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second attempt

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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second attempt

Unread postAuthor: jon_89 » Fri Oct 05, 2007 9:38 pm

okay my last attempt at a barrel sealer was unsucessful but i am trying again. this time i will use .5in for the chamber and a bb barrel. before i start i have some basic questions. does the piston have to be an air tight seal or do i want some room around it? next can .5in pvc hold 300 psi ( 600 written on pipe )? This may be a stupid question but can i fill it with a shock pump or do i havve to use high flow source? thanks.
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Unread postAuthor: Dornep » Fri Oct 05, 2007 9:51 pm

Your piston needs to be tight enough to be moved forward when you pressurize the pilot area. You can make it airtight but it will need an equalization hole to allow the chamber area to pressurize.

If the pipe says 600PSI it should hold 600PSI safely.

If your shock pump will move enough air to seal the piston against the barrel I don't see why it won't work. I have no experience using shock pumps on barrel sealing piston valves so I can't give you a definite answer.
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Unread postAuthor: noname » Fri Oct 05, 2007 11:10 pm

I would highly advise against using 1/2" Sch 40 (or 80, for that matter) above 150 psi. I've blown up 1/2" Sch 40 with 2x hybrid mixtures, which produce less than 150 psi.
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Unread postAuthor: jon_89 » Sun Oct 07, 2007 12:05 pm

i think 15O psi hybrid is different than 150 psi air. anybody have more suggestions?
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Unread postAuthor: jrrdw » Sun Oct 07, 2007 12:30 pm

How were you venting the pilot chamber on your 1st attempt?
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Unread postAuthor: jon_89 » Sun Oct 07, 2007 2:04 pm

a blow gun. i dont think it was that though. it was i couldnt get it to seal right.
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Unread postAuthor: jrrdw » Sun Oct 07, 2007 2:17 pm

I had simaler problems with mine at 1st. I figured it out by unscrewing the blow gun off the fitting so the pilot would vent basicly all at once. I cured it with a modded sprinkler valve from Bcarms. It was a worth while shot in the dark, but it worked!
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Unread postAuthor: Infernal Maveric » Sun Oct 07, 2007 4:08 pm

The piston needs to be a very good seal for the gun to pilot properly. If you are using O-rings, then as Dornep mention, you will need to add an equalisation hole, or have a check valve around the piston to allow air to reach the chamber.

I am no expert on PVC as I have never used it so I can't help you on that.

Shock Pumps can seal home-made pistons effectively, as long as the piston is well lubricated, as their flow is relativly low. A spring could help with this, depending on the volume of your chamber.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 07, 2007 4:11 pm

For a BB barrel, the piston only has to move back 1/16 inch so your pilot volume can be tiny enough to be piloted by the schrader you're using for filling.
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Is this equalization hole good size?

Unread postAuthor: gwoloshyn » Sun Oct 07, 2007 6:04 pm

Is this equalization hole good size?
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Unread postAuthor: jon_89 » Sun Oct 07, 2007 7:14 pm

i never got how to use the schrader for venting. i know you push down on it but how would i set it up in a trigger form. using a schrader would make it much more compact.
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Re: Is this equalization hole good size?

Unread postAuthor: jackssmirkingrevenge » Sun Oct 07, 2007 11:11 pm

gwoloshyn wrote:Is this equalization hole good size?


If your equalisation hole is too big, your launcher won't fire. It can't really be too small, if you're filling with a bike pump you can afford a pinhole sized hole and not realise the difference. If using a compressor, open your filling valve for several seconds to make sure your chamber is under pressure. In regard of the above, I would lean towards the smal - 0.5mm or so should suffice.

You can avoid the need for a hole by having 2 schraders, one in your pilot area and one in your chamber, of having a connector with a check valve in beteen pilot area and chamber.

how would i set it up in a trigger form.


It's a little complicated, you'd have to use a system of levers that at the same time are removable so you can still fill it. For easy ergonomics a blowgun is a better solution.
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Unread postAuthor: jrrdw » Mon Oct 08, 2007 6:46 am

I would have my equilization hole coming out the top of the piston just outside the barrel diameter. The barrel seal acts as a check valve. I use a 1/32nds hole in my piston.
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Unread postAuthor: jon_89 » Wed Oct 10, 2007 1:06 am

out of curiousity would .5 in work in 1 in for a coaxial?
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Unread postAuthor: jrrdw » Wed Oct 10, 2007 5:25 am

jon_89 wrote:out of curiousity would .5 in work in 1 in for a coaxial?


I must have missd something. What is .5 and 1?
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