HYDRAULIC DRIVE SYSTEM FOR CONSTRUCTION MACHINE

摘要

An object to achieve favorable operability, in a combined operation that involves a specific actuator and involves a great difference in load pressure and when an operation of an operating unit for the specific actuator is a fine operation, by reducing energy consumption arising from wasted restricting pressure loss of a pressure compensating valve and by flexibly varying a flow rate of a hydraulic fluid supplied to the specific actuator depending on the load pressure. To achieve this object, a boom cylinder (3a) is provided with an open center type flow control valve (6a) that controls a hydraulic fluid from a main pump (202) and a closed center type flow control valve (6i) that controls a hydraulic fluid from a main pump (102). The main pump (102) is subject to load sensing control. The flow control valve (6a) is opened to control a supply flow rate up to an intermediate zone of an operating range of an operating unit for the boom cylinder (3a). Both the flow control valves (6a) and (6i) are opened to control the supply flow rate after the intermediate zone.

主权项

1. A hydraulic drive system for a construction machine, comprising:
a variable displacement first pump device; a second pump device; a plurality of first actuators that are driven by hydraulic fluids delivered from the first pump device; a plurality of second actuators that are driven by hydraulic fluids delivered from the second pump device; a plurality of closed center type flow control valves that control flows of hydraulic fluids supplied from the first pump device to the first actuators; a plurality of open center type flow control valves that control flows of hydraulic fluids supplied from the second pump device to the second actuators; a plurality of pressure compensating valves that control differential pressures across the respective closed center type flow control valves; and a first pump control unit including a load sensing control section that controls capacity of the first pump device such that a delivery pressure of the first pump device is higher by a target differential pressure than a maximum load pressure of the first hydraulic actuators, wherein the first and second actuators include at least one first specific actuator as a common actuator, the first actuators include a second specific actuator that is used at relatively high frequency in a combined operation with the first specific actuator, the open center type flow control valves include a first flow control valve that controls a flow of a hydraulic fluid supplied from the second pump device to the first specific actuator, the closed center type flow control valves include a second flow control valve that controls a flow of a hydraulic fluid supplied from the first pump device to the first specific actuator, and the first and second flow control valves are set to have an opening area characteristic such that, when an operating unit for the first specific actuator is operated up to an intermediate zone of an operating range, only the first flow control valve opens to supply a hydraulic fluid from the second pump device to the first specific actuator and, when the operating unit is operated further from the intermediate zone, both the first and second flow control valves open to join and supply hydraulic fluids from the first and second pump devices to the first specific actuator.