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Piston Valve Questions

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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Piston Valve Questions

Unread postAuthor: Yonderknight » Mon Aug 21, 2006 11:26 pm

Hi everyone, I'm new here, and fairly new to spud gunning.

I recently finished a very simple pneumatic launcher with my friend, and I love it! We're planning on making another gun with a much bigger chamber and barrel, and a custom valve for fast opening.

I saw this video tutorial on bcvids.com for a "Barrel Sealing Valve."
http://www.bcvids.com/dload.php?action=file&file_id=4
Also, I think it's based off of these plans:
http://www.geocities.com/izzydoesit_99/index.html

After looking at the two, I had just one quesiton about the actual valve: don't you need some sort of hole in the piston to equalize the pressure when charging? I don't understand how the air chambers get filled otherwise, unless they're relying on the piston not sealing perfectly with the valve.

Help will be appreciated, thanks!
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Unread postAuthor: MrCrowley » Mon Aug 21, 2006 11:37 pm

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Unread postAuthor: A-98 » Mon Aug 21, 2006 11:41 pm

when you pump air into the back of the valve it forces the piston forward. then because the piston cannot move forward and the air has no-where to go it leaks around the piston thus presurising the chamber. you want your piston to have a bit of a seal on the valve walls and it should slide easily. the reverse is also true if your piston is too loose then it will not move forward thus not sealing against the barrel. and then when you vent the airs it will not move backwards.

hope this helped and ifnot some one else can better explain it
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Unread postAuthor: Yonderknight » Tue Aug 22, 2006 4:42 pm

Ah, I see. I thought if the piston had O-rings or if it was perfectly sealed you need some sort of equilization hole, but I guess not.

Thanks! =).
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Unread postAuthor: Shrimphead » Tue Aug 22, 2006 4:55 pm

Actually, you were right about that. Most of the time we just assume that it isn't a perfect seal and that the air will be able to equalize. But if you are putting o-rings on the piston, it might be a good idea to have a small equalization hole. If it is too big, the piston might not seal very well.
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Unread postAuthor: Velocity » Tue Aug 22, 2006 4:57 pm

yes... one of the few times where clumsey imperfections are a benefit :P
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Unread postAuthor: Yonderknight » Wed Aug 23, 2006 11:24 am

Hrmm ok, I'm a little confused again. What's the spring in the diagram used for? Before I thought it was for sealing the barrel, but then I figured the air pressure behind the piston would be doing most of that. Is it just for a quick return after the shot?
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Unread postAuthor: noname » Wed Aug 23, 2006 1:18 pm

The spring presses the piston lightly against the barrel so air doesn't leak out the barrel while the launcher is being pressurized.
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Unread postAuthor: Yonderknight » Wed Aug 23, 2006 1:53 pm

Ah ok. So I don't need some super powerful spring to keep the barrel shut.

thanks =)
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Unread postAuthor: Flinchy » Wed Aug 23, 2006 3:42 pm

you don't need a very powerful spring to do the job, just something strong enough to keep your piston in place. the spring is used to push the piston against the barrel to make a seal even when not pressurized. There are usually holes on the outer ring of the piston that go around the barrel and into the chamber, this allows you to pump air into the chamber while keeping a seal, when you pull the valve the pressure behind the piston will act as a vacuum and pull it back, and at the same time the pressure in the chamber is forced out the barrel very quickly. hope that helps a bit.

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