Operation of Dual 3 port Pneumatic Valves

You have probably seen the above valve symbols before in catalogues and on valves. Different manufacturers tend to call them different names. 5/4, Two 3 Port Valves Integrated, but Dual 3 port, 4 position is the most common term I have seen used. On the outside the valve looks the same as a 5/3 valve, but it operates as two 3/2 valves (like having two 3/2 valves in one valve body).

These can come in three combinations. Normally open/normally open (NO NO), where both vales are normally open, normally closed/normally closed (NC NC), where both valves are normally closed and Normally open/normally closed (NO NC), I think you get the idea now…

These can be assembled onto a nice compact manifold which only needs half as many stations as a manifold of 3/2 valves, which could save space in applications where 3/2 valves are normally used (for example, controlling single acting cylinders).

How do these valves function? I will use some cut away drawings to show how. The below picture can look complex, but I will try to explain using photos of a disassembled valve too.

Inside the valve body are two spools which act independently of each other. They are each controlled by a pilot valve. Air from the inlet port (1) of the valve is directed via an orifice inside the valve body to the inlet of the pilot valves.

The pilot valves are themselves small 3 port solenoid valves (as can be seen in the top photo above). These control air flow (from the pilot line) to pistons (seen above) which act on the individual spools. This is also why pilot operated solenoid valves need a minimum operating pressure to switch spool position. The piston can be seen in the above photo and is highlighted in the picture below. The valves in the photo and the cut away drawings are not exactly the same, so some components like the piston will look a bit different, but the two valves are a very close design so will work for the purpose of explaining the operation.

I am using a NC NC configuration to explain the operating principle (most valves from different manufacturers will use similar operating principles). When neither solenoid is energized, the air pressure at port 1 (inlet port, please click this link for an article which explains port identification: https://www.dhirubhai.net/pulse/pneumatic-valve-port-id-wayne-wilding/ ) is acting on the surface area on the land on the end of both spools, keeping them in a normally closed position (red line showing inlet pressure below).

In this position, both outlet ports (4 & 2) are connected with exhaust ports 5 & 3. See picture below showing flow paths from outlets to exhausts. As both sides are closed, air from the inlet port is not flowing to either outlet.

If we use solenoid 12 (on the right as you look at the picture) for this example, when this is energized, the pilot valve directs air into the chamber holding the piston (the photo below shows the orifice where air flows onto the piston). This acts on the surface of the piston, which in turn pushes the valve spool to the left (as you look at the picture), creating a flow path from port 1 to port 2.

As the surface area of the piston is greater than the surface area on the end of the valve spool, more force is generated by the piston, which allows it to push the spool against the inlet pressure, and open a flow path to port 2. Since air pressure from port 1 is directed to the pilots, the pressure acting on the end of the spool and the piston is the same, but the piston can produce more force output.

When this solenoid is de-energized, the pilot valve exhausts air from the piston section. When there is no pressure acting on the piston, the inlet air at port 1 forces the spool back into the normally closed position again (pushing the piston back), resetting the valve. Some manufacturers use a spring to reset the spools, or assist in resetting the spools.

Some of you may have noticed that the spool shown in the photos has a different shape to that seen in the drawings. The valve I disassembled was a NO NO type, I just used it to illustrate the construction of the valve body, pistons, pilots and spools (and because I just had it to hand). But, that shape does lead me into how the Normally open types work.

The image below shows the construction of a NC NO configuration. You can see in the image below the end of the spool on the right is thinner and allows flow from port 1 to port 2 in its normal position (so this side is normally open). When its solenoid is energized, this path will close. This valve has a spring assisted reset mechanism which can be seen in between the two spools, to put the spool back into its normally open position when the solenoid is de-energized.

Flow path 1 to 4 in the above valve is normally closed.

Hopefully I have been able to make a potentially complicated component easy to understand. When components such as these are understood, the value can be seen, enabling them to be selected for more applications.