We have two Fafnir 7216WN bearings (single row angular-contact thrust bearings) back to back on the coupling end of a pelleting unit (similar to a pump with a bladed hub instead of an impeller). Spacers for the inner and outer races lie in between the bearing. We found a cracked inner race after vibration analysis detected failing bearings. The crack begins on one side and propagates cirumferentially about 180 degrees before making a sharp turn to the other side. A large, deep spall occurred right at where the crack took a sharp turn toward the other side. As expected, gradual spalling occurs all along the crack. There was plenty of lubricant.

The unit runs at around 600 rpm, drive by a 150 HP motor. The bearings had just been replaced at the end of November. The entire unit was down for about 3 weeks before restarting. The next data was taken the first week of January 2006 and a large increase in vibration level was detected. Hence the scheduled maintenance to replace the bearings again.

What does circumferential cracking on an inner race indicate? What causes might we consider?

Mark

Mark,

This would be only my guess, but it its feasable to overload tandem bearings and twist them at that time. Proper preload will be defined by the spacers size, so make sure they are correct.

I am curious to see your vibration data.

If it were the outer race, I would say it wasn't supported in the housing well enough. With the inner ring, I would say they were too tight.

Dave

I think Dave is on the right track. There is probably a problem with the shaft fit that is not providing the proper support for the inner race. Although the bearings are hard, they follow the contour that they are mounted upon. Non-uniform roundness can give rise to stress points that are subject to premature failure. Check with Fafnir for the recommended shaft size and roundness tolerance.

a crack on the inner race means:
too tight of fit to shaft
out-of-roundness
make sure to measure the shaft properly before installing the bearing. If it's a big bearing, you can also measure the bearing and verify with the pump manufacturer what are the fit.

SKF's publication PI401E Bearing Failures and Their Causes, shows a photograph of an outer race of a deep groove ball bearing, which has circumferential cracking caused by fretting corrosion. I suppose this ties in with what other people have already said, that it could be an issue with correct fits.

I agree with the others. And oversized shaft if not corrected the first time would possibly explain repeat failures you have seen.

One other possibility... too little heat and too much hammer ;-) during mounting.