The frosted, fluted or corrugated pattern often found on the surface of the inner or outer race of an anti-friction bearing is often the result of electromagnetic forces. Due to advances in semiconductors, power electronic technologies have made adjustable-speed drives (ASD) with pulse-modulated technologies (PNP) available for many motor-driven elements. ASD use insulated gate bipolar transistors (IGTPS) as high-frequency switches and are most likely to cause race fluting. The high switching frequency and fast rise time of an inverter cause voltage in the rotor to be coupled to the motor shaft. This voltage can exceed the dielectric strength of the lubricant in the bearings. The resulting current flows from the shaftthrough the bearing lubricant and can lead to the premature failure of 52,100 chrome steel bearings.
Many manufactures minimize current flow through the bearings and gears by fitting passive grounding mechanisms that electrically grind the shaft through a mechanical device. The original shaft ground system is a result of years of research isolating the cause of electric motor bearing failure. Research demonstrates, however,that variable-speed motors often display shaft-to-frame voltage that can cause current flow across the bearings even with mechanical grounding devices. This conditioncan best be overcome by substituting noncondutive ceramic balls for the older chrome steel bearings, keeping the inner and outer race steel.
This construction is often referred to as "hybrid bearing" and has been offered by several bearing manufactures for high-speed spindle applications. A comparison of materials is shown in table 1. Besides eliminating race fluting, ceramic balls run considerably cooler than steel balls, due to a reduction inmicro weld adhesion. This can occur with marginal lubrication, due to the welding of peaks of steel contacting each other under extremely high pressure. Champion Bearing offers this type of construction with a steel or vibration-absorbing PTFE composite retainer trade-marked, "Beraloy."
by Richard Kay, CEO Champion Bearings, Inc., Published November 1999 in ELECTRICAL CONTRACTING & ENGNEERING NEWS
PROPERTY DENSITY (G/CC)
ELASTIC MODULUS (Gpa)
THERMAL EXPANSION COEFFICIENT
(10-6/C) (RT TO 800C)
MAXIMUM USE TEMPERATURE
FATIGUE LIFE, L10 (RELATIVE TO
WARE RESISTANCE (RELATIVE TO
M50 IN FATIGUE TESTING)