Thomas L. Carter
The effect of temperature using di(2-ethylhexyl)sebacate lubricant and dry powder lubricants on rolling-contact fatigue life of AISI M-1 tool-steel balls was investigated in the rolling-contact fatigue spin rig at maximum theoretical Hertz stress levels of 650,000 and 725,000 pounds per square inch in compression. In tests at temperatures of 100 degrees, 250 degrees, and 450 degrees F using the sebacate lubricant, it was found that life decreased with increasing temperature. Metallurgical transformation tended to increase in intensity with the total number of stress cycles and with test temperature. Tests were also made at 450 degrees F using dry molybdenum disulfide and dry graphite powders as lubricants. The molybdenum disulfide was carried into the test zone as a suspension in polyalkylene glycol which evaporated at 350 degrees F. Ball life at 450 degrees F with both molybdenum disulfide and graphite powders was significantly reduced from that observed with a fluid lubricant under the same test conditions. Failure was by fatigue spalling, but it had a different appearance than spalling previously observed with fluid lubricants. The failures appeared to be caused by stress raisers localized in the bands of pure rolling. The stress raisers probably were the dry lubricant particles. Since a complete bearing has better conformity between ball and race curvatures, hence greater relative sliding of the surfaces, the observed effect of dry lubricant particles acting as stress raisers may be reduced to a negligible role in normal rolling-contact bearing applications. At 100 degrees F the glycol suspension of molybdenum disulfide produced results for short lived balls in the same range as plain glycol and other fluid lubricants. An abrupt increase in slope of the fatigue life curve was observed at 2 x 10-to-the-eighth stress cycles. This change in slope appeared to be due to a combination of higher than normal surface shear stresses and corrosion cracking.
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