Patent attributes
A technique for improving a strength, especially a fatigue strength of a power transmission shaft provided with a serrated portion for torque transmission such as a spline shaft. A power transmission shaft is serration-fitted to an inner joint member of respective constant velocity joints. A driveshaft is provided with a serrated portion as well as a smooth portion having a cylindrical outer circumferential surface. A cut-up portion is formed at the end portion of the serrated portion. When applying an induction hardening to the smooth portion and the serrated portion, a hardening ratio of the smooth portion is set at a different value from that of the serrated portion, and such that a fatigue strength of the smooth portion becomes equal to or greater than that of the serrated portion.
