Pin point failure anaylsis / root cause 3

[JENNIFER429]

I have a 250hp 1800rpm 449ts TEFC GE motor in my shop. This is the second time. The first repair was performed in August of 2005. The DE bearing journal on the shaft was undersized .033 which allowed the rotor to poll dragging the stator iron and blowing the windings. Now here it is June of 2006 and the problem has occured again. I have checked the bearing for all possible failures, and have come to the conclusion that since this motor is in a direct coupled application, misalighnment may be the cause. I am looking for any other suggestions as to what may be a cause of failure.

 

[petronila]

Hello Jennifer 429,

I think is better you ask to your costumer about latest vibration analysis performed on the motor. This could clarify if the failure comes from missalingment.

The bearing failures could be produced by others factors like: poor lubrication. Defective fits. Preload. Contamination. Like an excercise you could cut the bearing in order to find some tippical pattern about this failure. The other think to do is a core test, to be sure about the stator core lamination condition, some cores with excesive watt/pound looses will produce more heat this excesive temperature will be nocive for the bearing´s grease.

Best regards
Petronila.

 

[aolalde]

What is the bering type and size?
Sometimes roller bearings with light radial load skid (instead of rolling), overheating and failing in short time. For that typical case, replacing the roller bearing with similar ball bearing could stop the bearing failure. I wonder, Is this your case?

 

[powerjunx]

i agree to petronila, procurement of vibration analysis would somehow tell you the roots. But common cause on a resort of undersized journals is due poor lubes where bearing overheats to the extent of its operation.

Can you figure out the MOTOR-LOAD configuration, in a concise manner? How well the Coupling? We'll be shooting this thing broadly, but if you can provide us a real set up maybe we'll hitting exactly the particular roots?

 

[electricpete]

I also agree vib will be a good gtool?
Did it rub during start or running?

Since this is TEFC I assume we are talking rolling element bearings?

"The DE bearing journal on the shaft was undersized .033"
Wow. That would certainly seem like a likely candidate to expalin the first problem. Total airap might be only 0.06 on some motors (or 0.15 on others). In any case large effective clearance can result in dynamics and magnetic pull playing bigger roles.

I assume the inner ring was spinning on the shaft and slowly increasing that clearance?

The question then is what makes a shaft spin on housing. I would think the prime suspects would be installation (proper fit at both the shaft and the housing).

What was the repair the first time? Build up the shaft and then grind down? Did you check both shaft and housing fits afterward?

I have heard of shaft-to-shaft misalignment accelerating movement of outer ring within housing (loose fit) but not movement of inner ring on the shaft. Then again I'm no expert.




=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

Can you tell us what type of machine this is? Also, can you describe the coupling type? Specifically, is there any resilience in the coupling or is it a firm fit where misallignment will load the bearing? Was the "August of 2005" repair the first repair on the machine or is it possible that another shop may have done earlier repairs?
Does the customer have an overzealous mechanic who may be overgreasing the bearings?
Are the bearings sealed or grease-able?
Thanks in advance for the information.
respectfully

 

[THooper]

If your rotor alignment is off when the motor is under load, it will try to pull and get mag center and cause pre-load on the bearing, rotor iron should be longer than stator iron.

Use the advise of cutting the bearing in half for inspection along with pre vib readings, temp readings etc.

check for shaft currents that cause arc pitting. note also that a direct drive should have a ball bearing on DE not a roller installed. Also note a belt drive should have a roller and not a ball.

Runout should be no more than .oo3 Most are .oo1 from the factory, rotor cores should not run out more than .003

Going back to the first failure it sounds like your customer had a poor alignment that was the cause of excessive wear on the de bearing seat, or a past poor shaft repair. If a shaft has been built up from a past repair it should be recommended to perform a complete shaft replacement on this repair.

Another thought is make sure you have the correct amount of endplay in the IE housing for thermal growth and expansion. I think the bearing inspection along with comparing it to skf failure discriptons and photos will tell the rest of the story.

kind regards

 

[electricpete]

I have a hard time seeing how the relatively small force from axial magnetic pull can play any role here.

I think it was a good point about endplay. As we know there should be room for at least one outer ring to move within the housing to accomodate endplay. If it doesn't exist or the outer ring gets bound and doens't slide, we could end up with substantial axial force transferred between inner ring and shaft which could be the beginning of movement at that location.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[THooper]

I should of elaborated a little more on my thoughts of possible mag center problems. Not real common but I have repaired probably a dozen motors in the past sixteen years where the rotor core had moved on the shaft enough to cause a constant preload on the load end bearing causing the bearing to reach excessive temperatures and eventually failing. Cutting the bearing in half showed wear ring marks on the outer bearing race, the root cause was the rotor lossing its fit to the shaft and moving off its normal mag center. This should be a standard check in all motor evaluation.