5/2 & 4/2-Way Pneumatic Valve - How They Work
Figure 1: 5/2-way pneumatic solenoid valve
The 5/2-way pneumatic valve has five connection ports and two states. It has one pressure port (P,1), two ports (A,2) and (B,4) that connect to the device that needs to be controlled, and two exhaust ports (EA,3) and (EB,5). The two states of the valve are:
- Pressure port (P,1) connects to port (A,2), while port (B,4) vents through exhaust port (EB,5)
- Pressure port (P,1) connects to port (B,4), while port (A,2) vents through port (EA,3).
Different manufacturers use different port designations. There is also a standard designation: ISO-1210-1. This standard we aim to maintain in our articles as much as possible. The ISO standard uses numbers to designate the different ports. Some manufacturers, however, use letters to somewhat clarify the port designations.
In the example, ISO port 1 is also designated with a letter P from pressure, which is the connecting point for the pressure line. The outlet ports (or working ports) with the ISO designations 2 and 4 are alternatively designated with the letters A and B. The corresponding exhaust ports have the ISO designation numbers 3 and 5. The alternative letter designations of these ports are respectively EA and EB, which means exhaust from A and exhaust from B. Valve symbols with a single exhaust port like, for example, 3/2-way or 4/2-way valves, the exhaust port number 3 (ISO) is alternatively designated with the letter R (relief)
Figure 2: Valve symbol of a mono-stable 5/2-way valve with ISO and alternative port designation
The 4/2-way pneumatic valve has four connection ports and two states. The difference between the two valve types is the number of exhaust ports:
- A 5/2-way pneumatic valve has two independent exhaust ports
- A 4/2-way valve has only one common exhaust port
Figure 3: Valve symbol of a mono-stable 4/2-way valve with ISO and alternative port designation
This means that both port (A,2) and (B,4) connect to exhaust port (R,3). The additional exhaust port of the 5/2-way valve offers extra control possibilities. For example, the speed in both directions of a double acting pneumatic actuator can be adjusted by controlling the flow rate through each exhaust port individually with throttle valves.
The valves can be mono-stable or bi-stable. Mono-stable 5/2-way valves return to their rest position when they are not actuated. The valve returns to its rest position by means of a spring. This means that mono-stable valves require constant actuation (pneumatic, electric or manual) to stay in the actuated position. Bi-stable 5/2-way valves keep their position during power loss, and require a separate action to switch the valve to a safe position.
The following circuit functions are possible:
- 5/2-way mono-stable
- 5/2-way bi-stable
- 4/2-way mono-stable
- 4/2-way bi-stable
5/2- and 4/2-way pneumatic valves can be actuated by different means:
- Electrically (solenoid valve)
The circuit function and actuation of the valve can be shown by a symbol. The circuit function symbols for both 4/2-way and 5/2-way solenoid valves are given below.
Figure 4: Circuit function symbols of 4/2 and 5/2-way pneumatic valves
Although the basic function remains the same, pneumatic 5/2-way valves are available in uncountable design variations with regards to size, material, color, connection interfaces, etc. This is necessary to meet a wide range of requirements, such as medical use, food processing, dusty environments, explosive environments etc.
Most 5/2-way valves have a movable spool with seals along the length in a cylinder. The valve ports connect to this central cylinder. By moving the spool through the cylinder, the valve ports are connected or blocked. Furthermore, the valve can be direct operated or pilot operated. With direct operation, the actuator is directly connected to the spool. With pilot operation, the valve uses the inlet pressure to help move the spool. The valve contains a small internal pneumatic cylinder that actuates the spool. The filling and emptying of this cylinder is controlled by the actuator, for example a solenoid. 5/2-way valves are usually pilot operated. The valve’s main parts are:
Many 5/2-way valve types are available with manual override, or even with a lock mechanism. The benefits of the lock mechanism can be seen during maintenance – the valve keeps the designated position until the lock is released – so the actuated pneumatic elements, such as cylinders or grippers retain their positions. One possible use for the manual override is testing the system; the valve doesn’t need to be electrically energized to be actuated. To switch the valve, it is enough to push the override button. A disadvantage of a lock mechanism is that operators might forget to remove the lock.
While selecting a solenoid valve, the type of electrical connector needs to be considered. Some manufacturers have a wide range of connector designs or use standard DIN connectors, such as DIN43650 type A, B and C. The connectors are available with different IP grades that indicate how well the connector protects against the ingress of water and dust. Another connector option is an indicator light which can be useful to detect valve malfunction or power loss.
Some 5/2-way valves are available with a NAMUR housing. NAMUR is a widely used interface standard to mount a control valve directly onto an actuator. By using a NAMUR valve, the number of fittings and tubing can be reduced.
Some valves can be mounted on a manifold. This is an easy way to group multiple pneumatic valves and save space, connectors and tubing. Manifolds can be equipped with pressure regulators, check valves, etc.
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A 5/2-way valve can be used as a 3/2-way valve, by blocking one inlet and one outlet (A-EA or B-EB) port. With two 3/2-way mono-stable valves, it is possible to build a 5/2-way bi-stable valve.
5/2-way valves are used to actuate double acting pneumatic actuators, such as pneumatic cylinders, rodless cylinders, grippers and rotary actuators. Double acting actuators require compressed air to move in both directions. To decide whether a mono-stable or bi-stable 5/2-way valve should be applied, it is necessary to know more about the system’s design and requirements.
The following information is needed to decide which valve can be used:
- How many actuators are in the system?
- How many I/O ports are available on the PLC?
- Actuator’s positions
- Normal working conditions
- Safety requirements
Figure 5: Schematic representation of a double acting cylinder with adjustable end-position cushioning at both ends, actuated by a 5/2-way mono-stable valve
The number of actuators determines the required number of valves: each actuator or group of actuators needs its own control valve. The PLC limits the number of coils that can be energized. Mono-stable valves (with one solenoid coil) require less wiring and PLC outputs than bi-stable valves (with two solenoid coils). Some manifolds have internal wiring and have one connector that connects to all valves, for example a 25 pin D-SUB connector. In this specific case, only 12 bi-stable valves could be installed compared to 25 mono-stable valves.
The choice between mono-stable and bi-stable valves is often motivated by safety requirements. In some machine designs, to prevent machine damage or injuries, it might be desired that the actuator returns to the base position during power loss. In such cases a mono-stable valve is recommended. In other designs, bi-stable valves are better suited if the requirements are to keep the actuator in the last position.