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Secondary QEV for Breech Action

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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Secondary QEV for Breech Action

Unread postAuthor: Backyard Pneumatic » Sun Aug 18, 2013 4:02 pm

Hi All, I've been working on a semi-auto airgun and I have a few design questions. It works on many of the same principles as the gun in wyz's semi auto tutorial video (thanks, by the way), however there are several modifications. I would like to use it to shoot nerf darts, but this makes the build more difficult. because nerf magazines are not airtight, I do not have the option that most people use: an airtight hopper which is then fed using a "bumper" which does not seal against the barrel and is air cylinder actuated. Instead, I plan to use a sort of bolt action breech, which seals off the barrel from the magazine and is externally actuated by the air cylinder.

because this design is more linear, I also hope it will improve air flow.

The main issue with this design is that one the gun is fired, as soon as the pressure in the chamber begins to drop, the air cylinder piston moves, and the breech cracks open, letting air escape before the dart leaves the barrel.

My solution to this problem would be a second QEV, which would be placed on the input of the air cylinder. It would be oriented so that the air cylinder would be connected to the pressure input, and the pilot area would be connected to the pressure chamber of the primary QEV. the output of the secondary QEV could be attached to a muffler, as shown, or rerouted to the primary chamber, to refill it part way. In theory, the air in the cylinder will only be exhausted via the QEV once the pilot area pressure has approached zero (if the diameter ratio of piston to output is very close). If the C:B of the gun is correct, the dart will be exiting the muzzle just as the chamber pressure (and therefore the pilot pressure) reaches zero, meaning that the breech will not open until the projectile is fired, and no air will be wasted. A spring would be used in the second QEV to make it semi-auto capable and fine tune the threshold on which the QEV opens.

hopefully the attachment photos worked. I think the diagrams I drew should help explain.

any comments and insights are welcome. Thanks!
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Attachments
smIMG_0906.JPG
the breech is constructed from 9/16" and 17/32" brass tubing
smIMG_0912.JPG
the whole system. note how the secondary QEV is oriented

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Unread postAuthor: wyz2285 » Sun Aug 18, 2013 4:16 pm

The secondary QEV will fire as soon as the primary QEV goes off, because the piston moves even with a slight pressure change.
I have solved the cylinder retract prematurely, by adding a flow reduce valve, but only reducing the flow from one direction, makes it retract slower. Make the bolt a couple mm longer and the problem is solved, nerf darts are light, you only have to keep the bolt sealed for a couple ms so...
If I were you I wouldn't bother with that at all, 20 psi will make the darts fly better than the best nerf gun on the market, the barrel pressure will be even lower. In the tutorial the magazine, valve/barrel connection non of them are airtight and I was shooting steel balls only with 150 psi
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Backyard Pneumatic » Sun Aug 18, 2013 5:52 pm

Ok, yeah, I had considered a mechanism similar to the one you described, and I even bought a flow control. the issue with that though is that the longer the bolt mechanism, the shorter the barrel length I have available, because the 9/16 brass I have is only in 1' lengths. I figure I could build some kind of coupler so that I could do that and still have a barrel longer than 3". I am planning to run around 20 psi, off of a regulated chamber I made of 4" diameter PVC, and it probably wont make a big difference, but JSR instilled in me a quest for maximum efficiency. I'll try the method you suggested and see how it works. Thanks!
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Unread postAuthor: wyz2285 » Sun Aug 18, 2013 6:49 pm

A commercial flow control won't work well as it limits the flow from both directions and you only want to slow the the flow when air exit the cylinder. If flow if reduced on both directions, it will take longer for the cylinded to extend slowing the reloading unnecessarily. You only have to male the bolt a couple mm longer, something like 5mm... The short barrel means even less time for the dart to exist from it, I'm pretty sure it will work without doing any of these extra you're thinking about.
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Gun Freak » Sun Aug 18, 2013 8:10 pm

Why not restrict flow to the cylinder so that pressure can remain in it long enough to let the dart leave the barrel before it opens far enough to vent the breech? You can simply restrict the flow down to a pinhole. I don't think extra complexities are required here. Besides, the speed of the dart should be high enough for it to leave the barrel before any significant amount of pressure is lost through the breech...
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Unread postAuthor: wyz2285 » Mon Aug 19, 2013 2:57 am

Gun Freak wrote:Why not restrict flow to the cylinder so that pressure can remain in it long enough to let the dart leave the barrel before it opens far enough to vent the breech? You can simply restrict the flow down to a pinhole. I don't think extra complexities are required here. Besides, the speed of the dart should be high enough for it to leave the barrel before any significant amount of pressure is lost through the breech...

That's exactly what I have just said lol
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Gun Freak » Mon Aug 19, 2013 5:07 am

I didn't bother reading the thread at all :lol:
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