Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Patent
1991-01-07
1993-09-21
Look, Edward K.
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
60445, 91459, 91518, F16D 3102
Patent
active
052458288
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The present invention relates to a hydraulic drive system for civil engineering and construction machines, and more particularly to a hydraulic drive system mounted on civil engineering and construction machines such as hydraulic excavators and constituting the so-called load sensing system in which a delivery flow rate of a main hydraulic pump is controlled so that a differential pressure between a delivery pressure of the main hydraulic pump and a maximum load pressure among actuators is held constant.
2. Background Art
Civil engineering and construction machines, for example, hydraulic excavators, each mount thereon a hydraulic drive system for driving a plurality of working members such as a boom, an arm and a swing. The hydraulic drive system generally comprises a prime mover, a main hydraulic pump driven by the prime mover, a plurality of actuators such as hydraulic cylinders and motors for driving the above working members, and a plurality of flow control valves for controlling flows of a hydraulic fluid supplied from the main hydraulic pump to the respective actuators.
Recently, it has been proposed and practiced to adopt the so-called load sensing system in that type of the hydraulic control system. The load sensing system is to receive a differential pressure between a delivery pressure of the main hydraulic pump and a maximum load pressure among the plurality of actuators, as a load sensing differential pressure, and control a delivery rate of the main hydraulic pump so that the load sensing differential pressure is held at a preset target differential pressure. For instance, JP, A, 60-11706 discloses a hydraulic drive system equipped with a pump regulator which comprises a control actuator for driving a swash plate of the main hydraulic pump, i.e., pump displacement volume varying means, and a regulating valve operated in response to the load sensing differential pressure for controlling operation of the control actuator. The regulating valve of the pump regulator includes a spring for setting a target value of the load sensing differential pressure.
In such a hydraulic drive system arranged to implement the load sensing system, the delivery rate of the main hydraulic pump is controlled so that the differential pressure between the pump delivery pressure and the maximum load pressure, i.e., the load sensing differential pressure, is held at a constant value in balance with a force of the spring of the regulating valve. When a single actuator is solely driven, for example, the flow rate of the hydraulic fluid passing through the associated flow control valve is substantially in proportion to the opening area of the flow control valve, and the delivery rate of the main hydraulic pump becomes equal to the flow rate of the hydraulic fluid passing through the flow control valve. Thus, the delivery rate of the main hydraulic pump is in substantially proportional relation to the opening area of the flow control valve. This is also true in the case of simultaneously driving plural actuators.
However, the above conventional hydraulic drive system equipped with the pump regulator has accompanied the problem as follows.
In civil engineering and construction machines, for example, hydraulic excavators, which mount thereon hydraulic drive systems, when jolting or vibrations of large amplitude occur upon application of some external load such as an impact, an operating lever may be shifted to vary the opening area of an associated flow control valve and the load sensing differential pressure may be fluctuated correspondingly, in spite of that the operating lever is continuously grasped fast by an operator to hold the opening area of the flow control valve constant with an intention of keeping an operating speed of an actuator. Further, even when a hydraulic fluid is supplied to the actuator with the opening area of the flow control valve held constant, a load pressure may be fluctuated owing to compressibility of the hydraulic fluid and so is the load sensing differential pres
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Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Nguyen Hoang
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