Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1989-12-21
1992-10-06
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60422, 60426, 60433, 60434, 60443, 60444, 60452, 60465, 60486, 91517, 91518, 91531, F16D 3102
Patent
active
051521431
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to hydraulic drive systems for construction machines such as a hydraulic excavator or the like and, more particularly, to a hydraulic drive system wherein hydraulic fluid of a hydraulic pump driven by a prime mover is supplied to each of a plurality of actuators in which respective differential pressures across them are controlled by a plurality of pressure compensating valves and wherein these actuators are simultaneously driven to conduct a desired combined operation.
BACKGROUND ART
In recent years, in hydraulic drive systems for a construction machine such as a hydraulic excavator, a hydraulic crane and the like, which comprise a plurality of hydraulic actuators for driving a plurality of driven units, delivery pressure of the hydraulic pump is controlled in synchronism with load pressure or requisite flow rate. Further a plurality of pressure compensating valves are arranged respectively in association with the flow control valves for controlling differential pressure across the flow control valves, whereby supply flow rates during simultaneous driving of the actuators are stably controlled. Of these hydraulic drive systems, load-sensing control is known from DE-A1-3422165 (corres. to JP-A-60-11706), U.S. Pat. No. 4,739,617 and the like, a typical example of which is the control of delivery pressure of the hydraulic pump in synchronism with load pressure. The load-sensing control is such that pump delivery rate is controlled so as to make the pump delivery pressure higher by a fixed value than the maximum load pressure among a plurality of hydraulic actuators. In these conventional examples, a swash-plate position of the hydraulic pump is controlled in response to the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure among the plurality of actuators, to conduct the load-sensing control.
Further, in these conventional systems, when the delivery rate of the hydraulic pump reaches its maximum so that the pump delivery rate is insufficient, the hydraulic fluid is preferentially supplied to the actuator on the side of the low load pressure during the combined operation. Thus, balance of the combined operation cannot be maintained. In order to solve this problem, a control force determined on the basis of the differential pressure between the delivery pressure of the hydraulic pump and the maximum load pressure of the plurality of actuators acts directly or indirectly upon each pressure compensating valve for controlling the differential pressure across the flow control valve, in place of a spring as one for setting a target value of the differential pressure. In this arrangement, the target value of the differential pressure across the flow control valve decreases in response to a decrease in the differential pressure between the pump delivery pressure and the maximum load pressure. The pump delivery rate is accordingly distributed in response the opening ratio (requisite flow-rate ratio) of the flow control valves. Thus, it is possible to maintain the balance of the combined operation.
Additionally, the hydraulic pump is driven by the prime mover. The delivery rate of the hydraulic pump is represented by the product of a displacement volume determined by the swash-plate tilting angle of the hydraulic pump and the rotational speed of the prime mover. The pump delivery rate decreases when the target rotational speed of the prime mover decreases. Further, in the conventional systems described above, a change in the passing flow rate of each of the flow control valves, with respect to a change in a stroke of a control lever, is constant regardless of the target rotational speed of the prime mover. Accordingly, in these conventional systems, when the pump delivery rate, at the time the target rotational speed of the prime mover decreases and the displacement volume is maximum, is reduced less than the requisite flow rate at the time the opening of the flow control valve is maximum, the following result occurs
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Hirata Toichi
Kajita Yusuke
Sugiyama Genroku
Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Mattingly Todd
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