[0030] The main section 22 of the main body 12 may be a polygon, whereas the cross sections are roughly square in shape. Referring to FIG. 2C, the main body 12 is a cylinder and has a main cross section 22a which is round. The first and second protrusion cross sections 24, 26 are still disposed on sides 221a, 222a of the main cross section 22a, respectively, and the sides 221a, 222a each has a predetermined radius of curvature.