Patent ID: 8141445

Claim:
A ball screw shaft supporting structure of a ball screw rotationally supported by a housing via supporting bearings comprising: a rotating screw shaft formed, on its outer circumference, with a fitting portion and a positioning shoulder; an inner ring of the supporting bearing being press fit onto the fitting portion via a predetermined interference; a steel-spacer-ring-is fit onto the fitting portion from outer side of the supporting bearing with the inner side of the inner ring abutted against the shoulder, the spacer ring abutting against approximately an entire surface of the outer side end face of the inner ring; and a plurality of independent and discrete caulking portions that are equidistantly formed and spaced along a circumference of the end of the fitting portion by plastically deforming the end of the fitting portion at the plurality of portions radially outward, the inner ring being axially immovably secured on the rotating screw shaft by the plurality of caulking portions via the spacer ring abutting the inner ring such that deformation of the inner ring is suppressed by the spacer ring relieving the radially outward force of the caulking.