After restoring my mine bolt-action potato cannon, I decided that the double-barrel cannon that resided in my garage would be interesting to get working again. It hadn’t been fired in several years, and one of the valves no longer worked. Additionally, it leaked badly, so I had my work cut out for me. Here’s a picture of the gun partially restored before any sort of leak testing or firing (Read on for another video at the end):
The principle behind this gun, or most pneumatic air cannons,is that one side of the cannon holds the pressure (marked in red squiggly lines below) while the other side holds your projectile, water in this case – marked in blue squiggly lines. Separating the pressure and projectile side is some sort of pneumatic valve – in this case a sprinkler valve. I’ve put a yellow (golden?) square around these two valves. (Be sure to see my article about spud gun range testing theory for more background)
Yes, two valves. Generally most pneumatic air cannons would only have one as they only have one shot per air charge. This design has two separate air and projectile chambers. Basically it’s two guns attached together. To aid in filling each chamber with air, a manifold was created with a PVC elbow and Tee. Air goes into a Schraeder valve shown by the leftmost red arrow and is split into two by this assembly. The charging air flow is illustrated by the two other red arrows.
In order to avoid discharging both sides at once, the two parallel flows go through a one-way valve. This way, during charging both pressures will be roughly equal, but after firing one chamber can be close to 0 PSI (gauge pressure for those that know about that) while the other will be charged to whatever it was pumped up to. The other end can then be discharged at will.
Which brings up the next question, how does one actually get these valves to fire? In normal operation, the valves used are powered by 24VDC, so if you happen to have a 24 volt battery laying around, that’s one option. The way this one is done is that three 9 volt batteries are chained together in series giving a combined voltage of 27 volts. A little more than recommended, but it seems to work well. Both solenoids were hooked up to individual normally open pushbuttons on the positive side. The other side was hooked up to negative as shown in the drawing below:
I used some DIN rail, terminal blocks, and grounding blocks to make a bank of positive and common out of the batteries. You can find this stuff at Automationdirect.com. I would definitely recommend using an additional safety switch with this assembly. Additionally, always keep the barrel pointed in a safe direction. Even after firing, there can be some air pressure still in the chamber. If there is a misfire, the valve can be manually triggered by twisting the solenoid. Definitely useful in some situations.
As for assembling the cannon, here’s a mechanical drawing that should illustrate things:
One thing that hasn’t been discussed is putting some sort of support on the cannon. As drawn here, two pieces of wood are cut to length and then a hole saw is used to drill a nice hole in it so it fits between the PVC pipe. In reality, as shown in the first photo, only one of these is used. It also doubles as a trigger assembly with a simple thru hole and counterbore for each of the trigger buttons.
Once you have everything together, it will undoubtedly leak. The way to fix this is, first of all to use enough pipe tape. If it’s still leaking, one way to figure out where is to submerge it in some liquid. Be careful not to submerge the electronics, but for the bottom part especially, this worked well as a diagnostic tool.
Once I worked most of the kinks out, it was ready to fire. So far I’ve only used water as a projectile, but this was mostly what it’s meant for anyway. As with all of my posts, this is simply what I’ve done and I don’t recommend trying it yourself. PVC is not rated for air pressure, so this adds an extra element of danger.
So regardless, here are some other views of how this cannon performed when shooting water. Special thanks to “Mario Mendoza” for helping produce these videos:
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