Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Patent
1994-03-28
1995-06-06
Look, Edward K.
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
60452, 60494, 91517, F16D 3102
Patent
active
054211557
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for hydraulic working machines, and more particulary to a hydraulic drive system which is incorporated in hydraulic working machines such as hydraulic excavators and includes a directional control valve having a center bypass passage.
BACKGROUND ART
As described in JP, B, 47-3927 and JP, B, 50-5354, for example, a prior art hydraulic drive system of the above-mentioned type comprises a variable displacement hydraulic pump, at least one actuator driven by a hydraulic fluid delivered from the hydraulic pump, a directional control valve of center bypass type, which has a meter-in passage provided with a meter-in variable restrictor and a center bypass passage provided with a bleed-off variable restrictor, for controlling a flow of the hydraulic fluid supplied from the hydraulic pump to the actuator, a low pressure circuit, a center bypass line for connecting the center bypass passage to the low pressure circuit at a location downstream of the bleed-off variable restrictor, a pressure generator, e.g., fixed restrictor, disposed in the center bypass line, and a pump regulator for controlling a displacement of the hydraulic pump by using, as a control pressure, the pressure generated by the fixed restrictor.
The pump regulator carries out well-known negative control in accordance with the control pressure generated by the fixed restrictor. More specifically, the pump regulator makes control such that the displacement of the hydraulic pump is increased as the control pressure lowers and the pump displacement is decreased as it rises.
In the prior art thus constructed, when the directional control valve is gradually stroked from its neutral position with an intention of driving the actuator, an opening area of the bleed-off variable restrictor of the directional control valve is gradually reduced and, to the contrary, an opening area of the meter-in variable restrictor thereof is gradually increased.
When the directional control valve is in its neutral position or at a start point of its stroke, i.e., when the bleed-off variable restrictor begins to close, the control pressure generated by the fixed restrictor is high and the hydraulic pump is kept at a predetermined small displacement to deliver the hydraulic fluid at a standby flow rate which is a small value corresponding to the above predetermined small displacement. As the bleed-off variable restrictor is gradually closed, a pressure of the hydraulic fluid delivered from the hydraulic pump, i.e., a pump pressure, rises. Assuming now that the load pressure of the actuator is Pa, the actuator starts moving at the time the pump pressure rises in excess of Pa. Upon the actuator starting to move and the hydraulic fluid from the hydraulic pump beginning to be supplied to the actuator, the flow rate passing through the center bypass passage is reduced accordingly. With this reduction in the flow rate passing through the center bypass passage, the control pressure generated by the fixed restrictor in the center bypass line is lowered. The pump regulator is thus driven in such a manner as to increase the displacement of the hydraulic pump. As a result, the delivery rate of the hydraulic pump is gradually increased, thereby providing a predetermined flow rate characteristic, i.e., metering characteristic.
DISCLOSURE OF THE INVENTION
However, the aforementioned prior art has the problems as follows.
When the load pressure of the actuator is a relatively small pressure P2, the delivery rate of the hydraulic pump is relatively slowly increased corresponding to an increase in a spool stroke of the directional control valve and, therefore, the flow rate of the hydraulic fluid supplied to the actuator is also relatively slowly increased in response to the spool stroke, with the result of a satisfactory metering characteristic.
But when the load pressure of the actuator is provided by a pressure P1 higher than P2, the actuator will not start moving until the bleed-off variable restrictor is close
REFERENCES:
patent: 3429123 (1969-02-01), Buroughs
patent: 4011721 (1977-03-01), Yip
patent: 4079805 (1978-03-01), Rau
patent: 4479349 (1984-10-01), Westveer
patent: 4553389 (1985-11-01), Tischer et al.
patent: 5101628 (1992-04-01), Yoshino
Hirata Toichi
Ishikawa Koji
Ochiai Masami
Sugiyama Genroku
Hitachi Construction Machinery Co. Ltd.
Look Edward K.
Nguyen Joang
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