| 摘要 |
PROBLEM TO BE SOLVED: To easily make ineffective a second boom overlifting device during operating a tower crane. SOLUTION: During operating a tower crane, a harness 42 for a tower is connected to a body relay box 40 provided on a revolving structure 3. Since the standing up of the tower is performed in state where the relative angle of a jib is fixed to a specified angle (110 degrees or 162 degrees), a jib releasing switch is turned on. Thus, a circuit with which a second boom overlifting relay 26 and a second boom overlifting switch 32 are short-circuited, is cut off, and a second boom overlifting relay 25 is not energized even when the second overlifting switch 32 is turned on. As a result, solenoids 61a, 62a of a solenoid selector valve for stopping the driving of a derricking winch, are not excited, and the second boom overlifting device is made ineffeictive.
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