White metal bearing cavitation

[kclim]

Hello folks

Recently had a failure involving a turbine white metal plain bearing that straddles a thrust collar.

On disassembly, it was found that much of the white metal surface was damaged. See the following images

http://img71.imageshack.us/img71/5899/dscf00752tc.jpg

http://img71.imageshack.us/img71/7951/dscf00799ay.jpg

http://img71.imageshack.us/img71/5466/dscf00809ed.jpg

It looks like a lot of white metal has been gouged out, with numerous cracks around the gouges.

(The bottom half of the bearing appeared to be fine, apart from a few minor scores.)

Has anyone seen this type of damage before? I'm trying to figure out what mechanism has effected it. I initially suspected cavitation, as the damage is on the top half (low pressure region). I imagine fatigue might be another possibility.

Any leads would be much appreciated.

 

[Carburize]

Take a look at the images in

http://www.machinerylubrication.com/article_detail.asp?articleid=638&relatedbookgroup=Lubrication

Your damage looks like mechanical fatigue rather than cavitation.

 

[cruzinonline]

While I agree that the site mentioned is very informative, I have seen this before and think you may have a grounding issue rather than fatigue. What is the condition of your grounding brushes or shaft riders?

 

[taylorg]

Carburize and Cruzinoline both make valid suggestions, although from the pictures I'm not sure you will have grounding bushes.

My own humble opinion is that your symptoms are that of fatigue failure, and as it is the upper half which is affected it may have been initiated by flexing of the bearing into a shallow cavity between bearing and housing; fatigue crack propagation would then occur.

A few recommendations:
1. Check the bore geometry, diameter and bearing crush to determine whether the design conditions had been met, particularly contact pressure.
2. Carry out an engineers blue check of the assembled bearing back to bore fit, using new bearings with any carbonised oil residue removed from the housing.
3. Check oil galleries and drillings for debris contamination.
4. Check for filter element disruption or bypass operation which may have introduced debris into the bearings.
5. Get the mode of failure confirmed by a lab, who will also check for abnormalities in the white metal to determine if a manufacturing fault was to blame.

Good luck!

 

[Tmoose]

Is any temperature info available? Near-Insert temperature would be useful. Bulk oil temp is much less so.

 

[kclim]

Thanks for info all.

Taylorg, I don't think bearing crush is an issue, as there is no shell (i.e. the white metal is directly bonded to the bearing). They do put shims between the bearing halves to get the joint clearances right though.

Analyses of the oil showed know unusual results, contaminant wise.

I have been told that there was nothing unusual in regards to oil temperatures and pressures.

Repairs have already been performed, so unfortunately no white metal samples could be obtained.

The bearing had been inspected in the past, with only minor porosity noted - certainly, not enough to be rejectable.

One thing I should have mentioned is that the damage was found while the turbine was being taken off-line, due to a turbine failure. Didn't mention this initially, because I was trying to explore the possibility that the failure of the bearing was responsible for turbine failure. I wanted the damage of bearing to be taken at face value.

Regards