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Strange Design Constraint.......

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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Strange Design Constraint.......

Unread postAuthor: VH_man » Wed Apr 09, 2008 8:32 pm

Well, on my quest to the .40 calibre bolt action dartgun, i have an interesting design constraint......

I want my gun to be as compact as possible. I have some REALLY NICE maple for the stock and i want to make the entire gun fit inside of it, and look like a Bullpup PCP air rifle......

Attached is a quicly sketched picture of what i want. Basically, i want to fill through one end, pilot through the other end, and have the flow go out the top. And most importantly, i DO NOT want to build a chamber sealer. this will be used at pressures ranging from 150 to 400 PSI.........

and also, if possible, the other design constraint is for it all to be INSIDE of a 3/4 inch Galvanized steel pipe............... nothing sticking out around the OD of the pipe besides the air outlet........

I have my ideas but id like to see what you guys can come up with. my idea is a little hard to do........

i also forgot to mention i plan on using epoxy..................... lots of epoxy.
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Unread postAuthor: Hotwired » Wed Apr 09, 2008 9:53 pm

Umm, the only example I'm aware of is in diaphragm solenoid valves where air from the fill side leaks into the pilot area to seal against a seat.

Making life hard for yourself eh?

Out of interest, where have you been reading that chamber sealers won't work at that pressure :wink:
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Unread postAuthor: VH_man » Thu Apr 10, 2008 5:26 am

Its just i dont want to have to deal with the annoyance of making sliding air seals with o-rings. to finniky.

my goal was to use a cylinder of Hard Rubber as the piston, negating the need for a "sealing face" that could come off without warning, ect.

The final result will be, damn sexy, to put it simply, so just bear with me... lol.
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Unread postAuthor: Hotwired » Thu Apr 10, 2008 6:43 am

Sound like you had an attack of the myths :wink:

O-rings are not required, neither is a sliding seal, in fact it can look EXACTLY like a barrel sealing piston with just the barrel and chamber swapped over.

Image

Oh and an equalisation hole.

Unfortunately as you've got a design requirement of chamber side filling you might need a spring to keep the piston held shut until the pressures are equalised.

Friction might be enough but I'd not bet anything on it holding if you dumped a compressors air tank in.
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Unread postAuthor: psycix » Thu Apr 10, 2008 6:53 am

Hotwired wrote:Sound like you had an attack of the myths :wink:

O-rings are not required, neither is a sliding seal, in fact it can look EXACTLY like a barrel sealing piston with just the barrel and chamber swapped over.

Image

Oh and an equalisation hole.

Unfortunately as you've got a design requirement of chamber side filling you might need a spring to keep the piston held shut until the pressures are equalised.

Friction might be enough but I'd not bet anything on it holding if you dumped a compressors air tank in.


If you havent got a sliding seal, wouldt it leak pressure from the pilot to the barrel?
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Unread postAuthor: Hotwired » Thu Apr 10, 2008 7:02 am

Blast, and the posts been quoted too :o

A flared piston end of another design derailed me on sealing ^_^

Nevertheless :D

It's still mostly good. The option is there to be creative with the rubber piston or cut a groove in to put an o-ring
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Unread postAuthor: VH_man » Thu Apr 10, 2008 9:08 pm

Speaking of wich, what are some good ways to make O-rings 100% "sealy"..... i really dont trust my grove cutting skills..... and remember its going to be taking HUGE amounts of pressure............
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Unread postAuthor: DYI » Thu Apr 10, 2008 9:42 pm

and remember its going to be taking HUGE amounts of pressure............


I thought you said your maximum operating pressure was 400 psi? Which is it? Because 400 psi doesn't really require anything beyond the norm, except maybe a bit more consideration of piston materials and design.
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Unread postAuthor: Gippeto » Thu Apr 10, 2008 11:25 pm

Go for the chamber sealer, cast the o-ring into an epoxy piston.

Use a greased 3/32" cross section o-ring for the casting.

Remove the o-ring after the epoxy has cured, and file the groove to the proper size for an 1/8" cross section o-ring.

o-ring chart here;

http://142.179.166.227/hitechseals/pdf/ORingChart.pdf
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Unread postAuthor: VH_man » Fri Apr 11, 2008 5:27 pm

Thanks for all your help guys, but im still interested in seeing how i could keep this a barrel-sealing gun.................

im going to try my hand at 100% sealing of things, but dont expect anything. im still a little hesitant on the 0-ring part, im not so great with o-rings..................
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Unread postAuthor: Hotwired » Fri Apr 11, 2008 8:24 pm

The problem is that you're wanting a barrel sealer in a chamber sealers configuration.

Short of having a 90* bend come in from the exhaust port for a piston to seal against it's not really possible to HAVE a barrel seal as it needs the exhaust to be opposite the pilot.
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Unread postAuthor: mopherman » Fri Apr 11, 2008 8:49 pm

what you could do is build it like a coaxial, but then bend the barrel out the top of the camber. This is nice because of compact-ness, but a requires creative construction.
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Unread postAuthor: VH_man » Sat Apr 12, 2008 6:50 am

Bah youve all convinced me to build this all in 3/4 inch copper pipe..... i have a GREAT idea to make it 100% take-downable......... if i can get O-ring seals perfect.......
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