Linear motion shaft for electric power steering device, electric power steering device, and methods for manufacturing them

A method for manufacturing a linear motion shaft for electric power steering device,

• the linear motion shaft including a rack shaft part having a rack toothed portion on an outer-circumferential portion thereof, a screw shaft part having a ball screw portion on an outer-circumferential portion thereof, and a joining part that joins the rack shaft part and the screw shaft part to each other,
• the method comprising:
• a step of manufacturing the rack shaft part;
• a step of manufacturing the screw shaft part;
• and
• a step of joining the rack shaft part and the screw shaft part to each other by friction welding;
• the step of manufacturing the rack shaft part including a step of forming the rack toothed portion on a material for rack shaft part by a plastic process;
• the step of manufacturing the screw shaft part including a step of forming the ball screw portion on a material for screw shaft part by a plastic process, and a step of forming a gripped portion for centering, with reference to the ball screw portion, on an outer-circumferential portion of an end portion in an axial direction on a side of the screw shaft part to be joined to the rack shaft part;
• and
• the step of joining the rack shaft part and the screw shaft part to each other by friction welding including a step of abutting an end portion in the axial direction of the rack shaft part and the end portion in the axial direction of the screw shaft part to each other while the screw shaft part is rotated by rotating a first gripping tool for centering in a state where the gripped portion of the screw shaft part is gripped with the first gripping tool, and the rack shaft part is held without rotation in a state where the rack shaft part is gripped with a second gripping tool for centering.