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semi auto piston 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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semi auto piston valve

Unread postAuthor: keep_it_real » Thu Jun 05, 2008 7:40 pm

I finished a semi automatic piston valve. The power is not good (probably 100 fps at 100 psi) but the concept works and I think with a couple changes I think I could get near 300 fps. The huge potential in this design is simplicity. The bolt both fires and loads which I'm a big fan off due to reliability.

Problems
Most of the air dumps out the pilot for these reasons (tell me if this sounds right):

1. The piston weighs a ton because it has to move the whole barrel along with it. Not very efficient but a major part in simplicity. Need suggestions...

2. The piston's diameter is too close to the diameter of the barrel. This was discussed in some other thread. I think we decided that the a small difference in diameter means it's less likely that the piston will open but if it does, it will happen faster. This problem is fixable with a bigger piston.

When you fire, there's a huge delay between when the pilot dumps and the gun shoots. This could be solved with springs pulling the barrel back but I think there are deeper rooted problems. The next design will have a bigger piston unless you guys have some different suggestions.
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Attachments
IMG_1387.jpg
close up of the valve. the cpvc tee is where you attach a clip or hopper. it shoots paintballs by the way
IMG_1391.jpg
the whole gun with a temporary tank on the bottom.
semi piston valve.png
semi piston valve.png (17 KiB) Viewed 1145 times

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Unread postAuthor: drex » Thu Jun 05, 2008 8:13 pm

thats very unique i look forward to seeing this thing develop in to a more powerful and efficient monster. :D
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Unread postAuthor: Hubb » Thu Jun 05, 2008 9:33 pm

You could make a lightweight sleeve to connect to the piston instead of the whole barrel. The sleeve would slide over the barrel.

This would make it much lighter (faster opening time) and make the barrel stationary (better accuracy).

It is a good design. It's sort of like an open bolt launcher. Nice job.
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Unread postAuthor: POLAND_SPUD » Thu Jun 05, 2008 10:51 pm

do I get it right? the valve fires through those holes?
If it's true then my question is - are they big enough?

I think that hubb017 is right it will help but first check if the holes are not too small
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Unread postAuthor: keep_it_real » Fri Jun 06, 2008 12:08 am

The holes should be big enough. Right now, it's not flow constriction that's the problem. The problem is delay.

I think sleeving is the best solution but the goal is to keep it simple. I'll tinker around with a new design and try it out.

Does anyone think the piston should be bigger?
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Unread postAuthor: Hubb » Fri Jun 06, 2008 12:15 am

Definitely. I didn't mention making the piston larger because you did. Also, make it as light as possible. There really isn't much you can do about piston travel, due to the ammo and all, so that's about all I could suggest.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 06, 2008 12:44 am

hubb017 wrote:It is a good design. It's sort of like an open bolt launcher. Nice job.


Seems a little familiar, n'est pas?

Image

props to KIR for making it work, some videos would be lovely :)
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Unread postAuthor: clide » Fri Jun 06, 2008 12:52 am

Galfisk from the Spudtech forums built a similar mechanism with the bolt and piston linked.

Planning stages: http://www.spudfiles.com/spudtech_archi ... hp?t=11200
After it was built: http://www.spudfiles.com/spudtech_archi ... hp?t=11396

Unfortunately the pics are broken.
Copperboy made a larger version of basically the same mechanism Galfisk used. Pics work on this one.
http://www.spudfiles.com/forums/marble- ... 10142.html


As for taking so long to fire. The closer your seat is to the side of your piston, the better your piston fit needs to be in order to not leak a bunch of air from the chamber out of the pilot. So yeah making the piston bigger would be one way to fix it, but it may also be able to be fixed if you can figure out a way to put o-rings on the piston. Although given the extra weight and friction on the piston because of the bolt/barrel it may be better to just make the piston bigger.

Nice work on the gun.
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Unread postAuthor: keep_it_real » Fri Jun 06, 2008 12:58 am

Seems a little familiar, n'est pas?


I was wondering why I hadn't seen something similar. Apparently I'm not keep up with the Jone's these days.

props to KIR for making it work


The delay needs to go before I can say it works. Every once in a while I'll get a shot out of the blue that goes fast. It makes me very happy. I'll take some video tomorrow. JSR do you have any suggestions for improvements?
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 06, 2008 1:03 am

keep_it_real wrote:JSR do you have any suggestions for improvements?


You definitely need a bigger diameter piston, also instead of moving the whole barrel, consider just moving a bolt as in the diagram above.
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Unread postAuthor: keep_it_real » Fri Jun 06, 2008 1:12 am

I'm assuming your diagram is in a closed position.

When it fires the bolt is back. Wouldn't that let air into the clip? Or would it have let out all the air by then?
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Unread postAuthor: Hubb » Fri Jun 06, 2008 1:21 am

Seems a little familiar, n'est pas?


I thought of that when I saw this thread. :lol:

When it fires the bolt is back. Wouldn't that let air into the clip? Or would it have let out all the air by then?


In the diagram, it's theory works just like your design. As the piston moves back, the bolt moves with it. The magazine is sealed by the front of the bolt, allowing air to only exit the barrel.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 06, 2008 3:09 am

As the man says:

hubb017 wrote:In the diagram, it's theory works just like your design. As the piston moves back, the bolt moves with it. The magazine is sealed by the front of the bolt, allowing air to only exit the barrel.


I'd draw it quickly in the open position but I have four simultaneous assays distilling at 200ºC to look after :? maybe when I get home.
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Unread postAuthor: THUNDERLORD » Fri Jun 06, 2008 10:16 am

Yeah, I was going to say bigger piston and sleeve.
Since your piston is travelling the same distance as diameter(round shot)

But I was certain some more knowledgable members would pick up on this interesting concept.

Dang it's good to see JSR back!!! :wink: 8)
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 06, 2008 10:59 am

Good to be here I can tell you :) at least now I have a PC at home I can browse through this forum without looking over my shoulder and pretending to work :)

Interesting to note that in spite of my hiatus, I'll still the top poster hehe
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