| 摘要 |
PROBLEM TO BE SOLVED: To lower the generation of noise by arranging three kinds of pitches, which have a different pitch length and in which a difference between the maximum pitch length and the minimum pitch length is set in a specified ratio range in relation to the minimum pitch length, so that a difference of pitch length between the adjacent pitches is set at a specified ratio or less in relation to the shorter pitch length thereof. SOLUTION: Three kinds of pitches P1 , P2 , P3 , in which a difference between the maximum pitch length l3 and the minimum pitch length l1 is set at 10-20% in relation to the minimum pitch length l1 , are arranged so that a difference between the adjacent pitches is set at 10% or less in relation to the shorter pitch length thereof. With this structure, generation of noise can be effectively lowered by the pitch variation of three kinds of pitches P1 , P2 , P3 , and a ratio of a difference between the maximum pitch length l3 and the minimum pitch length l1 in relation to the minimum pitch length l1 is set at 10-22% so as to prevent the generation of step-down abrasion, and lowering of the generation of noise is effectively realized. |
