OB thrust bearings discolouring oil 5

[rota1]

Hello all!
we have here a Byron Jackson two stage, between bearings, boiler feed water pump that discolours the oil upon minutes of being put in service,bearings are skf 7409BG, and the driver is a motor running on a magnetic center.Coupling is a Waldron gear type, the spool piece has axial movement of 3/16".All alignment settings are within tolerance.This pump failed recently, bearings appeared to be burnt and significant impacting on the balls were evident.Please advise

 

[ardeIII]

Depending on the thrust characteristics of the pump (which can vary depending on wear ring clearance, interstage bushing clearance if multistage, and point of operation on the pump curve, amongst other things), with a set of 40 degree contact angle thrust bearings with no preload it is highly likely that at least one of the bearings is skidding. The thrust load is typically primarily in one direction only, and depending on balance characteristics of the rotating element there may be little thrust load in either direction. A 40 deg contact angle bearing requires thrust loading in order to keep the balls spinning. In absence of this load the balls "skid" which results in wear of the balls and cages (the cage wear is probably what is darkening your oil-look for brass contaminants in the sample).

If this is a new problem you might have internal clearances opening up or have changed the operating point on the curve, which has changed your thrust loading. You'll want to get with the pump manufacturer and determine the normal thrust direction and loading of the pump at several points on the curve. Armed with this information you can make a bearing selection that will better fit your conditions (pump manufacturer or bearing manufacturer can help you pick the proper setup). An MRC pumpac (40/15 degree combination) or diamondpac (15/15 degree) is a "drop in" replacement that might well be the solution to your problem.

Also you can send your damaged bearings to SKF for analysis, although they almost universally blame "poor lubrication" for the failure.

 

[bingopin]

Of course, many things could be going on here but I think the comments on the possibility of ball skidding and potential fix with the 40/15 setup are good ones. If skidding is occurring, you typically only see indications on one of the two bearings...the unloaded one (unless you run to complete failure). Even after only a few hours runtime, the balls will apperar "frosted".
If this is a new problem that appeared suddenly with existing bearings, might want to check that the bearing locknut is being properly tightened. If it is not tight enough, the results could be a skidding bearing. If the nut is very loose, excessive axial shaft movement will be present. Since these bearings are relatively small, it's probably not a bearing nut issue, but I have seen a loose nut cause skidding on a 7900 series bearing.

 

[rota1]

Thanks guys!
How will we know that we have assembled the bearing correctly? is there a procedure? I have never seen one! if there is, one can you guys supply me with it!
Thanks again guys!

 

[JJPellin]

I have a few other suggestions in addition to the excellent points made above.

First, I would verify that the bearings are flush ground for back-to-back mounting. I apologize, but I am not familiar with the SKF suffixes.

Then I would verify the radial and axial fits. The shaft fit should have an interference of at least 0.0001". The housing fit should have clearance of at least 0.0005". The inner races should be tightly clamped together by the shaft nut. We don't usually torque this nut, but that is a very good practice. The outer races should have axial clearance. We prefer an axial bump of 0.002" to 0.004" on a pump of this style. Some manufacturers recommend more.

Next I would verify that appropriate mounting methods are being employed. The bearings should be heated (preferably with induction or an oven) and slipped onto the shaft hot. In your original post, you mention signs of impacting which could suggest mounting problems. The bearings should not be pressed or driven onto the shaft. Likewise, the coupling hub should be installed or repositioned by striking with a hammer.

Next I would verify lubrication. I am picturing a small bearing housing like a model GSJH or similar with oil ring lubrication. Verify the oil level and oil ring size. Some of these have different size oil rings on the radial and thrust ends. If they were reversed, they might not deliver proper oil to the bearing.

Most of the other likely culprits should show up in the vibration: imbalance, bent shaft, cavitation, misalignment, looseness, etc. For shaft alignment I would check the method used. On more than one occasion, I have found mechanics trying to align a gear coupling with the laser heads mounted on the shrouds rather than the hub or shaft. This will indicate perfect alignment every time, regardless of the actual alignment condition.

Based on your original post, choosing from the options I mentioned, I would be most suspicious of incorrect bearing (not flush ground), abusive mounting technique, inadequate lubrication (oil level, oil rings function). Only after ruling out all of these possibilities would I convert to 40/15 or 15/15 bearings.

 

[JJPellin]

Excuse me. I should have said the coupling hub should not be stuck with a hammer.

 

[jet1749]

Hi, had similar recent problems on recently overhauled David Brown multistage crude oil charge pumps. Problem seems to be skidding row of balls in double row angular contact bearing and also wear of the pressed steel bearing cage (SKF Explorer bearing)See this article for more explanation

http://www.noria.com/learning_center/category_article.asp?articleid=652&relatedbookgroup=OilAnalysis

regards
John

 

[rota1]

Hello guys!
Thanks for all the invaluable information.This is a follow up question, we have verified the thrust bearing on this pump is skf 7409 BCBM and discolouring oil, the OEM says it should be skf 7409 BG, we have now changed to this and the discolouring has stopped.Our research says that the BCBM bearings can be mounted in a back to back arrangement(even without the designation "G").The question is what then is the difference between the skf 7409 BG and skf 7409 BCBM.

 

[JJPellin]

There is a thread on another forum that may help. I do not know how to link to that thread.

It is on the Bearing Design, Manufacture and Maintenance forum. It describes the suffix G as indicating flush ground bearings that are ground for face to face, back to back with any bearing of the same type. Other bearings are only ground for one orientation or are ground only be mounted in matched pairs and cannot be mated with any other bearing.


Johnny Pellin

 

[electricpete]

I think this is the one you're referring to:
thread821-183480

(to create a link on eng-tips, just cut/paste the thread number exactly as given at the beginning of the thread)

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