Patent ID: 7399230

Claim:
A power transmission shaft coupled to one of an inner joint member and an outer joint member of a constant velocity joint, and provided with a smooth portion having an outer circumferential surface of a cylindrical shape and a serrated portion for torque transmission having a cut-up portion at an end portion thereof, wherein a core portion of the shaft is formed to have a dual phase structure of a ferrite and a martensite, and wherein said smooth portion and said serrated portion for torque transmission are respectively induction-hardened, a hardening ratio of said smooth portion is set at a different value from a hardening ratio of said serrated portion; a fatigue strength of said smooth portion is equal to or greater than a fatigue strength of said serrated portion; the hardening ratio of said serrated portion is equal to a depth of hardening of a minor diameter portion divided by a radius of the minor diameter portion; and a diameter ratio of an outer diameter portion of the smooth portion with respect to the minor diameter portion of the serrated portion is set within a range of 0.95 to 1.05.