of how and why failureshappen puts you in a better position. You are less likelyto mount or store the bearing incorrectly,you are more likely to adhere to an effectivemaintenance and monitoring program and, above all,you will be in a better position to correctly diagnose aproblem if a bearing does fail.”The quality of a bearing is a key factor that determineswhether it reaches its calculated service life, so stickwith a reputable supplier. Other key factors includehow the bearing is stored, whether it is correctlymatched to the application, whether it is mountedcorrectly, and the lubricant that is used.Estimates vary, but lubrication problems areamong the most common causes of bearing failure.Lubrication problems are more complex than theysound. There is more to it than simply having theappropriate choice of lubricant. It is vital that thecorrect quantity of lubricant is applied, that it isapplied in the correct way and that it is reapplied atthe right intervals.5

Monitoring and inspectionThere is usually a gap or window between the onsetof damage and the failure of the bearing. This willvary depending on the application and the qualityof the bearing. In fast moving bearings, failure couldquickly follow. In equipment that rotates more slowly,a bearing might remain serviceable for many monthsafter the onset of damage.Due to this gap, it makes sense to monitor and inspectbearings regularly. If damage or the symptoms ofdamage can be detected early, a bearing can bereplaced or alternative remedial action implementedin time to avert a failure. Ideally, a bearing could bereplaced during scheduled downtime, removing thecosts of unplanned halts in production.Given what is at stake, it is no surprise that manymanufacturers invest in high tech equipment to helpmonitor the condition of their bearings.Various monitoring technologies can spot damage at afar earlier stage than would be apparent using normalhuman senses. Vibration analysis is the most commonof these but other technologies, such as infraredimaging or ultrasound, are also used.If you cannot afford such equipment, or if theinvestment is not practicable, then more traditionalmethods of inspection are still recommended. Beforefailure, many bearing problems show up with thefamiliar symptoms of heat, noise and vibration.67

Heat, noise and vibrationHeat, noise and vibration are the keyparameters of any basic monitoringregime. While using your own senses maynot match up with the latest monitoringequipment, it can still give some warning.Just be aware that due to the limitationsof the human senses, by the time thesesymptoms are observable the level ofdamage may already be extensive.Listen to your bearings. Bearings in goodcondition make a soft purring noise.Squeaking or grinding sounds are usuallyindicative of poor condition. Pay attentionto vibration. Although all machinesvibrate, mechanical problems are usuallyaccompanied by increases in vibration.Temperature is perhaps the key variable astemperature increases are symptomatic ofso many bearing problems.You might notice a natural increase intemperature after first starting a machineor when first using it after regreasing. Inmost other instances though, temperatureincreases should prompt furtherinvestigation.Checking the lubrication is a key part ofthis investigation. Check the seals forlubricant leaks or for signs of damagethat might have permitted the ingressof contaminants. Relubricate accordingto schedule and ensure reservoir levelsare correct. If you are using systems toautomatically relubricate, make sure theseare functioning as intended.8A more frequent regrease is a good fix formany common problems. For example,if the lubricant is contaminated, thenrelubricating more frequently but withsmaller amounts will help purge thelubricant of contaminants.Bearings are not always accessible, butvisual checks should be performed duringroutine shutdowns of equipment. Themachinery should be cleaned to reduce therisk of contaminants entering the bearing.Lubricant samples can be taken. It is oftena good idea to keep samples of the originallubricant for comparison. You can spreadthe lubricant over a thin sheet of paper andexamine it under a light to check for signsof contamination.Having checked the lubricant, you want toexamine and clean any exposed externalsurfaces. These should be cleaned with alint-free cloth. Look for signs of corrosionor cracks. If practicable, you can rotate theshaft. If there is an uneven level of frictionor resistance, this may also indicate aproblem with the bearings.The seals are a common culprit in manybearing problems, so be extra vigilantwhen inspecting these for signs of wear ordamage.Even if the seals themselves are not theissue, they are only effective on a smoothcounterface. Check to make sure the latteris not damaged, worn or corroded.9

FatigueAll of these checks should be performedsystematically, as part of a routinecondition monitoring exercise for allyour equipment. Be warned though thatthese symptoms, although indicative ofproblems, could be the result of secondarydamage. In other words, they mighttell you a problem exists, but they willnot necessarily allow you to diagnose itcorrectly.Bearing failures can be diagnosed tohelp prevent a recurrence. Most failuresleave trademark imprints or clues. Whendiscussing these, it is useful to refer to theISO classification system, as this providesa common terminology for describingbearing failures.The ISO 15243, which was last updatedin 2017, sets out six main categories ormodes into which bearing failures can al erosionPlastic deformationFracture and crackingIn the following section, we briefly describeeach of these failure modes and thesubcategories within them. We also lookat some of the clues or imprints each ofthese failure modes leaves behind, whichcan help inform diagnostic assessmentsand remedial actions that could PlasticdeformationFracture andcracking11

FatigueWearStress changes can produce damageat both the surface level and the subsurface level of the rolling elements.Fatigue can be initiated at thesubsurface level, meaning the cracksbegin below surface level and are notimmediately visible. This will often leadto increases in noise and vibration.The crack will usually work its way tothe surface over time. The bearingmay remain serviceable after spallinghas begun, but the deterioration willgradually increase.Abrasive wear refers to the progressiveremoval of material. Abrasive particleswear down the rotating elements over time.Often, the metal cage is the firstcomponent to begin suffering the impactof this.Fatigue can also be initiated at thesurface level. This is most often aconsequence of damage to a rollingcontact surface resulting frominadequate lubrication.The second type of wear is adhesivewear. Also known as smearing, this iswhere material is transferred from onesurface to another. The process is usuallyaccompanied by friction and heat andis uncommon under normal operatingconditions. It is a vicious cycle because ithardens the material and thins the oil film.Although the damage is at the surfacelevel, it is often not visible to thenaked eye but will be visible undera microscope. Microspalls are only afew microns in size, so the surface willsimply look dull and grey.12Bearings will experience some lightwear during the initial running in-stage.More severe wear is usually the result ofinadequate lubrication or the presence ofsolid contaminants. The visible signs of thisform of damage are usually dull surfaces.13

CorrosionElectricalerosionMoisture corrosion can be caused byineffective sealing arrangements.Marine applications face environmentalconditions where the risks of this arehigh. It is also a common form of bearingfailure in paper machines and in the foodand beverage sector.When electrical current passes from onering to another via the rolling elements,damage can occur. The heat damages thematerial and the signs of this are oftenvisible in the form of discolouration orshallow craters where material has brokenaway. Sometimes, you might spot zig-zagshaped burn marks on the raceways.Frictional corrosion can be further brokendown into either fretting corrosion orfalse brinelling. Small particles can breakaway from a bearing’s surface and whenexposed to air they will oxidise quickly.Fretting corrosion is visible in the form ofrust on the outside surface of the outerring or the bore part of the inner ring.Current leakage erosion is a similarproblem. In this instance, the craters will beshallower during the initial stages, becausethe current is smaller. Over time, thisform of damage often leaves a trademarkwashboard pattern.False brinelling can occur in the contactareas due to micromovements. Whenthis takes place, rust will usually beobservable in the indentations. Thiscan often result from vibration thatequipment is subjected to during standstill.1415

PlasticdeformationFracture andcrackingShock loads, improper handling or staticoverloading can all cause overloaddeformation. Whichever of these three isthe cause, the resulting damage will lookthe same.If concentrations of stress exceed thetensile strength of the material, the resultcan be a forced fracture. Rough treatmentis a common cause of forced fracture. Forexample, someone might wrongly attemptto mount their bearings using a hammerand chisel.If a new bearing is making noise, in additionto the possibility of incorrect mounting,it might be that it was damaged in transitor storage. The importance of storage isoften underestimated, but it is critical atevery stage.During manufacturing, in storage andin transit – the bearings must be storedand cared for correctly to keep themprotected. This is another reason forworking with a trusted supplier.Deformation can also take the form ofindentations from debris. Contaminantscan enter the bearing via the seals orthe lubricant. Even soft particles can beproblematic if they are large enough.This reinforces the importance of cleanlubricants and careful handling.16Fatigue fractures, in contrast, occur whenthe fatigue strength of the materialis exceeded under cyclic bending. Therepeated bending causes a hairline crack toemerge.Another process to be aware of in thiscategory is known as thermal cracking.When two surfaces slide against oneanother the frictional heat producedcan sometimes lead to cracks. These willusually be at right angles to the sliding.17

Frequently asked questionsWhat makes aquiet bearing?Noise in a bearing is caused by severalfactors that increase vibration. Roughor damaged balls or raceways, poor ballor raceway roundness, contaminationinside the bearing, inadequate lubrication,incorrect shaft or housing tolerances andincorrect radial play can all contribute tonoise.Will relubricationmake rough bearingssmooth again?Relubrication cannot make rough bearingssmooth. Roughness or excessive vibrationindicates wear. Removing old or dirtylubricant can help to a degree but theunderlying problem will remain. Thebearing will feel smooth with fresh greasebut be aware this will only be temporary.18What are corrosionresistant bearings?As the name suggests, corrosion-resistantbearings can be used in applications wherecorrodible materials would be unsuitable,such as food processing, chemicalmanufacturing, pharmaceutical productionand marine applications. Stainless steel isthe most obvious material choice for thistype of bearing, but ceramic bearings canalso be used.Can you relubricatebearings with shieldsand seals?Relubricating bearings with shields or sealscan increase operational efficiency, andenergy efficiency, as well as extendingthe service life of the bearing. SMB’sexpert relubrication system enables us torelubricate ball bearings without removingshields.19

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a bearing might remain serviceable for many months after the onset of damage. Due to this gap, it makes sense to monitor and inspect bearings regularly. If damage or the symptoms of damage can be detected early, a bearing can be replaced or alternative remedial action implemented in time to avert a failure. Ideally, a bearing could be