The valve control spools on three way valves are, with the exception of very large nominal diameters, placed in the centre position and maintained there by two springs as soon as the solenoid force or the pilot pressure is zero (Figure F 3). These springs define, along with the flow forces, the switching power (switching speed) of the valve. At the same time they are used to reset the outer switching positions. Alternative: pressure centring.
Figure F 3: Spring centred directional valve