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Increased clearances between Bearing outer race and Housing of motors

Increased clearances between Bearing outer race and Housing of motors

Increased clearances between Bearing outer race and Housing of motors

(OP)
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

RE: Increased clearances between Bearing outer race and Housing of motors

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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RE: Increased clearances between Bearing outer race and Housing of motors

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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