Valves come in a number of “styles”. I don’t know if “valve style” is an official term, but that is what we will call it. I mentioned before that we don’t see many valves undersized. We see the wrong style valves all of the time. In fact, this is probably the most common error we see in pneumatic systems. (perhaps because there are a lot of ways to mess it up.)
The parameters that make up the valve style are:
- Positions (2 or 3)
- “ways” (ports) - typically 2, 3, 4, or 5
- Detented or non-detented
- Spring or Solenoid return
- Blocked or Vented Center
I’m not sure this could be called an exhaustive list, but it covers most of the variations.
A valve will have either 2 or 3 “positions”. A two position valve is either shifted one direction, or the other, and there is no “center” position. A 3 position valve has a “center” position that the valve can move to when it is neither being told to be in the left or right position.
When the valve is properly connected to an actuator and an air supply, moving (actuating) the valve to one side causes the actuator connected to the valve to move one direction. Actuating the valve to the other side causes the actuator connected to the valve to move the other direction. If the valve is a 3 position valve, and neither the left or right position is selected, the valve will move to the center position. (by spring force, normally) What happens then is a function of how the center position is specified.
Ways, or Ports
Valves have some number of ports, which are essentially locations where air lines will be connected to them. An old fashioned term for ports is “ways”. It is common to hear “4 way valve”, which is just another way calling out a valve with 4 ports.
The most simple valve has 2 ports. One where the air enters the valve, and another where the air exits the valve. A “2 way” valve only has 2 positions. The valve is closed in one position, and open in the other position. This means that in one position air can pass from one port to the other, and in the other position air cannot pass from one port to the other.
A 3 way valve has 3 ports. If we call the ports a, b, and c, in one position the valve will pass air from a to b, and in the other position it will pass air from port a to c, for example. A 3 way valve could be plumbed by putting pressure on port a, and using the valve to apply pressure to port b or c. It could also be plumbed with pressure on port b, atmosphere on port c, and something on port a that will either be pressurized or vented to atmosphere depending upon which way the valve is switched.
A 4 way valve has 4 ports. Typically this style of valve will have pressure and exhaust plumbed to one side, and the inlet and outlet of an actuator to the other two ports. In one position, the valve will apply pressure to one of the outlet ports, and exhaust the other outlet port. In the other position, it will reverse the connections.
A 5 way valve has 5 ports. It works similar to a 4 way valve, in that it usually reverses pressure and exhaust to the outlet ports when it is shifted from one end to the other. It has a 5th port, which is normally plumbed on the inlet side. The 5th port allows two different pressure sources to be connected on the inlet side. When a two position 5 way valve is in one position, inlet port 1 and 2 will be connected to the outlet ports, and in the other position inlet ports 2 and 3 will be connected to the outlet ports. This allows one pressure to be used to move the actuator one direction, and another pressure to move the actuator the other direction.
On a 3 position 5 way valve, the 3rd inlet port might be used to control what happens to the load in the center position. For example, the center port might be connected to both outlet ports. If the center port is connected to exhaust, the actuator will vent to atmosphere in the center position. It might be desired in some instances to pressurize both sides of an actuator when the valve is in the center position, in which case pressure would be applied to the 3rd inlet port.
Blocked or Vented Center
When a valve has a center position (3 position valve), the center position function must be specified. The valve can either “block”, or “vent”. A blocking center position does just what it sounds like. It attempts to block the ports to the actuator, which has the effect of trapping the air in the cylinder as it was prior to shifting to the center position. A venting center position allows the air from the actuator to flow through the ports on the valve. Where that air goes is a function of how the valve is plumbed, but typically the ports will be vented to atmosphere through a muffler, hence the name “vented center”.