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Schrader question

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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Schrader question

Unread postAuthor: daxspudder » Sat Aug 30, 2008 6:14 pm

Okay, I'm taking on the challenge of building a piston valve for the first time. I'm using wood, with a neoprene washer, but my real problem is getting the piston to seal before the chamber begins to fill... I'm thinking of using a schrader in the back of my piston to create enough pressure in the pilot volume to seal the chamber before it fills... I believe I have heard here before that a schrader will open at 90psi, if this is true, I really need to know soon so I can begin construction... I included a few pictures, the black oval is connected through to the hole in the back of my piston, where the schrader valve is shown...
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Unread postAuthor: SpudFarm » Sat Aug 30, 2008 6:18 pm

short answer, that was one helluva good idea!
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Unread postAuthor: Insomniac » Sat Aug 30, 2008 6:54 pm

Yeah nice idea, its basically a check valve that doesn't open untill a certain psi, so the piston can still move fowards. Can't believe no-one has already thought of this.
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Unread postAuthor: daxspudder » Sat Aug 30, 2008 7:29 pm

:? glad you guys thought it was a good idea... but I took it out and built a spring check valve, that will open at a lesser psi, 15-20, since it only takes about 10 to seal the piston... ill put up a picture soon...
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Unread postAuthor: QuaziGNRLnose » Sat Aug 30, 2008 10:35 pm

im not exactly sure what youre doing? are you trying to fill the chamber via the piston?. a diagram would be nice just for readability issues.
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Unread postAuthor: daxspudder » Sun Aug 31, 2008 12:53 am

...um not exactly... the schrader valve was just gonna be a way to get a positive differential pressure in the pilot volume, i.e. the valve would be completely inside the gun, with another valve behind it to fill the gun with...
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Unread postAuthor: ALIHISGREAT » Sun Aug 31, 2008 3:02 am

it really depends how tight fitting your piston is, and maybe you need to think about a softer sealing face before overcomplicating things, its a good idea though, if i ever build a larger bore piston cannon then i have thought about putting a check valve in the piston .

edit: thinking some more, a check/schrader should be unecesary as long as the piston is tight fitting and the sealing face is soft enough because a piston should seal with the same pressure on both sides because there is more surface area on the back than the front so more force is 'pushing' it foreward regardless of pressure differential.

edit 2: i assume you have made the piston tight fitting anyway or the shrader would have no effect? so maybe the sealing face is at fault?
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Unread postAuthor: CasinoVanart » Sun Aug 31, 2008 5:46 am

I used this same idea on my 4 inch piston, simply threaded a hole in the front of the machined piston and screwed in a schrader valve, it worked flawlessly but the chamber leaked. :D It would also lower the max PSI that your chamber gets too i think (in an o'ring piston).
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Unread postAuthor: POLAND_SPUD » Sun Aug 31, 2008 6:23 am

casinovanart is right... if a shreader is used then the pressure on the pilot side would always be higher than the max pressure in the chamber... thus shreader would limit both the power and piloting would take longer
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Unread postAuthor: daxspudder » Sun Aug 31, 2008 12:03 pm

Not if the pressure in the pilot and chamber were both greater than the pressure needed to open the schrader, then there would only be a minimal difference between the pilot and the chamber, but with a higher pilot chamber the chance of leaking are greatly decreased...
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Unread postAuthor: POLAND_SPUD » Sun Aug 31, 2008 12:58 pm

no.... since the pressure on the other side of the shrader acts on the shrader then the pressure on the pilot side will always be higher....
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Unread postAuthor: Gippeto » Mon Sep 01, 2008 12:04 am

My .02 :)

In the search for increased efficiency, controlling air flow within the valve is a worthwhile idea.

I use a "floating o-ring" on the piston for my copper gun. It is simply a slightly loose o-ring groove with a hole drilled into the side of the piston.

The drilled hole is half in, and half out of the groove on the chamber side.

The piston fits closely, and seals the barrel immediately upon pressurizing.

Pressure then fills the chamber to the same level as the pilot.

Upon firing (piloting), the o-ring on the piston is forced (by pressure differential) to seal the pilot volume off from the chamber volume.

Although, it would likely work without the o-ring, I doubt it would work as well.

And it does work very well IMO. :)
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Unread postAuthor: PotatoNick » Mon Sep 08, 2008 11:35 pm

Gippeto wrote:My .02 :)

In the search for increased efficiency, controlling air flow within the valve is a worthwhile idea.

I use a "floating o-ring" on the piston for my copper gun. It is simply a slightly loose o-ring groove with a hole drilled into the side of the piston.

The drilled hole is half in, and half out of the groove on the chamber side.

The piston fits closely, and seals the barrel immediately upon pressurizing.

Pressure then fills the chamber to the same level as the pilot.

Upon firing (piloting), the o-ring on the piston is forced (by pressure differential) to seal the pilot volume off from the chamber volume.

Although, it would likely work without the o-ring, I doubt it would work as well.

And it does work very well IMO. :)



However, upon the initiation of the pilot volume dump, your pilot volume is still connected to the chamber volume.

I'm not sure how much difference there will be between a check valve design that would be normally closed as opposed to a check valve design that would be normally open and then closed upon an action such as a change in piston position or a change in pressure differential. I believe that the more perfect design would be superior, but only testing can tell by how much.
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Unread postAuthor: ALIHISGREAT » Tue Sep 09, 2008 1:40 am

i can't see a schrader making much of a difference to be honest, in the time it takes for a piston to open, my relitivley heavy piston opens in 1ms according to GGDT, so thats no alot of time for the pilot to start emptying the chamber :wink:

so the problem only occurs, if you have a loose piston that is overly heavy, then the air can leak around as you are piloting because it would take longer to open.

so as i said before, i still think the sealing face is at fault or the piston is not tight fitting enough?

edit: after looking at your piston pics again, one factor causing the problem could the the small piston diameter compared to the barrel diameter? because then not much gas can get infront of the piston to 'push' it back when you pilot it.
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Unread postAuthor: CasinoVanart » Tue Sep 09, 2008 3:39 am

I used the schrader as a check valve within the piston, the chamber was filling through only the schrader.
The valve needs pressure to open, hence less pressure in the chamber upon filling, only like 5psi but it all counts :)
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