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Concentric auto-cocking spool / piston hybrid valve

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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Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: mrfoo » Sat May 02, 2015 2:02 pm

So, been toying with this idea for a while. Elements of the balanced spool quick-dump valve with a QEV-style piston, incorporating a trigger mechanism. Should autocock (and, if you hold the sear away, go full-auto ratatatatatatatata).

Here it is cocked and ready to go. The spring exerts pressure on the piston, pushing it forwards, but the sear holds it in place.

Image

Here we've triggered the mech - the piston springs forwards, and the rear spool allows venting of the pilot volume via annular holes.

Image

From here on in, it's a standard QEV - the piston is propelled violently back, dumping air into the barrel.

Image

the pressure then builds in the pilot volume either via leakage around the piston or a hole drilled through it, and the spring can drive us back to the cocked state (or, if the sear is held open, to the venting state, and full auto)

Apart from being crap at drawing, have I missed something here?
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: cammyd32 » Mon May 04, 2015 3:50 am

The design looks really solid for a semi auto, or single shot.
But I think that the design might have some severe balancing issues if you try and get it to run full auto.
A short stroke would probably mean it would oscillate so fast it would be unusable,and a too long stroke would mean it wouldn't cycle or kick back properly.

However if you are confident you can tune it right, it looks like a really promising design, definitely would be one to machine though.

My word of advice would be to design a system which allows you to modify spring pressure and gas flow rate easily without having to take it apart, allow you to fine tune it a lot easier.
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: ToasT » Mon May 04, 2015 5:03 am

Cool design, I have often thought about mixing springs and pistons but never made anything.

I wonder if you would get an air spring effect reducing the valves power when the vent reseals as the piston begins to move back? (between 2nd and 3rd diagrams)
Although this will probably be easy to work around with some tuning.
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: mrfoo » Tue May 05, 2015 10:39 am

ToasT wrote:I wonder if you would get an air spring effect reducing the valves power when the vent reseals as the piston begins to move back? (between 2nd and 3rd diagrams)

I think the answer to that is "yes", but I'm not sure how bad (or, indeed, good) it will be - it's gonna depend on how much air gets vented out of the pilot volume, and probably on a whole host of other variables - chamber pressure, piston and barrel diameters, and so on. If things work out as I hope, I think it will work to my advantage, and stop the piston slamming into the back of the chamber. Knowing my luck, piston and chamber seal will come flying loose on the first shot :)
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: ToasT » Wed May 06, 2015 5:29 am

It sounds like you know what you are talking about. Hopefully you are able to make this and get it to work.
It will make an interesting cannon or gun.
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: mrfoo » Sat Jul 02, 2016 1:27 pm

Got back onto this today. Only - ummm - 13 months later.

Busy turning valve parts from some blocks of teflon recovered from work. Photos later this weekend, with some luck.
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Re: Concentric auto-cocking spool / piston hybrid valve

Unread postAuthor: Gun Freak » Thu Jul 07, 2016 10:39 pm

Excited to see the progress on this!
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Re: Concentric auto-cocking spool / piston hybrid valve

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