It’s very important that the correct grease and the correct amount is used for each motor. This is vital to prevent damage to the motor bearings and other components. Too much or too little grease is a very common problem. In either case, the motor can be damaged by either “popping” the seals with too much, or running the bearing dry with no grease. Ultrasonic tools can help determine the correct amount of grease to apply.
Figure 1 shows a bearing with a well-defined colored raceway. The inner race is still intact, but the effect of high heat can clearly be seen.
These elevated temperatures are typically caused by the bearing running with boundary lubrication or with no grease.
Having excessive amounts of grease can eventually result in a chemical reaction and breakdown of insulation on motor windings. (Figure 2)
Dirt can also become trapped in grease on windings and can cause shorting and overheating.
The bearing housing contains the recommended grease, but it is filled beyond capacity.
The bearing ran hot during operation because of the high volume of grease.
After being in service for a short time, elevated noise levels were detected. (Figure 3)
After replacement of the bearing, markings were found on the rollers indicating wrong selection of grease for this particular application.
This bearing (Figure 4) ran only for a few hours at high-speed (6,000 RPM) with the bearing housing filled with grease. After a short time, abnormal sounds were detected and the bearing was dismantled.
Cause: under-loading of the bearing together with massive amounts of grease caused sliding of the rollers.
Most large AC motors are equipped with sleeve bearings. Although very reliable, some specific problems can exist.
The most common problem is direct contact between the rotor shaft and sleeve bearing, causing abnormal vibration, oil discoloration, and melted Babbitt metal particles in lubricating oil. (Figures 5 & 6)
Causes for this type of damage
- Abnormal Loading
- Incorrect lubricant or sudden changes in viscosity
- Severe Misalignment
Vibration and oil analysis are the main technologies to assess the condition of the sleeve bearings of the motor.
Information courtesy of Emerson Process