US 4,355,510 AGranted

Unloading means for flow-pressure compensated valve

AssigneeCaterpillar Tractor Co.

InventorWalter Z. Ruseff

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Abstract

A "load-plus" valve (34) is oftentimes utilized in the servo-system (20) for a variable displacement pump (11) to maintain pump discharge pressure (P.sub.D) above a minimum pressure level and above a load pressure signal (P.sub.L) generated in a fluid actuator (13). Valves of this type normally include a margin spring (39) for biasing a modulating spool (36) against the opposing force of pump discharge pressure (P.sub.D) to maintain a constant margin over the load pressure signal (P.sub.L) throughout the working range of the system. When the load pressure signal (P.sub.L) exceeds a maximum pressure level, it is normally vented to tank (12), or continuously bled to tank across an orifice, resulting in horsepower losses. The control package employed in systems of this type are also somewhat bulky. The improved fluid circuit (10) of this invention employs an unloading arrangement (50/50a) in association with the above type of "load-plus" valve (34) which functions to inactivate the margin spring (39) to reduce the above margin between the pump discharge pressure (P.sub.D) and load pressure signal (P.sub.L) to zero in response to the load pressure signal (P.sub.L) exceeding a maximum pressure level.

Claims12

Dependent claims

In a fluid circuit (10) having a fluid actuator (13), a variable displacement pump (11) including a control member (24) movable between first and second displacement positions, first biasing means (26) for urging said control member (24) towards its first displacement position, second biasing means (27) for urging said control member (24) towards its second displacement position in opposition to said first biasing means (26) and in response to a variable control pressure (P.sub.C), first means (35) for varying said control pressure (P.sub.C) in response to variations in the discharge pressure (P.sub.D) of said pump (11), second means (37) for controlling said first means (35) to modulate said control pressure (P.sub.C) in response to variations in a load pressure signal (P.sub.L) received from said fluid actuator (13), and third biasing means (38) for applying a margin force to said first means (35) to maintain a pressure differential between said pump discharge pressure (P.sub.D) and said load pressure signal (P.sub.L) during a predetermined range of said load pressure signal (P.sub.L), the improvement comprising:

unloading means (50/50a) for unloading the margin force applied to said first means (35) in response to said load pressure signal (P.sub.L) exceeding a maximum pressure level and for inactivating said third biasing means (38) to at least substantially equalize said pump discharge pressure (P.sub.D) and said load pressure signal (P.sub.L) in response to said unloading.

A fluid circuit (10) comprising

a source of pressurized fluid, including a variable displacement pump (11) having a control member (24) movable between first and second displacement positions,
a fluid actuator (13),
a directional control valve (14) interconnected between said pump (11) and said actuator (13),
first biasing means (26) for urging said control member (24) towards its first displacement position,
second biasing means (27) for urging said control member (24) towards its second displacement position in opposition to said first biasing means (26) and in response to a variable control pressure (P.sub.C) in a control chamber (33) thereof,
a "load-plus" valve (34) including modulating valve means (36) for modulating communication of pressurized fluid from said pump (11) to said control chamber (33), and
third biasing means (38) for applying a force to said modulating spool means (36) and maintaining a pressure differential between pump discharge pressure (P.sub.D) and said load pressure signal (P.sub.L) during a predetermined range of said load pressure signal (P.sub.L), and
unloading means (50/50a) for unloading the force applied to said modulating valve means (36) in response to said load pressure signal (P.sub.L) exceeding a maximum pressure level and for inactivating said third biasing means (38) to at least substantially equalize said pump discharge pressure (P.sub.D) and said load pressure signal (P.sub.L) in response to said unloading.

5 more elements

Dependencies are inferred from the claim text.