HYDRAULIC CONTROLLING APPARATUS FOR EXCAVATION MACHINE

PURPOSE: To improve operability by selectively moving a spool of a directional changeover valve to a first position or and a second position according to the differential pressure between the loading pressure which act on a hydraulic actuator and a delivery pressure of a variable displacement pump. CONSTITUTION: A load pressure works in a load pressure actuation chamber 42b of a load sensing valve 42 from a hydraulic actuator 18 via a pressure compensator 32 and a delivery pressure works in a pump pressure actuation chamber 42c of the valve 42 from the variavle displacement pump 10 via a main supply line 16. When a pressure difference between the load pressure and the delivery pressure is smaller than a reference value a shuttle piston 42a moves in the direction separating from the directional changeover valve 38 and a spool 38a is shifted to the second position. The hydraulic oil flows in a servo chamber 36d of a servo cylinder 36, and the piston rod 36 is contracted to increase the inclined angle of a swash plate 14 of the pump 10 and increase the discharge volume of the hydraulic oil. When the pressure difference is larger than the reference value, the spool 38a is shifted to the first position and the delivery volume is reduced.