Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1997-05-16
1998-11-03
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60422, 60429, 60430, F16D 3102
Patent
active
058292527
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a hydraulic system for driving a plurality of actuators by a plurality of pumps in a hydraulic excavator or the like.
BACKGROUND ART
Hydraulic systems for driving a plurality of actuators by a plurality of pumps comprise so-called open center circuits as disclosed in JP-B-2-16416, for example, and so-called closed center circuits as disclosed in JP-A-4-194405. The open center circuit is a circuit having a center bypass line, and a pump delivery flow is bled to a reservoir through the center bypass line when each directional control valve is in a neutral condition. An opening of the center bypass line located in each directional control valve is gradually throttled as the directional control valve is shifted by a larger amount, whereupon a pump pressure is produced and a hydraulic fluid is supplied to each corresponding actuator through a meter-in circuit.
In the open center circuit, independence of plural actuators is maintained by providing a preference circuit in the form of a so-called tandem connection or arranging a plurality of hydraulic pumps so that hydraulic fluids are joined together selectively.
On the other hand, the closed center circuit is a circuit having no center bypass line. As disclosed in the above-cited JP-A-4-194405, spools are connected to a hydraulic pump in parallel. There are also known a load sensing system for controlling a differential pressure between a pump pressure and a load pressure to be fixed when each directional control valve is in a neutral condition, and a system for reducing a pump delivery rate through a bleed circuit including a bleed valve as disclosed in JP-A-7-63203 when each directional control valve is in a neutral position.
DISCLOSURE OF THE INVENTION
In the open center circuit, as mentioned above, independence of plural actuators is maintained by providing a preference circuit in the form of a so-called tandem connection or by arranging a plurality of hydraulic pumps so that hydraulic fluids are joined together selectively. However, it is required not only to form a center bypass line in each directional control valve, but also to provide a plurality of directional control valves for one actuator. The valve structure is, therefore, complicated and increased in size. Also, because the preference circuit is made up by using the center bypass line, a preference degree and metering characteristics cannot be set independently of each other during the combined operation of actuators.
In the closed center circuit, the valve structure is relatively simple because the center bypass line is not necessary and only one directional control valve is usually required for one actuator. However, the closed center circuit is basically a parallel circuit and hence has a difficulty in realizing a preference circuit.
A first object of the present invention is to provide a hydraulic system in which a joining circuit and a preference circuit are realized in a closed center circuit with a simple structure.
A second object of the present invention is to provide a hydraulic system in which a preference degree and metering characteristics can be set independently of each other during the combined operation of actuators in a closed center circuit.
(1) To achieve the above first object, the present invention is constituted as follows. A hydraulic system comprises first and second hydraulic pumps, first and second actuators, a first directional control valve of closed center type connected to the first and second hydraulic pumps for controlling a flow rate of a hydraulic fluid supplied to the first actuator, and a second directional control valve of closed center type connected to at least the first hydraulic pump for controlling a flow rate of a hydraulic fluid supplied. The second actuator, the hydraulic system further comprises first and second feeder lines respectively connecting the first and second hydraulic pumps to a pump port of the first directional control valve, and first and second reverse-flow preventing valves dispos
REFERENCES:
patent: 3991571 (1976-11-01), Johnson
patent: 4136600 (1979-01-01), Heiser
patent: 4517800 (1985-05-01), Karakama
patent: 5048293 (1991-09-01), Aoyagi
patent: 5148676 (1992-09-01), Moriya et al.
Hirata Toichi
Sugiyama Genroku
Toyooka Tsukasa
Hitachi Construction Machinery Co. Ltd.
Karimi Bijan N.
Look Edward K.
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