Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Patent
1991-02-06
1992-10-06
Look, Edward K.
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
60452, 60469, 91446, 137516, F16D 3102
Patent
active
051521407
DESCRIPTION:
BRIEF SUMMARY
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 include a pressure compensating valve to control the differential pressure across a flow control valve for controlling operation of an actuator.
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, thus 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 actuator cylinder for controlling the displacement volume of the hydraulic pump, and a control valve operated responsive to the differential pressure between the pump pressure and the load pressure for controlling operation of the actuator 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 keep the force of the spring in balance 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 cope with fluctuations in the differential pressure between the pump pressure and the load pressure.
The pressure compensating valve generally comprises a valve spool slidably disposed in a valve housing 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 load pressure of the actuator (the outlet pressure of the flow control valve) is introduced to the first control chamber for urging the valve spool in the valve-opening direction, and the inlet pressure of the flow control valve is introduced to the second control chamber for urging the valve spool in the valve-closing direction. A spring for urging the valve spool in the valve-opening direction is disposed in the first control chamber to provide a target value for the pressure compensation.
When 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., the differential pressure across the flow control valve, becomes larger than the setting value of the spring, the valve spool is caused to move in the valve-closing direction so that the differential pressure across the flow control valve is controlled to be held at the setting value of the spring, i.e., the target pressure. As a result of the differential pressure across the flow control valve being controlled in this way, the flow rate of hydraulic fluid passing through the flow control valve, i.e., the flow rate of hydraulic fluid supplied to the actuator, is adjusted to a value proportional to the opening area of the flow control valve, thus permitting stable control of the actuator.
One pressure compensating valve of this type is disclosed in U.S. Pat. No. 4,688,600, for example.
However, the hydraulic drive system
REFERENCES:
patent: 3777773 (1973-12-01), Tolbert
patent: 3987623 (1976-10-01), Biachetta
patent: 4617854 (1986-10-01), Kropp
patent: 4688600 (1987-08-01), Kreth et al.
patent: 4967557 (1990-11-01), Izumi et al.
patent: 5025625 (1991-06-01), Morikawa
Hirata Toichi
Kajita Yusuke
Nakamura Kazunori
Sugiyama Genroku
Tanaka Hideaki
Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Lopez F. Daniel
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