The stability of a pressure compensating circuit used to regulate the output pressure in a variable displacement radial piston pump is considered here. A throttle spool placed in the pump inlet controls the flow into the pump to match the flow requirement of the system. A pilot, solenoid operated, with a hydraulic main stage, pressure-regulating valve in the pump outlet regulates the system pressure. The discharge flow of the pressure regulator controls the inlet throttle spool position. Too large of a flow spill through the pressure regulator denoting excess pump flow results in flow and pressure build up in the inlet throttle control cavity so as to throttle the inlet flow. Hydraulic instability may arise if the control pressures applied to the inlet throttle device over restrict the inlet flow. Stability is jeopardized when the inlet and outlet flows are too closely matched. Excessive pressure build-up in the actuating volume acting over the inlet throttle piston yields a phase-shift between the piston motion and the system pressure response. This phase-shift clearly affects the stable operation of the control system Tightening the reaction between components in the system requires limiting the pressure acting over the inlet throttle control piston. Lower pressures are obtained by opening up the bleed orifice in the control circuit. Other design elements including the inlet throttle flow area vs. stroke schedule, main stage area gain and pilot geometry are considered. The advantages of an overland over a poppet design in the main stage are illustrated in the correction of a rolling instability. Experimental tests monitored torque, pressures in both the pilot and main stages of the valve, pressure at the pump outlet and in the system accumulator.

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