| 摘要 |
<p>Convex parts are formed on portions abutting locking pieces (20, 21) at the peripheral edge of a pressure plate (2a), and the peripheral edges of the locking pieces (20, 21) are left floating from the peripheral edge of the pressure plate (2a). The peripheral edges of the locking pieces (20, 21), which are sharp ends, are prevented from strongly rubbing against the peripheral edge of the pressure plate (2a).</p> |
