| 摘要 |
PROBLEM TO BE SOLVED: To reliably prevent backward overturning of a jib by raising/lowering the jib by a raising/lowering rope extended to a sheave arranged on the tip of an A frame, and contacting the jib and the A frame in the first place when exceeding a raising/lowering upper limit of the jib. SOLUTION: When a jib 1 is positioned on a raising/lowering upper limit, even if the jib 1 tries to rotate further backward from this state by misoperation of a raising/lowering rope 2 and a hoisting rope 7, since a distance L between the jib 1-pulling direction and a foot pin 3 is short, only slight torque can be obtained. This slight torque is supported by an A frame 4 through a shock absorber 8 to prevent backward overturning of the jib 1 to thus previously prevent an accident such as the jib overturns backward due to a damaged to a backstop without requiring a front side fastening rope of the jib.
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