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Piston valve theory

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 theory

Unread postAuthor: squeaks » Sat Jan 06, 2007 8:38 pm

Okay, so a lot of people make piston valves by pushing the barrel pipe past the part of the tee that leads to the chamber such as in valves like Schmanman's Swat Gun. My post here is concerning valves with porting close to that of the tee they're housed in. Take a 3 inch porting valve in a 4 inch tee. In this case if the barrel pipe is pushed past the chamber port then you would not get full flow off of the valve because of the surface area the air is allowed around. The 3 inch pipe would cut 3.08 pi of surface area away from the chamber inlet allowing only .92 pi of surface area of air around it. Therefor in pistons like this you would need to leave the barrel pipe before the chamber inlet, or at least only partially around it.

This problem arises with several different sizes such as .75" and 1", 1" and 1.5", 1.5" and 2", 2" and 2.5", 2.5" and 3", and 3" and 4".
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Schmanman's Swat Gun Piston Design!(nicely made I might add)
Last edited by squeaks on Sat Jan 06, 2007 9:01 pm, edited 1 time in total.
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Unread postAuthor: PVC Arsenal 17 » Sat Jan 06, 2007 8:50 pm

Wow. I never though of that. This creates a HUGE restriction.
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Unread postAuthor: Bluetooth » Sat Jan 06, 2007 9:59 pm

Yeah but if you only have it halfway through the tee it wouldn't have to travel all the way back there.
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Unread postAuthor: squeaks » Sat Jan 06, 2007 10:01 pm

Exactly what I was saying. I'm just warning people that would attempt making a valve like schmanman's and end up getting performance like a ball valve. As long as they have it only about halfway past the opening it should be plenty fine.
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Unread postAuthor: MrCrowley » Sat Jan 06, 2007 10:12 pm

While we are on a topic of pistons...has anyone found a good piston for a 3" tee? Alot of people who use 3" tee's like gort.....Make there pistons,But my piston in my Dam Buster breaks everynow and then, so I was wondering has anoyone found a good fit piston for a 3" tee.
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Unread postAuthor: Scope » Sat Jan 06, 2007 10:18 pm

Isnt that called a barrel sealing valve...
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Unread postAuthor: squeaks » Sat Jan 06, 2007 10:21 pm

A whole bag of hot glue sticks works well. If you can get ahold of a lathe, even for a little while, a 4x4 is turned into a piston really quick. Just don't use really soft woods like redwood. Some steel cans from canned food work good such as baked beans or fruit coctail. Or, if you want a really heavy and really weak piston just melt some lead down and use it a s a piston. I'm not sure about bondo but it might work. A flat piece of pvc strung up on some all-thread would work. I guess you might be able to use a hole saw since those come in vastly different sizes, just put it on a piece of all thread. Need any more ideas?

oh, and scope, I'm talking about a specific kind of barrel sealing valve. I'm not that dumb........well actually I'm not that dumb all the time.......
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Unread postAuthor: MrCrowley » Sat Jan 06, 2007 11:16 pm

Ive tried cans,bondo/molding,and Ill give glue stick another go.
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Unread postAuthor: Velocity » Sat Jan 06, 2007 11:49 pm

In all of my piston valves, I always put the barrel portion of the seal halfway through the tee. Farther from the modified bushing, and it will create restriction (as you said). Closer to the modified bushing, and the piston will not seal properly.

I noticed you listed a lot of barrel sizes with corresponding chamber sizes that will cause this restriction. Just imagine the valve as being coaxial rather than tee'd. You would never have a tennis ball barrel inside of 3" pipe; there would be barely any chamber. This is somewhat unrelated, but it is a good rule of thumb if you like to build barrel-sealers with the barrel portion extended far backwards
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Unread postAuthor: MrCrowley » Sun Jan 07, 2007 2:56 am

I Might have to extend the barrel further then the opening like schmanman because of the small 'lip' inside all pressure tees that stops the pistons so you have to make 2 parts to the piston like the supah valve.
I dont have a lathe so at the moment I have no way of over-coming this lip thingy.
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Unread postAuthor: Scope » Sun Jan 07, 2007 11:51 am

Haha sorry man my bad..
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Unread postAuthor: squeaks » Sun Jan 07, 2007 12:57 pm

Oh, that's simple, all you have to do is remove the stop in the tee and sleeve a piece of pipe all the way through it. Then your piston can slide the whole way easily. Just use a dremel or a hole saw. The later would probably be the best idea.
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Unread postAuthor: MrCrowley » Sun Jan 07, 2007 3:18 pm

Yeah, But 80mm wholesaws are around $80NZD and no one will sell me an off-cut of 80mm pressure pipe which I need to sleeve the tee and it only comes in 6m lengths at $200NZD
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Unread postAuthor: Bluetooth » Sun Jan 07, 2007 3:22 pm

I have hole saws from 1" to 3".
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Unread postAuthor: MrCrowley » Sun Jan 07, 2007 3:34 pm

jeez....and my dad owns a hardware store! I know I have atleast a 60mm one so Ill double check our garage is a mess.
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