Predicting Bearing failure due to inner race slipage 4

[DieselGuy]

I am involved with a root cause investigation on a fan motor (100HP) bearing failure. Vibration Analysis is periodically performed (at least every 6 months) on this fan. Motor bearings had been replaced approximately 1 year prior to the failure, fan runs approximately 6 months out of a 12 month period. Failure analysiis of the motor indicates tha the inner race on the inboard bearing slipped on the shaft resulting in heat buildup, galling and an eventual bearing lubricant fire. My question is should the failure of this motor bearing be predictable with vibration technlogies, or would you have to be lucky to identify this prior to it being into a short term failure mode? Plant operators observe the fan at least twice daily. Control Room would see low flow or train trips only.

 

[alexit]

Depending on arrangement and bearing type you would see a loose inner race as loss of preload and resultant high vibration. Was the bearing lock nut not installed or installed incorrectly? Were these permanently lubed deep-groove bearings?

 

[DieselGuy]

This is an interference fit bearing to shaft (approx 3.125"Diam). Bearing is heated and slid into place. No locking collars or set screws. We are looking into work history to find tolerances of the shaft and bearing to see if there was adequate interference fit. Most likely not. Question is, is this typically a rapid failure mode or one that predictive technologies should detect before failure?

 

[imok2]

http://www.materialsengineer.com/CA-scc.htm

Some of the factors that may lead to bearing failure are improper lubrication, impact loading, vibration, excess temperature, contamination, excessive loading, and misalignment. In addition, some common characteristics of bearing failures are: Wear, Fretting, Corrosion, Indentations, Electrical pitting, Smearing , Cracking , and Flaking

 

[imok2]

Question: by heating the inner race to fit could you be distorting it. Is it posible to cool the shaft to fit the bearing? Just a thought!

 

[electricpete]

Heating the bearing is common. I think SKF says it's ok as long as you stay under 250F.

We have never seen this particular failure mode at our plant.

It's easy to imagine that it can be rapid because the rotating nature of the load on the inner/ring shaft fit (I'm assuming vertical machine) will cause relative movement once looseness occurs... leads to more looseness and more movement. In contrast looseness of outer ring in housing will not lead to much movement.

I would suspect that the symptoms might be 1x, possible harmonics, possible raised noise floor.

If this is a vane axial fan within the duct, you have measure very far away from bearings anyway and ability to detect anything is severely hampered.

I would recommend you post this question on maintenanceforums.com

There are a lot of rotating equipment vibration specialists there and I'll bet you get a good response.

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

 

[MikeHalloran]

You might want to track down whoever fitted the bearing and provide them either a bottle of Loctite, or a pink slip.



Mike Halloran
Pembroke Pines, FL, USA

 

[dgallagher]

Looseness in general will exhibit stong running speed harmonics in a FFT and the vibration waveform may be "noisy".

We typically see a dominant 3X peak when the bearing is slipping on the shaft and 4X when the bearing is slipping in the housing.

http://www.reliabilitydirect.com/brg_loose.htm

If heating the bearing, make sure to use an induction bearing heater so as not to damage the bearing:

http://www.maintenanceproductsdirect.com/category_s/87.htm

Dave G.

http://www.ReliabilityDirect.com