FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2023 46 R olling-element bearings (REBs) are machine components that use rolling elements (either balls or rollers) placed between two hardened and ground raceways. The relative motion between the inner race and the outer race is achieved by the rotation of these rolling elements. To separate and guide the rolling elements but also to reduce the overall useless frictional losses, a cage, a retainer, or some separators of given technical materials (often bronze, brass, polymers, or low-carbon steels) are usually used. Internal designs and assembly techniques allow most bearings to sustain axial, radial, and/or combined loads. Figure 1 gives a standardized overview of the large number of internal structures[1]. BEARING MATERIALS There are five main groups of REB steel families. These steels are specifically heat treated to a surface hardness of 58 + 4 HRC over a sufficient thickness to resist the triaxial Hertzian stresses built under the surface of the raceways and the rolling elements; the triaxial Hertzian stresses are generated by the external normal compressive load. Under repeated contact loads generated by the ball/roller passing over a given material element, the PREVENTING ROLLING-ELEMENT BEARING FAILURES THROUGH HEAT TREATMENT An examination of a bearing failure due to mismatched and incomplete flame hardening shows the need to work with heat treaters for proper selection and application. maximum shear stress, τ, develops beneath the depth of the material. The repeated load cycles potentially lead to nucleation of cracks under the surface at stress raisers such as impurities or discontinuities, providing what is usually called a true rolling-contact fatigue mechanism. For materials that would not reach those hardness levels after heat treatment, correction factors for the static and dynamic load-carrying capacities may be used. FAILURE TYPES Failures of REBs can occur for a variety of reasons. Accurate determination of the cause of a bearing failure depends to a large extent on the ability of the analyst to re- cognize and distinguish among the various types of failures. In most instances, this recognition will enable the analyst to determine the primary cause of failure and to make suitable recommendations for eliminating the cause. The major factors that, singly or in combination, may lead to premature failure in service include incorrect fitting, excessive preloading during installation, insufficient or unsuitable lubrication, overloading, impact loading, vibration, excessive operating or environmental temperature, contamination by abrasive matter, entry of harmful liquids, and stray electric currents. HEAT TREATMENT AND HARDNESS OF BEARING COMPONENTS The most common through-hardened ball-bearing materials are AISI 52100 (100Cr6 or WN 1.3505) low-alloy steel and M50 (80MoCrV42-16 or WN 1.3551) high-speed tool steel. Reference 2 lists average hardness, amount of retained austenite, and typical austenite grain size for eight ball-bearing materials (three heat treatment lots of each material). Hardness is an important variable in rolling-contact fatigue and can significantly affect bearing life. In general, bearing hardness should be at least 58 HRC for adequate bearing life. Investigations have shown that, within limits, rolling-contact fatigue life increases as hardness of the bearing components is increased[3]. Significant differences in hardness between rings and balls can also affect bearing life. Experimental results[4] indicate that balls should be approximately 1 to 2 Rockwell C points harder than rings. 10 Fig. 1 — Main loading directions of rolling-element bearing structures. Adapted from Ref 1.
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