More about electric-motor rolling-element bearings: lubrication & maintenance

Q: What are the four most common problems I’m likely to encounter when servicing motor bearings?

A: It should be no surprise to anyone who has experience with rolling-element motor bearings that the two most frequent sources of trouble come from improper lubrication — too much or not enough.

Overlubrication causes excessive temperature which can cause bearing failure, or grease leakage endangering the electrical life of the motor. Inevitably, high lubricant temperature increases the oxidation rate and shortens lubricant life.

Next to lubrication problems, careless handling of bearings is the biggest headache. Careless handling and the introduction of foreign matter into a bearing during overhaul impose severe duties on the lubricant and cause undue wear and premature bearing-component failure.

Finally, incorrect assembly during overhaul can cause rough or otherwise poor operation of the motor and shorten bearing life.

Q: What are the proper materials for cleaning and flushing motor bearings?

A: There are several high-grade cleaning and flushing materials, under various trade names, specifically for this purpose. In general, they are mineral spirits with flash points high enough for safe use as general-purpose solvents.

I do not recommend kerosene or commercial carbon tetrachloride; they are likely to be contaminated and could do more harm than good. Also, carbon tetrachloride can hydrolyze to hydrochloric acid and cause severe corrosion within the bearing.

Q: I could really save some time if I were able to clean electric-motor bearings without disassembly from the housing. How do I know if this is possible, and how do I proceed?

A: Examine your motor housings. If they have drain plugs at the bottom of their end caps, you will be able to clean the housings without disassembly. You can still do this even if your equipment doesn’t have drain plugs, if it is possible to drill the end caps to permit drainage.

Remove the pressure nipple, loosen the drain plug, and with a small funnel, pour in solvent until it begins to bubble at the top. Run the motor for about 2 min and drain. It may be necessary to loosen hardened grease from the drain hole to permit flow. Repeat this until the cleaning fluid runs clear. Fill the housing with light oil, be sure to drain off the oil thoroughly, and refill with grease. Most bearing housings should be cleaned at least every 3 years.

Q: Although I frequently flush the motors in a particularly dusty area of the plant with a solvent-type oil recommended for electric-motor bearings, I still have contamination problems. Any suggestions?

A: Sounds familiar. I’ve run into situations where the dust was such that it would not stay suspended in the solvent. Rapid precipitation caused the dust to fail to move out to the drain opening. It would pile up in a solid mass at the bottom of the bearing or housing, and lie in wait to be picked up in large chunks by the new grease. Result: early bearing failure.

One thing you might do is flush with SAE 20 oil. The dust will float or stay suspended in this relatively heavy oil long enough to be carried away with it. This oil won’t have the usual cleaning effect of solvent, but it will get rid of the accumulated contaminants.

The best method, provided your housings are adapted to it, is to use the pressure- relief system of regreasing. Experience will dictate how often you must disassemble the unit for proper cleaning.

Q: The motors in an unusually hot area of our plant operate normally at about 200 F. We use a high-temperature grease which (it says on the container) can be used at temperatures to 300 F. No problem, right?

A: Practically speaking, there have been no recent revolutionary advances in hightemperature lubricant technology; I think you have to take some lubricant advertising with a grain of salt. For instance, I’ve seen claims that a certain grease will never deteriorate, with the inference that it will stand just about anything. And I’ve seen publicity claiming a particular grease can be used to 300 F — with no mention of time limits.

The situation is that a high rate of lubricant oxidation develops above 180 to 190 F. And keep in mind that, beginning at about 140 F, the oxidation rate doubles for every 15 F temperature rise. Evaluate commercial claims accordingly.

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