Crack in NDE Bearing greasecap

[Guardiano]

Hi forum experts,

We recently did the first overhaul of our 6,0 MW 11 k V wound induction motor. This is a quite new motor, three years of service.Attached is a picture of the NDE bearing greasecap. You will notice traces of cracks close to the grease line and grease exit.Also visible are traces of uncleaned paint in the cavity of the grease exit.We believe this dated back to factory.
We did not notice any abnormal vibration during the operation of the motor. It has been running trouble free since installation. We overhauled the motor because we were pushed to do so by the insurance company.
I would like to have the views of the forum on the possible cause of these cracks.

Guardiano

 

[edison123]

As a motor repairer, I see it all the time in the cast grease-cup components which I attribute to poor casting/defective design (insufficient thickness etc.). I always replace them with fabricated mild steel.

6 MW machine on anti-friction bearings ? What is the speed and what are the bearings ?

 

[Guardiano]

Hi Edison123,

Thanks for your reply. It`s a 8 pole motor and there are one roller and one ball bearings on the NDE. I tend to agree with you about the poor design.

Guardiano

 

[edison123]

Hi Guardino

Only a ball bearing + a roller bearing ? Hmmm. I would have expected at least (a roller + a ball bearing + a roller bearing) or (a spherical roller + a roller bearing) combo.

What are the rotor weight, rotor speed and the bearing nos. ?

 

[itsmoked]

In that picture it looks like the area between the cracks is not the same radius as the rest of the area. It looks like it was flattened from above. Is that for real or just a camera lens artifact?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

Was the bearing spinning a lot? Looks like pretty heavy tracks from spinning.

Is there in fact some kind of very deeply worn groove in that bore, for instance most visible just below the left crack?

If so I'm thinking improper housing diameter allowed bearing to spin which wore a groove which weakened the housing.

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[Guardiano]

Thanks all for the replies. Itsmoked, you hit it the first time. Yes, you are right, a little cavity can be seen close to the cracks area.I suspect it`s a very poor casting design.

Both vibration and temperature monitorings are done on this motor and nothing suspect were seen.Electricpete,we have not yet verified the bearing housing but the tolerance of the roller bearing is ok. Basically, there is no axial load on the greasecap because the latter acts merely as a shield.

Guardiano

 

[electricpete]

The cavity you're talking about comes from spinning of the bearing within the housing... don't you agree?

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[electricpete]

Electricpete,we have not yet verified the bearing housing but the tolerance of the roller bearing is ok. Basically, there is no axial load on the greasecap because the latter acts merely as a shield.

My scenario had nothing to do with axial load.

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[electricpete]

My previous two posts - just wanted to make sure you understood my comment. Whether this is the case or not, can't tell without a closer look.

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[macmckim]

The cavity shown in the picture is right beside the
grease discharge retangular slot. Cavitity is bad casting that made it through QC. Looks like a TECO bearing cap as they use that blue paint on machined surfaces. I can see the grease discharge tube below the cap that connected to the grease cleaning tube. The cracks I have seen before, when brg cap is assembled a bit off center onto motor end bracket using a air impact gun to tighten the brg cap bolts.

 

[Guardiano]

Yes, macmckim, it`s a teco motor. I agree that this is the result of bad casting. My question is could we run the risk to operate this motor with this bearing cap ? What are the posible consequences ?

Guardiano

 

[macmckim]

Guardiano

I think you can operate the motor without any problems.
Does the NDE bearing have a snap ring on the shaft that acts as backstop to the bearing? If so, then the bearing cap is just keeping the grease in place and dust etc out.

Mac