Hitachi Construction Machinery Co., Ltd.

Yukiaki Nagao

Yasutaka Tsuruga

Junya Kawamoto

Takashi Kanai

Yasuharu Okazaki

Satoshi Hamamoto

Kenichiro Nakatani

Kiwamu Takahashi

In a hydraulic drive system in which a target compensated differential pressure for each of pressure compensating valves is set in accordance with a differential pressure between a pump delivery pressure and a maximum load pressure, and a target LS differential pressure is set as a variable value depending on a revolution speed of an engine a fixed throttle and a signal pressure variable relief valve are disposed in a maximum load pressure line A relief setting pressure P of the signal pressure variable relief valve is set so as to satisfy P &equals;P &minus;P &plus;a (where a is a value smaller than P ) with respect to a target LS differential pressure P and a setting pressure P of the main relief valve

Hydraulic drive system

An actuator lock switching valve is provided which communicates a drain line and a pilot line with each other when the valve is in a position C, and which communicates pilot lines with each other when it is shifted to a position D. The pilot line is connected to a delivery line of a hydraulic pump , and the pilot line is connected to pressure receiving sections provided at ends of the pressure compensating valves on the side acting in the closing direction. The actuator lock switching valve has a pressure receiving section connected to the output side of a pilot lock switching valve , and is switched over in interlock with shifting of the switching valve . In a hydraulic drive system including pressure compensating valves controlled by an LS system, an actuator can be locked with a simple construction and can be prevented from malfunctioning in an inoperative condition while an engine is being driven, even when the system includes a mechanically shifted directional control valve, or even when a mechanically shifted directional control valve is retrofitted to the system.

Hydraulic driving device

A hydraulic drive system having a swing control system, includes a pump control unit for controlling a pump delivery rate such that a pump delivery pressure is held a predetermined value higher than a maximum load pressure among a plurality of actuators. Pressure compensating valves are each constructed to set, as a target compensation differential pressure, a differential pressure between a delivery pressure of a hydraulic pump and a maximum load pressure among the actuators. A pressure compensating valve for a swing section is given such a load dependent characteristic that when the load pressure rises, the target compensation differential pressure is reduced, the load dependent characteristic being set so as to provide a flow rate characteristic simulating constant-horsepower control of a swing motor.

Hydraulic drive device

A hydraulic drive system includes a pump control unit for controlling a pump delivery rate such that a pump delivery pressure is held at a predetermined value higher than a maximum load pressure among actuators . Pressure compensating valves are each constructed to set, as a target compensation differential pressure, a differential pressure between the delivery pressure of a hydraulic pump and the maximum load pressure among the actuators . The pressure compensating valve is given such a load dependent characteristic that the target compensation differential pressure is reduced when a load pressure rises. A lower limit setting spring for limiting the target compensation differential pressure from becoming smaller than a predetermined value is provided in the pressure compensating valve for the swing section.

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Toyama, Japan

Yukiaki Nagao