Patent ID: 6453585
Filing Date: 2002-09-24
Classification: E02F,F15B

Abstract:
A hydraulic drive system for a work machine, said hydraulic drive system being provided with an engine, a first and second variable displacement hydraulic pumps drivable by said engine, a first group of center-bypassed directional control valves connected to said first hydraulic pump, a second group of center-bypassed directional control valves connected to said second hydraulic pump and including a flow-combining directional control valve, a flow-combining valve connected to a most downstream directional control valve of said first group of directional control valves via a center bypass passage to supply pressure fluid from said first hydraulic pump, in combination with pressure fluid from said second hydraulic pump, to said flow-combining directional control valve in said second group of directional control valves, a flow-combining circuit communicating said flow-combining valve and a supply port of said flow-combining directional control valve with each other, a combined-flow-driven actuator controlled by said flow-combining directional control valve, and a variable displacement controller for performing overall power control such that a total value of an input torque to said first hydraulic pump and an input torque to said second hydraulic pump does not exceed an output torque of said engine, comprising:a canceling valve for canceling said flow combination by said flow-combining valve when a load pressure on said combined-flow-driven actuator becomes higher than a predetermined pressure.