| 摘要 |
The outside diameter phi of a land portion 1 c is set to be larger than a BCD. Thus, when a ball 3 goes into a deep portion (a portion close to an axis line O of a screw shaft 1 ), the land portion 1 c of the screw shaft 1 guides the ball. Consequently, the circulation of the ball 3 can smoothly be performed.
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