Electric-motor rolling-element bearings: lubrication & maintenance

Q: I’ve been told the pressure-relief system of greasing electric-motor bearings is the preferred method. Just how do I do this?

A: That’s good advice. When properly applied, the pressure-relief procedure for regreasing ensures that the motor bearings have been charged with the right amount of fresh, clean lubricant. The procedure is not hard to follow. First, take out the lubricant drain plugs (the motor housing must have drain plugs for the pressure-relief system), then run the motor at full speed while pumping in new grease until the new grease appears at the drain opening.

Be sure to leave the plug out while the motor runs long enough for proper purging of excess grease — usually about 1/2 hr. Otherwise, excess lubricant may churn and cause overheat.

In a variation of the pressure-relief system the grease enters the back side of the housing, goes through the bearing, and comes out at the drain opening at the bottom of the closure.

You’ll probably notice a slight temperature rise immediately following use of the pressure-relief procedure. However, it will not last long and is of no consequence. The higher the lubricant viscosity, the longer the temperature rise.

Q: The various electric-motor housings in my plant have several different drain-opening configurations. How does this affect lubrication?

A: To lubricate motor bearings properly, you must consider the housing design you are dealing with. For instance, some housings have no drain opening. The only way to lubricate bearings in this type of housing is to fill the bearing cavity onethird full and add a little grease every 6 months. It is impossible to clean these housings without disassembling.

You can use the pressure-relief greasing system with housings with offset openings, but it is impossible to clean without disassembling, because a quantity of cleaning oil and solvent would remain in the housing.

Consider yourself lucky if your motor housings have openings at the bottom where they are readily accessible. This is the best design for pressure-relief greasing and cleaning without disassembling. It minimizes lost production due to maintenance requirements and reduces the possibility of bearing abuse during cleaning and relubrication.

Q: Some motors in my plant have housings with large grease reservoirs and small drain holes and they tend to overheat when I try to use pressurerelief greasing. What can I do?

A: Such housings can be headaches. Because of the large grease reservoir, not enough pressure is exerted to force grease through the small drain hole, but enough pressure can be developed to cause a fluid-friction temperature rise within the grease pack. This can lead to bearing failure unless the excess grease can quickly purge itself through the shaft opening.

The best way to lubricate such a motor is to fill the housing manually about 1/3 full and add a little grease every 6 months, never letting the housing get more than half full. A good idea is to attach a record tag to such a motor with instructions on the quantity of grease to be added at each interval, the date of application, and the initials of the person doing the work.

Next: More on motor bearings.