Power plants – Pressure fluid source and motor – Condition responsive control of motive fluid flow
Patent
1990-09-24
1992-09-29
Look, Edward K.
Power plants
Pressure fluid source and motor
Condition responsive control of motive fluid flow
91446, F16D 3102
Patent
active
051505746
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a hydraulic drive system for civil engineering and construction machines such as hydraulic excavators, and more particularly to a hydraulic drive system for civil engineering and construction machines which includes a pressure compensating valve to control the differential pressure across a flow control valve for controlling operation of an actuator.
2. Background Art
There is known a hydraulic drive system for use in civil engineering and construction machines such as hydraulic excavators, typically called a load sensing system, wherein the delivery flow rate of a hydraulic pump, i.e., the pump delivery rate, is controlled so as to hold the delivery pressure of the hydraulic pump, i.e., the pump pressure, higher by a fixed value than the load pressure of an actuator, causing the hydraulic pump to deliver a hydraulic fluid only at the flow rate necessary for operation of the actuator. As disclosed in JP, A, 60-11706, for example, the load sensing system includes a pump regulator for load sensing control (LS control), which comprises an actuating cylinder for controlling the displacement volume of the hydraulic pump and a control valve actuated responsive to the differential pressure between the pump pressure and the load pressure for controlling operation of the actuating cylinder. The control valve is provided with a spring for urging the control valve in a direction opposite to the differential pressure between the pump pressure and the load pressure. The control valve is operated so as to balance a force of the spring with the differential pressure between the pump pressure and the load pressure. The pump delivery rate is thereby controlled such that the above differential pressure is held at a fixed value corresponding to the spring force, i.e., a target differential pressure.
Furthermore, the load sensing system generally has a pressure compensating valve disposed upstream of a flow control valve to control the differential pressure across the flow control valve, thereby ensuring a flow control function to fluctuations of the differential pressure between the pump pressure and the load pressure.
The pressure compensating valve generally comprises a valve spool disposed in a valve housing in a slidable manner and having a flow control section which serves as a variable restrictor, and first and second control chambers formed in the valve housing in facing relation to each other and accommodating the opposite ends of the valve spool respectively. The inlet pressure of the flow control valve is introduced to the first control chamber working in the valve-closing direction, and the load pressure of the actuator (the outlet pressure of the flow control valve) is introduced to the second control chamber working in the valve-opening direction. A spring for urging the valve spool in the valve-opening direction is disposed in the second control chamber to provide a target value for the pressure compensation. The spring of the pump regulator and the spring of the pressure compensating valve are set into the relationship that the target differential pressure for the LS control is larger than the target value of the compensated differential pressure. Then, the valve spool is operated based on the differential pressure between the inlet pressure of the flow control valve and the load pressure of the actuator respectively introduced to the first and second control chambers, i.e., so as to balance a force of the spring with the differential pressure across the flow control valve, for controlling the differential pressure across the flow control valve.
Stated otherwise, until the differential pressure between the pump pressure controlled by the pump regulator and the load pressure, i.e., the LS differential pressure, reaches the fixed value corresponding to the spring force of the pressure compensating valve, i.e., the target value of the compensated differential pressure, the valve spool is held in a fully open state by that spring force. When the LS d
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Hirata Toichi
Kajita Yusuke
Sugiyama Genroku
Tanaka Hideaki
Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Nguyen Hoang
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