| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a rack assist type motor-driven power steering improved in manufacturing easiness and operability while adopting relatively simple constitution. SOLUTION: Many external thread grooves 51 are formed at a rack shaft 21, and a plurality of internal thread grooves 53 are formed at a ball nut 41. Three circulating elements such as pieces are provided in the ball nut 41. The circulating elements, the external thread grooves 51 and the internal thread grooves 53 form first to third circulating passages 61-63 each with one line, and first to third steel ball groups 67-69 composed of many steel balls 64-66 are built in the first to third circulating passages 61-63. The steel ball 65 constituting the second steel ball group 68 has a diameter D2 set larger than the diameter D1 of the steel ball 64 constituting the first steel ball group 67 and the diameter D3 of the steel ball 66 constituting the third steel ball group 69, and interposed between the external thread groove 51 and the internal thread groove 53 with specified pre-load quantity.</p> |
