Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1992-07-13
1994-05-31
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60426, 91517, 91532, 91446, F16D 3102
Patent
active
053158261
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a hydraulic drive system and a directional control valve, and more particularly to a hydraulic drive system and a directional control valve for use in construction machines, such as hydraulic excavators, each having a plurality of actuators.
BACKGROUND ART
A hydraulic drive system for use in construction machines such as hydraulic excavators comprises a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, and a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic source to a plurality of actuators.
From the standpoint of reducing energy consumption primarily, it is proposed in a hydraulic drive system of that type to employ a load sensing control technique for controlling a delivery pressure of the hydraulic pump dependent on the load pressure. As examples of such a hydraulic drive system, there are known GB 2,195,745A, DE 2,906,670A1, U.S. Pat. No. 4,939,023, etc. To carry out the load sensing control, those examples of the prior art employ a pump flow controller for controlling a delivery rate of the hydraulic pump so that the delivery pressure of the hydraulic pump is held higher by a fixed value than a maximum load pressure among the plurality of actuators. The plurality of directional control valves each comprises a pump port, a pressure chamber capable of communicating with the pump port, a feeder passage capable of communicating with the pressure chamber, an actuator port capable of communicating with the feeder passage, a reservoir port capable of communicating with the actuator port, a first meter-in variable restrictor disposed between the pump port and the pressure chamber, and a pressure compensating valve having a pair of opposite ends, one of which is subjected to a pressure in the pressure chamber and the other of which is subjected to the maximum load pressure among the plurality of actuators. With the pair of opposite ends respectively subjected to the pressure in the pressure chamber and the maximum load pressure, as mentioned above, the pressure compensating valve serves to control the pressure in the pressure chamber dependent on the maximum load pressure for holding the differential pressure across the meter-in variable restrictor at a fixed value, during the combined operation in which plural actuators are driven simultaneously. The differential pressures across the meter-in variable restrictors of all the directional control valves are thereby made equal to one another so that the flow rate of the hydraulic fluid from the hydraulic pump is distributed in accordance with the ratio of opening area between the variable restrictors to perform the desired combined operation.
Of the prior art, the apparatus disclosed in U.S. Pat. No. 4,939,023 is arranged such that one of the directional control valves comprises a pressure reducing valve disposed between the pressure compensating valve and the actuator port for reducing the pressure of the hydraulic fluid supplied to the associated actuator, a load line for leading out the load pressure via a fixed restrictor, and a proportional pressure relief valve of which relief setting pressure is regulated by a pilot pressure from a control lever unit to limit the pressure in the load line, the pressure in the load line being led to act on a setting sector of the pressure reducing valve to thereby control an outlet pressure of the pressure reducing valve dependent on the setting pressure of the proportional pressure relief valve.
The above examples of the prior art have, however, the following problems.
In the hydraulic drive systems disclosed in the above-cited GB 2,195,745 and DE 2,906,670A1, when a control lever for the directional control valve is manipulated to operate the associated actuator, the hydraulic fluid is momentarily forced to flow at a flow rate corresponding to the resultant opening of the meter-in variable restrictor of the directional control va
REFERENCES:
patent: 4361169 (1982-11-01), Williams
patent: 4617798 (1986-10-01), Krusche et al.
patent: 4787294 (1988-11-01), Bowden
patent: 4938023 (1990-07-01), Yoshino
Hirata Toichi
Sugiyama Genroku
Hitachi Construction Machinery Co., Inc.
Look Edward K.
Lopez F. David
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