Once again you will start by drawing a square on your paper, 2" x 2" is lots. You will find it easier if you use graph paper to help keep the lines straight. Make it easy on yourself and use a light pencil, and have an eraser handy. You will probably need it.
Once you have drawn the basic box, the box represents the valve body, you will insert the lines as shown in Fig. 27.
Remember, the four in a 4/2 valve means that there will be four air ports, and the two in a 4/2 valve means that the valve will have two positions.
Each of the two positions will be represented by a box like that in Fig. 27, and each will show that one same valve in one of its two positions.
In Fig. 27 the supply is traveling from the compressed air source to port four. Port two is open from the air actuator to exhaust.
The black circle identifies the compressed air source - the line from the air mains, or the compressor and compressor receiver / air tank.
Therefore, the two ports shown at the top of the drawing are air cylinder / actuator ports, the port in the bottom right is the single supply port for this 4/2 valve, and the port at the bottom left represents the single exhaust port for this valve.
When this 4/2 valve is shifted, the air flow paths switch. See Fig. 28.
This is the same 4/2 valve but it has now been shifted, or actuated, and Fig. 28 represents the flow paths in this same air valve now.
The supply of air is still at the same port, but the inner mechanism of the valve has shifted so that the flow paths inside the valve change as shown.
In a 4/2 air valve the supply port never gets shut off, nor does the exhaust port. They simply alternate one from to the other as the internal valve spool is shifted one way or the other.
With the air supply alternately flowing from port two to four, then four to two, and with the air alternatively exhausting, you can see how this compressed air valve would effectively supply a double acting air cylinder or double acting rotary actuator.
To finish drawing the valve, we simply bring the two drawings together as shown in Fig. 29. This is the complete valve schematic as it would be placed on the completed air circuit drawing.
As in all valve drawings, to simplify, I have not included any of the valve actuators that would be necessary to make this valve shift.
Even though there's two drawings put together, the two are the same valve, just showing the internal flow paths of that same valve when it has been shifted to either of its two positions.
Note that only one supply is shown, depicted by the circle. Since both sides of the schematic show the same valve, regardless of the position of the internal spool, there is still only one supply to that valve.
Without the actuators being shown in the above valve schematic, and since it is not in an actual valve circuit, a viewer cannot tell where this valve should be when it is at rest. In that case the supply circle is drawn on the right side of the valve schematic.
Sometimes when an air circuit deliberately stops functioning, one or some of the valves may be left in an actuated position. The supply line may be depicted on the left in these circumstances. The circuit schematic will show the details if that is the case.
You will need valve actuators to complete the drawing of the 4/2 valve schematic. Here's how to draw valve actuators.