What Do the Symbols on Pneumatic Valves Mean?

NC

Normally closed

NO

Normally opened

Monostable

A monostable valve, when non-actuated, will always return to one position by means of built-in return springs (or return air). It has only one actuation side (electrical, pneumatic).

Bistable

A bistable valve, when non-actuated, maintains its position and can hold two stable positions. The valve has two actuation sides (electrical, pneumatic).

Tristable

A tristable valve, when non-actuated, maintains its position and can hold three stable positions (three-position valve with mechanical locking).

Internal pilot air

The electrically operated valve moves the piston using air taken from port 1. This pressure must be at least 3 bar; otherwise, internal friction cannot be overcome and the valve will not switch.

External pilot air

At low system pressures or under vacuum, the pressure is insufficient to switch a valve. By supplying air at sufficient pressure to an additional port in the pilot head, the valve can still be actuated despite the low system pressure.

Dominant

If a valve is actuated at both control ports simultaneously, it will not switch. By enlarging one side of the valve, this side becomes dominant: it will switch under simultaneous actuation, while the other side will not.