The MOOG D634 is a direct-driven hydraulic servo (proportional) valve from the D633/D634 series, designed for precise fluid flow control in industrial hydraulic systems. It uses a permanent-magnet linear force motor to directly actuate the spool — eliminating the need for pilot oil — which results in high dynamic performance, pressure-independent behavior, and low power consumption. Typical applications include hydraulic presses, injection-molding machines, die casting presses, test benches, and other industrial machinery requiring accurate, stable flow and pressure control.