The 771/2/3 Series flow control servovalves are throttle valves for 3- and preferably 4-way applications.They are a high performance, two-stage design that covers the range of rated flows from 1 to 15 gpm at 1000 psi valve drop.

The output stage is a closed center, four-way sliding spool.The pilot stage is a symmetrical doublenozzle and flapper, driven by a double air gap, dry torque motor. Mechanical feedback of spool position is provided by a cantilever spring.The valve design is simple and rugged for dependable, long life operation. These valves are suitable for electrohydraulic position, speed, pressure or force control systems with high dynamic response requirements.

Principle of operation An electrical command signal (flow rate set point) is applied to the torque motor coils, and creates a magnetic force which acts on the ends of the pilot stage armature.This causes a deflection of the armature/flapper assembly within the flexure tube. Deflection of the flapper restricts fluid flow through one nozzle which is carried through to one spool end, displacing the spool. Movement of the spool opens the supply pressure port (P) to one control port, while simultaneously opening the tank port (T) to the other control port.The spool motion also applies a force to the cantilever spring, creating a restoring torque on the armature/flapper assembly. Once the restoring torque becomes equal to the torque from the magnetic forces, the armature/flapper assembly moves back to the neutral position and the spool is held open in a state of equilibrium until the command signal changes to a new level.

In summary, the spool position is proportional to the input current and with constant pressure drop across the valve, flow to the load is proportional to the spool position.

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