Patent ID: 7322256

Claim:
A ball screw mechanism for an electric power steering apparatus, comprising: a ball nut having an outer circumference and an inner circumference, wherein the ball nut is fitted around a ball screw portion of a steering shaft; the ball nut further including a screw groove formed on the inner circumference, a first circulating hole forming a passageway between the outer circumference and the inner circumference of the ball nut, and a second circulating hole forming another passageway between the outer circumference and the inner circumference of the ball nut; a ball screw track in communication with the ball nut inner circumference, having respective opposing ends at the first circulating hole and the second circulating hole, being formed by the engagement of the screw groove and the ball screw portion of the steering shaft, wherein balls travel in the ball screw track; a circulator having a first opening and a second opening, the first opening of the circulator attached to the outer circumference of the ball nut approximating the first circulating hole, and the second opening of the circulator attached to the outer circumference of the ball nut approximating the second circulating hole, wherein the circulator guides the balls from the first circulating hole to the second circulating hole; and a step in the screw groove adjacent to at least one of the first circulating hole and the second circulating hole, wherein the step is a change in depth of the screw groove at least one of the respective opposing ends of the ball screw track.