| 摘要 |
PROBLEM TO BE SOLVED: To keep the lock state of a drop preventing frame without special members for locking. SOLUTION: The drop preventing frame 20 is formed by a frame body 21 fitted to a cover receiving frame 10 and an engagement frame 22 fitted to the forward end of the frame body 21 to move forward and backward. Either the frame body 21 or the engagement frame 22 is provided with a plurality of guide parts 31, 32, etc., for guiding the forward and backward movement of the engagement frame 22, and the other is provided with a plurality of passing parts 31', 32', etc., for passing the guide parts 31, 32. An engagement element 34 is rotatably provided on at lest one guide part 31, 32, etc., and a passing hole 36 allowing the engagement element 34 to pass through it is formed in the corresponding passing part 31'. With an engagement frame 2 moved forward, the engagement element 34 is rotated to cause engagement with the frame body 21 so that the engagement frame 22 is restrained to the locking state not to move backward.
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