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In a hydraulic drive device for vertical pivoting movement of a load carrying platform, a first directional control valve has only three switchover positions, including a neutral position relating to operation of stopping and holding a load carrying platform, a switching position relating to operati

In a hydraulic drive device for vertical pivoting movement of a load carrying platform, a first directional control valve has only three switchover positions, including a neutral position relating to operation of stopping and holding a load carrying platform, a switching position relating to operation of lifting the load carrying platform, and a switch over position relating to operation of forced lowering of the load carrying platform. A second directional control valve has only three switchover positions, including a neutral position, a switching position, and a switchover position relating to operation of allowing the load carrying platform to fall by its own weight. The device includes a first on-off valve, a second on-off valve, and a third on-off valve that are proportional control solenoid valves for switching an appropriate one of the first and second directional control valves.

대표청구항 ▼

The invention claimed is: 1. A hydraulic drive system for vertical pivoting movement of a load carrying platform, said hydraulic drive system being mountable on a dump truck and being provided with a hoist cylinder for vertically pivoting said load carrying platform, a hydraulic pump for feeding pr

The invention claimed is: 1. A hydraulic drive system for vertical pivoting movement of a load carrying platform, said hydraulic drive system being mountable on a dump truck and being provided with a hoist cylinder for vertically pivoting said load carrying platform, a hydraulic pump for feeding pressure oil to operate said hoist cylinder, a directional control valve arranged between said hydraulic pump and said hoist cylinder to control operations of said hoist cylinder and having switch positions for performing a platform raising operation to pivot said load carrying platform in an upward direction, a platform forced-lowering operation to forcedly pivot said load carrying platform in a downward direction by pressure oil delivered from said hydraulic pump, a platform free-fall permitting operation to pivot said load carrying platform in a downward direction by its own weight and a platform stay holding operation to enable holding of an aerial stay of said load carrying platform, respectively, and a switching control capable of outputting a pilot pressure to switch said directional control valve, wherein: said directional control valve comprises: a first directional control valve having a neutral position relevant to the platform stay holding operation and two other switch positions, and a second directional control valve having a neutral position relevant to the platform stay holding operation and two other switch positions; four switch positions, which consist of said two other switch positions of said first directional control valve and said two other switch positions of said second directional control valve, are assigned to said switch position relevant to the platform raising operation, said switch position relevant to the platform forced-lowering operation and said switch position relevant to the platform free-fall permitting operation; and said switching control comprises: a first on-off valve having an ON position capable of outputting a pilot pressure to perform said platform raising operation and an OFF position enabling said platform stay holding operation, a second on-off valve having an ON position capable of outputting a pilot pressure to perform said platform free-fall permitting operation and an OFF position enabling said platform stay holding operation, and a third on-off valve having an ON position capable of outputting a pilot pressure to perform said platform forced-lowering operation and an OFF position enabling said platform stay holding operation. 2. The invention as described above in claim 1, wherein: said two other switch positions of said first directional control valve are said switch position relevant to platform raising operations and said switch position relevant to platform forced-lowering operations; said two other switch positions of said second directional control valve are said switch position relevant to platform raising operations and said switch position relevant to platform free-fall permitting operations; said ON position of said first on-off valve is an ON position capable of outputting said pilot pressure, which acts to perform said platform raising operation, to a control port of said first directional control valve and a control port of said second directional control valve; said ON position of said second on-off valve is an ON position capable of outputting said pilot pressure, which acts to perform said platform free-fall permitting operation, to another control port of said second directional control valve; and said ON position of said third on-off valve is an ON position capable of outputting said pilot pressure, which acts to perform said platform forced-lowering operation, to another control port of said first directional control valve. 3. The invention as described above in claim 2, further comprising: a pull-out restraining means for restraining occurrence of a pull-out, which is a devastating extension of said hoist cylinder that may occur during said platform raising operation. 4. The invention as described above in claim 2, further comprising: a load adjusting means for adjusting a depressing load on a truck frame of said load carrying platform during said platform forced-lowering operation. 5. The invention as described above in claim 1, further comprising: a pull-out restraining means for restraining occurrence of a pull-out, which is a devastating extension of said hoist cylinder that may occur during said platform raising operation. 6. The invention as described above in claim 5, wherein: said pull-out restraining means comprises a restrictor arranged at said switch position relevant to platform raising operations in corresponding one of said first directional control valve and said second directional control valve. 7. The invention as described above in claim 1, further comprising: a load adjusting means for adjusting a depressing load on a truck frame of said load carrying platform during said platform forced-lowering operation. 8. The invention as described above in claim 7, wherein: said load adjusting means comprises a relief valve arranged in a line connecting a bottom chamber of said hoist cylinder and a reservoir with each other.