The main function of pneumatic valves is to direct and distribute compressed air within a pneumatic circuit. They regulate the pace or slow it down.
Pneumatic directional valves build the path that the fluid must travel. But, at the moment of commanding them, the variables are many and it is necessary to know them in more depth to obtain good results in the process.
To classify them, we can name 4 different types of valves:
- Regulating and
Out of four, each one has a particular operation that differentiates them from each other. In this article, we are going to discuss about directional valves or also called distributors, precisely because their main objective is that, to distribute.
Pneumatic directional control valves are mechanical devices that regulate the air flow direction in pneumatic systems through a series of channels in the body of the valve.
These valves have a different number of ways and positions. The most common combinations are the following:
2/2 valves (2 ways and 2 positions):
They act only as a stopcock. One way is the input and another way is the output.
When in the open position, the two paths connect with nothing in between and the compressed air flows freely. When closing, logically the step is cut.
These valves can be normally closed or normally open, depending on whether they close or enable the passage, respectively, in their rest position. The most common is that they are normal closed.
3/2 valves (3 ways and 2 positions):
Normally they are used to handle simple effect cylinders. the air flow can go in two different directions and exhaust in its closed position.
4/2 valves (4 ways and 2 positions):
It has the same number of positions as the previous one, but since it has one more way, it is usually used to handle double-acting cylinders. One position draws the air into the piston and the other takes it out, causing the stem to rise and fall depending on the location of the air.
4/3 valves (4 ways and 3 positions):
They are similar to the two positions, but have an additional center position. According to this central position, these valves can be:
closed center or
Open center means that in the center position of the valve there is no pressure in any of the ports and the exhaust ports are open. In this way, a pneumatic cylinder (for example) is stopped and could be moved manually, because there is no pressure to block it.
Closed center means that in the center position all the ways are closed. The cylinder would be blocked by making escapes impossible.
Center under pressure maintains pressure in both ways, which allows a rod less cylinder to be precisely stopped, compensating for any air loss in the circuit.
A very common example of this type of valve is activation by means of a lever. When the lever is in its home position, the valve remains in its center position. When the lever is moved forward or backward, the valve passes to the other positions, allowing (for example), to carry out the forward or backward movements of a cylinder.
5/2 valves (5 ways and 2 positions):
It is like the 4/2, although in this case it has two exhausts, one for each position. Having two exhausts helps to better manage and regulate speed.
5/3 valves (5 ways and 3 positions):
They are similar to the two positions, but have an additional center position. Depending on this central position, these valves can be: open center, closed center or pressure center.
Types of actuation of Directional Control Valves
In addition to taking into account the number of ways and positions that a valve has, there are different types of actuation (or control). The most common drive types are described below.
- Manual activation
- Mechanical activation
- Electrical Activation
The main characteristic of these valves is that the operator decides when he wants the air to flow. It is not the most widely used, precisely because the main objective of pneumatics is to automate processes and limit the work of the human being. The options for this type of activation are by means of a button, a lever or a pedal.
If a button is used, with or without detent, the valve has two positions (one when it is pressed and one when it is not). On the other hand, if you want 3 positions, the most common option is the lever, which can be moved forward or backward to take it out of rest.
In this type of actuation there is some mechanical action that activates the valve when it makes contact with something. Many ways can be created to achieve this, even in some cases it can be made that when I hit from one side it changes position and when it hits from the other it doesn’t.
A fairly common example of these valves are those that have cams or rollers to function as a limit switch. The roller advances until it stops with something and then produces the change of position on the valve.
As its name suggests, these valves are directed by pneumatics, that is, they need compressed air to be controlled. If air enters, it works one way and if it leaves another only with pressure.
There are also cases in which it is activated with two different inputs, acquiring better working comfort.
These valves require an electrical circuit to activate them. The switching of the valves is obtained by some electrical device that has sent that order. It is important to know what voltage and type of current you need in your process, since it is not the same to use 12, 24, 110 or 220 volts and it is not the same to use alternating or direct current.
It is also important to know what the “rest position” of the valve is, that is, the position when the valve is not being activated. If the valve is a 2-position valve, it can be normally open or normally closed.
If it is 3-position, the rest position is generally the center position ( 4/3 valve description, for a description of the three alternatives: open center, closed center and pressure center).