| 摘要 |
PROBLEM TO BE SOLVED: To prevent such a possibility that a live mast is pulled up, insomuch that the live mast has been shouldered instead in consequence at a time when this mast is raised up for attaching the bottom boom to a body. SOLUTION: When a pulling-up angleθof a live mast 13 is reached to an upper limit pulling angle 60 deg., an actuator 23a of a first limit switch 23 works, therefore the energization of an electric current to a derricking drum is interrupted. Accordingly, any winding of a derricking wire rope is stopped, and a pull-up motion of the live mast comes to a stop. In addition, when a bottom boom is installed in a turning frame 21, an actuator 24a of a second limit switch 24 works, whereby the current energization to the derricking drum is reopened irrespective of the pulling angleθof the live mast 13. Therefore, the derricking wire rope is wound up irrespective of the pulling angleθof the live mast 13.
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