Reading failed bearings
Nine damage types: how to read a failed bearing
Almost every failed bearing falls into one of nine damage families. Learn to separate the cause from the result and the root cause usually shows itself.
Bearings are rarely the root cause
Bearings are made from very clean steel under tight quality control, so a genuine manufacturing defect is extremely rare. When a bearing fails, the useful question is what abnormal condition overloaded or weakened it. The job of the person looking at the parts is to separate the cause from the result.
| Damage family | What you see | Usual root causes |
|---|---|---|
| Contact stress fatigue (spalling) | Flaking or pitting of raceway or rollers, often in a defined zone | Overload, wrong adjustment, shaft or housing out of shape, misalignment, thin oil |
| Plastic yielding | Dents, smeared metal, heat colours, deformed cage and rollers | Heat with too little oil film, heavy shock loads, wrong lubricant quantity or quality |
| Adhesive wear (smearing) | Dull, torn, smeared surfaces where parts slid instead of rolled | Rollers skidding because of cage damage, light load at high speed, starvation |
| Abrasive wear | Matte, satin or scratched surfaces | Dirt or sand in the lubricant, failed seals, dirty grease |
| Corrosion | Rust, black etched patches | Water, long idle periods in moist air, poor storage |
| Fretting and creep | Brown rust-coloured dust or polished bands on bore or outside diameter | Fit too loose, soft or worn seat, ring free to turn |
| Electrical pitting and fluting | Pits or evenly spaced axial ridges | Current passing through the bearing |
| Handling and installation damage | Dents, chips, cracks, scuffs, smears | Impacts, wrong tools, cocked rings, heat removal methods |
| Fractures | Cracked rings, split rollers, broken ribs | Heavy overload, shock, misalignment, hard press fits |
Four habits that save the diagnosis
Work through these every time.
- Most abnormal conditions end up as high load or high temperature on raceways and rolling elements. That disturbs the oil film and leads to wear, spalling and yielding, so the final picture can look similar for different causes.
- One damage type can create another on the same surface. Two or three types are often present, with the later damage laid over the first. Look for the oldest, least developed area.
- Investigate as early as possible. A bearing pulled at an early stage tells the story clearly; a bearing run to destruction hides it.
- Ask the double-check question before you write the report: is there any possible way the other party (supplier, installer, operator) could have caused this? Consider heat, load, application, operation and maintenance facts, not only the part in your hand.
Finish the job
Naming the root cause is not the end. The result has to be communicated to whoever owns the cause, the repair done properly, and the follow-up checked so the next failure does not repeat it.
Takeaway. Treat the bearing as a witness. Record where the damage is, how it is shaped and what else is on the part, then ask what abnormal condition produces exactly that pattern.
Put it to work in the tool