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3/4" 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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3/4" piston problems

Unread postAuthor: subsonic_karet » Sun Feb 17, 2008 7:13 pm

I've recently been trying to build a piston valve in a copper tee. I'm using a 3/4"X3/4"X1/2" tee. The barrel is 1/2" copper. My piston is 3/4" oak dowel with piece of neoprene for a sealing face. I have it all together and now have a problem I can't sort out.

The problem is, I can pressurize the gun to 250 psi but the piston doesn't move when I pull the trigger. I'm piloting it with a modded blowgun. I've even tried with a 1/4"NPT ball valve. Same results, just the air coming out the pilot.

The piston sits in the 3/4" female threaded fitting with less than a 1/16" inch gap. It is greased very well. I have tried different piston lengths.

Is 1/16" too big of a gap?
Should my sealing face of my piston be bigger? I can only go as large as the female threaded fittng ID.
Will moving my barrel back farther in the tee make a difference?

I have searched the forum and the only thing I can think off would be the sealing face is too small. If this is the case I will need to use something else for my piston, hot glue or epoxy.

There seems to be several wooden doweled, 3/4" chamber and 1/2" barrel guns, so this should work. Right?

In the image below all parts are lined up where they go

Image
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Unread postAuthor: jrrdw » Sun Feb 17, 2008 7:33 pm

Is your barrel sealing face in the middle of the tee?
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Unread postAuthor: dongfang » Sun Feb 17, 2008 7:57 pm

Hi,

Is there any frontal area on the piston with pressure on it? In other words, is the piston diameter larger than the sealing face diameter?

If yes: Does the chamber pressure drop away when you trigger the gun, or does nothing happen at all (except that you lose pilot pressure, as intended)?

Regards
Soren
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Unread postAuthor: subsonic_karet » Sun Feb 17, 2008 7:59 pm

jrrdw: Are you asking center of ID or lineal center? If lineal, the image posted shows where the piston sits when fully pressurized. If center of ID, yes.

dongfang: Yes, there is some frontal area, but not much. When the pilot valve is triggered the entire gun is emptied through the pilot valve. The piston does not move back.
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Unread postAuthor: jrrdw » Sun Feb 17, 2008 8:10 pm

The sealing face needs to be in the center of the tee. That way pressure can be applied to the face of the piston to push it open.
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Unread postAuthor: subsonic_karet » Sun Feb 17, 2008 8:15 pm

So, I should move the barrel further back into the tee.
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Unread postAuthor: jr » Sun Feb 17, 2008 8:50 pm

or could he just cut a large groove in the midle of the piston so that the chamber pressure will move it open

__ ___
[ \____] ]
[ ____ ]
[__/ ]___]


so it looks like this!!!



I have the best CAD program in the world....LOL





Edit:
why does this doddle get changed on the forum?
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Unread postAuthor: lockmanslammin » Mon Feb 18, 2008 1:26 pm

you need a perty darn decent seal between the piston and the tube that the piston slides in, I had the same problem with my gun, except the ball valve worked but the blow gun didn't. I caused my problem myself by putting an equalization hole in my piston, and fixed it with a modified grease zerk. Hey at least you know you sealing face is working well...hehehe.
Maybe you could cut o ring grooves in your piston.... or you could simply try putting some electrical tape around it for a temporary test. and hey, if the tape works and stays on, then leave it.
If you want something pertier than tape try some shrink tubing. If the tubing works but slides off after a few shots just slide it back and use a little super glue on it.
I'm a bit of a noob myself, but I figured I would respond since I had a similar problem.
Good luck, Lock
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Unread postAuthor: SpudFarm » Mon Feb 18, 2008 1:52 pm

if i where you i would put a piece of neoprene(?) in the back of the piston with a perfect fit in the tee as well. cut the piston in half put in a nice fitting piece of neoprene and screw it back together with a wood screw. then you get a seel behind the chamber outlet and it WILL push the piston back if you get a nice fit :D
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Unread postAuthor: subsonic_karet » Sat Feb 23, 2008 5:24 pm

So, I moved the barrel farther back into the tee and it worked. Thanks for the help guys. :D
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Unread postAuthor: MrCrowley » Sat Feb 23, 2008 5:46 pm

subsonic_karet wrote:So, I moved the barrel farther back into the tee and it worked. Thanks for the help guys. :D

'Tis what we're here for :)
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Unread postAuthor: pizlo » Sat Mar 01, 2008 6:58 pm

Really sorry to awake this topic from its peaceful repose, but can anyone confirm that this problem can be solved by cutting a large groove into the dowel?
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Unread postAuthor: Sticky_Tape » Sat Mar 01, 2008 7:37 pm

I'm just guessing that the air in the chamber is passing around the piston when you try to pilot the piston keeping the presssure the same on both sides of the piston. This problem can be solved by putting exactly 1 layer of duct tape 1.5 cm in width near the back of the piston. This is my piston. by the way did you put any padding on the back of the piston yet because if youdidn't the piston could crack when you fire the gun.
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Attachments
100-0023_IMG.JPG
sealing face is at the left padding to the right piston also has duct tape seal.
100-0022_IMG.JPG
this is the setup of the valve.
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Unread postAuthor: pizlo » Sat Mar 01, 2008 8:15 pm

Well I've never actually piloted mine I still need a few more fittings. I was just expecting g to encounter this problem because there might not be much piston face for the pressure to hit. Anyways, thanks for that solution, I'll use it if needed.
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