Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1989-11-22
1991-10-15
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60427, 60444, 60452, 91517, 91518, 91531, F15B 1100, F15B 1105, F15B 1116
Patent
active
050563122
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hydraulic drive system for construction machines such as hydraulic excavators, and more particularly, to a hydraulic drive system for construction machines which includes distribution compensating valves for controlling differential pressures across respective flow control valves, and in which a control force in accordance with a differential pressure between the discharge pressure of a hydraulic pump under load-sensing control and the maximum load pressure among a plurality of actuators is applied to each of the distribution compensating valves to thereby set a target value of the differential pressure across the flow control valve.
2. Description of the Prior Art
Recently, in a hydraulic drive system for construction machines, such as hydraulic excavators and cranes, each equipped with a plurality of hydraulic actuators for driving a plurality of driven members, it is customary to control the discharge pressure of a hydraulic pump in response to load pressures or demanded flow rates, and to arrange pressure compensating valves in association with flow control valves for controlling the differential pressures across the flow control valves by the associated pressure compensating valves, so that the supplied flow rates are steadily controlled when simultaneously driving the hydraulic actuators. Load-sensing control is known as a typical example of controlling the discharge pressure of the hydraulic pump in response to the load pressures.
The load-sensing control is to control the discharge rate of a hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher a fixed value than the maximum load pressure among a plurality of hydraulic actuators. This control increases and decreases the discharge rate of the hydraulic pump in response to the load pressures of the hydraulic actuators, thereby permitting economical operation.
Since the discharge rate of the hydraulic pump has an upper limit, i.e., available maximum flow rate, the pump discharge rate will become not enough, when the hydraulic pump reaches the available maximum flow rate in case of simultaneously driving the plural actuators. This is generally known as saturation of the hydraulic pump. If saturation occurs, a hydraulic fluid discharged from the hydraulic pump will flow into the actuator(s) on the lower pressure side in preference to other actuator(s) on the higher pressure side, the latter actuator(s) being hence supplied with the deficient hydraulic fluid. This results in that the plural actuators cannot be driven simultaneously.
To solve the above problem, with a hydraulic drive system as described in DE-A1-3422165 (corresponding to JP-A-60-11706), two drive parts respectively acting in the valve-opening and -closing directions are provided on each pressure compensating valve for controlling the differential pressure across a flow control valve, in place of a spring conventionally provided for setting a target value of the differential pressure across the flow control valve. The discharge pressure of a hydraulic pump is introduced to the drive part acting in the valve-opening direction, and the maximum load pressure among plural actuators is introduced to the drive part acting in the valve-closing direction. This causes a control force in accordance with a differential pressure between the pump discharge pressure and the maximum load pressure to act in the valve-opening direction for setting a target value of the differential pressure across the flow control valve. When saturation of the hydraulic pump occurs in the above arrangement, the differential pressure between the pump discharge pressure and the maximum load pressure is reduced correspondingly. Therefore, the target value of the differential pressure across the flow control valve for each pressure compensating valve is also reduced and the pressure compensating valve associated with the actuator on the lower pressure side is further restricted, so that the hy
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Aoyagi Yukio
Hirata Toichi
Izumi Eiki
Kajita Yusuke
Nakamura Shigetaka
Hitachi Construction Machinery Co. Ltd.
Kapsalas George
Look Edward K.
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