Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1989-11-16
1992-08-04
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60452, 91518, F16D 3102
Patent
active
051348535
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
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 suitable for reliably distributing and supplying hydraulic fluid from a hydraulic pump to a plurality of hydraulic actuators including a swing motor for driving a swing body and a boom cylinder for driving a boom of the hydraulic excavator, by way of example, which actuators are subject to a relatively large difference between their load pressures, for the combined operation of the driven members.
BACKGROUND 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. Commonly known as a typical example of controlling the discharge pressure of the hydraulic pump in response to the load pressures is load-sensing control.
The load-sensing control system controls the discharge rate of the hydraulic pump such that the discharge pressure of the hydraulic pump becomes higher by a fixed value than the maximum load pressure among the 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 be insufficient, 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, the 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 insufficient rates of hydraulic fluid, with the result 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 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 the plural actuators is introduced to the drive part acting in the valve-closing direction. Thus, a control force in accordance with the differential pressure between the pump discharge pressure and the maximum load pressure is caused 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 foregoing 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 hydraulic fluid from hydraulic pump is prevented from flowing into the actuator on the lower pressure side with preference. This allows th
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Hirata Toichi
Kajita Yusuke
Sugiyama Genroku
Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Nguyen Hoang
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