Patent ID: 7530283

Claim:
A ball screw mechanism comprising: a screw shaft having a spiral-shaped screw groove on its outer peripheral surface; a nut member having a spiral-shaped screw groove on its inner peripheral surface mutually facing said spiral-shaped screw groove of said screw shaft; a plurality of load balls disposed in a spiral-shaped screw groove circulation passage formed by said mutually facing screw grooves and also in a return circulation passage which is formed in said nut member and is continuously connected to said spiral-shaped screw groove circulation passage, wherein said load balls taken out from said screw groove circulation passage can be returned again into said screw groove circulation passage through said return circulation passage; a plurality of retaining pieces each interposed between every two mutually adjoining ones of said load balls, said retaining piece having two concave surfaces respectively facing said two load balls and also an outside diameter dimension in the range from 0.5 to 0.9 times the outside diameter dimension of said load ball, wherein at least one of said retaining pieces, in cross section, further comprises rounded corners; wherein at least one of the retaining pieces is in a free state in which clearances exist between said at least one of the retaining pieces and respective two adjoining load balls interposing said at least one of the retaining pieces.