Patent ID: 7546730

Claim:
A hydraulic system for operating a double-acting hydraulic actuator including a cylinder and a piston to which a piston rod is connected, the piston separating an extend chamber of the cylinder from a retract chamber of the cylinder, the hydraulic system comprising: a first bi-directional pump and a second bi-directional pump that together can be coupled to and operatively driven bi-directionally by a prime mover, each pump having extend and retract ports that function as pressure and suction pumps depending on the direction of operation of the pumps; a first extend circuit portion for fluidly connecting the extend port of the first pump to the extend chamber of the hydraulic actuator; a first retract circuit portion for fluidly connecting the retract port of the first pump to the retract chamber of the hydraulic actuator; a second extend circuit portion for fluidly connecting the extend port of the second pump to the first extend circuit portion; a second retract circuit portion for fluidly connecting the retract port of the second pump to the first retract circuit portion; a non-return check valve connecting the second extend circuit portion to the first extend circuit portion for blocking reverse flow from the first extend circuit portion to the second extend circuit portion while permitting flow from the second extend circuit portion to the first extend circuit portion; an unloader valve connected to the second extend circuit portion between the non-return check valve and the extend port of the second pump, the unloader valve being responsive to fluid pressure in the first extend circuit portion such that unloader valve will open to a drain path when the fluid pressure in the first extend circuit portion increases above a predetermined value; a pressure responsive return check valve connecting the second retract circuit portion to the first retract circuit portion for normally blocking flow reverse flow from the first retract circuit portion to the second retract circuit portion while permitting flow from the second retract circuit portion to the first retract circuit portion, the return check valve being responsive to pressure in the second extend circuit portion to allow reverse flow when the pumps are being operated to extend the actuator; and an excess fluid dump valve connected to the first extend circuit portion for diverting to a drain path excess fluid exiting from the extend side of the actuator during actuator retraction, the excess fluid dump valve being normally closed during actuator extension and open in response to pressure in the first retract circuit portion during actuator retraction.