Grease degredation

[edrush]

I am using a grease comprised of synthetic hyrodrcarbon oil, mineral oil, and a calcium soap in a sealed, reciprocating linear bearing application. The service environment is outdoors. It can be submerged occasionally, sometimes in saltwater. The bearing is greased on assembly and can not be lubricated again unless the unit is dissambled. The unit operates at a pretty consitant low frequency which is not enough to heat up the body of the unit significantly (i.e. no more than +20 deg F).

I can run a fatige test equivalent to three years of service life and everything looks good. However, I get an occasional unit back from the field in approximately 1 year where the grease has turned into a sludge, sometimes a very hard sludge.

The last unit had sludge in an area which is very well sealed. I do not understand what is causing this sludge to form. It difficult to imagine any dirt or water getting in this area, but I suppose contamination is a possibility.

Any ideas? Does water turn grease to sludge?

 

[btrueblood]

How sure are you that users are not applying "spray lube" (WD-40 or similar).

 

[edrush]

I believe spray lube application is unlikely. It may occur on rare occasions. I would think WD40 would tend to solvate the grease, not turn in into sludge.

Are you thinking WD40 would solvate the oil and let it drain out leaving only the thickener? I don't think the oil would escape. I have considered using oil for lubrication, but if a seal leaks it would create a big mess resulting in unhappy customers. The product is used both indoors and outdoors.

 

[IRstuff]

One of the caveats of life testing is the temperature conditions.

You probably need to analyze the sludge to see if it's cooked grease

TTFN

 

[btrueblood]

If the unit is cycled enough, then yes, the oil may wipe out with each stroke of the rod past the seal. The other, similar problem, is dirt wiping past the seal and entering the cavity.

Finally, I'd agree with IRstuff, you need to verify operating temperatures to assure yourself of having selected the proper lubricant. Take one of your devices, and leave it in the hot sun for several hours, in a freshly paved parking lot, or maybe on the dashboard of a car...then measure surface temperature with a thermocouple.

 

[IRstuff]

Actually, I was thinking more along the lines that "user" rhymes with "abuser." Your own estimates of life and operating temperature may be perfectly reasonable, but users are abusers and may be operating the equipment well beyond what you intended.

TTFN