Patent ID: 8042451

Claim:
A hydraulic control apparatus for a single-acting 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 single-acting cylinder; a switch valve line connected to the switch valve; an adjustment valve arranged between the cylinder line and the switch valve line for selectively connecting and disconnecting the cylinder line and the switch valve line with respect to each other; and a valve control device for controlling operation of the adjustment valve, wherein the adjustment valve includes a fluid chamber, a valve body movably received in the fluid chamber, and a back pressure chamber into which a fluid pressure acting on the valve body is introduced, a restrictor being formed between the valve body and a wall defining the fluid chamber for connecting the cylinder line to the switch valve line, an opening degree of the restrictor being changed in correspondence with movement of the valve body, and 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 valve body in a direction for disconnecting the cylinder line from the switch valve line, 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, wherein the wall defining the fluid chamber forms a valve seat with which the valve body is brought into contact, the cylinder line being disconnected from the switch valve line when the valve body contacts the valve seat, wherein the fluid chamber includes a first opening communicating with the cylinder line and a second opening communicating with the switch valve line, the restrictor being defined at a position corresponding to the first opening, the valve seat being formed at a position corresponding to the second opening.