Heat Treatments to Improve the Rolling-Contact Fatigue Life for Crank Pin of Motorcycles

The present work was intended to increase the rolling-contact fatigue life of the crank pin for a motorcycle. The small motorcycle uses a needle roller bearing at the big end of the connecting rod. The needle roller exerts high Hertzian stress on the crank pin. This stress sometimes causes rolling-contact fatigue failure such as pitting. The contaminated oil accelerates the rolling-contact fatigue failure. In order to increase the life of the crank pin, not only the quality of the steel but also the casehardening treatment plays an important role. In the present work, a high quality Cr-Mo steel containing oxygen concentration below 10 ppm was chosen as the base steel. The rolling-contact fatigue life was compared in four types of casehardening: normal carburizing, carbonitriding, super-carburizing and super-carbonitriding. A thrust-type testing machine was used during these comparison tests. The oil containing a spherical powder of high-speed steel was used to simulate the contaminated oil. The life of super-carbonitriding was the longest and was 1.6 times longer than that of normal carburizing. The long life of super-carbonitriding is thought to be due to the increased resistance to temper softening. The established super-carbonitriding is currently being used in the mass-production of motocross-motorcycle YZ450F.