Patent ID: 8109198

Claim:
A hydraulic control apparatus for a cylinder, comprising: a switch valve for controlling supply and drainage of a fluid with respect to the cylinder, the switch valve being switched among a supply position for supplying the fluid to the cylinder, a drainage position for draining the fluid from the cylinder, and a neutral position for preventing the supply and the drainage of the fluid with respect to the cylinder; a cylinder line connected to the cylinder; a switch valve line connected to the switch valve; a valve support chamber arranged between the cylinder line and the switch valve line; a flow control valve movably located in the valve support chamber, the flow control valve selectively connecting and disconnecting the cylinder line and the switch valve line with respect to each other, the flow control valve including a communication path chamber; an on-off valve movably located in the communication path chamber, the on-off valve defines a back pressure chamber in the communication path chamber, a fluid pressure acting on the on-off valve is introduced into the back pressure chamber, the on-off valve selectively opening and shutting off a communication path between the cylinder line and the switch valve line; and a valve control device for controlling operation of the flow control valve and the on-off valve, wherein a restrictor is formed between the flow control valve and a wall defining the valve support chamber, the restrictor connecting the cylinder line and the communication path chamber to each other, an opening degree of the restrictor being changed in correspondence with movement of the flow control valve, wherein, when the switch valve is located at the neutral position or the supply position, the valve control device applies a fluid pressure in the cylinder line to the back pressure chamber for urging the on-off valve in a direction for shutting off the communication path, and when the switch valve is located at the drainage position, the valve control device applies a pilot pressure lower than the fluid pressure in the cylinder line to the back pressure chamber, thereby moving the on-off valve in a direction for opening the communication path.