Ball screw mechanism and steering system

A ball screw mechanism comprising:

• a screw shaft having an outer circumferential surface provided with a helical screw groove;
• a cylindrical nut having an inner circumferential surface provided with a uniform diameter portion, a first enlarged diameter portion adjoining a first axial end of the uniform diameter portion, a second enlarged diameter portion adjoining a second axial end of the uniform diameter portion, and a helical screw groove formed in the uniform diameter portion and corresponding to the helical screw groove of the screw shaft;
• a plurality of balls;
• and
• a cylindrical retainer provided between the screw shaft and the nut,
• wherein:
• the retainer has a retainer groove that retains the plurality of balls to allow the plurality of balls to avoid contact with each other,
• the nut is threadedly engaged with the screw groove of the screw shaft through the plurality of balls,
• an axial force is applied to the screw shaft in accordance with rotation of the nut, the nut being configured to rotate about an axis,
• an inside diameter of the uniform diameter portion, excluding the helical screw groove formed therein, has a uniform diameter,
• an inside diameter of each of the first and second enlarged diameter portions is larger than the inside diameter of the uniform diameter portion,
• the uniform diameter portion and the first enlarged diameter portion face the retainer,
• the first enlarged diameter portion includes a first tapered portion having a first inclination angle and a second tapered portion having a second inclination angle, the first inclination angle being different from the second inclination angle,
• the second enlarged diameter portion includes a third tapered portion having a third inclination angle and a fourth tapered portion having a fourth inclination angle, the third inclination angle being different from the fourth inclination angle, and
• the first inclination angle of the first tapered portion and the third inclination angle of the third tapered portion are tapered at an angle to allow the retainer to be inclined with respect to the axis of the nut and to reduce contact and friction between the retainer and the nut.