Why bearings fail: the common failure modes

A bearing that reaches the end of its natural fatigue life is the exception. Most fail early from a cause outside the bearing itself — and if you replace the bearing without fixing that cause, the new one fails the same way. Each mode leaves a signature; reading it is how you stop the repeat.

Contamination — the most common killer

A bearing runs on a lubricant film microns thick, so a speck of grit is enormous. Hard particles dent the raceways and act like grinding paste; water breaks down the lubricant and rusts the steel. The signature is dulled, scratched or pitted raceways all around. The fix is upstream: better seals, cleaner mounting, cleaner lubricant — see how to replace a bearing for keeping the seat clean, and closure choice for keeping dirt out.

Lubrication failure

Wrong grease, too little, too much, or grease that has aged out. Starvation and over-greasing both end in overheating — discoloured (blued) rings and smeared, glazed surfaces are the tell. Match the lubricant to the duty and don't over-pack: see grease vs oil.

Misalignment

If the shaft and housing are not square, the load concentrates on one edge of the raceway instead of spreading across it. The signature is a wear path that runs diagonally or sits to one side. Where misalignment is unavoidable, a self-aligning or spherical bearing is designed to absorb it — a rigid bearing is not.

Fitting damage

• False brinelling — evenly spaced dents at the ball pitch, from pressing the bearing on through the balls instead of through the fitted ring. Entirely avoidable with the right method.
• Fit too tight — the interference closes the internal clearance to zero, the bearing preloads, overheats and fails. See how fit and clearance interact in the fits guide.
• Fit too loose — the rotating ring creeps on its seat, fretting both surfaces and scoring the shaft.

Overload and shock

A load beyond what the bearing was chosen for, or a sharp impact, dents the raceway (true brinelling) or accelerates fatigue flaking (spalling). The fix is a correctly sized bearing and, often, understanding a duty that changed since the machine was designed.

Electrical erosion

Increasingly common on motors driven by variable-frequency drives: stray current passes through the bearing and arcs across the lubricant film, pitting the surfaces. Its signature is fine, evenly spaced washboard marks called fluting, and a greyish frosted track. The cure is electrical — insulated bearings, shaft grounding — not a bigger bearing.

Fatigue — the natural end

If none of the above kills it first, a bearing eventually fails by fatigue: the raceway surface flakes away (spalling) after millions of stress cycles. This is the failure the bearing's rated life predicts. Reaching it means everything else was done right — which, in practice, is rare.

The point

Before fitting a replacement, work out which of these happened. A bearing that "keeps failing" almost never has a bad bearing to blame — it has a cause that the new part will meet all over again. Getting the right part is only half the job; see what "interchangeable" really means.

General engineering reference only. Failure analysis for a specific machine should be done against the equipment and bearing manufacturer's guidance.