Advanced motors can electrically damage steel bearings.
Champion Bearings Inc.
Palm Springs, Calif.
The inner and outer races of antifriction bearings often become frosted, fluted, or get a corrugated pattern imprinted on them. These are not mechanical scars but are due to electromagnetic forces and can lead to bearing failure. They are usually found in modern systems that routinely feature pulse-modulated adjustable-speed motors and inverters with high switching frequencies and short rise times. These designs can transmit the rotor voltage through the motor shaft because voltage exceeds the dielectric strength of lubricants on the bearings. The resulting current flows from the shaft and lubricant, causing the frosting, fluting, and generating the corrugated pattern.
One long-used method of minimizing current flow through bearings and gears is to passively ground the shaft with a mechanical device. Unfortunately, newer variable-speed motors often generate shaft-to-frame voltages that send current across bearings despite the grounding device.
A better solution substitutes ceramic bearings for the more traditional, chrome-steel counterparts, while retaining the steel inner and outer races. These hybrid bearings, as they are known, eliminate scarring and also run cooler due to less microweld adhesion.
Comparing Ball Bearing Materials
|Elastic modulus (Gpa)|
|Coefficient of thermal expansion (10-6/C) (RT to 800C)|
|Maximum use temperature (°C)|
|Fatigue life, L10 (relative to M50)|
|Wear resistance (relative to M50)|