Failure analysis
Electric pitting and cage failure
Quick answer
Sparks through the oil film leave pits and washboard marks; a cracked or loose cage can fail a bearing quickly.
Electric pitting
When electric current passes through a bearing, sparks jump across the very thin oil film at the contact. The surface melts in small spots. The result is either pitting or ridge marks across the rolling path (fluting). Under magnification craters can be seen, which shows the surface has melted. Significant pitting leads to flaking, and the surface hardness falls so it wears quickly. If you can feel fluting by hand or see pitting by eye, the bearing is not reused.
Countermeasures for electric pitting
- Improve earthing, or the upkeep of the earthing.
- Insulate the bearing or the part near it.
Cage failure: five types
| Type | What it is | Usual cause |
|---|---|---|
| Cracks and chips | A seriously cracked cage fails under heavy duty | Careless handling; abnormal load or vibration impact |
| Flaw and distortion | Soft cage material deforms easily and disturbs the rolling elements, so check the size and place of the flaw | Careless handling |
| Rust and corrosion | If the cage rusts, assume rust on rings and rolling elements too | Poor sealing; storage without rust prevention |
| Wear | Pockets or guide surface worn so the rolling elements cannot turn properly | Wrong or too little lubricant; dirt; poor sealing |
| Loose or badly closed rivets | Looseness caused by poor mounting, moment load, variable load or vibration; a bearing with bad riveting cannot be used again | Poor mounting, tilt; severe load or vibration |
Cage failure: countermeasures
- Handle carefully.
- Improve sealing and check the lubricant regularly.
- Protect from rust in storage.
- Review lubricant and method; reduce bearing tilt.
- For severe load or vibration, discuss the duty with the bearing maker.
Takeaway. If the current finds a way through the bearing, it will leave fluting. A cage that looks cracked, bent, rusty or loose says the rest of the bearing is in trouble too.
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