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A load responsive system employing a fixed displacement pump and a variable displacement pump in a parallel circuit as a source of fluid in an open-center load sensing system. The load response control valve includes an unloading spool which is flow responsive in the operative positions of the contr

A load responsive system employing a fixed displacement pump and a variable displacement pump in a parallel circuit as a source of fluid in an open-center load sensing system. The load response control valve includes an unloading spool which is flow responsive in the operative positions of the control valve and maintains a constant Δρ across the control valve by dumping the pump flow to reservoir; the control valve Δρ being greater than the Δρ of the pump compensator means so that the variable displacement pump will remain destroked until the system utilizes the full flow of the fixed displacement pump at which time the variable displacement pump strokes-out upon demand.

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1. An open-center load sensing system supplied by multiple pumps for driving at least one motor (40) which in turn drives a load comprising: a fixed displacement pump (10); a variable displacement flow compensated pump (12) connected in parallel with the fixed displacement pump including a flow

1. An open-center load sensing system supplied by multiple pumps for driving at least one motor (40) which in turn drives a load comprising: a fixed displacement pump (10); a variable displacement flow compensated pump (12) connected in parallel with the fixed displacement pump including a flow responsive compensator means (14) with a predetermined Δρ for controlling the displacement of the pump; a load responsive control valve means (18) including a control valve spool positioned between said pumps and said motor which senses the load on said motor in its operative positions, the control valve means includes an unloading spool means (22) which is flow responsive in the operative positions of the valve and maintains a constant Δρ across said control valve spool by dumping pump flow to reservoir (50); said control valve Δρ being greater than the Δρ of said pump compensator means (14); the unloading spool means (22) is capable of dumping to reservoir a portion of or the entire flow from said pumps; a first sensing passage (16, 32 and 31) connecting the load caused pressure on said motor downstream of said control valve means to the unloading spool; and a second sensing passage (16a) connecting the pump compensator means with said load. 2. A system as set forth in claim 1, wherein the unloading spool means (22) includes servo chambers (26 and 24) at opposite ends of the spool means which sense a pressure drop across said control valve means (18), and a spring means (23) urging the unloading spool means toward a closed position in conjunction with one of said servo chambers. 3. A system as set forth in claim 1, wherein the unloading spool means (22) includes oppositely-acting first and second servo chambers (26 and 24) at opposite ends of the spool means which sense a pressure drop across said control valve means (18), and a spring means (23) urging the unloading spool toward a closed position in conjunction with the first servo chamber (26), and the first sensing passage is connected to the first servo chamber of the unloading spool means. 4. A system as set forth in claim 1, wherein the unloading spool means (22) includes oppositely-acting first and second servo chambers (26 and 24) at opposite ends of the spool means which sense a pressure drop across said control valve means (18), and a spring means (23) urging the unloading spool toward a closed position in conjunction with the first servo chamber, and the first sensing passage is connected to the first servo chamber of the unloading spool means, and said control valve means (18) having a neutral position which drains the first servo chamber of the unloading spool means. 5. An open-center load sensing system supplied by multiple pumps for driving at least one motor (40) which in turn drives a load comprising: a fixed displacement pump (10); a variable displacement flow compensated pump (12) connected in parallel with the fixed displacement pump including a flow responsive compensator means (14) with a predetermined Δρ for controlling the displacement of the pump; a load responsive control valve means (18) including a control valve spool positioned between said pumps and said motor which senses the load on said motor in its operative positions, the control valve means includes an unloading spool means (22) which is flow responsive to neutral drain flow across a first fixed orifice (30) with a Δρ less than the Δρ of the pump compensator, the unloading spool means includes servo chambers (26 and 24) at opposite ends sensing the pressure drop across said first fixed orifice to position said unloading spool means, the unloading spool means is capable of dumping to reservoir a portion of or the entire flow from the fixed and variable displacement pumps; a first sensing passage (16, 32 and 31) including a second fixed orifice (42) therein connecting the load on said motor downstream from said control valve means to one of the unloading spool servo chambers sensing downstream pressure, the second orifice being sized such that the Δρ from the pump discharge to load is greater than the Δρ of the pump compensator; and a second sensing passage (16a) connecting the pump compensator means to the load. 6. A system as set forth in claim 5, wherein the servo chambers (26 and 24) are oppositely-acting first and second chambers including a spring means (23) urging the unloading spool toward a closed position in conjunction with the first servo chamber and the first sensing passage is connected to the first servo chamber of the unloading spool means. 7. A system as set forth in claim 5, wherein the servo chambers (26 and 24) are oppositely-acting first and second chambers including a spring means (23) urging the unloading spool toward a closed position in conjunction with the first servo chamber, and the first sensing passage is connected to the first servo chamber of the unloading spool means, and a third sensing passage (38) connecting the first fixed orifice to the control valve spool, and said control valve means having a neutral position which drains the third sensing passage to reservoir, and operative positions which block the flow in the third sensing passage.