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Piston Valves Explained Visually

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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Unread postAuthor: Spooky » Sun Aug 23, 2009 6:30 am

I'm working on a coaxial at the moment, just successfully cast my epoxy piston.

The problem I'm having is that my copper barrel is sagging inside inside the chamber. This will obviously cause sealing problems as it wont line up correctly with the piston.


Thought about sleeving the barrel with something inside the chamber to keep it rigid but this will decrease my chamber volume too much. I also thought about a drop of epoxy between the outside of the barrel and the inside of the chamber at the piston end, but I'm worried about how this will affect airflow during firing.

Do I have any other options?
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Unread postAuthor: FORE!!!! » Sun Aug 23, 2009 7:15 am

Spooky wrote:I'm working on a coaxial at the moment, just successfully cast my epoxy piston.

The problem I'm having is that my copper barrel is sagging inside inside the chamber. This will obviously cause sealing problems as it wont line up correctly with the piston.


Thought about sleeving the barrel with something inside the chamber to keep it rigid but this will decrease my chamber volume too much. I also thought about a drop of epoxy between the outside of the barrel and the inside of the chamber at the piston end, but I'm worried about how this will affect airflow during firing.

Do I have any other options?



something like this maybe== :wink: click to enlarge..
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Last edited by FORE!!!! on Sun Aug 23, 2009 7:35 pm, edited 1 time in total.
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I've used various things like cutting oil, silicone lubricant, even butter

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Unread postAuthor: MrCrowley » Sun Aug 23, 2009 2:17 pm

Spooky wrote:I'm working on a coaxial at the moment, just successfully cast my epoxy piston.

The problem I'm having is that my copper barrel is sagging inside inside the chamber. This will obviously cause sealing problems as it wont line up correctly with the piston.


Thought about sleeving the barrel with something inside the chamber to keep it rigid but this will decrease my chamber volume too much. I also thought about a drop of epoxy between the outside of the barrel and the inside of the chamber at the piston end, but I'm worried about how this will affect airflow during firing.

Do I have any other options?


Most people use some sort of barrel support with either holes in it or only making the support take up a small cross-sectional area to support the barrel. I cut a cork in half, trimmed it a bit and glued it at the end of the barrel inside the chamber.

Some people make a circle from some rigid material and then cut out 80% of the middle, leaving only necessary supports, so air can still flow past it.
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Unread postAuthor: Technician1002 » Tue Aug 25, 2009 4:05 am

MrCrowley wrote:
Spooky wrote:I'm working on a coaxial at the moment, just successfully cast my epoxy piston.

The problem I'm having is that my copper barrel is sagging inside inside the chamber. This will obviously cause sealing problems as it wont line up correctly with the piston.


Thought about sleeving the barrel with something inside the chamber to keep it rigid but this will decrease my chamber volume too much. I also thought about a drop of epoxy between the outside of the barrel and the inside of the chamber at the piston end, but I'm worried about how this will affect airflow during firing.

Do I have any other options?


Most people use some sort of barrel support with either holes in it or only making the support take up a small cross-sectional area to support the barrel. I cut a cork in half, trimmed it a bit and glued it at the end of the barrel inside the chamber.

Some people make a circle from some rigid material and then cut out 80% of the middle, leaving only necessary supports, so air can still flow past it.


Because air expands, the highest velocity in the chamber is near the valve. I provide barrel supports in coaxials about 6 inches from the valve. It prevents sagging and is mostly out of the airflow path. There is no need for a support right at the area of the valve.
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Unread postAuthor: MrCrowley » Tue Aug 25, 2009 4:10 am

Good advice, never thought of that though it makes perfect sense. :)
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Unread postAuthor: Technician1002 » Tue Aug 25, 2009 4:54 am

MrCrowley wrote:Good advice, never thought of that though it makes perfect sense. :)


I guess a picture helps. Here is a view into the breech of the Mouse Musket. The barrel supports are back about 6 inches so they are not visiable near the breech.

Image
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Unread postAuthor: pipeboyswe » Thu Sep 03, 2009 8:41 am

thanks now i understand how a coax works :D what is the best material for the piston body?
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Unread postAuthor: Willdebeers » Thu Sep 03, 2009 9:16 am

Whatever you can get hold of. They range from baked bean cans to solid PVC.
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Unread postAuthor: qwerty » Thu Sep 03, 2009 10:25 am

It depends what you have access to if you have a machine shop you could make a solid nylon piston or if you dont have any machinery you could make one out of hot glue or epoxy. I made mine out of some random toy car wheel so you can use almost anything cylindrical shape.
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Unread postAuthor: Brian the brain » Thu Sep 03, 2009 10:36 am

A bolt and washers works well..
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Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Unread postAuthor: toogers » Sat Oct 24, 2009 5:38 pm

I'm wondering, instead of the air compressor pressure pushing the piston forward, can i put a spring behind the piston, because i don't have access to an air compressor. my idea is that opening the pilot would create vacuum pulling the piston back letting the pressure in the chamber go through the barrel.
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Unread postAuthor: MrCrowley » Sat Oct 24, 2009 5:42 pm

toogers wrote:I'm wondering, instead of the air compressor pressure pushing the piston forward, can i put a spring behind the piston, because i don't have access to an air compressor. my idea is that opening the pilot would create vacuum pulling the piston back letting the pressure in the chamber go through the barrel.

Yes that should work, and by the way, the piston doesn't open because of a vacuum. I suggest you look at the drawings again. It opens because when you remove the force behind the piston, the force in front of the piston pushes it back.
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Unread postAuthor: bobsaget » Sat Dec 19, 2009 12:16 pm

If i am molding a piston out of the connector that the piston will slide in, i will need to make a slight equalization hole because the piston is the same exact size as the gun itself, correct? And how large should my equalization hole be if the piston is 3/4 in wide or about two CM. I am planning on making a very small hole and making it larger untill the piston moves back. The piston should move back if i have a very small hole right, something around 2 mm?
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Unread postAuthor: mrsteel » Sun Jan 03, 2010 5:58 pm

im building a piston valve airgun and i want to know if the diameter of the exhaust port should be bigger or smaller than that of the barrel, whichone woold give more power to the barrel?
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    Unread postAuthor: c11man » Sun Jan 03, 2010 6:03 pm

    i assume you mean the porting size by exhaust port and it should be the same or larger than the barrel for more power. in no way do you need to make it larger but i generly gives more airflow resulting in more projectile velocity. also you will be supprised by the power of a piston valve

    edit: ok now i know what you were talking about
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    Last edited by c11man on Sun Jan 03, 2010 6:30 pm, edited 1 time in total.
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