Hydraulic Presses - Articles & Resources

Defining the setup of dual-pump control
The problem with valve control of cylinder motion is that it does its work by converting (consuming) unused power to heat in the hydraulic fluid. The advantage of valve control is that it accommodates differential rod and cap end flows. Pump control, on the other hand, is efficient because it can deliver power only when and where it is needed. The disadvantage is that it works best with equal area actuators, and works not at all with unequal area actuators.

Attempts have been made to use pump control on single—rod cylinders. However, they incorporated some form of valving to deal with the unequal flow, thus introducing an inefficiency. The simplified schematic shown here depicts pump control without any valves, except those needed to control the displacement of the two pumps.

This system is called the two—pump control method, and it uses a pump on each end of a cylinder. Put simply, the two pumps have displacements that are matched to the respective cylinder areas. To extend the cylinder, pump A displacement increases to route flow into the cap end of the cylinder. At the same time, displacement of pump B increases to absorb flow from the rod end of the cylinder. Obviously, pump B's displacement must increase at a lower rate than pump A's. This is the function of the block labeled Ratioed stroke synchronizer.

Suppose the cap-end to rod-end area ratio of the cylinder is 2:l. The synchronizer is a mechanism (electrical, hydraulic, or mechanical) that causes pump A's displacement to be twice that of pump B's. If pump A displaces 4 in. 3/ rev, pump B will displace 2 in.3/ rev. Clearly, the pumps must be capable of over—center operation. The least expensive means for displacement synchronization would be a simple mechanical linkage between the two pumps. However, the most flexible solution would occur with computer control.

Setting up the system
It would be impractical to try to set up the displacement synchronizer before the system is built. The system can be tuned approximately during final assembly and testing, but the final adjustment has to be done in the system. The tuning of the synchronizer involves a slight misadjustment of one displacement or the other (it doesn't matter which) to prevent cavitation during system operation. The two pumps must work against each other at all times, but only to a degree. This conflict elevates both pressures, and the adjustment is aimed at ensuring that neither pressure drops below atmospheric, thereby preventing cavitation during normal operation.

How the system works
The circuit in the illustration has the equipment connected in a closed positional servo loop. Preliminary adjustments will set the two displacements at zero when the cylinder is unloaded and not moving. The two cylinder pressures will both be zero, or nearly so. This results in a lack of stiffness in the servo loop a condition that must be corrected. When the cylinder is at rest, adjusting the offset control causes the displacement of one pump to increase. If this is done to the rod end, fluid from pump B will tend to retract the rod. However, if the piston rod actually does retract, the position loop takes over and increases the displacement of pump A to push the rod back.

Source: All Inputs From “Hydraulics & Pneumatics” Magazines (A Penton Publication).