Bi-rotational, two-stage hydraulic system

A hydraulic system including a double acting hydraulic cylinder with expand and retract chambers and ports into the expand and retract chambers,

• first and second reversible pumps, each of which has first and second ports which are capable of assuming pressure and suction functions depending on the direction of operation of the respective pump;
• a prime mover for operating the pumps in one direction or another;
• a reservoir for supplying fluid to the pumps and for receiving excess fluid;
• a fluid circuit for directing fluid between the reservoir, the ports of the pumps and the ports of the actuator, the first ports of both pumps fluidly connected via a first circuit portion to the first port of the hydraulic actuator, and the second ports of both pumps being fluidly connected via a second circuit portion to the second port of the hydraulic actuator;
• first valve means in the first fluid circuit portion operationally responsive to fluid pressure in the first circuit portion, for controlling flow from the first circuit portion through a first drain path to the reservoir when the fluid pressure in the first circuit portion increases above a predetermined value;
• second valve means in the second circuit portion between the second ports of the first and second pumps operationally responsive to fluid pressure in the first and second circuit portions for controlling flow between the second pump and the hydraulic actuator, with the second valve means:
• a) in an open position allowing flow between the second pump and the hydraulic actuator when i) the pressure in the first circuit portion in conjunction with the pressure in the second circuit portion at the second port of the second pump is greater than ii) the pressure in the second circuit portion at the second port of the first pump, such that flow is provided between both pumps and the hydraulic actuator;
• and
• b) in a closed position preventing flow between the second pump and the hydraulic actuator when i) the pressure in the first circuit portion in conjunction with the pressure in the second circuit portion at the second port of the second pump is less than ii) the pressure in the second circuit portion at the second port of the first pump such that flow is only provided in the second circuit portion between the first pump and the hydraulic actuator;
• and
• third valve means operationally responsive to fluid pressure in the second circuit portion and fluidly connected to the second pump, for controlling flow through a second drain path from the second pump to the reservoir when the fluid pressure in the second circuit portion at the second port of the first pump is above a threshold value.

In a system for operating a hydraulic actuator in extend and retract modes, where the actuator has internal extend and retract chambers and extend and retract ports for the extend and retract chambers, respectively, the system including:

• first stage and second stage reversible pumps, each of which has first and second ports which are capable of assuming pressure and suction functions depending on the direction of operation of the respective pump;
• a prime mover for conjointly operating the pumps in one direction or another;
• a fluid circuit for directing fluid between the ports of the pumps and the ports of the actuator, the first ports of both pumps fluidly connected via a retract circuit portion to the retract port of the hydraulic actuator, and the second ports of both pumps being fluidly connected via an extend circuit portion to the extend port of the hydraulic actuator;
• first valve means in the retract circuit portion operationally responsive to fluid pressure in the retract circuit portion to compensate for different fluid capacities of the extend and retract chambers, the first valve means directing flow from the retract circuit portion through a first drain path to reservoir when actuator is in the extend mode of operation;
• second and third valve means operationally responsive to fluid pressure in the retract and extend circuit portions for i) allowing flow between both pumps and the actuator when the load on the actuator is below a second predetermined value;
• and ii) allowing flow between the first stage pump and the actuator but isolating the actuator from the second stage pump when the load on the actuator is above the second predetermined value, in both the extend and retract modes of operation.