Ball bearing splits in two in VFD motor

I have seen this discussed on this site before, but never directly experienced it. I would check for shaft surrent and verify that ground of the motor is proper. I'm certainly no motor expert, but there is more to proper grounding that just a copper wire.

There was a discussion about this on the vibration forum about a year ago and someone spelled out proper grounding practices. You might try searching that forum and posting this there, too. There is much more traffic on that forum.

That previous discussion of shaft current was very informative, I followed it closely at the time. These motors indeed have a motor to ground cable of common (unshielded) ground cable. Unfortunately the visiting drive "expert" suggested chokes on the power cables. What!???

Thank you,

The deformation under load of a rolling element is tiny, or referred to as Hertzian.

Surface or deeper flaws allow the "egg to crack" so to speak: many bearing components are case-hardened, leaving a hard shell of some thickness over softer material. Bearing quality or manufacturing practices might be at issue.

Beware of insulating the bearings. This tends to send the problem elsewhere (through the coupling and into the driven component).

The VFD may partially be at cause. Power quality is a likely issue (the VFD can only do so much with what it receives, and if many UNFILTERED VFDs are present, the waveform for the power line must be polluted in the extreme).

Finally, the "ground to brush on shaft-mounted copper or brass ring" might extend life (it never solves the problem), but a conductive lubricant may be a solution in this case (avoids inner lightning across element to race gap by providing easy and evenly distributed channels).

Are the raceways affected?

From a machine design standpoint, the "egg cracking" may also have much to do with bearing load: is the ball bearing sustaining belt tension (should be roller instead)? Is the coupling oversized for some reason?

I agree it's a pretty rare failure for a ball to split open. We have never had it. I have browsed through all the bearing failure analysis documents linked in the other thread in the failure analysis forum, and I don't remember seeing anything like that.

I agree that current through the bearings from vfd or welding is an important possibility to consider.

To go a little bit out on a limb into the realm of unknown and weird failures... I have a general suspicion from experience that "craters" in bearings balls might be associated with a loss of lubrication (although I have no explanation for the mechanism).

I have three data points that support this conclusion to varying extent:

1 - A bearing from a pump at our plant - showed up good on vibration, then had an event where oil leak caused loss of oil level, then went in to repair the seals and found bearing balls with craters as shown in the powerpoint file attached to this post. Not much damage on the races. All of this happened within a few weeks. More details here at this link: (thanks Oli)

http://www.vtab.se/PHP-NBoard/html/images/materiali/Forum2/HTML/002539.html

2 - Andy goenner posted some details in response to my thread above which identified that several experts had examined failed bearings with an identical appearance to mine on bearings from an ammonia compressor and conclded was caused by a "lack of lube". He sent me photos by email and they are almost identical to ours.

3 - Roy G made a post here on maintenanceforums.com with identical symptom. He did not reach a conclusion about the cause, but he mentioned that the grease on this machine was prone to being washed out by oil:

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...681084221#4681084221

Again I don't have much of an explanation... particularly why loss of lub would end up with these craters and yet not show a lot of evidence of overheating (in our case there was some discoloration of the races, but as you can see no rainbow pattern on the rolling elements). And the failed rolling elements you show do not look particularly the same as ours (ours were like a crater with an island in the middle... yours are like a crater with no island).

I am curious in your case whether inspection showed grease within the bearing itself (not just in the housing)

http://maintenanceforums.com/eve/forums/a/tpc/f/7161085912/m/4391082213

Hello,

Could someone please explain in simple terms what's electrical discharge in the case of VFDs...how and why it occurs. Thank you.

Rennie,
Check the thread above your last reply.

RMiller,
I had a split roller last year in a VFD motor. Mine had fluting in the outer race and the inner race had holes at the spacing of the rollers. The inner race holes had to have happened while stopped. Was it grounding through a stopped bearing or a grounded welding machine? I guess I'll never know, but the outer race proved that the grounding issue was certain.

David Eason