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Bearing failure investigation

Bearing failure investigation

(OP)
Dear all,

We have faced a bearing failure at one of the overhung centrifugal pumps installed at our plant. The failure was picked quite early using vibration spectrum and was replaced at early stages saving secondary damages.

In order to reach out to the root cause, the bearing was disected and flaking was observed on half of inner race and a couple of balls. This was a SKF 5306 ball bearing. The bearing is lubricated through Regal R&O 68 oil(Chevron).

Only one half of DE bearing is affected while the other half along with NDE bearing which is lubricated thorugh same oil has remained unaffected. Request members to help out!

Regards,
SSA.

RE: Bearing failure investigation

Depending on your relationship with your bearing supplier, and the supplier's relationship with SKF you might get SKf to analyze if for free, or reduced cost.

During the bearing replacement the shaft and housing features should have been carefully qualified (measured) to answer questions that will likely emerge from the failure analysis.

If you provide pictures comparison could be made to the several bearing failure analysis documents on line from various manufacturers.
I Google "bearing failure analysis pdf"

https://ec.kamandirect.com/content/resources/2010/...

http://www.schaeffler.com/remotemedien/media/_shar...

http://www.bartlettbearing.com/pdf/Bearing%20Handb...
http://www.skf.com/group/products/bearings-units-h...

http://www.koyousa.com/brochures/pdfs/catb3001e.pd...

http://www.emersonbearing.com/technical-toolbox.ht...

RE: Bearing failure investigation

In my opinion, the first thing to determine is:

Is the spalling caused by surface initiated fatigue or subsurface initiated fatigue?


Surface initiated fatigue is linked to abrasive wear and surface distress typically caused by inadequate lubrication or contamination. It will look like the surface is flaking off and the damage texture will be constant and continuous around a large portion of the raceway.

Subsurface initiated fatigue is the result of regular fatigue caused by strain within the ring(s) and rolling elements. Spalling originating from subsurface initiated fatigue typically looks deeper, sharper and more discrete than surface initiated fatigue.
Subsurface initiated fatigue is unavoidable and will happen to each bearing over time (as long as the load applied to the bearing surpasses the fatigue load limit). Accordingly, a higher load will reduce the fatigue life of the bearing and subsurface initiated fatigue will manifest itself faster.


Once you determine which type of damage you have, you can try the following:

If surface iniated:
- Check kappa ratio and determine if the oil viscosity/additive package is proper for the temperature and speed of the bearing.
- Conduct oil sample analysis to determine the extent of contamination or degradation to the oil.
- Review the method of lubrication of the bearing. If oil circulation, check if the flow rate is adequate and filters are OK. If oil or splash ring, make sure that it is not damaged and is picking up the oil correctly. If oil fligner or oil bath make sure that the oil level is at the proper height.
- Verify if there is an issue with the sealing that may have led to reduced lubricant or may have the ingress of contamination.


If subsurface iniated:
- Verify that the pump flow rate is near the best efficiency point (+/- 15%). If it isn't, this could be creating an overload in the pump and accelerating the fatigue in the bearing.
- Check for misalignment or shaft deflection which could pinch the raceways leading to a higher load.
- Verify that the pump is adequate to pump the fluid that you are pumping. For instance, if you are pumping a slurry or a dense fluid, is the pump able to handle this?
- If the drive end bearing is the one that is damaged then perhaps this is caused by the belt or coupling system that is driving the pump (depending on the type of pump you have).

RE: Bearing failure investigation

Quote:

flaking was observed on half of inner race and a couple of balls

I would suspect the damage occurred to the inner race first and by the pattern reported this would imply you had significant thrust loading favoring one side of the roller bearing.

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