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custom auto 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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Unread postAuthor: POLAND_SPUD » Tue Jan 20, 2009 5:54 pm

hmmmmmmmmmmmmmm (thinking)

why don't you take a QEV and replace its piston to build a full sized high pressure verison of this ??

should be fairly easy... and you could avoid using epoxy (correct me if I am wrong)
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Unread postAuthor: Ragnarok » Tue Jan 20, 2009 6:08 pm

POLAND_SPUD wrote:and you could avoid using epoxy (correct me if I am wrong)

JSR? Avoid using epoxy? Surely that's impossible...
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Unread postAuthor: Brian the brain » Tue Jan 20, 2009 6:09 pm

He is máde of the stuff.
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Unread postAuthor: Dave_424 » Tue Jan 20, 2009 6:49 pm

Either that or he was born inside of it and had to slowly chip himself out with a specially designed and concealed beak :P
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Unread postAuthor: Lentamentalisk » Tue Jan 20, 2009 7:35 pm

the reason why you can't just mod a QEV to do this is that the important part is the ratio of the piston area to the area of the face when the piston is set on the pipe. QEVs generally don't have nearly as good of a ratio as this. The other issue is that manufactured QEVs don't have an air tight seal between the front and back of the piston, like this does.

edit: wait! Forking Genious! just turn a QEV backwards. Have the part that usually is attached the the barrel be attached to the chamber, and the part that goes to the chamber usually, goes to the barrel. Then in the pilot port, put and adjustable spring, to vary the cracking pressure.
You will need to do some serious mods on the QEV to change the surface that the piston touches, but that could work!
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Unread postAuthor: inonickname » Tue Jan 20, 2009 7:43 pm

Jsr never stops..watch as soon as he perfects the design, then half of spudfiles does exactly the same thing. Eg: pen gun. I think I have a syringe somewhere, so I might quickly try this design and fit it into my pb gun. Kudos for the pneumatic spring, I can never get a decent spring in the right size.

Good work :wink:
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Unread postAuthor: POLAND_SPUD » Tue Jan 20, 2009 8:08 pm

yeah that's what I meant... the moment I saw JSR's pics I thought 'ohhh so he spent several years building a reversed QEV' :lol:

and yes I know that pistons in QEVs are made to act as one way valves... that's why I mentioned that they would have to be replaced...

I don't really see any reason for not using an air spring here... I suppose it would be needed for easy adjustment...

we would have to wait for JSR to comment on that... but I don't see any reason why it won't work
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Unread postAuthor: Gippeto » Tue Jan 20, 2009 10:09 pm

Can actual "taka-taka-taka" be on the horizon?

Seems like it is but a pour and cure (or two) away at this point.

Kuddos Jack. :)
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Unread postAuthor: Hotwired » Tue Jan 20, 2009 10:22 pm

POLAND_SPUD wrote:
I actually suspect it's the friction of the syringe plunger that's keeping the ROF down
silicone oil should help with that...


Well no, you don't necessarily want a higher rate of fire, I'm saying that because of the resistance of the plunger in moving at all it has a longer dwell time.

If that was a slick and well lubricated PTFE plunger I'd bet the dwell time would be rather shorter

A useful product of a longer dwell time is more gas per burst. But if it's derived from friction you also get heat and wear.
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Unread postAuthor: Lentamentalisk » Tue Jan 20, 2009 10:36 pm

make the whole system water cooled, like the pros :D
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Unread postAuthor: POLAND_SPUD » Tue Jan 20, 2009 10:46 pm

A useful product of a longer dwell time is more gas per burst
or longer dwell time means that air is wasted when ammo has already left the barrel.....
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Last edited by POLAND_SPUD on Tue Jan 20, 2009 11:04 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Tue Jan 20, 2009 10:54 pm

POLAND_SPUD wrote:
A useful product of a longer dwell time is more gas per burst
or longer dwell time means that air is wasted when ammo has already left the barrel.....

There's a fine line somewhere, it will take awhile to figure it out so JSR may as well just lube it and see what happens. If it's worse, take the lube off. Or if you increase the barrel length...
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Unread postAuthor: Hotwired » Tue Jan 20, 2009 11:12 pm

POLAND_SPUD wrote:
A useful product of a longer dwell time is more gas per burst/quote] or longer dwell time means that air is wasted when ammo has already left the barrel.....


You can have a low ROF and still have a very short dwell time.

The piston in that instance would snap open and shut with a relatively long recharge time.

A reason for increasing the weight of the hammer in hammer valved pneumatics is to increase the time the valve is knocked open for and so increase the force on the pellet by means of a larger volume of gas being behind it. This isn't a hammer valve but it has the same capability to increase or decrease valve dwell time.

The gas is never wasted, it still increases the energy of the pellet. Otherwise why are we going round with cannons using massive air chambers irregardless of the pressure?

If it was a commercial design then yes, there are acceptable limits for the use of gas per shot.
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Unread postAuthor: jackssmirkingrevenge » Tue Jan 20, 2009 11:37 pm

Thanks for all the feedback gentlemen :)

I'm not too keen on adding lube, for starters it's all sealed up so it would have to be added through the schrader, but more importantly I would be afraid that the seal would absorb some of it, expand and just gum up.

I'm not too concerned about cooling, the air has quite a cooling effect in itself and the piston tube doesn't get hot at all, and besides I won't really be using this for long bursts anyway.

I'm not sure if it's friction keeping the ROF down. I mean, if I take a similar syringe, block off the nozzle with my thumb and pull the plunger back, in spite of the friction it will return to its origial position with alarming speed if I let go of the plunger, and that's with 14.7 psi behind it. In my design, the air spring is at around 15-20 psi.

For these pop-off designs, ROF is controlled simply by the rate of air entering the chamber - the faster the chamber reaches pressure, the faster it will pop the piston.

Do you think it could be stepped up to a paintball size? or would the air required just be too ridiculous?


Sure, it can be done, but you'd need a large port - say one inch - and a piston twice that size to give you decent power, so it would be quite a substantial device.

What happens behind your piston, as in, do you need to re-adjust the balast pressure every once in a while? I would think it would vary, seeing as almost nothing is 100% airtight


I haven't used it enough to know but I imagine that yes, it would need regular top-ups.

As to using a QEV with a reversed piston as Lentamentalisk suggested, it could very well work - however, it's important that the diameter of the piston is significantly bigger than the seat. I would also be inclined to use a coil spring, it takes the bother of pressurising the rear out of the equation.

and you could avoid using epoxy (correct me if I am wrong)


HERESY! - but yes, that would be cool, because it would make it within reach of most builders who seem reulctant to use epoxy.

I'll have a play with it after work to see what the power's like for single shots and if it's worthwhile, work on a reloading mehanism.

I do by the way apologise for the completely sloppy workmanship, I made this in a half hearted "let's see if this works but it probably won't" mood and in one epoxy "session", ah well.
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Unread postAuthor: POLAND_SPUD » Tue Jan 20, 2009 11:45 pm

the piston is significantly bigger than the seat
QEVs have significantly bigger piston than the seat... do you happen to have one already ?

(and It was my idea to use a QEV not lentamentalisk - yes I want to get all credit for it - hell you can even call it poland valve :D )

fullautos are not exactly what I aim at but kudoz for making it work
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