Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
How many times have you been part of the following event? A piece of equipment fails. It is taken out and sent to the shop for repairs. The shop machinist opens up the oil drain and out comes black goo.
The machinist says, "Yep, the oil was bad, it looked like hell. You should've seen the bearing; it looked like hell too. Looks like your lube mechanic was sleeping on the job again." You proceed to talk with the lube mechanic and ask him questions. All of his answers seem to be correct and it appears that he had just changed the oil in the pump; or at least, that's his story. The lube mechanic already got the word that the pump had failed; he knows that high profile machines always attract attention, and when they fail he is always the first person to be rounded up in the "usual line of suspects." He knew that you would be coming.
The lube mechanic doesn't offer anything else, and all that you've seemed to accomplish is to upset him. Now that you've done that, let's take a close look at the evidence that you should have checked in the first place, the evidence that was thrown away in the metal dumpster. Looking at the bearing in the lower left hand picture, it's obvious that the bearing got so hot that it lost its temper and the metal became soft enough to deform. The subtle colors on the raceway to the left indicate that the surface temperature reached between 4500F. (dark straw) to 6000F. (medium blue). The dark purple color is 5000F. That's pretty hot. Most bearings will begin to lose their temper at 3000F., and discolor at 3500F. (light straw). The discoloration holds an important clue because it shows exactly where the oil was under severe mechanical distress.
Now looking at the lower left picture, take a look at the color of the raceway to the right. It has a nice dull gray color to it, signs of good lubrication. There wasn't a boundary lubrication condition on this raceway; there was a good film of oil. It did not get hot. This observation is perplexing because if the cause of the bearing failure was due to a lack of lubrication, or the wrong lubricant, why then does the raceway on the right look so good?
|Take a closer look at the raceway in the upper right hand picture. The purple and medium blue colors are unmistakable. This bearing got very hot at the fragile interface between the raceway and the ball.
Look at the balls in the picture to the left, that came out of this bearing. The left ball came from the left raceway and the right ball came from the right raceway. There is a remarkable difference between the two balls, especially for two balls sharing the same lubricant in a static reservoir and less than a couple of inches apart. It is obvious that even with good oil, the film was doomed to fail because of the high surface temperatures. But which came first in the sequence of events? Was it the oil that degraded until the oil film could not support the balls and the friction forces generated greater heat, or did outside forces overpower the oil film that subsequently caused the same sequence of events? If the failure was due to degraded oil then the raceway on the right should have shown greater discoloration from the distress, and it doesn't. If the radial and axial loading was normal then the bearings in Figure 3 should have an equivalent appearance, and they don't. What then, went on here? The sad thing is that nobody usually asks these questions, and nobody usually breaks a bearing apart to see why it failed. Most of the time the evidence lands up in the metal dumpster. All anybody ever sees is the grossly discolored oil.
Fortunately in this case, the question was asked, and the answer was found. It wasn't the oil that caused the bearing failure; it was cavitation of the pump. Cavitation sets up tremendous and unstable thrust loads in the axial direction. The thrust load in this bearing was carried by the left raceway. The right raceway carried the radial load. Although the radial load was not as great as the thrust load, it was still substantial during cavitation because of the tremendous unbalance forces that were generated. The findings were issued to management and were reviewed with the lube mechanic. The mechanic got in a well deserved "I told you so."
One part of performing a failure analysis investigation consists of determining whether the equipment was operating outside of its design parameters. It is important to seek out and find corroborating evidence during the investigation. A well-lubricated bearing will usually protect a lot of the corroborating evidence that would normally be destroyed. The amazing part is the amount of heat that the oil can withstand in the final hours of a failing machine. It is a losing battle for the oil, but it is an amazing battle. If the time is taken to recover the bearings, there may be a surprise in store. The forensic evidence usually reveals the true cause of the failure.
An old paradigm that is hard to kill is that when bearings come out looking black and disfigured, the problem HAD to be caused by a "lack of lubrication." If you've ever wondered why some lubrication mechanics have "bad attitudes" it's because of these armchair quarterback failure analyses. Give 'em a break and look at the bearings first. You just might be surprised. You'll end up gaining the respect of your lube mechanic who is your best ally in the front line of preventive maintenance.
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