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Reducing pilot volume and piston size

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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Reducing pilot volume and piston size

Unread postAuthor: Biopyro » Thu Mar 05, 2009 9:59 am

I always thought that the barrel should stop at the point circled in green for maximum flow, but perhaps I was wrong. If it moves back to where it is in that diagram, surely the performance would be no worse than a coax in terms of flow, and the piston could be smaller along with the reduced pilot volume?

Does a coaxial perform worse of better than a barrel sealer if you ignore the fact that the air in a barrel sealer has to make a 90 degree turn?
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Unread postAuthor: jeepkahn » Thu Mar 05, 2009 10:18 am

in theory a coax is not as effecient, but I personally think that because of flow dynamics performance differance is negligible... with your diagram however, you will be using the worst of both worlds, because you are going to have an offset turbulence effect AND restrictive flow from the air having to turn a 90 and then a 180... you would actually be better off moving the barrel farther to the left in your diagram...
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Unread postAuthor: IlovePsi » Thu Mar 05, 2009 11:31 am

looks like a industrial qev or _?
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Unread postAuthor: MRR » Thu Mar 05, 2009 12:43 pm

I build on of these, bad flow but really good as a pilot valve.
You can operate them with a blow gun (piston needs a spring) due to the little pilot volume.
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Unread postAuthor: POLAND_SPUD » Thu Mar 05, 2009 12:46 pm

that's a good question bio.. but yeah it seems that Ilove psi is right (if that's what he meant) QEVs look similar to your drawing so either:

1. there is no loss of performace
2. performence drop is lower than the gain resulting from faster opening time
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Unread postAuthor: SubsonicSpud » Thu Mar 05, 2009 5:09 pm

I think it depends on the diameter of the tee relative to the valve and barrel. If you have a large tee with a narrow barrel there is a lot less flow restriction, the barrel position will not affect it too much. If you have a tee with a diameter not much greater than the barrel then you are going to be chocking the flow and will want to put the barrel opening as far up the front of the tee as you can.

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Unread postAuthor: Biopyro » Fri Mar 06, 2009 3:52 am

Yeah, I thought industrial QEVs were laid out like this. I reasoned that not only are you cutting the pilot volume, but also you could get away with a much shorter (shorter even than the pipe containing it) piston, making it much lighter too.
Notice that in a QEV, the piston is less than 1cm long, even though the pipe containing it is 17mm wide. (Admittedly it isn't a flat shape, but I only think that helps it to seat)

I was thinking of using a rigid rubber disc, with a dip cut into it where the barrel sill seat, so that every time the piston seats it will realign itself which sould prevent it rotating too far during travel.

I did think that jeepkhan, but I think that it's a small sacrifice for the huge loss of piston weight and volume. Also, the cannon i'd be applying it to has a 40mm barrel in a 60mm tee, so there's quite a lot of flow allowed there. (I will have to use a 40mm coupler in the tee to extend the barrel into the pilot port of the tee.

When I model stuff like this in GGDT, things like piston weight and volume/(pilot valve diameter), seem to make almost no difference to the performance. Do you think this is because of the difficulty in calculating such effects? (Perhaps D_hall would be best suited to answer this :P)
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Unread postAuthor: jeepkahn » Fri Mar 06, 2009 10:56 am

Biopyro wrote:When I model stuff like this in GGDT, things like piston weight and volume/(pilot valve diameter), seem to make almost no difference to the performance. Do you think this is because of the difficulty in calculating such effects? (Perhaps D_hall would be best suited to answer this :P)


If I'm not mistaken, ggdt gives you performance #'s based on projectile speed, etc, performance of the valve(from weight and pilot size) plays relatively little role in that aspect, performance of the valve shows up more in seat of the pants type improvements, and you'll see this if you ever have a too small pilot, the gun will take forever to fire, but when it does the performance differance of the projectile is relatively small...

GGDT can tell you if the gun will fire with given parameters, but as long as the gun fires, it won't really tell you how well your valve is performing speedwise...
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Unread postAuthor: Biopyro » Fri Mar 06, 2009 11:20 am

Actually my current configuration ( with the piston in the centre of the tee ) fires very slowly an with poor power, but I think it's because a lot of the air is going past the piston, even though it was a fairly tight.
I will have a play with GGDT.

I see your point about the opening time not making an awful lot of different between piston valves though. Do you think the flow makes that much difference too though?
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Unread postAuthor: jeepkahn » Fri Mar 06, 2009 12:20 pm

Flow characteristics will make a lot of differance...

And lots of times, the valve will open quickly, but seat of the pants it seems slow because there is a delay before the piston starts moving, even though the movement is equivelant once it unseats...
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Unread postAuthor: Biopyro » Fri Mar 06, 2009 12:38 pm

So keep it how it is and get a new piston then?
I'm thinking of getting a HDPE rod for the new piston because it's light, machinable and has a low coefficient of friction, but I've never seen anyone else use it - any idea why?
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Unread postAuthor: psycix » Fri Mar 06, 2009 3:17 pm

For normal spudguns, the difference in performance isnt that big. But when approaching the sound barrier, the flow of the valve becomes more and more important.
The lower pilot volume can also be achieved in the "maximum flow" way by having a longer piston.

even though it was a fairly tight.

When talking about pistons, fairly tight usually means "just tight enough".
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Unread postAuthor: ramses » Fri Mar 06, 2009 4:33 pm

however, a longer piston is a heavier piston, so make it hollow, if possible.
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Unread postAuthor: Biopyro » Fri Mar 06, 2009 6:13 pm

So firing at 150psi shouldn't give a problem as far as flow is concerned in that valve because the air isn't fast enough. I was thinking of doing 2 discs separated by a bolt, but I always worry that when the piston is opening or closing this might happen?
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How thick does the back disc have to be to prevent this? Or does it move quickly enough not to matter?
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Unread postAuthor: sgort87 » Fri Mar 06, 2009 8:12 pm

Yeah, that's definitely gonna jam up on you like that. You're going to have to design a track that it will slide along.
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