The first number in the 4/2 air valve, the four, refers to the number of "working" air ports that are found in the valve body. That is, the number of ports that supply air to the valve, and channel the compressed air from the valve to the application. That takes care of three of the four ports. The fourth port is for exhaust.

The 4/2 valves will have numbers or letters etched, cast or painted near each of their four "working" air ports. If there are numbers near the ports, the number 1 would be the supply port to bring the compressed air to that valve.

Port numbers 2 and 4 would be the working ports from which air would flow to accomplish whatever task that you wanted that valve to do.

The other port, usually adjacent to the supply port, is the common exhaust port.

A 4/2 air valve is used to power a double acting air driven device. By double acting, I mean that the device is powered by compressed air sent to one of two ports on the device and the device reacts by alternatively moving one way or the other, depending on which air line to it is charged with compressed air.

A typical example of a double acting air device is the air cylinder. If air is injected into the port at one end, the piston that is inside the cylinder is ?pushed? to the other end of the cylinder barrel, and the rod that is attached to the piston moves along with it.

When it's time to move the cylinder piston and rod in the opposite direction, air is supplied to the other port on the cylinder, the piston moves back along with the piston rod, and air exits the other port.

A 4/2 air valve is the valve of choice for a double acting air cylinder.

When compressed air is brought to the supply port of the 4/2 valve the compressed air will flow through and out of one of the two actuator ports. The other actuator port will be open, through the valve, to atmosphere.

When the 4/2 valve is shifted, compressed air will flow through it and out the second actuator port, and the port that was formerly supplying air to the cylinder, is now opened to exhaust.

A 4/2 compressed air valve will have only one exhaust port. Each time the valve shifts, one of the two actuator ports on the other side of the valve are connected internally to this exhaust port.

As the valve shifts back and forth, compressed air is supplied alternatively to one or the other of the two actuator ports. Since each of the valve's actuator ports is connected with an air line to one of the ports in the air cylinder, as the compressed air flows alternatively to each actuator port, it too flows through that particular air line to the cylinder, which move the piston one way or the other inside the cylinder barrel.

Air goes in one end of the cylinder, and the air that was in the other end, escapes down it's line, through the valve to exhaust, allowing the actuator piston to move. If the air couldn't escape, the cylinder piston wouldn't move.

4/2 air valves will usually have an internal spring to ?reset? , or shift, the valve to it's resting state when it's external valve actuator isn't being operated. An external valve actuator could be a lever, a push button, a whisker switch, a solenoid, a foot pedal or a host of other actuators that are used to operate an air valve.

Much more information about compressed air, valves, actuators, specialty components etc. can be found at my site. If you don't get an answer to your question there, please feel free to contact me via my site contact page.