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Mixing oil and grease in a gearbox

Mixing oil and grease in a gearbox

(OP)
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

RE: Mixing oil and grease in a gearbox

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.

RE: Mixing oil and grease in a gearbox

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.

RE: Mixing oil and grease in a gearbox

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.)

RE: Mixing oil and grease in a gearbox

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.

RE: Mixing oil and grease in a gearbox

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.)

RE: Mixing oil and grease in a gearbox

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."

RE: Mixing oil and grease in a gearbox

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.)

RE: Mixing oil and grease in a gearbox

(OP)
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.

RE: Mixing oil and grease in a gearbox

(OP)
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.

RE: Mixing oil and grease in a gearbox

> "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)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Mixing oil and grease in a gearbox

(OP)
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.

RE: Mixing oil and grease in a gearbox

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.

Quote:

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.

RE: Mixing oil and grease in a gearbox

(OP)
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.

RE: Mixing oil and grease in a gearbox

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
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Mixing oil and grease in a gearbox

(OP)
And that is quite likely the direction we will head.

Thank you all.

RE: Mixing oil and grease in a gearbox

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.

RE: Mixing oil and grease in a gearbox

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.)

RE: Mixing oil and grease in a gearbox

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

RE: Mixing oil and grease in a gearbox

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

RE: Mixing oil and grease in a gearbox

What does the OEM O&M manual have to say about // how // to run with grease lube, beyond a chart or table containing rpm?

RE: Mixing oil and grease in a gearbox

(OP)
Manual? We don't need no stinkin Manual.
At least that appears to be the opinion for the manufacturer.
The closest would come with this portion of the dimensional drawing we were provided. There is little there other than the shaft with two bearings. And there is a grease zerk shown in the top of the manufacturer's drawing mid-way between the two bearings which are about 3" apart. In correspondence they indicate they pack the bearings before sending the unit to us. The units that failed never had oil in them, When we made the switch we started with new production units and simply used the manufacturer recommended grease to fill the housing rather than the oil we had been using.

RE: Mixing oil and grease in a gearbox

If it ain't broke...

RE: Mixing oil and grease in a gearbox

2

Quote:

In correspondence they indicate they pack the bearings before sending the unit to us. The units that failed never had oil in them, When we made the switch we started with new production units and simply used the manufacturer recommended grease to fill the housing rather than the oil we had been using.

Well, that's interesting. A grease gun can easily develop enough pressure to blow the seal right out of a one-side sealed bearing, and/or displace the bearing itself, depending on the details of the assembly, so far not in evidence.
That's the sort of bearing one might specify for an oil-lubricated gearbox, or one might pre-pack for use with grease.
... but if the gearbox is intended to be filled with grease through a zerk fitting, it should have one-side shielded bearings, or one-side sealed bearings plus some aperture through which excess grease can be forced out without pressurizing the cavity.

Of course zerk fittings and grease extruding everywhere is sort of a last-century or earlier fashion; more modern assemblies might use double-sealed bearings, pre-greased for life at assembly.

Just out of curiosity, how long has this unidentified OHLA manufacturer been in business?



Mike Halloran
Pembroke Pines, FL, USA

RE: Mixing oil and grease in a gearbox

(OP)
As to:
If it ain't broke, Believe it or not sometimes you need to reduce manufacturing steps to keep your costs in line. We had been providing a small oil reservoir with hose lines to keep oil in this unit. Going to grease the way the manufacturer was suggesting would reduce quite a bit of cost and time. We thought going to how the manufacturer recommended would save cost, guess not in the long run.

And I checked, the manufacturer which shall remain nameless has been in business 68 years.

RE: Mixing oil and grease in a gearbox

Has the nameless manufacturer been approached for comment as to why the bearings failed?

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.)

RE: Mixing oil and grease in a gearbox

MikeHalloran brings up a great point about proper design of the re-lubrication provisions for grease lubed rolling element bearings. It is not a simple matter of installing a zerk fitting at some convenient location near the bearing and squirting in some random quantity of new grease. First, the bearing installation should have an arrangement of shields/deflectors/cavities/etc that retain the correct volume of grease around the bearing both during and after operation, but also allow excess grease to be expelled within the housing enclosure where it won't create a problem. Second, the circuit for applying new grease should control the flow so that it efficiently displaces the old grease from the bearing space. The old grease is usually full of debris and contaminants, and simply adding some new grease to the old grease is not that helpful.

RE: Mixing oil and grease in a gearbox

Being intended for grease, how did you add and remove the oil prior to replacing it with grease?

Are all the adaptors working in horizontal position?

How did you measure the amount of grease that was injected in each overhung load adaptor?
Because it is a relatively small cavity, the manufacturer usually recommends a max/min volume.

How frequently is the removing and repacking of grease recommended by the manufacturer?

"Where the spirit does not work with the hand, there is no art." - Leonardo da Vinci

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