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pilot volume vs. chamber volume

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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pilot volume vs. chamber volume

Unread postAuthor: cfb_rolley » Thu Sep 09, 2010 6:39 pm

so im venturing out today to build my first pneumatic, a 40mm barrel sealing piston valve with a 30 mm barrel. i plan to have the barrel half way into the tee where the piston will meet the back of the barrel. from what i have learned, if i have the piston move back 15mm, that should be plenty open enough. my chamber will hold between 600 to 750 cubic centimetres, and my pilot volume about 60, at 80 - 120 psi, my pipe is rated to 175 psi.

3 questions, will a blowgun be enough to dump the pilot volume? is my pilot volume too big? and what would the best way to install a schrader valve as a fill valve?
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Unread postAuthor: velocity3x » Thu Sep 09, 2010 6:58 pm

I think a blow gun will (if it works at all) will provide a very slow full piston retract.

I have 6in^3 pilot volume and use a power actuated ball valve. The ball has an internal bore size of .630". A small pilot valve will seriously diminish the power produced in the barrel.
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Unread postAuthor: cfb_rolley » Thu Sep 09, 2010 7:14 pm

What if I used a larger chamber and/or added some filler to the pilot volume? I can get the pilot down to about half that I think
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Unread postAuthor: Gun Freak » Thu Sep 09, 2010 7:41 pm

Use a larger ball valve like a half inch or upgrade to a sprinkler valve pilot like I did here. A small pilot valve can be used if your piston has a built-in backflow-preventer, also known as a check valve. You can see an example in the link above...
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Unread postAuthor: saefroch » Thu Sep 09, 2010 7:44 pm

I believe there was a previous experiment with putting water into the pilot volume to reduce the total pilot volume at atmospheric pressure. It worked... but cause some safety problems. It sounds like you're using PVC, so I would avoid using a liquid to fill your pilot volume.

That said, if you can "fill" your pilot volume, be very careful not to restrict flow beyond the limitation already posed by your pilot valve. You want the valve port to be the limiting factor. Your pilot volume shouldn't be too large (if my reasoning in metrics is correct) to function well.

To install a schrader as a fill valve, drill a hole that fits the threads, but not the rubber part of the schrader. Then slowly use the same bit to widen the hole until you can get the first part of the rubber in. Then jam it into the hole (being sure you're putting it in the right side) as far as you can (schrader valves widen towards the base) and if you press it in well, it should seal to the PVC. Epoxy, as always, will help, but with my method is not needed.

And just a note, Ragnarok may comment on this as well, the "size" of a piston valve is measured by the diameter of the port of the valve, which in this case is probably the diameter of your barrel, so you have a 30mm piston valve, but it's just housed in a 40mm tee.
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Unread postAuthor: MrCrowley » Thu Sep 09, 2010 7:55 pm

On another note, it may be better if your barrel didn't go so far inside the tee. I would probably have it so it only rested 1/4 of the way over the bottom port, not 1/2. If you don't understand what I mean, at the moment you will have the barrel so it is halfway inside the tee. This will mean that it rests halfway over the bottom port of the tee (which goes to your chamber), I would suggest to bring it back a little bit so it only over-hangs 1/4 of this bottom port.

It wouldn't do much harm to have the piston go a little further back than 1.5cm as well, to account for piston bounce. If the piston travel was 4cm, it's not going to kill performance or anything. As long as you have a decent pilot valve, you'll be fine.
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Unread postAuthor: cfb_rolley » Thu Sep 09, 2010 7:55 pm

I see. Another option I thought of was using a qev actuated by the blowgun to exhaust it. I've heard of people modding blowgun valves too, but does that only work on smaller guns? To fill the pilot area I was going to use a round block of rubber with a hole in the centre that leads straight to the blowgun.

Edit: yeah I figured 4 or 5 cm is not a long way for the piston to puck up speed, plus having the rubber filler would probably cusion it anyway
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Unread postAuthor: saefroch » Thu Sep 09, 2010 8:05 pm

A QEV actuated by a blowgun would be a very good pilot valve. Just be very careful with that rubber... don't let the hole in the middle be any smaller than the port in whatever pilot valve you chose to use. I made that mistake and was puzzled for almost a week, then amazed at my own stupidity how I was choking my pilot valve.

Don't know about modding blowgun valves.
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Unread postAuthor: cfb_rolley » Thu Sep 09, 2010 8:12 pm

Thought so. Main reason why I was going to use a blowgun is because I already have 3 haha.

Gun freak, that's the same cannon that made me want to start building a pneumatic! That thing is awesome!
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Unread postAuthor: Gun Freak » Thu Sep 09, 2010 8:22 pm

Thanks dude! That is so awsome to hear!!!!!! :D :D

I modify all of my blowguns. There is a good tutorial in the Hubb Library. It is the best one I have seen. It can greatly increase the flow and it is a must if you will pilot large volumes with it. Just click on the Sprinkler Valve section and download it, open it up and it has its own section, I believe it is on page 16 if I recall correctly. It is a great tutorial.
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Unread postAuthor: MrCrowley » Thu Sep 09, 2010 8:25 pm

Though if you're using it to pilot a sprinkler valve/QEV which pilots the piston valve, there is no need to mod the blowgun.
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Unread postAuthor: Gun Freak » Thu Sep 09, 2010 8:26 pm

MrCrowley wrote:Though if you're using it to pilot a sprinkler valve/QEV which pilots the piston valve, there is no need to mod the blowgun.

That is true, but you could still do it for the fun! :P
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Unread postAuthor: cfb_rolley » Thu Sep 09, 2010 9:44 pm

Have to say I never really modded anything until I became a spudder... Now nothing is safe from my need to mod stuff!
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