Patent ID: 6526747
Filing Date: 2003-03-04
Classification: E02F,F15B

Abstract:
A hydraulic drive system comprising a variable displacement hydraulic pump (10), a plurality of actuators (4a,4b) driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of directional control valves (20a,20b) for controlling respective flow rates of the hydraulic fluid supplied from said hydraulic pump to said plurality of actuators, a plurality of pressure compensating valves (21a,21b) for controlling respective differential pressures across said plurality of directional control valves, and pump control means (12) for performing load sensing control to hold a delivery pressure of said hydraulic pump higher than a maximum load pressure of said plurality of actuators by a target differential pressure, said plurality of pressure compensating valves including a first pressure compensating valve (21a) provided in association with a particular one (20a) of said plurality of directional control valves and a second pressure compensating valve (21b) provided in association with the other directional control valve (20b) than said particular one, wherein said hydraulic drive system further comprises:a first lock switching valve (50; 50A; 110; 110D) having first and second shift positions (C,D; E,F; G,H) and outputting a pressure of a hydraulic supply source when said first lock switching valve is shifted from the first position to the second position; and a first pressure receiving section (28a) provided at an end of said first pressure compensating valve (21a) on the side acting in the closing direction, and connected to the output side of said first lock switching valve, said first pressure compensating valve being fully closed when said first lock switching valve is shifted to the second position and the pressure of said hydraulic supply source is introduced to said first pressure receiving section.