Ball screw device, method of manufacturing ball screw device, and steering system including ball screw device

A ball screw device comprising:

• a screw shaft having an outer peripheral surface provided with a helical outer peripheral rolling groove;
• a nut formed in a tubular shape, the nut having an inner peripheral surface provided with a helical inner peripheral rolling groove, the nut being disposed on an outer peripheral side of the screw shaft, and the nut having a plurality of mounting holes extending between two different single-turn rolling grooves of single-turn rolling grooves aligned in an axial direction thereof and an outer peripheral surface thereof, the inner peripheral rolling groove being defined as a collection of the single-turn rolling grooves aligned in the axial direction and each having a single turn about an axis thereof;
• rolling elements arranged in a rollable manner in a rolling path defined between the outer peripheral rolling groove and the inner peripheral rolling groove;
• a return path that short-circuits between openings of the plurality of mounting holes that are open to the rolling path and are respectively formed in the two different single-turn rolling grooves, the return path allowing the rolling elements rolling in the rolling path to circulate endlessly;
• and
• a deflector accommodated and fixed in a mounting hole of the plurality of mounting holes, that defines a part of or the entire return path therein, the deflector being configured to guide the rolling elements rolling in the rolling path to the return path and return the rolling elements to the rolling path;
• wherein:
• the inner peripheral rolling groove of the nut includes, in a range between the openings of the mounting holes;
• a constant pitch circle diameter region spaced apart from the openings and formed such that a pitch circle diameter of the rolling path is constant, and
• gradually-changing pitch circle diameter regions formed such that the pitch circle diameter gradually increases from the constant pitch circle diameter region in ranges from opposite ends of the constant pitch circle diameter region to the respective openings;
• and
• at least the constant pitch circle diameter region has a surface hardness greater than or equal to a predetermined value, among the constant pitch circle diameter region and the gradually-changing pitch circle diameter regions excluding edges at boundaries between the respective openings and the inner peripheral rolling groove, and the edges at the boundaries between the respective openings and the inner peripheral rolling groove have a surface hardness less than the predetermined value.