Several cases of failure analysis found shaft worn out right under where bearing seated. Is there any specific mechanism to cause this kind of defect? How to prevent it? Case pictures attached.

Thanks, Kevin Ku.

Shaft_wron_out.pdf (552 Kb, 115 downloads) Pictures

Kevin,
Surely the metal is being worn away because of movement between the bearing and shaft. I have seen it happen when bearings were not properly fastened to the shaft--for example, set screws were not torqued properly. Vibration and temperature changes can put stress on these mating surfaces, and on the bearings. A bad bearing can also damage the shaft.
Are floating bearings used, if needed, for thermal growth? Could you use a bigger shaft? Wider bearing? One with a better locking mechanism? Consider using Loctite 609 or a similar substance under the bearing, and a threadlocker compound on the set screws.

Seems, inner race is rotating on the shaft causing the damage. May be due to less interference, high temperature, bearing inner race gets loosened on shaft.
Regards

Kevin, In my experience, slight clearance of the inner race bore is the initiator of wear on a shaft. The SKF book has a chart of fits to suit conditions.
Good luck
Joe Mc Cormack

Thanks all of your valuable inputs.

Kevin.

On SKF website you can find information regarding recommended fits between the shaft and relevant bearing. In your case, may be shaft dia is on the lower side. If you provide recommended interference between inner race and shaft, this problem is going to be solved.

Kevin, i also had similar experience, these type of occur when we have frequent replacements of brng over the same shaft. so i suggest that brng loctite should be used and while assembling it shd be with brng induction heater so that after getting cooled proper fits will be obtained considering the tolerance limits.

Kevin,

Do you have vibration data associated with the presented cases which could be posted?

Was this damage sdiagnosed based on vibration data?

Thanks,
David

Correct responses above. The result is called "fretting", leaving a brown deposit on the mating surfaces. In this case it has gone beyond fretting and started to wear the shaft.

After evaluating the interference fit between inner race bore and shaft, next time you mount a bearing in this application put a mark on the shaft and a corresponding mark on the inner race. This way, when the inner ring starts walking, you can catch this condition early before it ruins your shaft.