| 摘要 |
A tank 16 and a main hydraulic pump 17 are connected to head side actuator chambers 13 and rod side actuator chambers 14 of hydraulic actuators 12 via conduits 21, 22. A direction control valve 18 and a quick drop valve 36 are connected to the conduits 21, 22. A hydraulic circuit is provided with an electromagnetic switch valve 42, which switches the quick drop valve 36 from a non-quick drop position 37 to a quick drop position 38. When switching the direction control valve 18 from a neutral position 23 to a lowering operation position 25, if the manipulation speed of a manipulation lever 33 exceeds a criteria speed, an excitation signal is output from a control device 32 to an electromagnetic switch valve 42 to quickly drop a blade 11, and the quick drop valve 36 is switched from the non-quick drop position 37 to the quick drop position 38.
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