How is the spool shifted in jet pipe and flapper nozzle type servo valve?

Supply pressure (the setting of the relief valve or the setting of a pressure compensated pump) is supplied to the points identified with Ps . Fluid flows across the fixed orifices and enters the center manifold. Orifices are formed on each side between the flapper and the opposing nozzles. As long as the flapper is centered, the orifice is the same on both sides and the pressure drop to the return is the same. Pressure at A equals the pressure at B, and the spool is in force balance.

Suppose the torque motor rotates the flapper clockwise. Now, the orifice on the left is smaller than the orifice on the right, and the pressure at A will be greater than the pressure at B. This pressure difference shifts the spool to the right. As the spool shifts, it deflects a feedback spring. The spool continues to move until the spring force produces a torque that equals the electromagnetic torque produced by the current flowing through the coil around the armature.

At this point, the armature is moved back to the center position, the flapper is centered, the pressure becomes equal at A and B, and the spool stops. The spool stays in this position until the current through the coil changes. Because of the feedback spring, the spool has a unique position corresponding to each current through the coil ranging from 0 to rated current. At rated current, the spool is shifted to its full open position.