Not everyone uses this same valve terminology as a norm, so when you are talking to other folks about compressed air components, in particular air valves, make sure they define their terms to help you better understand, and for your mutual understanding. Misunderstood valve or air component names is an ongoing problem.
So, the operator is the person that moves the valve lever, pushes the valve button or flicks the switch to actuate the valve. An operator will actuate the valve using the valve actuators.
One is usually installed inside the valve, is usually a spring, and I call this the internal valve actuator. When the external valve actuator for a particular valve is not in use, it is the internal spring actuator that returns that valve to its un-actuated, resting state. A valve at rest is one that is not being acted upon by an external force or external actuator.
The actuator that an operator uses to operate an air valve is what I call an external valve actuator. It can be a solenoid, a lever, a push button, a mechanical switch of some sort, an air signal, and all of these external actuators come in a variety of shapes and sizes depending on the manufacturer and the valve.
In the instruction pages on how to draw air valves (See site map under valves ) for purposes of simplicity all of the schematics show those valves without any actuators.
Fig 41 shows a 3/2 valve with an internal spring actuator,drawn on the right side of the schematic.
It is customary to draw an air valve with the internal valve actuator shown on the right, and, if the valve is at rest, to show the air supply on the right side of the valve drawing as shown in Fig 41.
Drawn in this fashion, the valve shown is a 3/2 NC (normally closed) valve. When the valve is at rest, the internal spring is in control, and the valve is not passing compressed air through it.
Regardless of the type of valve actuator that might be drawn on the left, if the air supply is shown on the spring side, then that is the position that valve will be in its resting state.
The dashed line is the generally accepted symbol for an air pilot signal.
When an air pilot signal arrives at the air signal port of the compressed air valve, the valve internal air seal (a poppet or a spool type) is shifted by the incoming air, compressing the spring as it shifts the valve innards.
In Fig 42, you can see that if the valve is shifted by the air pilot signal air, the flow path through the valve will be opened, and the exhaust port will be blocked. The drawing depicts both sides of the valve, but both sides of the schematic are the same valve. One side shows the flow path through the valve if it has been shifted in one position, and the other side shows the flow paths when that same valve is shifted into its second position.
In this case, as long as air is present at the air signal port of the valve, the valve will remain shifted, and air will continue to flow through the valve to the air-using application.
When the air pilot signal is dropped, and the air pilot signal line loses pressure, the valve internal spring actuator once again takes over control of the valve, shifting the internal spool or poppet back. The supply of air to the application is blocked, and air from the application can now exit through the valve to the exhaust port and to atmosphere.
Fig 43 is a photo of a 3/2 valve, with 3 working ports (one on the bottom cannot be seen). Since I took the photo of the valve, I know that this one has 2 positions, but that cannot be determined by looking at the outside of an air valve. This one is a single air piloted, spring return 3/2 NC air valve, to be precise. It is typical for an air valve to have a label which shows a schematic identifying the flow paths of that valve.
What is controlling the valve in Fig 43 is the internal spring actuator located under the thin cap on the left end of the valve. The wide cap on the right contains the air port for the air pilot signal.
Should the air circuit require it, this same valve could have double air pilot valve actuators, with the omission of the internal spring actuator, and another air pilot end cap attached to the other end.
This double air pilot controlled air valve can be used as a latching valve in air circuits. When an air pilot signal reached one of the end ports the valve would shift. Even if that air pilot signal was dropped, the valve internals, and hence the compressed air path through that valve, would not change, but would stay in its last position until such time as another air pilot signal reached the other air port, to then shift the valve back.