One of my customers had a 1500 HOP Z-Mill motor making a strange "growling" noise once in a while that was resonating the steel grating out at the mill. This has went on for some time, and last month, due to meggar readings going to 1 meg, it was decided to remove the motor (no easy task) and have it rebuilt, dipped and baked.

A small spall was found on the one of the balls when the bearings (SNR 6240MJ30) were cut open.

This message has been edited. Last edited by: RRS_Dave,

The thing I found strange though was this ball, which had been spinning in a direction perpendicular to the radial spin of the bearing. Has anyone seen anything like this before?

There wer no marks on the races which would indicate any heavy axial loading.

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I have turned the ball 90 deg. so you can see the witness marks on it

Here's the way I see it as having been in the bearing.

Has any one seen anything like this before, or care to hazard a guess as to how this ball could spin against the cage in this fashion?

We mic'd the balls, and they were all at 1.625" which is what SRN says they are supposed to be. This one does not feel any looser in the cage than any of the others.

It may have went on the inside of the cage as well. Pattern fits there also.

I've seen it before on a pressed steel cage. Didn't have any grooves or rough surfaces, just mainly smooth marks. ?? Lube problem? What type of lube is used? Mine was grease.

Grease was used on this one too, Don.

Dave

Just a shot in the dark......maybe the balls get "polished" from rolling in the cage area, slight lack of grease might allow this.

I have seen this on some synchronous belt tensioner pulleys with 6210 bearings also grease lubricated. I have not been able to figure out the reason.

Here is another picture. Same bearing different ball

Crop Circles is what we called them. This SKF 6222 bearing was taken out of a 350 hp 1800 rpm motor caught during our motor test run. Data taken on this bearing showed FFT (cage)and the critical location of this motor we couldn't chance it so we sent it back to the motor shop and this is what we found. I agree with the group in the call of lack of lube, but could it also be a tight fitting cage?

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Interesting photos.

I don't have any idea what caused that, but my guess would be that the ball was spinning the regular direction when the damage occurred. The rubbing would have occurred for maybe 1/4" on either side of that split. Either due to lub breakdown or foreign material or perhaps due to one ball chipped and tried to stop... other balls had to transfer force to the cage to continue to push that ball against friction... who knows.

Why does that damage only go 1/4" on each side of the cage split and then stop? That's a good question and I can see how that led you to suspect ball spinning in perpendicular direction.... but I just can't imagine a ball spinning in the perpendicular direction. Maybe the reason the damage is limited to that sector of the ball has to do with foreign material accumulated in that gap near the split. Or maybe the geometry of the cage pocket... tighter clearances near the split than 90 degrees away. Examination of the cage might tell the difference. If the rubbed part of the cage is just on either side of the split, it supports my theory... if the rubbed part of the cage is all the way around the pocket, would seem to support your theory.

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