Patent ID: 7568977

Claim:
A constant velocity universal joint shaft for use with a constant velocity universal joint, a fastening member, and a shaft external to the joint, wherein the constant velocity universal joint includes: an outer member with a plurality of first raceways formed on an inner surface thereof; an inner member with a plurality of second raceways formed on an outer surface thereof, each second raceway of the plurality of second raceways corresponding to a respective first raceway of the plurality of first raceways; a plurality of balls located in respective tracks formed by the plurality of first raceways and the plurality of second raceways; a retainer disposed between the outer member and the inner member so as to retain the plurality of balls; and an abutment member attached to the retainer, said constant velocity universal joint shaft comprising: a shaft body formed of a medium carbon steel, said shaft body having a minor diameter portion shaped to avoid interference with the outer member of the constant velocity universal joint; a spline provided on said shaft body for fitting to the constant velocity universal joint; a pressing member attached to said shaft body; an elastic member disposed abutting said pressing member; and a yoke portion formed of a medium carbon steel, said yoke portion being formed integrally with said shaft body and having a ring-shaped fitting portion, wherein said ring-shaped fitting portion is configured so that a diameter of said ring-shaped fitting portion is reducible by a fastening force applied via the fastening member to fit said ring-shaped fitting portion to the shaft external to the joint, wherein said elastic member is configured to elastically abut said pressing member and the abutment member so that each ball of the plurality of balls constantly remains in contact with a first raceway of the plurality of first raceways and a second raceway of the plurality of second raceways, wherein said shaft body portion is subjected to an induction hardening process, and wherein d/4≦γ≦d/2 is satisfied, where a depth of a hardened layer formed on said minor diameter portion of said shaft body by said induction hardening is denoted by γ, and a diameter of said minor diameter portion is denoted by d.