Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|It doesn't take a lot of grease to lubricate a bearing ... really. Usually, a bearing housing is over greased if the bottom half of the housing is totally full. Here is a classic example of what every bearing manufacturer is striving to achieve when they specify just a few pumps, or fractions of an ounce, of grease between each interval.|
|Sometimes problems don't exist unless we look for them, and if we look hard enough we'll find one, and if we don't find one, we'll make one. Sounds like the opening words of an AA meeting. There wasn't anything wrong with the split roller bearing shown to the left. It is a "floating" bearing, meaning that there are no grooves cut into the race to restrain axial movement of the rollers. It has a flat contour so that the rollers can move freely in the axial direction.
PLANT HISTORY - The counter part to this bearing, the fixed bearing, runs hotter than most bearings because the rollers contact the grooves and generate frictional heat. The bearings never fail, they just run hot. Production personnel have never been comfortable with this style bearing because of the fact that it runs hotter than other style bearings. The old adage "if you can't put your hand on it then it's running too hot and something is wrong," while true in the old days, doesn't hold enough truth today when everything is designed to push the material to its maximum potential. For example, motors run hotter today because of better winding insulation. Nobody has taken the time to educate plant personnel who are use to the old motors that did run cooler simply because the advanced insulation technology didn't exist back then. Taking that same train of thinking, and expanding it to bearings, one can see the similarities. Now, getting back to bearings, during the winter you never hear about these bearings from the Production people. It's only during the summer that there is a "problem." Consider the fact that if the bearing was truly running hotter because of a lack of lubrication, then the bearing should fail just as easily in the winter as it supposedly does in summer, yet there is never a "problem" during the winter. Actually there is enough load on these bearings to justify a circulating oil system to handle the heat load, but these bearings do quite well without such a system. It's just the plant personnel that are "the problem."
|THE REST OF THE STORY - Therefore, as usual and right on time (i.e. a hot summer day), there was concern about this bearing and it was opened up for inspection. A close up of the outer race is shown in the picture to the left. This housing was not packed full of grease. There was grease accumulated on one side of the bearing. The grease built up from the centrifugal forces generated by the rollers that expelled it when injected through a grease hole in the center of the outer race. This rare glimpse of nearly undisturbed grease showed how the rollers had created a channel through the grease. The step in the grease, highlighted by the red line in the picture, is where the roller cage traveled.
The first question on everyone's mind was whether the bearing was bad. Looking at the surface condition, it is a dull gray, with no shine and no discoloration from running hot. The dull gray appearance indicated that the lubricating film thickness developed by the grease was proper. If it was marginal then the surface would have a shine to it where the rollers actually contacted the higher asperities of the outer race. A film could be detected by rubbing a finger across the surface.
HOW IT WORKS - As the rollers heat up, the oil in the grease comes out of suspension and builds up next to the contour of the grease buildup. The rollers pick up the oil and distribute it uniformly. Notice how the grease channel acts like a dam. As the oil comes out of suspension the oil is restrained within the outer race. It has nowhere to go but into the path of the rollers. If there was too much oil then it would be expelled on the opposite side of the race where there is no grease buildup. Which brings us to another point. If there was grease built up on the right side of this bearing, then the excess oil could not be expelled and the rollers would start generating excessive hit from fluid friction. The excessive heat would cause more oil to come out of suspension and the heat problem, from fluid friction, would get worse because now the rollers are sloshing through a deeper and deeper trough of oil. Which brings us to yet another point. Grease has to be periodically purged from a housing. This is because as the oil comes out of suspension the only thing left is the "filler." This filler material is useless when the oil is depleted, and what's worse, it will actually cause problems. How? Imagine this same bearing shown above, but this time imagine that the grease buildup contour is no longer grease, but only filler. Where are the rollers going to get their source of oil when they start heating up. At some point, the new grease won't be expelled in this region because it will be increasingly difficult to displace the ever hardening filler. As the housing fills up with spent grease filler material, the grease is forced to locations less desirable and less affective. At some point, if the filler has not been purged, the lubrication mechanic will start to pump his usual allowance of grease into the housing only to feel resistance on his hand pump. This usually indicates that the bearing has enough grease in it, when in fact in this particular case the housing is actually full of filler, and not good grease. A bearing failure will eventually occur. I have never seen a perfectly lubricated bearing, until this one turned up. I doubt that I will ever see one again. So I had to write about it.
Take a good look because you may never see one yourself.
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