Bearing failure analysis: why bearings fail early, the six ISO 15243 failure modes, the common causes and their signs, and examining a failed bearing
Rolling bearing failure analysis: why most bearings fail long before their rated life, the six failure modes of ISO 15243 and what each looks like, the common causes from lubrication and contamination to installation and electrical current with their signs and prevention, and the steps for examining a failed bearing so the cause is corrected. With a drawing of the failure modes.
Key takeaways
- Most rolling bearings are removed long before the fatigue life they were rated for. Lubrication, contamination, installation and misalignment end far more of them than metal fatigue does.
- ISO 15243 classifies bearing damage into six modes: fatigue, wear, corrosion, electrical erosion, plastic deformation, and fracture and cracking. Naming the mode from its appearance is the first step to the cause.
- The damage itself is the best evidence. Photograph and mark the bearing before removal, keep a lubricant sample, and do not wash it.
- The position of the damage against the load zone, and its spacing, often says more than the damage itself: dents at element spacing point to installation or false brinelling, an angled wear path to misalignment.
- A failure analysis is only finished when the cause is corrected on this machine and the others like it, and the mode and cause are recorded in the maintenance history.
A pump's drive end bearing is replaced for the third time in a year. Each time the old one is thrown in the scrap bin, a new one goes in, and the work order says "bearing failed, replaced". Nobody has looked at the old bearings. When the fourth is finally kept and examined, the raceway shows grey washboard fluting, the mark of electrical current. The pump was moved to a variable speed drive two years earlier without insulated bearings or shaft grounding, and every bearing since has been destroyed by the drive.
This guide covers rolling bearing failure analysis: why bearings fail early, the six failure modes of ISO 15243 and what they look like, the common causes with their signs and prevention, and the steps for examining a failed bearing so the cause is found rather than the bearing replaced again. It sits beside the guides to root cause analysis and condition based maintenance. The standards named are reference points, and the bearing maker's guidance comes first.
The six failure modes
ISO 15243 classifies bearing damage by its appearance into six main modes, each with sub-modes, and gives the likely causes of each. The value of the classification is that it starts from what can be seen. A raceway that is flaked, smeared, rusted, fluted, dented or cracked is a different problem in each case, and naming the mode correctly narrows the causes to a few.
A failed bearing usually shows more than one mode, because the first damage leads to the rest. Contamination dents a raceway, and the dents start surface fatigue. Poor lubrication leads to smearing, then heat, then cracking. The analysis has to find the first damage, which is the one that points to the cause.
Find the first damage. Everything after it is a consequence, and replacing the bearing fixes only the consequence.
Why bearings fail early
A bearing's rated life under ISO 281 is a fatigue life: the life that 90 per cent of a large group of identical bearings would reach or exceed under the stated load and speed, with good lubrication and clean conditions. In service, most bearings are removed long before that, because the conditions behind the rating are not met. The bearing does not wear out. Something in its installation, lubrication or environment ends its life.
That is good news for maintenance, because the common causes are within the site's control. Each leaves recognisable damage, and each has a known prevention.
| Cause | What it is | What it looks like | Prevention |
|---|---|---|---|
| Lubrication | Too little, too much, the wrong grade or degraded grease or oil | Discoloured or glazed surfaces, smearing, surface distress leading to flaking | Lubrication routes with the right lubricant, quantity and interval, and oil analysis where it is worthwhile |
| Contamination | Dirt, process dust or water past the seals | Many small dents, abrasive wear, rust and staining | Seals suited to the environment, clean handling of lubricant, breathers and filtration |
| Installation | Wrong fits, hammering, misalignment, damage when mounting | Dents at element spacing, fretting on the bore or outer ring, an off-centre wear path | Fitting to the manufacturer's method with the right tools, heaters and alignment checks |
| Misalignment | Shaft and housing not in line, soft foot, bent shafts | A wear path that runs across the raceway at an angle | Laser alignment after installation and after any disturbance |
| Overload | Loads beyond the bearing's rating, shock loads | Early fatigue spalling, plastic deformation | Bearing selection checked against the real duty, shock sources removed |
| Electrical current | Current through the bearing, often from a variable speed drive | Grey frosted raceways with washboard fluting | Insulated bearings, shaft grounding and correct drive cabling and earthing |
| Vibration when stationary | Vibration from nearby running plant while the bearing stands still | Marks at element spacing with no rolling wear (false brinelling) | Isolate or rotate standby equipment, transport with the shaft locked |
Examining a failed bearing
Most of the evidence is lost in the first hour after a bearing comes out. It is washed, thrown in a bin with others, or left in the rain. A short procedure, part of the work instruction for changing critical bearings, keeps it. The examination itself needs good light, magnification and the bearing maker's damage guide, and for important failures the bearing maker or a laboratory can section and analyse the material.
| Step | What it involves |
|---|---|
| Record before removal | Photograph the bearing in place, mark the load zone and orientation, note the fits, lubricant condition and any signs of heat or rubbing |
| Keep the evidence | Take a lubricant sample. Do not clean or wash the bearing, which removes the debris that shows the cause |
| Collect the history | Operating hours, lubrication records, vibration and temperature trends, recent work on the machine, and any change in duty |
| Examine the parts | Rings, rolling elements and cage, in good light and with magnification, describing the damage and its position against the load zone |
| Classify the mode | Match the damage to an ISO 15243 failure mode, and separate the first damage from what followed it |
| Find the cause | Work from the mode to the cause with the history, then confirm it on the machine: fits, alignment, lubrication, seals, current |
| Act on it | Correct the cause on this machine and others like it, and record the mode and cause in the maintenance history |
Reading the damage
Where the damage sits often says more than the damage itself. The load zone, the part of the raceway that carries the load, should show an even, centred running path. A path off to one side points to axial load or misalignment. A path running across the raceway at an angle points to misalignment. Marks at exactly the spacing of the rolling elements point to an event while the bearing was not turning: an impact when mounting, an overload, or vibration while standing. Damage all around a ring that should only be loaded in one zone points to a fit problem or an unbalanced load.
Lubricant condition is part of the evidence. Dark, hard or dry grease, water in the oil, or metal particles in a sample point to lubrication and contamination causes, and an oil analysis of the sample can separate them.
From one failure to the fleet
A cause found on one bearing usually applies to others. A wrong grease on one route is wrong on every machine on that route. Insulation missing on one drive-fed motor is missing on its neighbours, as the motor starters guide notes for variable speed drives. Recording the failure mode and cause in the maintenance history makes these patterns visible across assets, and turns repeated bearing failures into a bad actor list with causes rather than a parts bill.
Where bearing failure analysis goes wrong
- Failed bearings scrapped without being looked at, so the same cause destroys the next one.
- Bearings washed before examination, removing the debris that shows the cause.
- The last damage taken for the cause, rather than the first.
- No record of orientation or load zone, so the position of the damage cannot be read.
- A cause found and corrected on one machine but not on the others like it.
- Electrical erosion missed on drive-fed motors because nobody checked for fluting.
- Failures recorded as "bearing failed" with no mode or cause.
Common questions
What are the main causes of bearing failure?
Poor lubrication, contamination by dirt or water, poor installation, misalignment, overload, electrical current passing through the bearing, and vibration while the bearing is stationary. Between them these end the life of far more bearings than material fatigue under normal load, which is what the bearing's rated life is based on. Each leaves recognisable damage, which is why examining the failed bearing leads to the cause.
What are the bearing failure modes in ISO 15243?
ISO 15243 groups bearing damage into six main modes, each with sub-modes. Fatigue is subsurface or surface initiated. Wear is abrasive or adhesive. Corrosion comes from moisture, or from friction as fretting and false brinelling. Electrical erosion comes from excessive current or current leakage. Plastic deformation comes from overload, debris or handling. Fracture and cracking is forced fracture, fatigue fracture or thermal cracking. The standard describes the appearance and likely causes of each.
What does fluting on a bearing mean?
Fluting is a pattern of evenly spaced grey lines across the raceway, like a washboard, caused by electrical current passing through the bearing. It is common in motors fed by variable speed drives, where the drive's switching induces shaft voltages. The cure is to stop the current: insulated bearings or rolling elements, shaft grounding, and the drive cabling and earthing the drive and motor makers specify. IEC TS 60034-25 covers bearing currents in drive-fed machines.
What is false brinelling?
False brinelling is wear at the spacing of the rolling elements, caused by small oscillating movements or vibration while the bearing is not rotating, such as a standby pump next to a running one, or a machine vibrating on a truck. It looks like brinelling, the dents an overload leaves, but the marks are worn rather than plastically pressed. Rotating standby equipment, isolating it from vibration and locking shafts in transport prevent it.
How do you do a bearing failure analysis?
Record the bearing in place before removal, with its orientation, load zone and fits, and keep a lubricant sample. Do not clean it. Collect the operating and maintenance history. Examine the rings, rolling elements and cage, and describe the damage and where it sits against the load zone. Classify it against the ISO 15243 modes, separate the first damage from what followed, work back to the cause with the history, confirm the cause on the machine, and correct it.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
Related case studies and tools
- Reliability engineering and asset strategy program (case study)
Related reading
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