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Coaxial Piston 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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Coaxial Piston Question

Unread postAuthor: surfbum » Tue Apr 17, 2007 12:37 am

i have a few technical questions about the design of a coaxial piston valve

1. how much movement does the piston need to move (i.e. in a barrel sealing design, how far from the barrel does the piston need to travel to provide the best flow)?

2. how much air needs to be behind the piston to prevent the chamber from moving it and firing the launcher?

my piston design is going to be machined delrin with o-rings, so there will be no equalization around the piston (main reason for the o-rings is to cut down of friction)
3. for equalization of the main chamber and the air behind the pistion, is it necessary to make/install a check valve in the piston or is just a small hole or two enough? how large should the hole be?

4. what is some good high strength hose that is rated to at least 200 psi?

thanks for the help
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Unread postAuthor: paddo » Tue Apr 17, 2007 1:10 am

i have just made one myself and

1) i would say the piston has to move at least its own length. mine is 50mm long (2 inches) and moves just over that amount backwards.

2) as soon as a certain amount of pressure is behind the piston it will seal and no matter how hard you shake it it should not move. mine seals at about 25 psi and if it has that amount in it you can shake as hard as you want and itit'lltay put. because of physics and stuff the pressure inside the chamber is equal on every part of it.

3) if its a perfect seal you would need some type of check valve OR equalisation holes. if the sealing face is a piece of sheet rubber just drill a hole through the piston and the rubber will act as the check valve. i think that if it seals perfectly against the chamber and has some type of check valve it would be most efficient because no pressure from in front of the piston could escape through the barrel.

4) sorry mate not sure i just use airline hose which is for big compressors aswell. it should be printed on the side of the hose somewhere of what the max. pressure is.

good luck mate.
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Unread postAuthor: hi » Tue Apr 17, 2007 1:11 am

1. maybe and inch.. it really depends on the gun, the larger the chmaber is in diameter, the more space it needs, but usually for little ones about 3/4'' to 1'' is ok, bigger ones, like 2'' or greater need more like 2-4 inches, but usually no more than the barrel, so if you have a 2'' barrel, then it wouldnt make much sense for it to move 5 inches. so for a 2'' barrel, about 2''.

2. dont worry about it, as long as the piston has room to slide its ok.

3. you cant install a check valve, it would fall apart... but drill one (1) hole that is as small as you can possibly make it.

4. i dont know what you need hose for, but go to an auto shop, they have hose rated for 380+ psi...
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Unread postAuthor: surfbum » Tue Apr 17, 2007 2:40 pm

thanks for the help guys, if anyone else has any comments feel free to post

got another question:
what is a good material to use in order to:
1. seal the barrel
2. use as a good bumper
3. use as a check valve
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Unread postAuthor: lukemc » Tue Apr 17, 2007 2:56 pm

1. neprem rubber is a good sealer

2. i dont know i need the same question

3.check valves a not entierly nececary the pilot valve releases air too fast for air to move around the piston

by the way what kind of piston are you useing?

for the second question you asked in your original post- the piston wont move becaus the barrel is sort of acts like a suction and holds the sealing face to it untill the piston moves
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Unread postAuthor: goathunter » Tue Apr 17, 2007 6:05 pm

1. Marine grade rubber gasket material(marine parts supplier)mine is 1/4" thick
2.a thick o-ring or make a buffer from something squishy like a mousepad.
3.don't bother with a check valve you want equilization at all pressures and backflow isn't an issue.Try to make the hole as small as possible.I used the smallest dremel bit I could find.
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Unread postAuthor: keep_it_real » Tue Apr 17, 2007 7:22 pm

I'm pretty sure 1/4" braided vinyl tubing is rated at 220 psi.

do what paddo said for the equalization holes. Definitely don't use a check valve. That will the piston heavier which is bad.
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Unread postAuthor: spudthug » Tue Apr 17, 2007 7:34 pm

u need no such thing as a check valve..either drill a hole through it or make a 1/32" groove with a metal file on the side if u dont have a dremel or a really really really small drill bit
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Unread postAuthor: rad14701 » Tue Apr 17, 2007 7:46 pm

In answer to your first set of questions, your piston valve only needs to move 1/4 to 1/2 the diameter of your barrel... You will not gain performance with additional piston travel, you will only invite delayed piloting problems due to the excess air behind the piston...

As for the "O"-rings, they will only reduce friction if your piston is within specific tolerances of the chamber... If the piston is too tight then the "O"-rings will increase the friction and it it is too loose then there is more chance of the "O"-ring blowing out of its seat and binding the piston than decreasing resistance...
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Unread postAuthor: surfbum » Tue Apr 17, 2007 8:12 pm

someone asked what i am using for a piston

im going to machine a piston out of delrin (lightweight/strong plastic material) using a lathe so it is going to fit perfectly in the chamber, along with the o-rings its going to create a good low friction seal
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Unread postAuthor: Hawkeye » Tue Apr 17, 2007 9:54 pm

it will actually make a difference if your piston leaks to fill but seals totally and prevents backflow when fired.
There are a few simple ways of achieving that. The easiest is to have short piston travel and a sealing surface on the back as well.
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Unread postAuthor: surfbum » Wed Apr 18, 2007 12:30 am

should the piston be a light as possible (i.e. is lighter better)?
or is there a cutoff where too light negatively effects performance?
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Unread postAuthor: hi » Wed Apr 18, 2007 12:42 am

surfbum wrote:should the piston be a light as possible (i.e. is lighter better)?
or is there a cutoff where too light negatively effects performance?


if its to light it wont seal all the time, if it is really heavy it opens kinda slow. just make it so that it slides well and it will be ok. it takes alot of work to make a piston too heavy or to light. even it you made it out of carbon fiber or lead it would still work.

the lighter the better isn't true, there is such a thing as too light. i would say no less than an ounce.
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Unread postAuthor: paddo » Wed Apr 18, 2007 6:46 am

i'm trying to figure out what would be the best piston material and i think i can get my hands on some turning nylon tomorrow. has anyone used it before ? we have some at work for the lathe and its very solid and i think it would be very well suited. what is anyones thoughts ?
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Unread postAuthor: Scope » Wed Apr 18, 2007 1:11 pm

I would use delrin... its been tested 1000000 billion times as being extremely good acting as pistons and bolts...
just dont get it wet... because it will swell..

o yea and delrin is self lubricating
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