Mixing oil and grease in a gearbox 4

[paintballJim]

Not sure if this is the correct place for this but the Petroleum engineer section seemed to be for exploration so here goes.
We have a gear box that we have been using for a while now. We had been using oil in it but the manufacturer suggested using grease. We switched to the grease he suggested and started seeing bearing failure. We are planning to go back to oil but are wondering if there is a problem leaving the grease in there with the oil, or if the grease needs to be cleaned out. The thought is that there should be no problem with them both in there together but I have not been able to find anything to confirm or deny that idea.
Can someone point me in the right direction?

Thank you

 

[racookpe1978]

It must be cleaned out thoroughly.

More important, did you find out from the manufacturer WHY he wanted you to change to grease, and what the advantage was supposed to be? Clearly, you have introduced some warrantee issues - because of that recommendation. Slow speed gear lube is usually grease, high speed at high loads is usually pressurized oil. Medium speed with low loads can work with "splashed" oil from the sump.

 

[3DDave]

Mechanical grease is a combination of thickener and oil. It is probably the thickener that is causing the problem, so removing that is a good idea.

To see what I mean, place a pea size glob of grease on sheet of brown paper (like grocery bag) and wait for a week to see what is left of the original pea size. You can also look at the spec for the grease or call the maker to determine what the thickener is, but the visual evidence is more informative for the nature of the material.

 

[Artisi]

Were the failed bearings analysed for failure mode, if not - why not.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[romke]

some gearboxes are lubricated with grease - notably those that are not completely oil tight and running in such a way that heat transfer to some place outside the gearbox is not necessary. examples are found in electric hand drills, household mixers etc.

also you can find grease lubricated gears in agricultural equipment and some industrial applications.

you did not mention what type of grease you used in the gearbox. given the fact that you observed bearing damage, the grease might have been to viscous. standard lubricating grease has a consistency of NLGI 2, for gear lubrication usually a much less consistent grease is used, like NLGI 0 or NLGI 00. those thinner greases will more or less be semifluid and thus can flow back into bearings once a rolling element has passed.

 

[Artisi]

Without knowing why the bearings failed, it's all guess work as to why. Maybe you should ask yourself why didn't they fail with oil lube. Any comment from the manufacturer?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

Who says the bearings failed with oil? No mention of failure until after grease was used. The way the original post reads, the manufacturer could have just walked in the door and said "hey put grease in there."

 

[Artisi]

Did anyone mention failing with oil lube?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[paintballJim]

No, there was no failing with the oil lube. This is with an overhung load adapter used to protect the bearings in a planetary. Previously we have run with a synthetic oil and a reservoir for several years with no problems. We wanted to simplify the design so looked to the manufacturer. We found out that they actually recommend a NLGI#1 synthetic grease so we switched to that. Our RPM is around 1450 which is well below their listed range of 1900 for the grease. Both units (2 of 8 total with grease) that have failed operated for close to a year before failure and when disassembled the failed bearing appeared dry of grease. Our plan is to return to using the 75W-90 synthetic gear lube that we were previously using with no problems. Just wondering what the ramifications of simply adding oil to the remaining 6 OHLAs without removing the grease would be.

 

[paintballJim]

I should add that the manufacturer has been contacted and can see no reason for the failures. Our thought was 1 failure could be customer misuse, two totally unrelated failures out of 8 total units indicates we need to go back to what was trouble free.

 

[IRstuff]

> "dry of grease" Was there ANY grease in the bearing at all, or was the grease simply not in the bearing track?

> How did the manufacturer envision the grease distributing correctly within the bearing? Were there recommendations that were missed or not presented to you?

> Ostensibly, it was the lack of grease that caused the failures. BUT, the issue is how close to failure are the remaining units and ought you not rebuild and repack the bearings to make sure? Dry running the bearings would imply the possibility of metal particles getting generated, and if they aren't removed, the oil will ensure that they go to where they'll do the most damage.

TTFN (ta ta for now)
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[paintballJim]

The bearing that failed showed a little burnt grease on it but not the packing on the other grease in the same cavity. The housing was equipped with a grease zerk, not sure how we were to evenly distribute it other than filling the cavity. You do make a good point about the potential for material in the grease for the remaining housings.

 

[MintJulep]

To sort of actually address the question being asked:
There is obviously no problem with thoroughly cleaning out all the grease.
There might be a problem with the mixture.
So the safe and easy course of action is clean them.

Beyond that, everyone is speculating and guessing because of a near complete lack of information.

For example. Are the bearings actually grease lubricated? There is a whole world of oil-lubed gearboxes that use oil lubricated bearings. There are features in the casting that catch the splashed oil and port it into the bearing cavities. If these gearboxes are of this type then the expected result of draining the oil and throwing in grease would be failed bearings in short order.

Our RPM is around 1450 which is well below their listed range of 1900 for the grease.

Grease can be finicky. Too fast or too slow and the film doesn't form right. Too hot or too cold and the oil doesn't come out to do its job. Running "well below listed speed" is not necessarily a good thing.

 

[paintballJim]

The manufacturer's data sheet shows a graph where 0 to 1900 is indicated as "use Grease", 1900 to 2700 is indicated as "Consult Factory". I am sure there are better methods of ensuring that the bearings are well greased, like taking them out and packing them, but when the manufacturer places a grease zerk in the housing and simply indicates grease unit, you assume you are intended to use a grease gun and fill the housing. And no there probably wouldn't be much of a problem cleaning out the housing if that is what needs to be done. We were just hoping to avoid sending the dealers 4-6 hours one way to get to the unit and then having to disassemble the unit to get to the OHLA before disassembling it and cleaning it.
If that is what needs to be done so be it,there are only 6 of them to do, we were just hoping the answer to the original question would be something along the lines of " No there is no problem leaving some grease in the housing when switching to oil. The grease and oil will mix to produce a thicker oil and thinner grease." The thought there was that since we have gone several years using oil in the same units with no problem we would be able to provide better bearing lubrication by going back to the oil that had been successful. Then possibly if the grease did thicken the oil somewhat it would still have the viscosity that would allow it to flow to the bearing as needed.

 

[IRstuff]

I get the sentiment that you don't want the hassle of cleaning and prepping for oil by leaving the grease in place. But, you have to seriously consider that the two failures are not flukes, and are harbingers of failures to come on the remaining units. The oil may prolong that eventuality, but I would consider all units to be damaged goods, at this point.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[paintballJim]

And that is quite likely the direction we will head.

Thank you all.

 

[tbuelna]

One issue with grease lubed rolling element bearings is the fine metallic debris produced at the onset of common failure modes gets trapped within the grease charge. Given the history of bearing failures using grease lubrication in these gearboxes, it would seem prudent to tear them down and check the grease for metallic debris. One benefit of using oil lubrication is that this metallic debris gets flushed out of the bearings and settles to the bottom of the oil sump, where it can easily be detected without disassembling the gearbox.

 

[Artisi]

Metallic debris is the reason why magnetic drain-plugs were invented for gearboxes and similar equipment.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

Did you pack the bearings full of grease first? Or just drain oil and fill with grease without opening anything up?

 

[dik]

Is there any part of the assembly that prevents grease from getting to where it is needed? Grease being less fluid, may not flow to the required location.

Dik