We had recently replaced two fan bearings due to indications of inner race bearing defects and there was an unbalance on the fan blades (silica build-up on the fan blades, which was cleaned off). On further inspection of the bearings, the inboard bearing show signs of electrical fluting. This fan is on a VFD drive, max rpm is 1200rpm, 25000cfm. Overhung fiberglassed fan, supported by two fan bearings FAG 22215E, coupled to the motor shaft via a rubber coupling. We ran a crude shaft current test using a Fluke multimeter. The PD across the shaft is about 0.2 V and the unit is well grounded. I've attached pre and post readings when the bearings were replaced (CSI 2120).

EF20-1-10.doc (519 Kb, 74 downloads) Picture

Attached is a picture of the bearing

Oh yea, that's fluting! Nice big picture too.
Pete

Forgive my ignorance, but how could I have electrical fluting on the fan bearing, when it should be isolated from the motor through the rubber coupling? Shouldn't I expect to see this at least on the motor bearings first?

I agree it looks like most likely fluting from electric current.

You probably wouldn't see damage on the motor bearngs because the motor probably has both bearings insulated for a vfd motor.

It's a very good question whether current can flow through this coupling. You said rubber couplign... do you have a brand or model name?

Just to keep an open mind to other possibilities, the SKF bearing failure analysis document suggests that bearing vibration while running can sometimes give a fluting pattern very similar to electrical fluting. See figure 22 on left side of first slide attached. (I have never seen this myself - I have only seen false brinneling from vib while shut down which produces different pattern with marks at roller spacing).

VibFluting.ppt (476 Kb, 48 downloads)

Is static build-up a possibility to generate EDM?

I am inclined to agree with Pete, it may be vibration induced. We have had damage to bearings on wet felt rolls on a paper machine that looks a lot like electrical fluting, but, given that these rolls are not connected to any drive motor (they driven by the screen), and have no chance of any static charge due to being in a wet environment, it would be highly unlikely that it was electrical fluting.