EDM on the outer diameter of the bearing

I have come across many bearings with bearing fluting (EDM) with the damage on the inner or outer bearing raceway. But this is the first bearing where I have seen it on the outer diameter of the bearing outer raceway. Has anyone else come across this?

This was on a AC soft start.

Images of the outer diameter.

This was the inner raceway.



Update:  8th August 2017.

Site Inspection: This motor drives a process blower and is isolated from the ground via rubber isolation mounts, there is no earth bonding on the motor or blower. The blower pipework is also isolated by expansion joints.

The motor  supply cables are three seperate SWA with a seperate earth cable that has 28.5 Amps on it, probably due to back EMF.

After onsite shaft current discharge tests also revealing no current discharge when direct on line this looks to be Static generated Electrical discharge damage.


Update:  13th August 2017.

Responding to some comments here are some more photos:

Image of the motor and blower.

DE Shaft.

DE Rolling element.

DE Zoom into rolling element.

DE Bearing Outside diameter.



16 thoughts on “EDM on the outer diameter of the bearing

  1. is it possible an insulating film may have developed around the other surfaces of the outer race? was there any unique characteristics in the vibration data you were able to single out and associate with this observation?

  2. I have several instances of quite bad electrical fluting on bearing races, but not on outer surface of ring. There must have been similar erosion on inside of bearing housing, corresponding to this outer ring damage? The it must be spark erosion with a defect in housing over period of time. Even though it is on rubber mount, there may be earthing on base frame somewhere?

    • Hi, thanks for the comment.

      The only earthing is the seperate one from the electrical drive to the motor terminal box, the one carrying 28.5 Amps. There is no bonding at all on the motor or blower. I will see if I have any photos.

      Thanks again.

  3. Hi, hmmm strange, it maybe an unbalanced fan turbine and the bearing due to excessive vibration has obviously got extremely hot. Due to the size of the motor I’d say this is a large blower? Vibration analysis and a full motor check would be my starting point.

  4. I have experienced several cases of fluting with the typical markings on the outer race, from what I see in the pictures my conclusion is that this does not look like fluting.

  5. The inner race surface seems to have been suffering from lack of lubrication (colourisation) which may have resulted in a (temporal) discharge path through the bearing (plus some HF noise). Static buildup and discharge may be a repetitive process as long as the electrical flow path exists. Were the bearings lubricated before you performed the discharge tests? Nice case.

    • These images were from the motor NDE that failed, the other bearing on the failed motor DE was like molten metal. This motor failed only a few months after previous repair.

      The discharge test was performed on a brand new motor with an insulated NDE bearing.

      I will try to get some more photos up on this later.

  6. James, do you have vibration data from the last weeks before the motor failure? Any signs of marginal lubrication?

    • Hi, unfortunately no data as it’s not under a monitoring programme. I only got called after the repeate failures! I would guess that there would have been huge signs of lubrication deficiencies.

      I was thinking that if it was random static build up and discharge that this will likely carbonise the grease and then cause the catastrophic failue.

  7. This sort of bearing damage/failure is prevalent in large double portal cranes i.e. ECL, that operate above earth in aluminium smelters. It can also be observed when welding on the structure of the crane has been carried out without proper isolation and grounding. It tends to be seen in the large slow rotating elements such s the long travel bogeys.
    Vibration analysis is not very effective in alerting to an imminent failure due to very low rotational speeds involved.

  8. James, I am fairly confident that you have an eddy current – very simple to fix.
    All you need to do is ground all of the components including the shaft in your circuit with a common ground (brushes on the shaft) and run it to ground; not to electrical earth. My guess is that you are generating a current on the fan and electricity will typically take the easiest path which as it builds up, is passing through the moving elements of dissimilar metals and is arcing across the bearings. A ground wire of say 6mm wire, connected to the feet of each component, motor, bearings, gearbox, shaft etc will equalise these components and prevent the current arcing.
    Do not run it to your electrical earth – completely separate.

  9. Based on past experiences I recommend that you follow the advice of Bob Smith. Sure looks like arcing where the current is seeking ground. Give it the path YOU want it to take.

  10. The pictures of the bearing do not appear to look like normal fluting to me. It looks more like damage resulting from a welding machine grounded through bearings while stationary. Could this be possible? I suggest you have the bearings inspected/analysed by a reputable vendor such as skf

  11. I came across multiple melting spots and irregular linear melting traces of different extent on the back of plain bearings. The same features were also found on the white metal running surfaces. The traces on the bearing backs were well visible due to the low melting temperature of a thin Sn-flash. In general, these patterns or defects had a destructive effect on the running surfaces by e.g. crack initiation. Often, thermally decomposed lube oil was detected within the melting spots or on the bearing backs. The only reason for such spot-like thermal effect is electricity, i.e. electric discharge across the oil film.

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