Patent ID: 6820355
Filing Date: 2004-11-23
Classification: E02F,F15B

Abstract:
A hydraulic control circuit of a boom cylinder for a work machine, which is provided with a boom cylinder extending and contracting to move a boom upward and downward, and a control valve that is changed to an operation position where pressurized oil is supplied to and discharged from respective oil chambers at the head side and rod side of the boom cylinder on the basis of operations of an operating member, and to a neutral position where no pressurized oil is supplied thereto and discharged therefrom, the hydraulic control circuit comprising:neutral holding means by which the control valve is held in the neutral position regardless of a descent operation of the operating member; and a communication line by which the head side oil chamber and rod side oil chamber of the boom cylinder are caused to communicate with each other, the communication line is provided with opening and closing valve means that open and close the communication line, and direction valve means that is changed to an unidirectional state in which, although an oil flow from a weight holding side oil chamber, for holding the weight of the boom, to the other oil chamber of the respective oil chambers of the boom cylinder is permitted, a reverse oil flow is hindered, and a bi-directional state in which oil flows in both directions between the respective oil chambers are permitted.