| 摘要 |
PURPOSE:To prevent the generation of the tear of a rib by forming an inclined surface at the edge part on the main-groove side on the surface of a shoulder rib between the shoulder part and the main groove immediately close to the shoulder part and forming said inclined surface lower by a prescribed amount than the opposed edge part having the main groove interposed. CONSTITUTION:A gentle inclined surface 11b towards a main groove 12 is formed on the edge part 11c on the main groove 12 side of a shoulder rib 11a, and a step difference (h) in 6-12% of the depth H of the main groove 12 is formed from the opposed edge part 11e having the main groove 12 interposed. By the formation of such step difference (h), the generation of cracks 5 which are formed, because of fatigue, from the bottom of the main groove 12 on the basic part of the shoulder rib 11a can be suppressed, and the tear of a rib can be prevented. |
