Cracks on shafts/rotors 4000HP 1

[nucferr]

We have tantum (shaft to shaft)4500HP motors for a feed pump. During a 5 yr PM a motor company reported finding cracks in the rotors of these motors in the area of the stainless steel shrink ring. Further inspection of the rotors found that the cracks extended under the surface of the shrink ring and into the rotor bar shorting ring assembly. It was also noticed that the shrink ring had moved toward the outer end of the rotor. This condition was present on both ends of the rotors for both motors. This should not have happened as the shrink ring has an interference fit to the rotor of approximately 0.025”. The cracks in the rotor bar shorting ring were causing a high resistance connection to develop between the rotor bar lamination and thus produced a large amount of heat. The motor shop has stated that they have not seen this type of failure before on this type of motor (Allis Chalmers)
Anyone have any ideas?

 

[aolalde]

The stainless steel of the retaining rings is subject to extreme tensile stress. This due to the centrifugal forces (is it 3600 rpm?) and the interference fit.
The material could be defective or with the wrong excessive interference fit. After it cracked, the bracing of the short circuit rings was lost, the short circuiting ring and bars fractured and the resultant high resistance paths of currents developed.
You should review the mechanical design of those retaining rings to ensure that have the required characteristics demanded by the speed, interference and supported mass.

 

[nucferr]

Aolalde
Thanks for the post. Yes it is a 3600rpm. The part that bothers me is that the motor shop (which we have do all of our motors-and has been a good source of knowledge) states they have never seen this before. We have 5 additional sets of these tantum motors in service now. I am on a root cause team that also needs to determine extent of condition (as related to the other 5 sets). I will not have access to the other sets therefore need as much input as I can get.
Thanks again!

 

[edison123]

In high speed generators, old 18Mn-5Cr steel retaining rings develop stress corrosion cracks in a moist environment due to their chemical composition. They are being replaced now with 18Mn-18Cr steel rings.

May be your motor has this 18Mn-5Cr steel rings?