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Piston actuation

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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Piston actuation

Unread postAuthor: Gun Freak » Sun Mar 07, 2010 11:41 am

Should a blowgun be a sufficient pilot for this design or should I go with a QEV?
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Unread postAuthor: psycix » Sun Mar 07, 2010 12:33 pm

If your piston is tight and you minimize pilot volume, then a blowgun is a good pilot valve.

With a setup like this, there will be a lot of pilot volume.

I don't see why you are absorbing recoil of such a small caliber. Imagine the extra barrel length (power!) you would get if you dropped the spring mechanism.
And even then, you won't need recoil absorption.
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Unread postAuthor: Hawkeye » Sun Mar 07, 2010 12:42 pm

Mount the blowgun directly onto the first T. Attach a street elbow to the rear of that T and have your fill sticking out the side of the gun.
You can get rid of a lot of pilot volume and fittings.
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Unread postAuthor: Gun Freak » Sun Mar 07, 2010 2:03 pm

Hawkeye, that is a good idea but I put it on the stock so it wouldn't blow back with the internals. And psycix, I know there is a small recoil but I think it will be cool. Besides, if I am going to model it after the Barret 50 ccal, then I might as well make it work the same way...
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Unread postAuthor: jackssmirkingrevenge » Sun Mar 07, 2010 4:32 pm

I see this obsession with large pilot volumes persists :roll:

Think how compact and efficient it would be with a bit of well applied epoxy, mounting the barrel in a threaded fitting would still allow the (also cast epoxy ;))piston to be servicable and by having the quick connect in the blowgun you remove the need to have a separate fill station.
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Unread postAuthor: Hawkeye » Sun Mar 07, 2010 4:57 pm

Sure it will still move back. The gun is just sliding within the stock tube so if there is a slot for the blowgun to move back in then you just need a comparable slot for the fill valve in the side. You'll just have to hang onto the blowgun a bit. Building a nice comfortable handle around it solves the problem.
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Unread postAuthor: Hotwired » Sun Mar 07, 2010 5:57 pm

jackssmirkingrevenge wrote:I see this obsession with large pilot volumes persists :roll:
If considered in the right light there is a lot of reason to. For single shot cannons I'd probably agree.

A large pilot volume is wasteful only if the pressurised gas it contains is released to atmosphere. Pressurised gas is gas with work yet to do.

Brian wants to get the primary gas blast to do the work of working the bolt in his auto design but it's fairly busy that end.

Meanwhile at the pilot end the gas is escaping freely.

I drew up plans for automatic mechanisms a while ago but making a strong locking bolt and operating it was too complex. But if a quick connect can be that part then























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Unread postAuthor: jackssmirkingrevenge » Sun Mar 07, 2010 6:05 pm

A large pilot volume is wasteful only if the pressurised gas it contains is released to atmosphere. Pressurised gas is gas with work yet to do.


Aside from the increased pumping effort, a large pilot volume means that pressure is decreasing at a lower rate and therefore the piston is opening slower, reducing performance.
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Unread postAuthor: Hotwired » Sun Mar 07, 2010 6:16 pm

To cycle a mechanism you have to steal energy from somewhere.

If you have the best performance benchmark being a single shot manually loaded design then any mechanical addition to that design will reduce performance.

If you don't make the trade there is no deal :)
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Unread postAuthor: jackssmirkingrevenge » Sun Mar 07, 2010 6:22 pm

Hotwired wrote:To cycle a mechanism you have to steal energy from somewhere


After the projectile leaves the muzzle there's a lot of air that's normally wasted, mechanisms like the one used by the FX Monsoon use no extra air to cycle the action compared to their bolt/lever action counterparts as the gas piston is effectively a suppressor with a "moving wall" so to speak.
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Unread postAuthor: Hotwired » Sun Mar 07, 2010 7:20 pm

I'm annoyed with myself.

Wasn't intending to return to designing cannons.

The gas ram is the wrong way round...
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Unread postAuthor: Gun Freak » Mon Mar 08, 2010 5:21 pm

@Hawkeye, I'll give it some thought if I do build the recoil reducing design.

Thanks jsr. Seems like all my guns have pilot volumes "sufficiently large to hold the contents of a blue whale's left bollock" :D I might just make a metal T piston or another coaxial. One question though... An online store said galvanized steel has a 150psi rating... Is it really that low? I might buy a shock pump so I want to know if a galvanized chamber would be sufficient for 300psi.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 08, 2010 11:47 pm

So this one will be made to hold the contents of the right one then ;)

I've used aluminium tubing from the inside of printer cartridges at 800 psi, you should be fine with galvansied steel of sufficient wall thickness at 300. If you're going to be using a shock pump, remember that you don't want your chamber to be more than a couple of cubic inches otherwise it will take too long to pump.

The good news is though that the more pressure you put in, the less chamber volume you need for equivalent power, have a look as this handy virtual model of how power varies with pressure and chamber volume.
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Unread postAuthor: Gun Freak » Tue Mar 09, 2010 4:31 pm

Thanks for the data. I was thinking about a 6" chamber of 1" galvanized... but I guess it would take too long to pump.
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Unread postAuthor: psycix » Wed Mar 10, 2010 6:48 am

Gun Freak wrote:And psycix, I know there is a small recoil but I think it will be cool. Besides, if I am going to model it after the Barret 50 ccal, then I might as well make it work the same way...

Could've mounted a longer, more powerful gun in the same space or you could've made the gun smaller by removing the useless recoil absorption.
Or, by making the barrel longer and the chamber smaller: a more air-efficient gun with the same power in the same package. Just by removing the recoil mechanism.

Recoil absorption is only usable in two cases: to make insanely heavy recoil bearable, or to dampen medium recoil for improved accuracy in rapid fire.
In your case, where neither apply, the recoil mechanism will just make it less accurate as the barrel isn't fixed to the stock. So much for you sniper rifle.

Well, its your loss.
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