Patent ID: 7740323

Claim:
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.