Unloading means for flow-pressure compensated valve

Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement

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Details

417222, F16H 3946

Patent

active

043555101

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION



TECHNICAL FIELD

This invention relates generally to a flow-pressure compensated valve for use in a servo-system of a variable displacement pump, and more particularly to means for unloading a biasing or margin force imposed on the valve in response to the load pressure in the fluid actuator exceeding a maximum pressure level.


BACKGROUND ART

A variable displacement pump is employed in a hydraulic circuit for construction vehicles to control a fluid actuator, such as a double-acting hydraulic cylinder. The servo-system employed with such pump oftentimes includes a flow-pressure compensated or "load-plus" valve which functions to modulate a discharge pressure signal and to maintain pump discharge pressure above a minimum pressure level and also above a load-pressure signal generated in the cylinder. This type of valve is fully disclosed in U.S. Pat. No. 4,116,587, issued on Sept. 26, 1978 to Kenneth P. Liesener, and assigned to the assignee of this application.
The load-pressure signal is communicated from the fluid actuator to the "load-plus" valve to automatically control actuation of the swash plate for the pump to maintain the desired pump discharge pressure. In addition, a margin spring is employed in the valve to maintain the pump discharge pressure at a "MARGIN" above the load pressure signal. The load signal is continuously in communication with tank through an orifice which results in a loss in horsepower.
The present invention is directed to overcoming one or more of the problems as set forth above.


DISCLOSURE OF INVENTION

In one aspect of the present invention, a fluid circuit has a fluid actuator, a variable displacement pump including a control member movable between first and second displacement positions, first biasing means for urging the control member towards its first displacement position, second biasing means for urging the control member towards its second displacement position in response to a variable control pressure, first means for varying the control pressure in response to variations in the discharge pressure of the pump, second means for controlling the first means to modulate the control pressure in response to variations in a load pressure signal received from the fluid actuator, and third biasing means for applying a margin force to the first means to maintain a pressure differential between the control pressure and the load pressure signal during a predetermined range of the load pressure signal.
The improved fluid circuit further comprises unloading means for unloading the margin force in response to the load pressure signal exceeding a maximum pressure level and for inactivating the third biasing means to at least substantially equalize the pump discharge pressure and the load pressure signal in response to such unloading.
The improved fluid circuit of this invention will thus function efficiently to first establish a margin pressure during the working range of the circuit which is thereafter overridden by the unloading means when the load pressure signal exceeds a maximum pressure level. The fluid circuit is thus enabled to minimize horsepower losses by preventing needless loss of the load pressure signal which is normally vented in conventional systems and also provides that the total control package may be contained within a minimum envelope size.


BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of this invention will become apparent from the following description and accompanying drawings wherein:
FIG. 1 schematically illustrates a fluid circuit employing an unloading means embodiment of the present invention therein for overriding the function of a "load-plus" valve;
FIG. 2 is a longitudinal sectional view illustrating the unloading means and "load-plus" valve associated with a pump;
FIG. 3 graphically illustrates a margin or differential pressure, during the working range of the fluid circuit, between a pump discharge pressure P.sub.D and a load pressure signal P.sub.L generated in a fluid actuator; and
FIG. 4 is a sectional view

REFERENCES:
patent: 3406850 (1968-10-01), Hancox
patent: 3444689 (1969-05-01), Budzich
patent: 3797245 (1974-03-01), Hein
patent: 4116587 (1978-09-01), Liesener
patent: 4174614 (1979-11-01), Dezelan

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