Hydraulic Presses

Defining the setup of dual-pump control
The valve control of cylinder motion does work by converting unused power to heat in hydraulic fluid. With the advantage of this valve control being of accommodating differential rod and cap end flows, on the other hand it also allows efficient pump control as well as adds to the capability to deliver power only when and where the same is required.

The disadvantages are

• It works best with equal area actuators
• Does not work at all with unequal area actuators

With attempts been made for using pump control on single - rod cylinders, these come incorporated with some form of valving so as to deal with issues of unequal flow. The simplified schematic as shown here also depicts pump control without any valves, except ones required to control displacement of two pumps.

Two-pump control method

• It uses pump on each end of cylinder
• The two pumps have displacements that are matched to respective cylinder areas
• To extend cylinder, pump
• A displacement increases to route flow into cap end of cylinder
• At same time, displacement of pump B increases to absorb flow from rod end of cylinder

In cases where cap-end to rod-end area ratio of the cylinder is 2:l, the synchronizer is mechanism (electrical, hydraulic, or mechanical) which is used and causes pump A’s displacement to be twice of that of pump B’s. If pump A displaces 4 in. 3/ rev, pump B would displace 2 in.3/ rev.

Clearly, these pumps must be capable of over-center operations. Further, the least expensive means for displacement synchronization would be in form of a simple mechanical linkage between two pumps. However, most flexible solution would occur with computer control.

Setting up the system
It would be impractical setting up displacement synchronizer before system is built. With the system possibility of getting tuned approximately during final assembly & testing, the final adjustment needs to be done in system. Further, the tuning of synchronizer involves slight maladjustment of one displacement or other so as to prevent cavitation during system operations. The two pumps also need to work against each other at all times but only to a defined degree. With the conflict elevating both pressures, the adjustment is aimed at ensuring neither pressure drops below atmospheric, thus preventing cavitation during normal operations.

How the system works
The circuit as shown in illustration has equipment connected in closed positional servo loop. With preliminary adjustments setting the two displacements at zero when cylinder is unloaded and not moving, the two cylinder pressures will be both be zero or nearly zero. This situation results in lack of stiffness in servo loop, which is a condition that must be corrected.

When cylinder is at rest, adjusting offset control also

• Causes displacement of one pump to increase
• If the same is done to rod end, fluid from pump B tends to retract rod
• However, if piston rod actually does do the retraction, the position loop here takes over and increases displacement of pump A so as to push rod back