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Pilot 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

Unread postAuthor: no-limit » Fri May 23, 2008 7:03 pm

How much pilot volume is needed for a piston valve to work?
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Unread postAuthor: potatoflinger » Fri May 23, 2008 7:09 pm

As little as possible.
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Unread postAuthor: no-limit » Fri May 23, 2008 7:12 pm

Thanks! -------- I thought so but wasn't sure.
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Unread postAuthor: BC Pneumatics » Fri May 23, 2008 7:13 pm

The volume required would be the minimum amount you can achieve while still allowing the piston to stay closed, and travel far enough to open. So, as flinger said, do your best to make it as small as possible.
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Last edited by BC Pneumatics on Fri May 23, 2008 7:17 pm, edited 1 time in total.
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Unread postAuthor: jrrdw » Fri May 23, 2008 7:15 pm

Enough to allow the piston to slide back 1/4 of the barrel diameter. Including room for a bumper.
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Last edited by jrrdw on Fri May 23, 2008 7:16 pm, edited 1 time in total.
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Unread postAuthor: no-limit » Fri May 23, 2008 7:16 pm

So I do need some. Oh yeah, to let the piston come back.
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Last edited by no-limit on Fri May 23, 2008 7:18 pm, edited 1 time in total.
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Unread postAuthor: BC Pneumatics » Fri May 23, 2008 7:18 pm

I edited my post... may be a bit more clear now.
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Unread postAuthor: no-limit » Fri May 23, 2008 7:21 pm

"still allowing the piston to stay closed" how do you figure that? Wouldn't any amount achive that?

"slide back 1/4 of the barrel diameter" So a 1" barrel would require 1/4" of pilot volume.
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Unread postAuthor: BC Pneumatics » Fri May 23, 2008 7:26 pm

For most people, yes. There is a point at which the volume could not physically decrease without the diameter shrinking, thus not providing the force needed to keep the piston sealed. This point doesn't exist from a strictly mathematical standpoint, nor would it likely be encountered here, but something in my very rigid mind makes me consider things like this situation, and mention them in posts.
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Unread postAuthor: no-limit » Fri May 23, 2008 7:29 pm

Well thanks again for your input.
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Unread postAuthor: jrrdw » Fri May 23, 2008 7:35 pm

For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.
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Unread postAuthor: psycix » Sat May 24, 2008 8:23 am

jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

And add a lil extra traveling to be sure.
Traveling 1/8" too much isnt bad, but traveling 1/16th" too less is.
The extra travel also makes you have less problems with bouncing problems.
Some pistons may bounce back a short way.
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Unread postAuthor: jimmy101 » Sat May 24, 2008 10:03 am

jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

That doesn't look right because the units are wrong. You're adding three lengths and getting a volume. That ain't possible. (Dimensional analysis to the rescue!)

Shouldn't it be, for a solid cylindrical piston;
pilot volume V<sub>1</sub> = (distance piston must move)(area of piston)
The "distance piston must move" is generally taken to be at least 1/4 of the barrel diameter.

For a cylindrical piston open at one end;
pilot volume V<sub>2</sub> = V<sub>1</sub> + internal volume of piston.

If there is a bumper then you need to know what the gas volume is in the bumper region.
pilot volume V<sub>3</sub>= V<sub>2</sub> + gas volume in the bumper region
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Unread postAuthor: jrrdw » Sat May 24, 2008 5:05 pm

jimmy101 wrote:
jrrdw wrote:For a 1" barrel, 1/4"+ piston lenth+bumper thickness=pilot volume.

That doesn't look right because the units are wrong. You're adding three lengths and getting a volume. That ain't possible. (Dimensional analysis to the rescue!)

Shouldn't it be, for a solid cylindrical piston;
pilot volume V<sub>1</sub> = (distance piston must move)(area of piston)
The "distance piston must move" is generally taken to be at least 1/4 of the barrel diameter.

For a cylindrical piston open at one end;
pilot volume V<sub>2</sub> = V<sub>1</sub> + internal volume of piston.

If there is a bumper then you need to know what the gas volume is in the bumper region.
pilot volume V<sub>3</sub>= V<sub>2</sub> + gas volume in the bumper region


Over complicating it a tad bit, is the reason it doesn't look right.

My formula is quick, easy and works.
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Unread postAuthor: MrCrowley » Sat May 24, 2008 5:38 pm

Doesn't T=1/4 D* apply more to the minimum travel of a piston in a co-axial for maximum flow, not how much pilot volume you should have?

Travel and pilot volume usually go hand in hand, but I've always thought of it as working out the travel of a piston you need to get the best flow, not how much pilot volume you should have.



*Obviously T = Travel, D = Barrel Diameter
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