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Piston valve not actuating....

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: King_TaTer » Wed Mar 18, 2009 12:16 am

I'm not sure if I'm seeing this correctly but are you sealing the tee to the pilot release with duct tape? Erm....... Correct me if I'm mistaken.

I wouldn't feel quite safe holding let alone operating that gun. If I were you I'd scrap the gun and look around this forum some more and gain some knowledge regarding piston valves.

Anyway it's a decent attempt creating a piston valve. Good luck in the future.
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Unread postAuthor: Metroid fan » Wed Mar 18, 2009 12:42 am

I'm only using duct tape on one place and I only got it up to pressures of 30 psi just to test, my dad is a aeronautic engineer and looked at it for a minute. He determined that the pilot volume hardly does anything for the gun's performance. (As stated previously) But I think that you are correct on the pistons barrel sealing end. I have zero area for the air to push back on and that's my main problem I believe. Sealing face entirely contacts the 1 inch coupler... So I'll test that out tomorrow! I'll keep you guys updated on progress of my apparently extremely ugly gun (really though, performance over prettiness. So long as the dang thing shoots I could care less what it looks like.) Thanks for all of the help Gippeto and Jook13!

P.S. Again, yes it's duct tape. No I'm not going to leave it there permanently it's for testing purposes, I can hear it leaking and whatnot. I'm only going to pressures of 30 psi. I'm not holding the gun while doing so. I will glue it in the future. And I will for sure not scrap this gun, I've put way too much time and effort into it to scrap it. I've put 25 man hours into this gun (mainly for researching piston valves, that was quite an experience!) and know mostly what i'm doing. Piston valves are not the easiest thing I've ever made. Especially since I didn't understand all of the physics behind them. I'm 18, I'm not a 12 year old kid with no brain. Again thank you guys for the help and the... Constructive criticism? Hopefully I will get this puppy up and running pretty quick.
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Unread postAuthor: jook13 » Wed Mar 18, 2009 1:05 am

As another side note, I have made a piston that would not fire at 30 psi but fired fine at about 50 psi and up. Try to do all you can to keep the efficiency up (ie. good fitting pistion, small pilot volume, quick piloting, ect.) Small factors can add up quick to equal failure.

Anyways goodluck!
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Unread postAuthor: Carlman » Wed Mar 18, 2009 9:06 am

Metroid fan wrote:my dad is a aeronautic engineer and looked at it for a minute. He determined that the pilot volume hardly does anything for the gun's performance. (As stated previously)


no offence to your dad and all but he knows squat diddly about the subject in that case.

pilot volume is directly proportional to performance and has been proved many times over.
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Unread postAuthor: Metroid fan » Wed Mar 18, 2009 12:05 pm

Um no. Look at the math, I see that you say a lot about how pilot size is proportional to performance. Have you looked at the physics behind it? Give me some equations and some tests to prove your theory. Honestly I can see how it could go either way, but I've been lurking on this forum for like 3 weeks looking at stuff and a lot of people are saying a lot about pilot size, but in GGDT I have changed the pilot size to gigantic amounts with very little change to the fps of a gun. Really, I'm willing to listen to anyone here but I have yet to see any proof of this pilot size statement. And by the way, I'm not saying my dad is one hundred percent correct. He just knows quite a bit more about physics and fluid dynamics than most people though and from what I'm seeing in GGDT he's correct. But really, I'm not trying to be ignorant, I just want to learn stuff like the rest of you guys. I've done quite a bit of research but I would love to be proven wrong if that's the case.
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Unread postAuthor: john bunsenburner » Wed Mar 18, 2009 12:18 pm

Well think about It, the larger the pilot voume the larger the vacume caused by the piston moving back, so that more air will be pulled backwards and thus will loose kinetic energy, and will, instead of going into the barrel go into the area infront of the piston.
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Unread postAuthor: mark.f » Wed Mar 18, 2009 12:33 pm

A barrel sealing piston valve, properly constructed, will pilot only at a certain pilot pressure. Pilot valve and volume are really only an issue when you're loosing too much air around your piston before this low pressure can be achieved.

So, if your piston seals exceptionally well, then the pilot volume and valve will not be that much of an issue. If it's wrapped in duct tape... then you better start worrying about it.

EDIT: <morehelpful>I would look at both the pilot volume, valve, and again the difference between the diameter of your piston and the outer diameter of the object the piston seals against. If they are too close, then your piston will only pilot at a ridiculously low pressure, and then actuate weaker than other pistons, if at all.</morehelpful>
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Unread postAuthor: jrrdw » Wed Mar 18, 2009 12:38 pm

You should be able to move the piston with your breath. Surface area around the outside of the barrel is needed for the stored air to push the piston back.

As for pilot size there is a 4D or D4 rule that states "basically" your piston only needs to move back 1/4 of the barrel diameter. It's a good rule of thumb for judging pilot size. Hope this helps. :D
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Unread postAuthor: Metroid fan » Wed Mar 18, 2009 12:45 pm

Now that makes sense. I can understand that air leaking around the piston before the pressure is reached would account for a drop in psi in the chamber. But depending on what that pressure was and how much volume and the size of the pilot valve and how fast the pilot valve actuated you could overcome that factor quite easily. I can see that the easiest way to do that would be to shrink the pilot volume. When I get this valve working I'll probably do some tests to check how much pilot volume really does for a gun. Thanks for the quick replies!

At the moment I can move the piston with my breath easy. And I didn't know about the D4 rule. My piston has a pretty large range of movement at the moment.
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Unread postAuthor: Fnord » Wed Mar 18, 2009 1:11 pm

Your dad is right, in a theoretical world, pilot volume doesn't matter, even with a very small vent.

However, in the real world there are several things that make it bad, even with an oring piston.
First, it's a huge waste of air.

Second, a piston allowed to travel a great distance can damage the valve when it slams back.

Third, your sealing face is not PERFECT. On paper, it should seal completely until there is enough pressure drop behind it to let it move back, then unseal all at once and let the sudden increase in surface area drive the piston back fast.

In reality, your sealing face will let air leak out the barrel a little when the forces are starting to equalize. This will result in a fffffsshoOOMP when you would otherwise get a BOOM!

As far as I know, this phenomenon has not been discussed before and is not covered by ggdt.


Though your problem is as others have stated, the valve doesn't have enough frontal surface area.
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Last edited by Fnord on Wed Mar 18, 2009 1:16 pm, edited 1 time in total.
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Unread postAuthor: MrCrowley » Wed Mar 18, 2009 1:13 pm

jrrdw wrote:As for pilot size there is a 4D or D4 rule that states "basically" your piston only needs to move back 1/4 of the barrel diameter. It's a good rule of thumb for judging pilot size. Hope this helps. :D

Remember the D/4 rule is only for coaxials. It's an alright 'guide' for barrel-sealers, but you generally want a little more room to stop piston bounce and flow restrictions.

Metroid fan wrote:Now that makes sense. I can understand that air leaking around the piston before the pressure is reached would account for a drop in psi in the chamber. But depending on what that pressure was and how much volume and the size of the pilot valve and how fast the pilot valve actuated you could overcome that factor quite easily. I can see that the easiest way to do that would be to shrink the pilot volume. When I get this valve working I'll probably do some tests to check how much pilot volume really does for a gun..

On my first piston valve cannon it shot a golfball about 80m, when it should shoot about 300m. I put on a larger pilot valve (even though my pilot volume was minmial) to counter the air flowing around the piston from the chamber, the cannon shot about 120m. I made a better piston and soon after i'm shooting 300m.

In your case shrinking the pilot volume will be efficient for your air supply and allow you to use a smaller, easier to open, pilot valve. Then your main concern is your piston, the D/4 rule (with a little extra), the piston's exposed frontal surface area and the fit inside the piston housing.
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Unread postAuthor: jeepkahn » Wed Mar 18, 2009 1:28 pm

And one point that i think has not been mentioned, when you have a leak in front of the piston(the ducttape joint), pressure is dropping in front of the piston and therefore takes longer for the pilot pressure to drop beyond the chamber pressure...
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Unread postAuthor: mark.f » Wed Mar 18, 2009 1:41 pm

_Fnord wrote:In reality, your sealing face will let air leak out the barrel a little when the forces are starting to equalize. This will result in a fffffsshoOOMP when you would otherwise get a BOOM!


Interesting. Have you seen clide's flipbook of his clear STV firing at 2000 frames per second? It appears that the piston sealing face clings to the outlet port when the piston moves back.

http://gbcannon.com/projects/flipbooks/clearvalve.html

It's all really a matter of how accurately his sealing face and port are cut, I guess.
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Unread postAuthor: Fnord » Wed Mar 18, 2009 2:00 pm

What I was describing would only be noticeable on an inefficient valve, so clide's gun probably doesn't have this problem.

But what clide has shown should probably be normal for a sealing face that isn't attached completely. If it was solid the piston wouldn't move until there was enough force to break the "cling".
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Unread postAuthor: Metroid fan » Wed Mar 18, 2009 2:39 pm

Okay so I have gotten the piston working. The problem was the frontal surface area :D I just put a smaller face for the neoprene to seal on and now the piston moves when I pilot it! I'll shrink the pilot volume when I get the money. Everything on this gun will screw on (Barrel, chamber and pilot) so I should be able to replace it with ease! Thanks a ton guys!
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