When the open-center system is in neutral, pump output flows through the directional control valve to tank. When this simple circuit handles only low flows and the directional valve spool has tapered lands, it proves to be very efficient.If several cylinders are used, the valve can be connected in series - that is, the tank port of one valve is connected to the pressure port of the next.
A pressure-compensated, variable-volume pump is controlled by system pressure. As pressure increases, displacement of the pump decreases so that pump output at the preset pressure is only sufficient to make up for leakage. Used with a closed-center valve, the pump is stroked to minimum (zero) displacement when the valve is centered.
Many systems require a high volume at low pressure for rapid traverse of a vise or tool, and then low volume, high pressure for clamping or feeding. This can be accomplished by a hi-lo circuit using two pumps. During rapid traverse, both pumps supply the system. When pressure rises during clamping or feed, the large-volume main pump unloads, and the small pump maintains pressure.
Output flow of the small pump is low enough to prevent heating of the oil. Instead of pilot operation, the unloading valve can be solenoid controlled and actuated by a pressure switch.