To prevent shock generation resulting from the operation of avoiding hazard for a jib hoisting rope to go in rupture, etc., when a boom is being raised or lowered and also prevent deterioration in the working effectiveness.
The tension of a jib hoisting rope is sensed when a boom is being raised or lowered, and a controller 21 issues a forecast command to a forecast/alarm device 27 if the rope tension has attained the forecast tension value which was set and stored previously. Decompression valves 22 and 23 are controlled and the speed is reduced within the range in which the jig hoisting motion or boom lowering motion can be maintained, and further when the rope tension has attained the preset limit tension, an alarm command is emitted, and the jig hoisting or boom lowering speed is set to a minimum value over zero.
ISHIHARA HIDEAKI
TODO OSAMU
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