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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