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Coax design sketch up

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Coax design sketch up

Unread postAuthor: Live1nce » Sun Aug 19, 2012 4:23 pm

I looked around to see if anyone has made a coax of this size, and haven't really found anyone making anything like this one. I know its not much different than any other coaxial, but I do use some different parts and have a couple specific goals I'm trying to achieve. Here is the design i'm thinking of building.

Some of my constraints are; to get something compact, (within 2" in diameter) but still be able to fire 2" Projectiles. I probably wont be filling this with more than 60-70 PSI Max, and only want the projectile to travel about 100 Yards at a 45 Degree angle. I will be using this on a pirate ship model for a lake and be firing 2" water balloons out of it. There will be at least 12 of these cannons made for 2 ships, 3 on each side, on each boat. The other constraint, (which I'm not sure if its even possible) is to make the exhaust valve about 1/8". If I could use the schrader valve as an exhaust valve, that would be great! but I'm not sure if that's possible yet.

I'm trying to scale down the size of the piston valve for 2 reasons...
1. I Haven't been able to get a 2" Piston valve to work properly. I can't get the O-rings to seal against the ID Surface of the 2" pipe because the 2" pipe I buy locally is never round enough. Its almost oval shaped inside. I found this out when i turned my 1 1/4" inch end cap piston on my lathe and put o-rings on it. I could see the 2" was not circular. (although, the o-rings where not free floating, i couldn't find 2" OD O-rings anywhere) :x So hopefully sizing down the cylinder size with smaller pipe will tighten up the tolerances. Any advice on this?
2. I would like to fire this without having to use a 1/4" release valve, I would make my release valve as small as an 1/8" if i can. I'm hoping that the reduced amount of chamber space behind the piston will exhaust the PSI to 0 Faster, without needing to have a bulky ball valve or tank sprayer.

Take note of the port holes that would be drilled behind the 3/4" wooden dowel and pipe. This would also help me reduce the size of the exhaust valve by letting me control and slow the amount of air flow that passes through the piston.

Do you guys think this would work? I would just like to let you guys look it over to see if there are any flaws before i go out and buy all the materials.

Also, should I use a solid 1" wooden dowel for the piston? I can use my lathe to cut out the o-ring slots. I chose to at least sleeve it inside 3/4" because I hear the wood pistons fall apart easily from the condensation. Is this true? I do want it to last as long as possible before needing repair work. At least 300-400 shots before it needs to be taken apart and repaired. Is that a realistic/achievable goal?

I do have the tools to make something nice, but just cant figure out how to keep it all within 2" diameter.
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Unread postAuthor: PaperNinja » Sun Aug 19, 2012 4:52 pm

Why so complicated on the outside there?
Why don't you just have a smooth casing?
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Unread postAuthor: MrCrowley » Sun Aug 19, 2012 5:30 pm

Some of my constraints are; to get something compact, (within 2" in diameter) but still be able to fire 2" Projectiles
I apologise in advanced if I have missed something here, but you can't constrain a coaxial to 2 inches in diameter and still fire 2" diameter projectiles :?

I assume the chamber is threaded so you can swap out the barrels, but why then is the piston only 1" in diameter? If you want to use multiple barrels, best make the piston the diameter of the chamber.

1. I Haven't been able to get a 2" Piston valve to work properly. I can't get the O-rings to seal against the ID Surface of the 2" pipe because the 2" pipe I buy locally is never round enough. Its almost oval shaped inside. I found this out when i turned my 1 1/4" inch end cap piston on my lathe and put o-rings on it. I could see the 2" was not circular
If your pipe is that non-circular, I'd be worried about using it at all. You'll never get a piston to work in it either, air will just leak around the sides. I would suggest buying from somewhere else.

O-rings aren't even necessary in these designs, a good fitting piston with lube is all that is required. If you really want to use o-rings, one will be enough and it wont even need to seal 100%.

In your diagram, I'd cut the piston a bit shorter so it can retreat fully in to the housing and allow more flow. While the smaller piston may be advantageous for a 1/2" barrel, a 2" chamber would render the design inefficient. You may as well just use a 1.25" chamber with a 1.25" piston if you're intent on using barrels 3/4" or less.
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Unread postAuthor: Live1nce » Sun Aug 19, 2012 9:33 pm

MrCrowley wrote:I apologise in advanced if I have missed something here, but you can't constrain a coaxial to 2 inches in diameter and still fire 2" diameter projectiles :?


I didn't draw up the entire plan of the bazooka, so i apologize for that. What i planned on doing to get it to fire 2" projectiles is bump the 1/2" up to 2" again with another bushing. So essentially its like a 1/2" ball valve, just faster, with a lot of dead space.

Is it bad to have a tight seal with o-rings? What should be the tolerance's between the piston and the cylinder?

PaperNinja wrote:Why so complicated on the outside there?
Why don't you just have a smooth casing?


The threaded adapters are so i can pull the system apart to fix the piston if there is ever a problem. I want this to last for a few years.
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Unread postAuthor: MrCrowley » Sun Aug 19, 2012 11:03 pm

Live1nce wrote:
MrCrowley wrote:I apologise in advanced if I have missed something here, but you can't constrain a coaxial to 2 inches in diameter and still fire 2" diameter projectiles :?


I didn't draw up the entire plan of the bazooka, so i apologize for that. What i planned on doing to get it to fire 2" projectiles is bump the 1/2" up to 2" again with another bushing. So essentially its like a 1/2" ball valve, just faster, with a lot of dead space.

Is it bad to have a tight seal with o-rings? What should be the tolerance's between the piston and the cylinder?

Oh, so the 1/2" barrel inside the chamber is always there, you just add a 2" barrel on to the end of it? Well if that's the case, performance will be pretty crappy but it will still send water ballons 50' without trouble I would guess.

As for the o-rings, a tight seal isn't bad but in this case it is largely unnecessary. It would require you to drill an equalization hole whereas a piston with an o-ring that isn't meant to seal 100% (i.e. an o-ring that just gives the piston a better fit) will still allow air to pass by and performance will be the same. You can even skip the o-rings and just build a piston that fits well. As for how well it should fit; it should be as close to the inside diameter of the housing without getting jammed. Always use a lubricant as well :wink:

I looked around to see if anyone has made a coax of this size
Well there are heaps of coax's this size, and much larger, just check the showcase sections.
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Unread postAuthor: Lockednloaded » Sun Aug 19, 2012 11:54 pm

Live1nce wrote:
MrCrowley wrote:I apologise in advanced if I have missed something here, but you can't constrain a coaxial to 2 inches in diameter and still fire 2" diameter projectiles :?


I didn't draw up the entire plan of the bazooka, so i apologize for that. What i planned on doing to get it to fire 2" projectiles is bump the 1/2" up to 2" again with another bushing. So essentially its like a 1/2" ball valve, just faster, with a lot of dead space.


I wouldn't recommend this sort of design for what you want to achieve. Generally when you want to launch a fragile projectile such as a water balloon, a lower pressure, higher flow valve is recommended. With your design you'll need such high pressures to overcome the dead-space and low flow that you'll pop your balloons.

The best design for your cannon IMHO is to use a Toolies Valve. That way, you can still keep the inline layout, but you'll break less balloons and shoot further with less pressure
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Unread postAuthor: Live1nce » Mon Aug 20, 2012 7:52 pm

I have been doing some studying into the toolies valve, but it doesnt seem to be a very common valve. Is it hard to get it working? Is there any clear cut plans on building a 2" toolie valve? I tried using the search function on here, but seem to be pulling up a lot of other topics that have nothing to do with toolie valves, even pneumatic's for that matter.

I also ran into something saying that a toolie valve has to be reset by pulling the string back between shots?
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Unread postAuthor: Lockednloaded » Mon Aug 20, 2012 8:23 pm

I'm actually working on a little one as a side project at the moment, I could take some pictures of all the parts pre-assembly
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 20, 2012 11:58 pm

Live1nce wrote:I also ran into something saying that a toolie valve has to be reset by pulling the string back between shots?


Nah, you're thinking of tampons mate.
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Unread postAuthor: dewey-1 » Tue Aug 21, 2012 10:31 am

Live1nce wrote:I also ran into something saying that a toolie valve has to be reset by pulling the string back between shots?


I think you are confusing the Toolie valve with Techs quick dump valve.(QDV)
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