Pneumatic valves

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.

Functions

With the various valves, different functions can be realised in a pneumatic system.

3/2 valve

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.

5/2 valve

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.

5/3 valve

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:

A 'closed' intermediate position closes both the supply port and the working ports. In other words, the compressed air supply cannot get in or out and the cylinder remains in its position. However, internal leaks can cause the cylinder to become depressurised after a long period of time.
A 'pressurised' mid position switches the supply pressure to both working ports. This causes the piston to be under pressure on both sides, so that a cylinder with an equal piston area is in balance. A cylinder with an uneven piston area will make a stroke movement in the mid position.
An 'exhausted' mid position vents the system when the signals are removed. Because both outlet ports are connected to the outside air, the piston is depressurised on both sides, which means that the cylinder can start moving with a small external force.

Operation

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

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.

Mechanical

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.

Electric

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.

Pneumatic

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 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.

Bi-stable valve

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).

Note

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.