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Semiauto WIP - blowback-study ...with homemade dcv

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 » Wed Feb 16, 2011 1:01 pm

In the Tech-QDV-style, the spool is moved away from the port, that has to be exposed to pressure, causing the pressure to be able to push on one side of the spool additionally to the force, which moved the spool at first, so it is not balanced anymore.
Was this understandable
Yeah, perfectly - hence
It should work like that (meaning open faster) at higher pressures

Spool valves are typically balanced, but not always. In this particular application it makes sense to have a valve that becomes unbalanced as soon as it opens. In hydraulics it would be a problem
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Unread postAuthor: Gun Freak » Wed Feb 16, 2011 8:59 pm

Looks very nice so far! Looking forward to seeing the final product.
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Unread postAuthor: nature-boy » Fri Feb 18, 2011 7:33 am

POLAND_SPUD wrote:In this particular application it makes sense to have a valve that becomes unbalanced as soon as it opens.

That's why I chose it over a balanced one. :)

@jhalek and Poland_Spud:
Thank you for your ideas for slowing down the bolt, but I have to admit that I don't really understand them.
Here is a diagram of the current blowback (I told you it's nothing fancy JSR...), feel free to add your idea's into it.

And Poland_Spud, the return stroke is not the problem, because the mag-port is already open at this point.
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Unread postAuthor: jackssmirkingrevenge » Fri Feb 18, 2011 8:02 am

nature-boy wrote:Here is a diagram of the current blowback (I told you it's nothing fancy JSR...), feel free to add your idea's into it.


Seems I'm of the same mind, but my idea was to eliminate the whole QEV/DCV setup by using an airtight o-ring detent which would remove the need for a fast valve.

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Unread postAuthor: nature-boy » Fri Feb 18, 2011 8:35 am

jackssmirkingrevenge wrote:my idea was to eliminate the whole QEV/DCV setup by using an airtight o-ring detent which would remove the need for a fast valve.


I even thought about using a o-ringed-bolt for the valve AND the loading mech. ...in fact that looks very similar to yours, only thing I have the seals on the bolt, and it would be semi-auto, yours full-auto.
(you would have to bleed air trough a pinhole into the chamber for it to work).

...ou..I forgot the manual-valve-actuator-rod in the diagramm
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Unread postAuthor: jackssmirkingrevenge » Fri Feb 18, 2011 9:58 am

That could work well, reminded me of a design I had thought up before I knew how "strafer" BBMGs worked, it looked something like this with air and BBs feeding through the same port:
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Unread postAuthor: POLAND_SPUD » Fri Feb 18, 2011 2:40 pm

And Poland_Spud, the return stroke is not the problem, because the mag-port is already open at this point
well if it isn't then just use a stronger spring...
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Unread postAuthor: nature-boy » Sat Feb 19, 2011 8:08 am

POLAND_SPUD wrote:well if it isn't then just use a stronger spring...


Jack already suggested that, but it won't move anymore at all when I do this, I already tried.

btw, just a thought: have you ever run tests at your qev-dcv-aircylinder-guns, if that happens there too, that the mag port opens, before the projectile left the muzzle? Because at the point where the pressure in the single-acting-cylinder and the barrel are the same (or the FORCE, in case your cylinder has a larger cross-area as the barrel) the spring will start pushing it back. The only question is, if the projectile is still in the barrel at this moment.
....just realized, that even if that would be the case, you can simply decrease the airflow to the cylinder to prevent that happening... :roll:
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Unread postAuthor: al-xg » Sat Feb 19, 2011 8:40 am

Yeah you'd need a slightly larger piston, just enough to counteract the return spring. Th problem with reducing the flow it that it limits the ROF. Chamber filling time was the main problem on my prototype. Getting suitable power from a large enough bore at the right ROF is tricky without very high pressure.

What you could do (and that has been done successfully) is use a forward sliding blow-through bolt instead. Then a tiny air ram can be used, but then dead space will be larger, although that doesn't always affect performance that much. Or even have the whole barrel move back, but that would probably get crazy in full auto :).
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Unread postAuthor: POLAND_SPUD » Sat Feb 19, 2011 8:56 am

just a thought: have you ever run tests at your qev-dcv-aircylinder-guns, if that happens there too, that the mag port opens, before the projectile left the muzzle? Because at the point where the pressure in the single-acting-cylinder and the barrel are the same (or the FORCE, in case your cylinder has a larger cross-area as the barrel) the spring will start pushing it back. The only question is, if the projectile is still in the barrel at this moment.
....just realized, that even if that would be the case, you can simply decrease the airflow to the cylinder to prevent that happening
lol you've already answered your own question :D

To some extent it does work like delayed blowback - since the air cylinder is blocked in extended position for a while
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Unread postAuthor: nature-boy » Sat Feb 19, 2011 12:28 pm

POLAND_KITTY wrote:lol you've already answered your own question :D

I know :roll: :)
Is the thin plastic pneumatic-tube/hose going to your air-cylinder enough flow-restriction or did you additionally slowed down the cylinder with a extra-valve in that line?

al-xg wrote:What you could do is use a forward sliding blow-through bolt instead. ... Or even have the whole barrel move back.

Yeah I thought about it. In that configuration only the pressure in the cylinder will have an effect on the cylinder's spring-driven back-movement, causing it to move back later. The air-cylinder-moving-barrel was done by PVC ARSENAL 17 in his "RSP-Skeleton" if I remember right. (I just noticed that the pictures in his thread are not working anymore, so I can't be sure.)
Another option I came up with would be to have a locked breech, where the loading bolt is separated from the cylinder-piton.
The loading bolt would be locked until the cylinder-piston moves back, unlocks the bolt and pulls it back.
That would have the same effect of the barrel-pressure not playing a role anymore in the forces that act onto the breech.
I even drew a diagram of a locked breech, that could be done with the limited tools I have at hand, but it would definitely be fiddly, and I'm not sure if it's worth it.
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Unread postAuthor: POLAND_SPUD » Sat Feb 19, 2011 1:31 pm

Is the thin plastic pneumatic-tube/hose going to your air-cylinder enough flow-restriction or did you additionally slowed down the cylinder with a extra-valve in that line?
yeah probably it does restrict the flow. Not that it is needed...

The air cylinder is connected to the chamber. You have to vent the chamber first to vent the air cylinder. It takes about 9ms for the bb to leave the barrel. the moment it leaves the barrel the chamber pressure is still greater than 50% of original pressure. Which means that the pressure in the air cylinder is even higher

I don't have a high speed camera so I can't check it but I guess that the air cylinder doesn't move at all for the first 15 - 50 ms.
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