Screw shaft and method for manufacturing same, and electric position adjusting device for steering wheel and method for manufacturing same

A method for manufacturing a screw shaft, comprising:

• a step of performing a rolling process on a work comprising a large-diameter shaft portion for rolling, and a small-diameter shaft portion for rolling arranged adjacent in an axial direction to the large-diameter shaft portion for rolling, and having an outer diameter that is smaller than an outer diameter of the large-diameter shaft portion for rolling;
• wherein the rolling process generates walking of the work using a plurality of rolling dies in order to form a male-side screw portion over an entire length in the axial direction of an outer circumferential surface of the large-diameter shaft portion for rolling;
• and
• in the process of performing the rolling process on the work, a helical rolling mark is formed on an outer circumferential surface of the small-diameter shaft portion for rolling using the rolling dies, while at the same time performing the rolling process for forming the male-side screw portion on the outer circumferential surface of the large-diameter shaft portion for rolling, wherein
• the male-side screw portion comprises a completely threaded portion having a specified thread height, and incompletely threaded portions not reaching the specified thread height arranged at edge portions on both ends in the axial direction of the male-side screw portion, and
• the entire male-side screw portion functions as a normal threaded portion and only the rolling mark corresponds to a non-threaded portion that does not function as the normal threaded portion.

A feed screw mechanism comprising:

• a screw shaft, having:
• a large-diameter shaft portion, and
• a small-diameter shaft portion;
• the large-diameter shaft portion having a male-side screw portion over an entire length of an outer circumferential surface thereof;
• and
• the small-diameter shaft portion being arranged adjacent in an axial direction to the large-diameter shaft portion, having an outer diameter that is smaller than an outer diameter of the large-diameter shaft portion, and having a helical rolling mark on an outer circumferential surface thereof that is in phase with an extension line of a spiral curve that is a root circle line of the male-side screw portion;
• and
• a nut having a female-side screw portion on an inner circumferential surface that engages with the male-side screw portion, and
• a part in the axial direction of the female-side screw portion capable of being arranged at a position in the axial direction deviated from the male-side screw portion, wherein
• the rolling mark does not engage with the female-side screw portion of the nut.