Patent ID: 6836981

Claim:
A hydraulic control circuit for a boom cylinder in a work machine, comprising: a boom cylinder that causes the boom to extend and contract in order to move the boom upward and downward; a first control valve that is freely changed to an operation position that controls supply of pressurized oil to respective oil chambers at a head side and a rod side of the boom cylinder and discharge thereof on the basis of operations of an operating member with a first pressurized oil supply source used as a pressurized supply source and to a neutral position where no pressurized oil is supplied thereinto and discharged therefrom; and a second control valve that controls supply of pressurized oil to a weight-holding side oil chamber of the respective oil chambers of the boom cylinder, which holds the weight of the boom, with a second pressurized oil supply source used as another pressurized oil supply source, wherein the hydraulic control circuit is provided with neutral holding means capable of holding the first control valve at the neutral position regardless of any operation of the operating member and a communication line for causing the head side oil chamber and the rod side oil chamber of the boom cylinder to communicate with each other, the communication line is provided with an opening and closing valve that opens and closes the communication line, and a direction valve that is freely changed to a unidirectional state where, while an oil flow from the weight-holding side oil chamber of the boom cylinder to the other oil chamber is permitted, a reverse oil flow is hindered, and to a bi-directional state where both oil flows are permitted, and the second control valve is provided with discharge means for flowing surplus oil, which is discharged from the weight-holding side oil chamber of the boom cylinder and supplied into the other oil chamber, into an oil reservoir when the first control valve is held in the neutral position of the first control valve by the neutral holding means.