Bearing Failure Analysis: Reading the Damage

A failed bearing is a record of what happened to it. The damaged surface preserves the sequence, the load direction, the temperature and the contaminant, if you read it before you clean it. The purpose of failure analysis is not to name a culprit quickly but to work from what you can see back to the mechanism, and from the mechanism back to the root cause - because replacing a bearing without changing the cause simply repeats the failure.

This field guide starts from visible damage and leads to the cause, using the failure-mode families of ISO 15243. It covers what to photograph and record, the checks on grease and debris, and the discipline of establishing which damage came first.

Start from what you see, not from the part number

The temptation is to conclude "bad bearing" from the brand or the price. That conclusion is almost never supported by the surface, and it leads straight to a repeat failure. Begin instead with a physical description - where the damage is, what it looks like, how it is distributed - and let that drive the classification.

The ISO 15243 failure-mode families

ISO 15243 is the vocabulary standard for bearing damage. It separates the modes into six families, and once you have placed the damage in a family, the plausible causes narrow sharply.

Reading the damage: symptom to mechanism

The following patterns are the ones a maintenance engineer meets most often. Read them together: the position and distribution of the marks are as informative as the marks themselves.

Fatigue spalling

Classic fatigue appears as pits and flakes that begin subsurface and break through, usually after the bearing has run for a large fraction of its calculated L10 life under a normal load. A bearing rated for 20,000 h at 1,500 rpm that fails at 15,000 h with clean, undamaged surrounds is behaving normally. It is the intended failure mode - the bearing did its job. Apply the life calculation to confirm that the hours run match the expectation before looking for another cause.

Brinelling, true and false

True brinelling is a row of evenly spaced indentations matching the rolling element spacing, caused by a static overload or by shock - a bearing dropped 300 mm, or hammered during mounting. On a 6205 the indentations sit about 10 mm apart, at the ball pitch. False brinelling looks similar but is wear, not deformation: it occurs when a stationary bearing vibrates so slightly that the elements fret against the raceway, and it carries a small amount of corrosion product. The distinction matters, because true brinelling calls for fixing the handling or the load, while false brinelling calls for isolating the machine from vibration.

Fluting from shaft currents

Regular, closely spaced parallel ridges across the raceway - fluting - are the signature of electrical erosion. In a variable-frequency-drive motor, shaft voltages of 5-30 V drive currents of 0.5-2 A through the bearing, discharging across the contacts and melting microscopic volumes of steel. The fix is electrical, not mechanical: an insulated or hybrid bearing and a proper shaft grounding ring.

Wear: smearing, adhesive, abrasive and corrosive

Wear damage comes in four recognisable forms. Smearing is a roughened, torn surface from sliding under poor lubrication, common on high-speed or lightly loaded bearings that skid. Adhesive wear transfers material between surfaces. Abrasive wear leaves fine scratches in the running direction from hard particles and destroys the finish that should be below Ra 0.1 micrometre on a bearing raceway. Corrosive wear combines material loss with chemical staining. In the field they often appear together, and the grease condition is the fastest way to tell which one dominated.

Overheating, discolouration and creep

Heat leaves a clear signature. A straw to blue tint along the raceway or the rolling elements shows the maximum temperature the steel reached: the temper colours run from about 150 degC for straw, through 200 degC for brown and 250 degC for purple, to 300 degC for blue. The correlation with hardness loss is well known - from about 150 degC the hardness starts to fall, and above roughly 200 degC a standard bearing is effectively softened. Discolouration on one side only points to a load or alignment problem rather than a general temperature problem.

Creep and rotating outer rings

Creep is the slow rotation of a ring on its seat when the fit is too loose. On the rotating outer ring of a worn housing it leaves a polished band and a wear ridge inside the housing bore, often after 0.5-2 mm of travel has accumulated over months of running. The damage is in the housing, so a new bearing will creep and fail the same way unless the housing is corrected. This is why the housing design checks are part of any analysis that shows creep.

The damage-sequence discipline

Almost every real failure shows more than one form of damage, and the order in which they occurred is the whole point. Apply this test: identify which damage is consistent with interrupted running. If the spalling is surrounded by fresh, unworn steel, the spalling ended the life and is the primary damage. If the spalling sits inside already-worn or corroded surfaces, the wear came first and the spalling is a consequence of it. On a position that failed within 2-3 weeks of a rebuild, suspect mounting damage before fatigue, because fatigue cannot develop that fast under a normal load. Getting the sequence right is what separates a correct corrective action from a plausible but wrong one.

Evidence to record and laboratory checks

Record before you clean. Once the grease is wiped away and the bearing discarded, the evidence is gone. The minimum record set is below.

Grease and debris analysis

Grease analysis is cheap and fast. Check for water content - above about 0.1% is a warning and above 1% is usually fatal to a mineral grease - for metallic particles, and for a colour or texture change from the original fill. A magnetic plug or an oil sample showing iron or copper particles tells you which component is wearing. Vibration severity can be graded against ISO 10816, where a velocity above about 4.5 mm/s RMS on a medium machine is a concern, and the defect-frequency interpretation is covered in the vibration guide.

Breaking the replace-and-repeat trap

The commonest maintenance failure is replacing the bearing without changing the cause. A new bearing in a bad position fails again in 6-8 weeks, and the second failure is often blamed on the supplier. Break the cycle by refusing to sign off until each of the following has been checked: the fit dimensions, the clearance class, the alignment, the lubrication type and quantity, the closure, and the electrical environment. The broader discrimination of similar-looking modes is covered in the failure mode discrimination reference, and motor-specific patterns in the motor bearing field guide.

Step-by-Step Failure Analysis Procedure

1. Photograph and document the bearing in place and on removal before any cleaning.
2. Collect the service data - hours run, load, speed, temperature and the full lubrication history.
3. Place the damage in an ISO 15243 class from its appearance and distribution.
4. Establish the sequence of the damage forms to identify the primary mode.
5. Check the fit and clearance - measure the shaft and housing, and confirm the clearance class.
6. Check the lubrication by inspecting the grease and any debris before disposal.
7. Check alignment and the electrical environment, including whether a VFD is present.
8. Write the root cause and the corrective action together, so the fix targets the cause rather than the symptom.

Common mistakes

• Declaring bad quality from a single spalled bearing without checking fit, clearance and lubrication - most spalling is a service problem, not a manufacturing one.
• Cleaning before photographing, which destroys the position and colour evidence.
• Confusing false brinelling with true brinelling and applying the wrong corrective action.
• Treating fluting as a bearing defect when the cause is a shaft current that a new bearing cannot survive either.
• Replacing the bearing inside a worn housing, so the replacement creeps and fails the same way.

Use the interchange lookup to compare an alternative only after the cause is understood, and consult ISO 281 for the life basis against which normal fatigue is judged. The grease condition and interval side is handled in the re-lubrication guide and the mounting damage in the mounting and dismounting guide. Damage terminology is also summarised in the ISO 15243 terminology note. For specific positions, review our deep groove ball bearing and cylindrical roller bearing listings, compare a SKF equivalent, or send photographs with the service data through the contact form.