US Patent 7322256 Ball screw mechanism for electric power steering apparatus

A first ball circulating hole and a second ball circulating hole for connecting a circulator are bored midway at a screw groove formed on an inner circumference of a ball nut, an intermediate groove path between the first and second ball circulating holes becomes the range of a ball screw track for circulating balls, and other ranges become a first outer track and a second outer track which are not the ball screw track. By making the groove depth of the intermediate groove path deeper than the groove depth of a first groove path of the first outer track and the groove depth of a second groove path of the second outer track, steps are formed between the ball screw track and the two outer tracks. These steps prevent an outflow of the balls from the intermediate groove path to the respective groove paths of the outer tracks.