I recently found heavy electrical fluting damage on the ODE bearing of my 700 hp DC motor. Any ideas why I had higher peaks of 21600 cpm (SCR firing frequency) showing up on my DE bearing?...especially in axial direction? I did have the 21600 CPM peak on the ODE, but they appear to be lower than the DE, yet this is where the damage is? There was no evidence of fluting on the DE bearing, just some circumferential scratches on the OD that indicate the OD spun in the housing.

Consider current as rain water on a leaky roof. It will sure find any hole and prefer to get through that way. Current finds the shortest - meaning techncially the lowest resistance path to ground. Where that bearing is, the insulatiing oil film was/is the thinnest and maybe has the most conductive particles, etc. You need to offer an even better hole - offer a path with even smaller resistance to ground. A brush can do wonders. Regards Arne

I understand that concept Arne, but I am still a little confused as to why the bigger indication was on the end with zero damage? Understand that this is a metal contact/conduction issue, but I cannot fathom how. Bearing fit? Bearing clearance? Are these conditions helping to amplify the 21600 cpm vibration while at the same time discouraging the current flow?

Mark

Hi Mark;
I think the 21600cpm peaks are probably irrelevant in the case of electrical fluting damage. Leaking current is leaking current regardless of the frequency or voltage. Just my opinion ofcourse.

I am in agreement with vibbase. 21600cpm may be present, but by themslf do not necessarily signify presence of fluting. Fluting on the races will manifest itself rather as a cluster of BPFI or BPFO harmonics in the higher frequency range of the spectrum. Mark, take a closer look for this pattern and post it please in normal and Db scale..

maybe firing pulse frequency shows up on de brg cause it is closer to the poles/core which is where the mechanical force you are measuring is being generated. The ode brg is normally twice as far away, through a less stiff structure. Is the frequency very directional, if so suspect pole fault, if not suspect drive fault.
Shaft current will result from unequal magnetic field