Pneumatic Cannon

DSCN1346This cannon is similar to many of the other pneumatic cannons you may have seen on the net: it uses a piloted diaphragm valve to vent the contents of a tank out the barrel. The unusual feature of this cannon is its construction: it is built from a 4” dia, 1/2” thick steel tube, with 1/2” thick end caps. With the 3/16 thick 1’ bore barrel, the total weight is about 56 lbs. It is designed to run on 3000 psi, but can handle around 5000 psi with a reasonable safety margin, if you trust the welds.

The light gray attachment on the front of the barrel adapts the straight, un-threaded barrel to a 2” water pipe. I use both a 2” and 1” steel water pipes for barrel extensions.pneumaticcannon-end

Like many other cannon designs, the 1” bore barrel extends through the tank to within a few inches of the back of the tank. The diaphragm valve sits between the barrel and the end of the tank, and the end of the barrel is capped with a machined seat that is soldered to the barrel. The seat is allways in compression, so the solder joint is not critical. In this view you can also see the crummy welds between the flange and the tank. The eight bolt holes hold grade 8 bolts. I usually only put four of them in, which is still sufficient for 3,500 psi with a 100% safety factor.

The piston and diaphragm seals off the barrel, pressed against the barrel seat by the gas pressure behind the piston. By reducing the pressure behind the piston – venting the gas through 1/2” ball valve – the piston is pushed back by the higher pressure on the front of the piston and diaphragm. This kind of valve is also used in electric sprinkler valves. A good description of how the valve works is available from
pneumaticcannon-diaphragmNote the Phillips head screw on the piston at the top of the photo. This screw has a #60 hole drilled through it; it is the leak for gas to move around the piston as the tank is filled. This leak allows air to slowly equalize between on either side of the piston.

Construction was simple, but tedious. Rather than buy the flanges on the ends of the tank, or just order a steam pipe, I cut the flanges from 6” square plate. I may have chosen this as an excuse to use a plasma torch at a local high school. The torch was a lot of fun, but was not built to sever 1’2” of steel. So, I got a quick education in using an Oxy-Acetylene cutter, and a very slow education in using an angle grinder, since my lathe is too feeble to turn a 6” disc. After making three discs, I gave up; the top end-cap, which has the barrel running through it is square, though I often claim it is intentionally square to keep the cannon from rolling

Since I finished another cannon , I rarely use this one. But, eventually I plan to fire it with helium at 3,000 psi and try to break the sound barrier.

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