When did it fail? The timing narrows the cause
A bearing that fails right after mounting points to different causes than one that fails after years of normal running. Use the moment of failure to shorten the list of suspects before you open the housing.
Why timing matters
Rolling bearings wear out by fatigue in the end, but many fail long before that. When you know exactly when a failure showed itself, the list of possible causes gets much shorter. A bearing that fails in the first hours is very rarely a fatigue problem. A bearing that fails after years of steady running is rarely a mounting problem.
Five moments and what to suspect
| When it failed | Look first at |
|---|---|
| Straight after mounting or within a short time | Mounting method and damage, wrong fit or clearance, bearing the wrong type for the job, shaft or housing out of shape or out of round, misalignment |
| Straight after an overhaul | Parts refitted dirty or damaged, wrong parts mixed back, housings and shafts not cleaned or checked, wrong lubricant or amount |
| Straight after new lubricant was supplied | Wrong lubricant or grease mixed with a different type, too much grease, dirt or water carried in with the lubricant, wrong method |
| Straight after repair or removal of a shaft, housing or neighbouring part | Alignment and fits disturbed, seals or labyrinths now rubbing, clearances changed, loose parts |
| During normal operation | Lubricant and seals gradually failing, contamination, overload or changed duty, vibration, current passing through the bearing, then end of fatigue life |
Six families of cause
- Bearing used for the wrong job (wrong type, size or clearance for the load and speed).
- Faulty design or installation: shaft, housing or fits out of shape or out of tolerance.
- Wrong lubricant, wrong method of lubricating, or a poor sealing arrangement.
- A defect in the bearing itself, which is rare.
- Mis-mounting of the bearing.
- Seal defects that let water, dust or other foreign matter in, or let the lubricant run short.
How to use it
Write down the moment of failure before you start the teardown. Then work through the families above for that moment and look at the parts for the signs in the photo guide. Also check the shaft, the housing and the lubricant, not only the bearing. A failed bearing is often the messenger, not the culprit.