Pneumatic valves are distinguished by the number of connections and the number of switching positions. Mono- or bi-stable / NO or NC.
How does a valve work?
A pneumatic valve is characterised by a number of connections (ports) and a number of switching positions. In the description of a valve, the number of connections is always stated first, followed by a slash and then the number of switching positions. Thus, a traditional 3/2 valve has three ports and two switching positions. Similarly, a 5/3 valve has five ports and three switching positions. The most commonly used valves are the 3/2, 5/2 and 5/3 valves.
With the various valves, different functions can be realised in a pneumatic system.
A 3/2 valve has three ports for supply (1), output (2) and exhausting (3) respectively, combined with two switching positions. As only one working output is available, this valve can only fill one chamber with compressed air. This means that the cylinder is single acting and the return stroke must be achieved mechanically, e.g. with a spring assembly.
A 5/2 valve also has two switching positions but provides two additional ports for working (4) and venting (5). With this, two chambers can be pressurised and exhausted which means full control of both sides of the actuator. With a 5/2 valve, a movement can therefore be fully pneumatic.
Finally, the 5/3 valve has the same number of ports as the 5/2 version, but an additional switching position. This so-called "middle position" offers the control system an additional position when no actuating signals are applied. This can be relevant in critical safety applications, for example.
Various versions are available for this intermediate position:
In addition to the number of connections and switching positions, pneumatic valves also differ in the way they are operated. There is a choice of: manual, mechanical, electric or pneumatic. Each type of operation has its own symbol which can be found in the pneumatic diagram for the relevant valve. Sometimes a combination is used, such as a manual operation to start a movement and a spring assembly to return the valve to its initial position. In this case, two symbols are shown next to the valve.
Manual operation is possible in many different ways and depends on the type of application. Should the control element be locked after use or remain in a certain position? Or should it return automatically and immediately to the starting position? By answering these types of questions, the right solution can ultimately be selected and the desired switching behaviour achieved.
A mechanical control is similar to a manual control but is used in places in a machine where no manual control is required. For example, a roller control when detecting an object on a conveyor belt.
Electrically operated valves are widely used in machines with a PLC control. The valves can be operated directly from the PLC. These valves are currently the most common in machine building.
Pneumatically operated valves are controlled by means of a separate pilot air connection. This makes it possible to realise complete pneumatic circuits.
Normally Closed, Normally Open
The difference between Normally Closed (NC) and Normally Open (NO) valves is important. The term indicates the position a valve takes when it is not energised. A NC valve does not allow any air to pass through it when it is not energised: the port is 'closed'. A NO valve, on the other hand, does allow air to pass through, exhausting the working port when it is not operated.
These terms are confusing in relation to the electrical version, where NC means that the current circuit is closed and thus activation takes place. In the case of pneumatic valves, therefore, think in terms of ports (the port is closed, no air can pass through it, so the valve does not switch) and in the case of electrical valves in terms of bridges (the bridge is closed, air can pass through it, so the component switches).
Mono- and bi-stable valves
In addition to the operating mode, the number of actuations also affects the function of a valve. In this context, we speak of a single (monostable) or double (bi-stable) valve.
Monostable valves have one signal (air or in the case of solenoids, electric) that switches the valve when it is energised. These valve signals must be operated continuously for the switchover time. If the actuation signal is removed, the valve automatically returns to its rest position by spring force or air pressure.
With a bi-stable valve, the return to the initial position is controlled individually. These valves switch when operated for a short time do not automatically return to the initial position; this requires a separate actuation signal. These are termed double pilot valvs (pneumatically operated), or doble solenoid valves (electrically operated).
Valves with two solenoid coils are sometimes considered to be bi-stable. But, for example, the 5/3 valve shown in the illustration below has two coils but is nevertheless a monostable valve. This is because the two return springs return the valve to its initial starting position (=middle position) when de-energised.