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Piston valve not opening: Help!

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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Piston valve not opening: Help!

Unread postAuthor: daniele tn » Mon Feb 08, 2010 2:55 pm

Hi, I have got a problem with the small metal piston gun that I built.
It was actuated by a ball valve, thanks to your help I upgraded it using a modded sprinkrel valve instead.
The proble was that the valve was too big for the gun (1 inch compared to the 1/2 inch gun) and that it could sustain only 15 bar.
So I thought to build a piston valve.
The problem is that the piston doesnt open when I open the pilot valve (ball valve)
This is a crappy diagram that I made in paint:

Image

What I think is the problem is that the washer that seals the barrel is large like the tee I use (1/2)
Making that modification could help?
(making the barrel enter the tee and using a smaller washer)Image
Or could the problem be a too small pilot valve(3/8 ball valve)?
Thank you
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Unread postAuthor: MRR » Mon Feb 08, 2010 3:06 pm

If the the diagram is accurate then you have a bad sealing face to piston diameter ratio.
The diameter of the piston has to be bigger than the diameter of the barrel that you are sealing.
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Unread postAuthor: tghhs » Mon Feb 08, 2010 3:06 pm

http://www.spudfiles.com/forums/piston- ... t8157.html

The piston needs to be bigger than the bore, because it needs a surface area for the opening force to act on.
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Unread postAuthor: daniele tn » Mon Feb 08, 2010 3:15 pm

Thank you so much.
I thought that was the problem but I wasnt sure.
Thank you both
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Unread postAuthor: i-will » Thu Feb 11, 2010 2:42 pm

That only half the problem. Your piston is backwards. the end with the bigger surface area is gonna be pushed by the pressure. In your first design the pressure is actually forcing the piston to seal against the barrel (it has the greater seal diameter). If the seal on both ends is the same diameter (like the second den) then its not gonna move at all (equal force on both ends). this is my favorite type of piston and I've made the same mistake long ago. it's all about the surface area of the actual seals.
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Unread postAuthor: daniele tn » Thu Feb 11, 2010 3:26 pm

It works exacly like that, just had to think a bit to find out.
I changed it and now it works, but I still have leaks from the barrel.
I use a neoprene washer as sealing face, but it doesnt seal properly, do you know a way to fix that?
And also, the bigger the difference in sealing surface and pilot surface the faster will be the opening?
If yes, do you think 15mm pilot and 10mm barrel diameter is enough?
Thank you
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