Failure analysis
Bearing failure types: what each one is and who is usually to blame
Quick answer
Eight families of failure and the one main cause behind each. Some wear is unavoidable; most of the rest is handling, lubrication or fitting.
Look at the whole system, not only the bearing
When any failure shows up, even a small one, find out why. Look at the shaft, the housing and the lubricant as well as the bearing. Judging a failure takes knowledge of bearings and lubricants and of how the machine runs and was installed.
Eight families and their main cause
| Family | Includes | Main cause |
|---|---|---|
| Rolling fatigue | Flaking, pitting | Unavoidable in the end |
| Wear | Wear, fretting | Mostly unavoidable |
| Fracture | Cracks, chips | Improper handling |
| Flaw | Brinelling, nicks, scratches, scuffing | Improper handling |
| Rust | Rust, corrosion | Improper handling and storage |
| Seizure | Seizure, discoloration, smearing | Improper lubrication |
| Creep | Ring creep | Improper fitting |
| Electric pitting | Pitting, fluting | Passage of electric current |
Pattern to cause: quick guide
| What you see | Usual cause |
|---|---|
| Flaking along one side of the raceway | Too much axial load |
| Symmetrical flaking on each side | Bearing mounted at an angle, or shaft or housing out of round |
| Flaking on one side or as an oblique line on the fixed ring | Shaft bent, poor centring, bearing not square on the shaft |
| Partial flaking on a thrust bearing | Eccentric mounting |
| Flaking in one small area only | Foreign matter, a flaw, or the start of fatigue |
| Scuffing on roller end and rib faces | Heavy axial load, poor lubrication |
| Scratches on the raceway | Grease too stiff, or too much acceleration at start-up |
| Scratches on a thrust bearing raceway | Rolling elements sliding under centrifugal force |
| Cracks or chips on a roller | Wrong material, big impact, or too much clearance in a cylindrical roller bearing |
| Cracks or chips on a ring | Advanced flaking, too much interference, badly designed housing |
| Chips on a rib | Impact while fitting, axial impact, heavy load |
| Cracks or chips on a cage | Wrong lubricant or method, high speed, strong vibration, advanced wear |
| Creep on a ring | Not enough interference |
| Wear on a ring | Sliding, soft material, dirt, short of lubricant |
| Wear on a cage | Dirt, wrong lubrication, bearing running tilted |
| Rust on the bore or outside surface | Fretting, water, humidity |
| Rust over the whole bearing | Poor washing oil or lubricant, water, humidity |
| False brinelling | Vibration while the machine is stopped |
| Fluting on raceway or rollers | Electric current |
| Discoloration | Heat or chemical action |
Can the bearing be used again?
As a rule, a bearing with fatigue flaking, cracks, seizure, or electric pitting or fluting is not reused. Minor flaws, light rust or staining may be dealt with by repair or cleaning, depending on how serious they are and on the use. Always decide with the cause removed, otherwise the same failure will return.
Takeaway. Wear and fatigue come with time; almost every other failure traces back to handling, lubrication, fitting or current. Fix that cause, not just the part.
Put it to work in the tool