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constant flow full auto :)

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: PVC Arsenal 17 » Wed Jan 07, 2009 3:21 pm

jackssmirkingrevenge wrote:I'm quite confident it will, it does move a lot of air - you can see the blowgun recoiling in the video - and we're talking 1/4" porting for a 6mm barrel.


Is that thing exhausting out the front? That'd be pretty convenient if it did! I guess it would have to if it's recoiling.
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Unread postAuthor: Sticky_Tape » Wed Jan 07, 2009 3:24 pm

I say make a 1/2'' shell ejector with a homemade barrel sealing piston 1/2'' with a spring behind the piston sortof like a homemade qev. I found out that those (I think marksman) glass sling ammo fits awesome in 1/2'' type M which would be arsome in full auto.
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You can tell how awesome a cannon is by the pressure used.
http://www.spudfiles.com/forums/high-pr ... 12803.html
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Unread postAuthor: iknowmy3tables » Wed Jan 07, 2009 6:07 pm

so the problem is that since the valve is made to relief excess above a certain amount with maybe a little extra depending on the flow and time it takes to reset in addition each burst get significantly reduced by the blow forward breech

so in order for a better auto valve you need it to stay open longer and dump more produce a higher pressure drop, what if you you had a spring loaded piston with a plug type seal that produced a lot of friction moving in the opening direction but very little in the closing/reseting direction, but in addition you put it on the back of the blow forward bolt
Image
it's basicly like brian's old sheet rubber and washer sandwich type seal, I've tried them and depending on how you make it you can get that very same friction effect. typically to make a good seal I would put it on backwards from the way it is in the diagram, but I'm wondering since the piston in this is free to move forward during filling and the majority of the surface area on the bolt and not loose rubber could the air pressure push the bolt forward thus press against the seal, or would presure always push it's way around the seal
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Unread postAuthor: CasinoVanart » Wed Jan 07, 2009 6:51 pm

Looking gooood Jack, hurry up and have a play!!
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Unread postAuthor: jackssmirkingrevenge » Thu Jan 08, 2009 2:20 am

I say make a 1/2'' shell ejector with a homemade barrel sealing piston 1/2'' with a spring behind the piston sortof like a homemade qev.


This is sounding like the more attractive option at the moment, something like the drawing below but replacing the sprinkler valve with a pop-off piloted piston valve.

Image

Is that thing exhausting out the front? That'd be pretty convenient if it did! I guess it would have to if it's recoiling


It does lose some air through the top but it mostly exhausts through the side ports. Capping the top also affects performance, as I said before the system is very sensitive to change and needs some very fine tuning.

so in order for a better auto valve you need it to stay open longer and dump more produce a higher pressure drop, what if you you had a spring loaded piston with a plug type seal that produced a lot of friction moving in the opening direction but very little in the closing/reseting direction, but in addition you put it on the back of the blow forward bolt


Friction is a killer in this case as far as I'm concerned. You want the least friction possible so you can use as weak a spring as you can, that will fly forward with ease then return only when the pressure has gone down completely. This is essential because with a stronger spring, the bolt will tend to flutter as opposed to completely simple back/forward motion.
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Unread postAuthor: iknowmy3tables » Fri Jan 09, 2009 4:01 pm

jackssmirkingrevenge wrote:so you can use as weak a spring as you can, that will fly forward with ease then return only when the pressure has gone down completely. This is essential because with a stronger spring, the bolt will tend to flutter as opposed to completely simple back/forward motion.

I guess I was a little unclear because I thought the major problem was the valve, but reading again I see that you feel the bf bolt to be more of the problem, but the valve is pretty limited so it could use improvement and design was suppose to act as both a valve and a bolt. please note that high friction is only when the bolt moves forward because when the rubber pushes against the walls it due to the wider surface instead of bending back it spreads, this will means that it would make the opening pressure higher without using more spring tension, because spring tension causes the piston to close prematurely
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 10, 2009 1:27 am

I understood your idea, my worry is that the bolt would not have returned to its "rest" position before the pop-off valve actuates for the next shot, as any sort of friction in the breech cycle introduces a delay.

I'm considering making your versionof bolt though, worth trying out :)
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Unread postAuthor: Ragnarok » Sat Jan 10, 2009 6:40 am

jackssmirkingrevenge wrote:I'm considering making your versionof bolt though, worth trying out :)

I'm not so sure that's a good design - after it's opened, the force from pressure (ignoring flow losses) will be equal in both directions on the bolt, so it will snap back under spring force.

So then you'll have a farting breech instead of a farting valve.
Really, the blow forward piston needs uneven pressure forces to hold it forward.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: jackssmirkingrevenge » Sat Jan 10, 2009 12:47 pm

Ragnarok wrote:I'm not so sure that's a good design - after it's opened, the force from pressure (ignoring flow losses) will be equal in both directions on the bolt, so it will snap back under spring force.


It's actually essentially the same as my design, only with the bolt never leaving the rear tube - and it works quite well :) I made a ghetto magazine to go with it, now that I've got so far it deserves a few runs on automatic :) videos to come soon-ish.
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Unread postAuthor: Leonard » Sat Jan 10, 2009 2:40 pm

Im going to put my two cents if you guys dont mind.

I have some experience with pop-off piloted pistons (i must have made five or six different prototype working on almost the same principle and had five or six different problems ^_^). The most successful set-up i came up with is a blowforward bolt fed by a pop-off piloted piston.

A finely tuned blowforward bolt with maximum flow would always close before the next burst of air, and im talking around 5-8 RPS. It's not all that fast, but i bet with a strong spring and a small chamber you could make it cycle very fast.

Howhever, i found out that the blowforward bolt would make it harder for the piston to actuate (?!) AND reduce power significantly. But it's reliable and easy to build. And one last thing, the bolt needs to be UBER strong, most of the problems i had was the bolt breaking when auto-firing.
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Unread postAuthor: iknowmy3tables » Sun Jan 11, 2009 8:04 pm

jackssmirkingrevenge wrote:I understood your idea, my worry is that the bolt would not have returned to its "rest" position before the pop-off valve actuates for the next shot, as any sort of friction in the breech cycle introduces a delay.

I'm considering making your versionof bolt though, worth trying out :)

there is no pop off valve, the bolt acts as a pop off valve
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 11, 2009 11:40 pm

iknowmy3tables wrote:there is no pop off valve, the bolt acts as a pop off valve


No, there is no spoon. But I made your version of the bolt and used it in conjunction with the pop-off, take a look;)
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Unread postAuthor: iknowmy3tables » Mon Jan 12, 2009 3:07 pm

okay but what do you think about making an auto piston that takes advantage of one directional friction, you can think of it as a roller-delay blowback mechanism only the rollers and grove are replaced by the rubber and washer assembly
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Unread postAuthor: jackssmirkingrevenge » Mon Jan 12, 2009 11:53 pm

Given my experience with blow forward valves, I don't think it would make a difference if used alone - as in, it wouldn't give you effective full auto.

It would improve performance if used as a semi-auto reloading breech though due to the greater pressure buildup.
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Unread postAuthor: iknowmy3tables » Tue Jan 13, 2009 3:43 pm

thats not going to makes sense if you keep thinking of it as a bolt and not a pop-off valve mechanism, if I modded the design wouldn't it at least work as a stand alone valve? if not why

edit:
okay normally for relief valves the relief pressure is dependent on the spring tension and only a small amount from friction, so ideally the valve is just suppose to just slowly and continuously bleed excess pressure above the spring tension, the popping affect only due to static friction in the seals, momentum in the piston and the elasticity of air, thus standard popoff valves will relive no more than 20psi of the input pressure. the opening pressure of the valve is determined by summing the forces holding the valve shut which would include spring tension and friction, to determine the pressure at which the valve will ideally you just add up the spring tension and the friction in the seal.
Image
with this design since friction is the dominant force in holding the seal shut instead of spring tension and is only very high when opening the opening pressure is much higher then closing pressure thus the valve is much more powerful
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