Patent attributes
A power transmission shaft includes a torque-transmission teeth portion formed on an outer peripheral surface on an end portion side, and a smooth portion formed on the outer peripheral surface adjacent to the torque-transmission teeth portion so as to have a stepwise reduced diameter. A ratio of a minimum diameter of the smooth portion to a minimum diameter of the torque-transmission teeth portion is set to 0.9 or more and 1.02 or less. Further, the torque-transmission teeth portion and the smooth portion undergo heat treatment so as to form a heat-treated hardened layer. The heat-treated hardened layer is formed to increase in depth from a side of the torque transmission teeth portion to a side of the smooth portion. At least a minimum diameter portion of the smooth portion undergoes full hardening.
