Patent ID: 8101031

Claim:
A method of producing a hollow power transmission shaft having a large-diameter portion formed along an axially central portion thereof and small-diameter portions formed along axially opposite lateral portions thereof, the small-diameter portions being located laterally with respect to the large-diameter portion, said method comprising: subjecting a pipe material to plastic working to mold a hollow shaft material having the large-diameter portion and the small-diameter portions; and performing induction hardening of the hollow shaft material with a frequency of a frequency current in a predetermined region thereof set different from a frequency of a frequency current in another region thereof, wherein the frequency of the frequency current in performing induction hardening of the large-diameter portion of the hollow shaft material is set higher than the frequency of the frequency current in performing induction hardening of the small-diameter portions of the hollow shaft material, and wherein a hardened layer is formed in an outer peripheral surface of the large-diameter portion and the small-diameter portions while leaving an unhardened layer in the inner peripheral surface of the large-diameter portion and the small-diameter portions, such that a hardening ratio is less than 0.6 in the large-diameter portion while being 0.6 or more in the small-diameter portions, and wherein the hardening ratio is defined as a ratio of a depth of the hardened layer having a Rockwell hardness of HRC40 or more to a wall thickness of the large-diameter portion and the small-diameter portions.