Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2001-09-12
2003-07-01
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S452000
Reexamination Certificate
active
06584770
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for a construction machine, such as a hydraulic excavator, in which load sensing control is performed to hold a delivery pressure of a hydraulic pump higher than a maximum load pressure of a plurality of actuators by a target differential pressure, and in which differential pressures across a plurality of directional control valves are each controlled by a pressure compensating valve. More particularly, the present invention relates to a hydraulic drive system in which a target compensated differential pressure of each pressure compensating valve is set by a differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators, and the target differential pressure in the load sensing control is variably set depending on an engine revolution speed.
BACKGROUND ART
A hydraulic drive system, in which load sensing control is performed to hold a delivery pressure of a hydraulic pump higher than a maximum load pressure of a plurality of actuators by a target differential pressure, is called a load sensing system (hereinafter referred to also as an “LS system”). Usually, in the LS system, differential pressures across a plurality of directional control valves are each controlled by a pressure compensating valve so that a hydraulic fluid can be supplied to the actuators at a ratio depending on opening areas of the directional control valves regardless of the magnitude of load pressure during the combined operation in which the plurality of actuators are simultaneously driven.
In connection with such an LS system, JP,A 10-196604 discloses a hydraulic drive system in which a differential pressure (hereinafter referred to as an “LS differential pressure”) between a delivery pressure of a hydraulic pump and a maximum load pressure of a plurality of actuators is introduced to pressure compensating valves for setting a target compensated differential pressure of each pressure compensating valve by the LS differential pressure, and in which a target differential pressure (hereinafter referred to as a “target LS differential pressure”) in the load sensing control is variably set depending on an engine revolution speed.
By setting the target compensated differential pressure of each pressure compensating valve by the LS differential pressure, when a saturation state, where a delivery rate of the hydraulic pump is insufficient for satisfying a flow rate demanded by the plurality of directional control valves, occurs during the combined operation in which the plurality of actrators are simultaneously driven, the LS differential pressure is lowered depending on a degree of saturation, and the target compensated differential pressure of each pressure compensating valve is also reduced correspondingly. Therefore, the delivery rate of the hydraulic pump can be redistributed at a ratio of flow rates demanded by the respective actuators. Such a system is based on the concept of the invention disclosed in JP,A 60-11706.
By variably setting the target LS differential pressure depending on the engine revolution speed, when the engine revolution speed is lowered, the target LS differential pressure is also reduced correspondingly. Accordingly, even when a control lever for the directional control valve is operated in the same input amount as in the rated state, the flow rate of the hydraulic fluid supplied to the actuator is reduced and the actuator speed is slowed down. As a result, the actuator speed can be obtained corresponding to the engine revolution speed and fine operability can be improved.
Further, in connection with the LS system, GB2195745A discloses a system in which a signal pressure relief valve is disposed in a maximum load pressure line for detecting a maximum load pressure as a signal pressure, a setting pressure of the signal pressure relief valve is set to be lower than a setting pressure of a main relief valve, and the maximum load pressure having an upper limit restricted by the signal pressure relief valve is introduced to each pressure compensating valve. By providing the signal pressure relief valve in the maximum load pressure line, even when a load pressure of any one actuator reaches the setting pressure of the main relief valve and a delivery pressure of a hydraulic pump becomes equal to the maximum load pressure during the combined operation in which a plurality of actuators are simultaneously driven, it is possible to prevent all of the pressures compensating valves from being fully closed and hence prevent all of the actuators from being stopped, because the signal pressure in the maximum load pressure line is reduced to a level lower than the delivery pressure of the hydraulic pump.
DISCLOSURE OF THE INVENTION
However, the prior-art systems described above have problems as follows.
In the prior art disclosed in JP,A 10-196604, as described above, the LS differential pressure is introduced as the target compensated differential pressure to the pressure compensating valve. During the combined operation in which a plurality of actuators are simultaneously driven, therefore, when the load pressure of any one actuator reaches the setting pressure of the main relief valve and the delivery pressure of the hydraulic pump becomes equal to the maximum load pressure, the LS differential pressure is reduced to 0 and the pressure compensating valves are all fully closed. Consequently, no hydraulic fluid is supplied to the other actuators as well, of which load pressures do not yet reach the relief pressure, and the actuators are all stopped.
By providing the signal pressure relief valve, disclosed in GB2195745A, in the maximum load pressure line of the hydraulic drive system disclosed in JP,A 10-196604, even when the delivery pressure of the hydraulic pump becomes equal to the maximum load pressure as mentioned above, the signal pressure in the detection line is reduced to a level lower than the delivery pressure of the hydraulic pump. It is hence possible to prevent all of the pressure compensating valves from being fully closed and prevent all of the actuators from being stopped. Such an arrangement, however, causes another problem.
In the hydraulic drive system disclosed in JP,A 10-196604, the target LS differential pressure is variably set depending on the engine revolution speed. Therefore, the target LS differential pressure differs between when the engine revolution speed is set to a rated value and when the engine revolution speed is set to a lower value. The target LS differential pressure is smaller in the latter case than in the former case, and the actual LS differential pressure is also reduced correspondingly. Accordingly, if the setting pressure of the signal pressure relief valve is set to be lower than the setting pressure of the main relief valve by a value corresponding to the LS differential pressure during the rated rotation, the following problem occurs. During the rated rotation, the LS differential pressure resulting when the load pressure of the actuator is low and the main relief valve is not operated is equal to the differential pressure between the delivery pressure of the hydraulic pump and the signal pressure in the detection line resulting when the load pressure rises up to the setting pressure of the main relief valve, and hence the target compensated differential pressure of the pressure compensating valve is not changed. However, when the engine revolution speed is set to a lower value, the LS differential pressure is reduced to a level lower than that during the rated rotation as described above, while the differential pressure between the setting pressure of the signal pressure relief valve and the setting pressure of the main relief valve remains the same as the LS differential pressure during the rated rotation. Accordingly, the differential pressure between the delivery pressure of the hydraulic pump and the signal pressure in the detection line resulting when the load pressure rises up to the setting pressure o
Hamamoto Satoshi
Kanai Takashi
Kawamoto Junya
Nagao Yukiaki
Okazaki Yasuharu
Hitachi Construction Machinery Co. Ltd.
Kershteyn Igor
Look Edward K.
Mattingly Stanger & Malur, P.C.
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