Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Reexamination Certificate
2001-01-23
2002-07-23
Look, Edward K. (Department: 3745)
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
C060S452000, C137S087040
Reexamination Certificate
active
06422009
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a pump displacement control system for a hydraulic drive apparatus provided with a load sensing system for controlling the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a set differential pressure. More particularly, the present invention relates to a pump displacement control system for controlling the displacement of a hydraulic pump in link with an engine revolution speed, and a valve unit for use in the pump displacement control system.
BACKGROUND ART
As one hydraulic system for controlling actuators of a hydraulic excavator, there is known the so-called load sensing system including a pump displacement control system wherein respective load pressures of the actuators are detected and the delivery rate of a hydraulic pump is controlled so that the delivery pressure of the hydraulic pump is provided by a pressure equal to the sum of a maximum one of the detected load pressures and a certain set differential pressure. The set differential pressure in such a load sensing system (hereinafter referred to also as the LS set differential pressure) is usually set to a certain constant value (e.g., 15 bar) by biasing means such as a spring.
Also, JP-U-2-149881 and JP-A-5-99126 each disclose a pump displacement control system which enables an actuator speed to be changed in link with an engine revolution speed in the above-described ordinary load sensing system.
In the pump displacement control system disclosed in JP-U-2-149881, a throttle is disposed in a delivery line of a fixed displacement pump that is provided as a hydraulic source of a pilot hydraulic circuit for operating equipment such as a group of hydraulic remote control valves. A pressure upstream of the throttle is detected as a signal pressure Pc, and the detected signal pressure Pc is introduced via a signal hydraulic line to a pressure bearing sector of a load sensing valve on the same side as a pressure bearing sector to which a load pressure Pls is introduced. Since the pressure upstream of the throttle changes depending on the revolution speed of the fixed displacement pump, this means that the detected signal pressure Pc contains information of the revolution speed.
The pump displacement control system disclosed in JP-A5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for performing displacement control such that, depending on a differential pressure &Dgr;PLS between a delivery pressure Ps of the hydraulic pump and a load pressure PLS of an actuator driven by the hydraulic pump, a pump delivery pressure is supplied to the servo piston so as to maintain the differential pressure &Dgr;PLS at a set value &Dgr;PLSref. The disclosed pump displacement control system further comprises a fixed displacement hydraulic pump driven by an engine together with the variable displacement hydraulic pump, a throttle provided in a delivery line of the fixed displacement hydraulic pump, and means for varying the set value &Dgr;PLSref of the tilting control unit depending on a differential pressure &Dgr;Pp across the throttle. The engine revolution speed is detected in accordance with change of the differential pressure across the throttle provided in the delivery line of the fixed displacement hydraulic pump, and the set value &Dgr;PLSref of the tilting control unit is varied depending on the detected engine revolution speed.
DISCLOSURE OF THE INVENTION
In a hydraulic drive apparatus provided with a typical conventional load sensing system wherein the set differential pressure of a load sensing valve is given by a spring, even when the engine revolution speed is lowered, the displacement of a hydraulic pump is not changed and the flow rate of a hydraulic fluid supplied to an actuator is also not changed. Accordingly, the actuator speed cannot be slowed down in link with the engine revolution speed. The working speed can be regulated by adjusting the throttle opening of a flow control valve, but to this end a control lever for adjusting the throttle opening of the flow control valve must be operated while holding a lever position within an intermediate stroke range. To improve fine operability, it is desired that, even with the control lever held at a full stroke position, when the engine revolution speed is lowered, the maximum actuator speed (maximum flow rate of the hydraulic fluid supplied to the actuator) can be reduced correspondingly for adjustment of the maximum working speed.
In the pump displacement control system disclosed in JP-U-2-149881, the set differential pressure of the load sensing valve is given by the signal pressure Pc that is obtained by detecting the pressure upstream of the throttle provided in the delivery line of the fixed pump. As a result, with a decrease of the engine revolution speed, the signal pressure (pressure upstream of the throttle) Pc is lowered, which in turn lowers the set differential pressure of the load sensing valve, whereby the displacement of the hydraulic pump is reduced and the working speed of the actuator is slowed down. It is hence possible to control the displacement of the hydraulic pump and adjust the working speed in link with the engine revolution speed.
In the disclosed pump displacement control system, the pilot hydraulic circuit is provided to produce a signal pressure for operating the equipment such as a group of hydraulic remote control valves, and the pressure downstream of the throttle for detecting the engine revolution speed is set by a relief valve for setting a primary pilot pressure. Letting Pa be the pressure set by the relief valve and Pb be the pressure loss caused by the throttle for detecting the engine revolution speed, the pressure (signal pressure) Pc upstream of the throttle is expressed by Pc=Pa+Pb.
Assuming, for example, that the set pressure Pa of the relief valve for setting the primary pilot pressure is 45 bar, the delivery rate of the fixed pump at the engine revolution speed of 2000 rpm is 35 liter/min (the set pressure Pa is assumed to be kept at 45 bar even upon consumption of the pump delivery rate), and the pressure loss Pb caused by the throttle for detecting the engine revolution speed is 15 bar, the pressure Pc upstream of the throttle is 60 bar. In the typical conventional load sensing system wherein the set differential pressure of the load sensing valve is given by a spring, an equivalent pressure applied by the spring is, e.g., about 15 bar. To provide the set differential pressure at a value equal to 15 bar in the pump displacement control system disclosed in JP-U-2-149881, the pressure bearing sector of the load sensing valve is required to modulate 60 bar of the throttle upstream pressure Pc down about ¼, i.e., to 15 bar. Providing such a function to modulate the pressure results in a more complicated structure of the load sensing valve.
In the pump displacement control system disclosed in JP-A-5-99126, the set value &Dgr;PLSref of the tilting control unit is varied depending on the differential pressure &Dgr;Pp across the throttle instead of the pressure Pc upstream of the throttle for detecting the engine revolution speed. The differential pressure &Dgr;Pp across the throttle coincides with the pressure loss Pb caused in the throttle, and is 15 bar in the above-mentioned example. This value is equal to the equivalent pressure applied by the spring, i.e., about 15 bar, which is provided in the typical conventional load sensing system. Accordingly, when the differential pressure &Dgr;Pp across the throttle is employed instead of the pressure Pc upstream of the throttle, the differential pressure &Dgr;Pp across the throttle can be directly introduced to act upon the pressure bearing sector of the load sensing valve and the structure of the load sensing valve can be avoided from being complicated. This prior art, however, has a problem as follows.
Wh
Nishimura Yoshizumi
Nozawa Yusaku
Takahashi Kinya
Tougasaki Mitsuhisa
Hitachi Construction Machinery Co. Ltd.
Lazo Thomas E.
Look Edward K.
Mattingly Stanger & Malur, P.C.
LandOfFree
Pump capacity control device and valve device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pump capacity control device and valve device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pump capacity control device and valve device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2857024