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 inspecton, 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?

If I can figure out how to do this you can see a photo.

How thick was the babbit before, and how thick is it now? How much has the clearance opened up? How fast do you need this back in service? You can get pads rebabitted and turned around in as little as a day at some of the shops here in Houston. Talk to TCE, or Turbocare.

I would UT the pads and make sure you have good bond. Assuming they pass the UT, I would replace as soon as possible in a future outage. Even if it passes the UT, there may be microcracking along the bond line that can grow and result in the surface flaking off under dynamic loading. If you lost a lot of babbit, you also risk having a piece of debris stick in the pad surface and cause a wire wool failure in the shaft.

Thanks Steve.

I am not sure the change in dimension of the babbit. I will check on that.

We are resetting the clearances so the missing babbit will not affect the clearances.

Based on schedule considerations and OEM recommendations we are on course to reassemble using these repaired pads without opportunity to have done UT.

Is it realistic that overheating can damage the bond? I thought bond problems came mostly from original manufacture. Can we draw any comfort from the fact no visible voids/cracks at the externally-visible bond surface?

Without knowing anthing about the particulars of the bearing service, I shall offer the following for what it is worth.

Scraping pads can change the geometry. If the OEM went to the trouble of a tilt pad, this may be critical.

If this is a horizontal machine, can you move the scraped pads to the upper positions (same pad geometry?)? If all pads have the same geometry, the upper postion will generally prove less critical (unless the residual force is up, like many gears).

I am attaching a procedure for determing the clearence in a tilting pad bearing. This may be of any help at this point but should come in handy in the future. By knowing the bearing running clearence prior to installing a new bearing, if this should ever happen again you will be able to determine if there is excessive clearence prior to reassembling the bearing.

Bearing_Inspection_Data_Sheets.xls (76 Kb, 29 downloads)

Hey Pete, depending on the steel used in the backing material the coefficient of thermal expansion may be a little as 6 E -6 in/in/F. Babbitt is around 13 E -6 in/in/F. Anytime there is differiential thermal expansion, cracking is a possibility, especially at a place like a bond line.

William and Steve - good point about possibly changing the geometry - radius of curvature and therefore "preload" of the pads. To my thinking that is a short term concern and would show up in temperature or vibration during the testing and if we pass testing, not a further concern. (OK, it could be bad radius on a pad without temp monitoring but I'm not going to worry about that... or should I?).

(btw we haven't had a representative run following repair yet.... very very long story which I will save for another day).

Skip - the way the clearances were adjusted here was to first center the rotor within the bearing bracket (rotor supported from overhead hoist), then adjust all pads into contact with the runner. Then put a dial indicator on the back of the adjusting device and back out until reach the desired radial clearance. Seems ok to me. I didn't follow the calcs in your spreadsheet to understand what it was doing but I'd be itnerested if you can explain a little more.

Steve - good point about thermal expansion. Coefficient of expansion is higher for babbit and temperature is higher for babbit so both factors would contribute to babbit expanding more than steel. Now I understand that concern a little better. I got a good email from Arne with an example case where debond did appear after overheating. Seems like it is something to watch for.

Any other comments?

Electricpete,

Only thinking about:

I think that if this system is working and you maintain conditions and loads are normal loads could be usefull for a long time, like a new bearing.

Next time, when it fails is the same thing if it fails because film is not bonded or it burns because a new high temperature.

If film is there and conditions are good since now (oil quality and maintenance) bearing is going to operate nice durin a long time.

Iván Pérez G.

BPI there in Houston has always given us great
service and quick turn around on bearing repairs.