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Coaxial piston problems

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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Coaxial piston problems

Unread postAuthor: JDP12 » Thu Apr 19, 2007 9:58 pm

Well I've been looking to upgrade to a coaxial piston gun now, but I have a couple questions.
Sorry if they may seem newbie-ish, I just want as much power as possible :twisted:

1) It is best to put your piston as far back in the chamber, correct?? Because that way, wouldnt you have more room for the actual air that powers the projectile?

2) If you are looking at this article on the spudwiki , it says that the air gets to the chamber (B) by a small equalizing hole or leak around the piston. Will you lose a significant amount of air when you fire it, due to the air releasing around the piston, or is it not significant enough??

K thanks a bunch guys, i hope to build mine soon :)

PS. I did use the search engine
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Unread postAuthor: rad14701 » Thu Apr 19, 2007 10:14 pm

1.) The pilot area behind your piston should be relatively small because the less volume it has, the more efficiently the gun will pilot and fire... Generally, a piston movement of 1/4 to 1/2 the diameter of your barrel will suffice...

2.) You can choose to either use a small equalization port in your piston or simply rely on air being forced past the piston itself... Either way you want the gun to pilot at a rate greater than the chamber air returning from the chamber to the pilot area... Both methods have their merits...
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Last edited by rad14701 on Thu Apr 19, 2007 10:19 pm, edited 2 times in total.

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Unread postAuthor: scatdawg » Thu Apr 19, 2007 10:14 pm

hi. i just completed my coaxial today. on my gun from the back of the chamber to the piston when its against the barrel was the same size as the inside diameter of my barrel. so my barrel id was .800 of an inch and this was the length also of the space between the back of the chamber and the piston. for the piston i just bolted a thin piece ot rubber on the surface that meets the barrel. then under this flap i drilled two 3/32 holes thru the piston. this allows the air into the chamber thru your piston. this worked great on my cannon
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Unread postAuthor: pyro6314 » Thu Apr 19, 2007 10:43 pm

One 3/32 hole is enough. Two in my opinion isn't necessary and will only hurt performance by allowing air to vent into the pilot area when firing.

Just my two bits...
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Unread postAuthor: AmYisroelChai » Thu Apr 19, 2007 10:49 pm

2) If you are looking at this article on the spudwiki , it says that the air gets to the chamber (B) by a small equalizing hole or leak around the piston. Will you lose a significant amount of air when you fire it, due to the air releasing around the piston, or is it not significant enough??


Ok my 2 cents here...

I believe there are ways to form a check valve to prevent backflow into the piloting area. Just put a washer over the pilot hole. this way the air moving towards the Cahber will push open the rubber. And then seal it when your done.[/quote]
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Unread postAuthor: pyro6314 » Thu Apr 19, 2007 11:09 pm

I agree, would have done that on mine but I had already put the hole on the side before I thought of using the front (barrel sealing) rubber for the check valve. I think thats my problem with mine, it still works though just with a bigger pilot valve.
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Unread postAuthor: JDP12 » Fri Apr 20, 2007 2:08 pm

1.) The pilot area behind your piston should be relatively small because the less volume it has, the more efficiently the gun will pilot and fire... Generally, a piston movement of 1/4 to 1/2 the diameter of your barrel will suffice...


so if my barrel would be 1" (its not going to be, just throwing that number out there,) then I would only want about a 1/2" pilot area?[/quote]
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Unread postAuthor: pyro6314 » Fri Apr 20, 2007 2:25 pm

We are saying you need 1/4 to 1/2 of your barrel diameter (so for a 2" barrel port you would need 0.5" to 1") for a minimum of "piston travel" to give you a full volume port to maximize the air flow out of the barrel. The piston travel is how far the sealing surface (on piston) will move away from the seat (barrel) when the pilot air is released (how far the piston will slide to each end). The pilot volume is up to you, it will still probably work if you have a large pilot volume but it will actuate your piston slower because it has lots of air to dump. Putting your piston assembly as far back in the chamber (for a Coaxial) as possible will help reduce your pilot volume and it will also increase the reservoir volume of the chamber.

PS. Volume is a cubic measurement. ie - 5 cu in or 5 inch^3
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Unread postAuthor: JDP12 » Fri Apr 20, 2007 9:05 pm

so when you say piston travel, are you suggesting that I stop it with a bumper before the chamber ends, as compared to stopping it at the end of the chamber??

EDIT: And would I be able to use wood for my piston??

anyone????
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