Radial tilted pad subjected to overheating - reliable after repair? 3

[electricpete]

We experienced a bearing pad overheating event due to lack of oil flow on radial tilted pad bearing for vertical motor. Lots of darkened oil residue and some babbit melting.

No replacement bearing pads available so the shop has cleaned up the bearings very well by cleaning oil residue with solvent, scraping and scotch-briting the babbit. Now the babbit surface is smooth.

My questions:
1 - Do we expect there may have been damage to the babbit/steel bond due to the overheating event? Is it necessary to do a UT to see if any damage was done? (no damage is visible at the externally-visible boundary between babbit and steel)

2 - Assuming no UT inspeciton, do we expect this pad to operate reliably or can previous overheating have an adverse affect (assuming repair was competent although I realize they can't precisely control things like radius of curvatuve during hand scraping).

3 - Assuming bearing passes initial testing (satisfactory performance on temperature, vibration , oil samples), should we have confidence the repaired pad is as good as new or would it be worthwhile to install new pads at some future convenient opportunity?

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[electricpete]

Here is link to a photo

http://maintenanceforums.com/eve/ubb.x/a/ga/ul/7091033121/inlineimg/Y/Dsc00055.jpg

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[unclesyd]

The link doesn't work as is.

 

[diamondjim]

Can you rotate the pads?
This might be a short solution.

 

[rnd2]

Hi electricpete. It really doesn't look like you have much option other than -3- and replace at the earliest convenient opportunity. A minor overheating event has occurred, not too serious it would seem but enough to melt the babbit.

 

[Tmoose]

http://www.kingsbury.com/library.shtml

"A General Guide To Hydrodynamic Bearings"

They talk about repairing some pretty severely damaged bearings.

 

[electricpete]

Thanks guys. Some good thoughts and perspective. Which article at the Kingsbury link should I look at?

I did come to understand a little better the nature of the debonding concern after overheating. The babbit temp coefficient of expansion can be almost twice as high as steel. Also the babbit temp can be higher than the steel. Both factors contribute to differential expansion which can stress the bond.

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[vanstoja]

For a severe rub incident, I would be more concerned about pad assembly thermal warpage than babbitt to shell bonding integrity. If you have changed the "machined" radius of the pad load surface by either thermal warpage or post-repair surface grinding, the pad load capacity could be affected (either adversely or beneficially) by altering the radial bearing pad's surface crowning feature known as "preload". Can you or have you measured the pad "machined" radius after repairs to the babbitt surface? If the pads were intentionally designed with a specific preload for load capacity or dynamic stability, then any deviations from bearing design conditions should perhaps be referred to the bearing designer for verification of loading reliability and adequacy of operating fluid film thicknesses.