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HELP : Piston valve help needed

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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HELP : Piston valve help needed

Unread postAuthor: POS » Wed Sep 27, 2006 12:43 pm

Yo guys,

I 'm building my first spud gun, a pneumatic over-under, 16 mm marble barrel and 1" galva chanmer of 50 cm.

Now when I pressurise the gun, no problem, but when I shoot it, all the air escapes trough the pilot valve. Due to construction problems and part problems, the chamber os the pilot valve is pretty big. Could this cause the problem.

Also, the barrel inside the T-part is 16 mm ID. I used a pipe to put the piston in, this pipe is 20 mm ID. The last pipe i tried was 25 mm ID. The piston moved forward when I pressurise the gun, but when I shoot it, It doesn't go back to open the barrel. When I take the gun apart, i can without any force puch the piston back.

What the hell is wrong here. :evil:
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Unread postAuthor: Torch » Wed Sep 27, 2006 2:42 pm

Im no expert on this but I think your pilot is to big, might wana try something like a blowgun. Dont hold me to it though :?
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Unread postAuthor: )DEMON( » Wed Sep 27, 2006 3:07 pm

Your pilot volume could be the cause of the problem, try to make it as small as possible. Make your piston much bigger than needed, so the air in the chamber can push the piston back easily.

Do you have an equalization hole in your piston?, you need one of those and o-rings around the piston. Also, try to get your barrel half way inside the fitting so the piston takes up the other half.

If your barrel is 16mm, and the tube housing your piston is 20mm then the psiton is at most 18mm in diameter. That is way too small for the air to push the piston. If your piston is too far forward then the air finds it easyer to move around/through it than pushing it back.

Hope this helps.
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Unread postAuthor: POS » Wed Sep 27, 2006 3:38 pm

Guys, thanks for the advice.

I have made the pilote chamber much smaler and used a balvalve instead of a blowgun. Now it works perfectly :D (I'm so happy)

The piston has a diameter of aproximently 25 mm, the barrel is 18 mmOD, 16mmID.

I had to make the pilote chamber a bit bigger now, cause I needed to implementate the vent, trough which i pressurise the gun. But I guess i could fill up the chamber with hot glue or something, or clay from my kids :twisted:

So thanks again for the adice. My first pistonvalve works, My breech works, the ammo shells work, I found marbles that fit the barrel ...

The only thing that withholds me from firing, is putting averything together. I just ran out of teflon :?

So one of these days, yoy get to see pics
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Unread postAuthor: )DEMON( » Wed Sep 27, 2006 3:46 pm

Ah, just a bit of confusion with the sizes. Glad we could help. You could fill the chamber with smaller pipe for a stronger alternitive to clay. The first time my piston worked I was also really happy. I was expecting a problem.
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Unread postAuthor: POS » Wed Sep 27, 2006 3:58 pm

I expected it, and got it :lol:

But I toyght all along the pilot chamber was to big, cause all the air got out trought the blowgun. This could mean four things,
one : the piston is jammed, but I knew that was not the case
two : there is to much space between the piston and the tube ont the back, I also new that was not the case
three : The piston is to small, snce the barrel is 18 mm
and four : the pressure in the polite chamber falls to slow, due to a small airgun (it has a hole of 6 to 7 mm and costed me 5 euro) or to a much to big pilot chamber.

So it seemed to be the last one. Figured it out, before I red your answers, so I am glad I could find it on my own, and afterwards read that you guys had the same sollution :)
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Last edited by POS on Wed Sep 27, 2006 4:10 pm, edited 1 time in total.
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Unread postAuthor: )DEMON( » Wed Sep 27, 2006 4:05 pm

Um, there is a typo in your post. You seem to have written you gays instead of you guys. :shock: :P , I also had this problem but figured it out.
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Unread postAuthor: schmanman » Wed Sep 27, 2006 4:09 pm

yep, I had a little problem with that at first, make your pilot smaller, and go and buy tfe paste and wd-40. paint on the tfe, then spray the inside where the piston slides with it. you can also paint the inside with tfe, in fact youshould paint the inside with tfe. It works.
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Unread postAuthor: POS » Wed Sep 27, 2006 4:12 pm

Sorry gUys, that was not meant to. I corrected it. I make some mistakes sometime, after all, English isn't my language. But indeed, that was a serieus fault. Funny though. :lol:
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Unread postAuthor: jrrdw » Wed Sep 27, 2006 4:20 pm

When i got mine together and tryed it for the 1st time, same thing happened. I think my problem was i didn't have the barrel through the 4way far enough, and all the pressure just pushed on the sides of the piston instead of the face forceing it into the pilot chamber. So i cut the barrel off leaveing enough to use a coupler and reducer, glued the 2" coupler and reducer together and then filed out the stop and measured the distence i needed to go through the 4way, glued the custom made reducer to a 1" inside dia. pvc pipe and now i'm working with a 1" bore. When i build my next one i'll be sure to get the barrel into the 4way far enough to get the air on the piston face where it needs to be, i won't make that mistake again!
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Unread postAuthor: POS » Wed Sep 27, 2006 5:17 pm

That's how we learn, by making mistakes, and correcting them ;)

Best school is practice and practice again
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