Quick Exhaust Valve for Air Launcher

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I wanted to build a QEV (quick exhaust valve) air cannon as seen online, but creating the key component - the sliding piston - was tough for me to make from raw materials. Many QEV air cannons use a valve design where the piston has a face seal to the barrel, but I found this design a bit leaky. This design is built on a PVC Tee junction and the center port is the output instead of the reservoir. An example of how to make one can be found here: https://makezine.com/projects/boom-stick-air-cannon/ This video shows the process: https://youtu.be/seDjQidoCUk Disclaimer: These air cannons are not toys. They can shoot a projectile at high speeds, even at low pressures, and may explode if not made correctly. I take no responsibility for injuries or damages caused by this project. The piston is sized to work with 1-1/4 inch schedule 40 PVC pipe. Be aware that different manufacturers have varying tolerances on their PVC pipes, especially the inside diameter. This piston slides on the inside of the PVC pipe and may need to be scaled slightly so that it and the o-rings fit properly. In my diagrams, I use screw-on PVC fittings for development purposes, and the unit can be easily disassembled and reassembled. Some of the PVC parts (bushings) had to be modified so that the inner 1-1/4 pipe could pass through. Theory of operation: The piston is the key component in the air cannon because it allows compressed air in the reservoir to be released very quickly into the output port - much faster than a person turning a valve can achieve. In the sectional drawing, the left port is the reservoir, the center top port is the output, and the right port is the switched filling port. Here's a video of the assembly: https://youtu.be/_z327hESdH4 Before the cycle starts, the piston is in an arbitrary position and all ports are at zero pressure (relative to atmosphere). This is when the projectile is loaded into the output port. The cycle starts when air pressure is applied to the filling port. This pressurizes the chamber on the right side of the piston, forcing it all the way to the left, seating the piston o-rings into the reservoir port. A small air passage through the piston allows the reservoir to be filled with air from the fill port until both are at equal pressure. You'll hear the flow of air slow and stop when this condition is met. The piston stays in the left position due to friction in the o-rings. The launch cycle starts when you quickly release the pressure on the fill port - that's why I called it "switched." There are multiple ways to achieve this action, and I prefer using a 5-way 2-position pneumatic valve because it can switch very quickly from fill to exhaust. When the pressure in the fill chamber suddenly collapses, the piston is forced to the right by the pressure in the reservoir. You'll see that as the piston moves to the right, it opens a large diameter passage between the reservoir and the output port. This is where the sudden burst of exhaust air is generated. An extra o-ring is placed between the piston and the rim of the PVC pipe on the fill port to cushion the collision of the piston and the fill port. The effect of this simple operation is quite effective, and I was surprised at its power. Even though the PVC is rated for 60 psi, I've only needed to use up to 30 psi. Do you really need to shoot a golf ball more than 300 yards? I don't remember what size o-rings were used, but print the piston so that it just fits inside the 1-1/4 PVC tube and try different sizes until they make a decent seal. It doesn't need to be tight. I also sealed up three of the pass-through ports on the piston to minimize losses when launching. You can experiment to see what works best.