Increased clearances between Bearing outer race and Housing of motors

[VMATHUR]

We have many 2-pole, 3-Phase induction motors ranging from FHP to 200 KW (on 415 Volts) in our complex. It has been observed that after 3-4 years of operation clearances between the outer race of the bearing and the Bearing-housing increases beyond the limits. We found that it is to the tune of 0.04-0.05 mm. This causes rotation of bearing in the housing and lead to high vibration. The phenomenon in more prominent in 2-pole (high speed) motors.

The driven equipment are normally centrifugal pumps.

Is it a normal phenomenon? Otherwise what could be the reasons for this problem?

Thanks

 

[electricpete]

Typical requirement for an electric motor is a k5 shaft fit and H6 housing fit. It is a tigher fit at the shaft-to-inner ring (slight interference) than the housing-to-outer ring (slight clearance).

A slight amount of clearance (as given by H6) is desirable to allow floating bearing function... bearing is free to move slightly within the housing to accomodate thermal expansion.

I believe it is normal that upon inspection of removed bearing you can see black fretting marks on the outer ring. I have seen that myself. I think it is indication of very small intermittent movement.

I don't believe it is normal that there should be continuous movement that will show up on vibration. At that point I think the clearance is excessive and movement will lead to large scale fretting and possible heating and action should be taken.

Why would it get that way after a few years? I can speculate that perhaps the original fit was not not H6 or the combination of specified fit and bearing was not proper for the application. Another thing: I would inspect the removed bearing and housing to see if there are any clues.

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[electricpete]

Another thing that can help a bearing work loose I think is rotating load. Either misalignment or severe unbalance can cause rotating load which tends to help it spin and keep it spinning.

I believe that is the reason the inner ring is specified tight. On a normal horizontal machine the load (weight of the rotor) is always rotating with respect to the shaft/inner ring fit. This facilitates sliding and is why we need a tighter fit on the inner ring.

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