| 摘要 |
A diagonal inner-face curved portion is formed on an inner face of a diagonal portion of a pyramid-shaped yoke portion. The diagonal inner-face curved portion is formed to make at least one of partially inner-face curved portions at the major axis side and the minor axis side of the diagonal axis swell to the outward side. The partially inner-face curved portions at the major axis side and the minor axis side are formed by circular arced portions with different radii to each other. |
