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full-auto piston valve and bolt combination revisit

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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Unread postAuthor: POLAND_SPUD » Tue Feb 16, 2010 7:00 pm

hmmm

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doesn't it look similar ??
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Unread postAuthor: iknowmy3tables » Tue Feb 16, 2010 8:39 pm

not exactly very similar to my diagram, but it's similar to what we are attempting, and I admit thats pretty darn good and has the power I'm looking for
I think I could build one I'll just need to get a 3way control valve with 3ports

I have a diagram but it I might make the real deal with a bigger piston,
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Unread postAuthor: POLAND_SPUD » Tue Feb 16, 2010 10:09 pm

IMO it's not a good idea...

First of all, the main valve is piloted via the 3 way valve but it can't exhaust air freely so it will lower performance
Secondly, the air cylinder won't return to its original position. Iknow you could not put an o-ring there to fix it but it really isn't the way things should be fixed
What is more, the way in which the bolt is attached to the air cylinder can cause some troubles. it wouldn't be a big deal if you machine it but I assume that's not how you plan to build it
And finally, the 3 way valve is exposed to the same pressure as the main valve (though I understand you don't mind it)


Have you considered using the same design as I used on the semi auto? It really is pretty much as foolproof as it gets... lol even my lack of manual skills was not enough to prevent it from functioning the way it should


if you replace its manually actuated 3 way valve with a pneumatically actuated one you can convert it into a fullauto gun... more on this here
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 17, 2010 12:14 am

POLAND_SPUD wrote:JSR but what kind of flow coef can you get with it ? and where's efficiency in it??


It can easily have equivalent flow to the barrel porting, with a little dead space of course. Still, air is going to start flowing through it well before the bolt is closed. With an airtight magazine this shouldn't be much of a problem though, considering I was getting very good results with no bolt at all. It's better to have a bolt setup which starts to open as the piston moves as opposed to starting to close, ideally something like this.
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Unread postAuthor: iknowmy3tables » Wed Feb 17, 2010 12:52 am

First of all, the main valve is piloted via the 3 way valve but it can't exhaust air freely so it will lower performance

not as freely, and I admit the restrictions on the pilot do reduce performance, but once the bolt is pushed forward there is a direct pathway from the pilot to the atmosphere, and remember it is a barrel sealing piston deliberately built with a large seat area to diameter ratio to delay the opening of the the piston until the pilot pressure has dropped significantly and once it does open there is plenty of surface area for a fast opening,

Secondly, the air cylinder won't return to its original position

oh wait I see, I'd like to fix that some how because I like how this design
perhaps I could use double acting control valve which shouldn't cost much more, then a plain 3way

I'm familiar with the design you used but I'm always afraid the the cylinder will bolt will begin to open while it's fireing, and what if I wish to use a cartridge design that depend on the bolt being firmly pressed against the cartridge while firing, so I can't afford to have the pressure on the bolt rapidly decrease
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Unread postAuthor: POLAND_SPUD » Wed Feb 17, 2010 5:51 am

I'm always afraid the the cylinder will bolt will begin to open while it's firing


if you use a single acting air cylinder the pressure inside the air cylinder is the same as the pressure that acts on the bolt to push it back (in fact it's slightly higher due to the fact that the air cylinder and the chamebr are joined via a 2mm ID tubing)

so assuming that the area of the bolt exposed to the pressure of air upon firing is the same or smaller than the area of the air cylinder it won't open

of course, there is also friction and inertia at work if you take that into consideration you'll see that this isn't a problem...


in case of double acting cylinders this isn't as simple to understand but it works too. Air can't instantly rush from the air cylinder to the chamber and from the DCV valve to the other side of the air cylinder and again there is friction and inertia at work too

I hope this helps.

I wish to use a cartridge design that depend on the bolt being firmly pressed against the cartridge while firing

that shouldn't be a problem.
Even if you use an air cylinder with really a small bore piston when compared with the bolt area exposed to pressure upon firing you can always put one of those one way check/flow restricting valves that would allow full flow the the air cylinder but restrict flow from the air cylinder to the chamber



(ohh BTW I don't know if you noticed that your design on this page will have problems with it this... )
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