| 摘要 |
PROBLEM TO BE SOLVED: To suppress deterioration in buckling strength caused by formation of circumferential grooves.SOLUTION: A plurality of circumferential grooves 15 that continuously extend over an entire circumference are formed on a body part 13 with intervals in a vertical direction. Each of the circumferential grooves 15 forms a wave shape extending regularly along the circumferential direction while curving in the vertical direction viewed from the side of the body 13, respective phases of the circumferential grooves 15 that are adjacent to each other in the vertical direction are displaced from each other. |
