Valve Switching Positions

For each of the switching positions provided by a valve, the graphic symbol shows a square, sometimes called a box. This means the valve on the left provides two switching positions and the valve on the right three. The non-actuated position of a valve is assumed by its moving parts (valve mechanism) when the valve is not connected, not pressurized, and not actuated. In most hydraulic control circuits all valves are depicted in this non-actuated position, and by convention two-position valve symbols should normally be connected on the right-hand square .

Three-position valves should be connected to the center square, which again depicts the valve's non-actuated position (sometimes also called the neutral position). However, it is the practice of some circuit designers to draw valves in circuit diagrams in their actuated or so-called initial position. This position is assumed when the valve is installed in the machine and levers, cams or machine parts are pressing onto the valve's operating element, so that the valve's mechanism is no longer in the non-actuated position. Since this is not a standard practice, it is imperative to state on the circuit diagram that all valves are shown in their initial position. Also, each of the valves which are actuated must be shown with a cam, and all pressure lines are to be connected to the left-hand square .

Two-position valves which are not spring biased and can freely assume both switching positions (bi-stable), may be drawn in whatever flow-path configuration demanded by the actuator position. But here again, the pressure lines must be connected to the right-hand square.