Hydraulic circuit for construction machines

A hydraulic circuit for construction machines, formed so that a variable displacement pump is connected to actuators for driving working units corresponding thereto respectively via a plurality of direction change-over valves, a relief valve for setting a maximum discharge pressure being connected to a discharge pipe line of the variable displacement pump, the cutoff of the volume of the variable displacement pump being controlled via a cutoff valve adapted to reduce a discharge rate of the variable displacement pump gradually and have the same rate reach a minimum level when a discharge pressure of the variable displacement pump becomes close to a set pressure of the relief valve, the hydraulic circuit comprising:

• a suspension mode switch for setting a working mode to a suspension mode, a lift detecting sensor for detecting a predetermined actuator which has been operated to a load lifting side, a relief valve control unit capable of freely setting an object pressure of the relief valve to a normal set level and a high set level higher than the normal set level, a cutoff valve control unit capable of freely switching the cutoff valve to a cutoff operation executing mode and a cutoff operation disengaging mode, and
• a controller adapted to increase a set pressure of the relief valve by outputting an instruction to the relief valve control unit when the controller receives a suspension mode signal and a lift detected signal from the suspension mode switch and lift detecting sensor respectively, and disengage the cutoff function of the cutoff valve by outputting an instruction to the cutoff valve control unit.