摘要 |
<p>A plurality of grooves 32 are arranged on the inner circumferential surface of a first sleeve 19 so as to extend in the axial direction of the first sleeve 19. The plurality of grooves 32 are arranged at equal intervals around a center axis of the first sleeve 19 in the same sectional area of the first sleeve 19. Each groove 32 is formed with a circular cross section, and each groove 32 has a sufficient depth in the inner circumferential surface of the first sleeve 19 to reliably collect foreign matter in the groove 32. Each groove 32 has the same length as that of the first sleeve 19 in the axial direction of the first sleeve 19, and both ends of each groove 32 are opened on both end surfaces of the first sleeve 19. <IMAGE></p> |