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surface area for air to push against?

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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surface area for air to push against?

Unread postAuthor: JDP12 » Fri Sep 26, 2008 7:31 am

Well my gun is 90 percent done, with everything sealing and the chamber filling up properly, but there's one problem and thats that the piston isn't getting pushed back at all when i fire it- so all the air is going around the piston and out the pilot. me and my dad worked on it for a bit and calculated some things out and realized that there was BARELY ANY surface area on the piston for it to get pushed back by.

so we were going to reduce the size of the barrel in the tee down to 2" instead of 3" to give it more surface area, I just want some quick opinions on if you think it will work or not
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Unread postAuthor: Ragnarok » Fri Sep 26, 2008 7:44 am

When you say barely any, what percentage are we talking about, and what's your piloting mechanism?

Pistons can actually work better with a restricted surface area - and indeed, HEAL uses the trick, with only about 15% of the piston area to push on, which actually improves opening time.
Although, it's worth noting that the lower area pistons are best complimented by high pressures.
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Unread postAuthor: JDP12 » Fri Sep 26, 2008 7:56 am

um... we're talking about a ring of the piston about MAYBE 3/16" wide, so its nothing at all, and only about 80 PSI right now if that much.

it's piloted by a QEV which is piloted by a 3/4" ball valve
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Last edited by JDP12 on Fri Sep 26, 2008 8:30 am, edited 1 time in total.
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Unread postAuthor: Sticky_Tape » Fri Sep 26, 2008 8:10 am

What size is the qev you are useing? I suggest to get a tighter piston and lube.
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Unread postAuthor: JDP12 » Fri Sep 26, 2008 8:17 am

3/4" QEV.


maybe, but my problem is its so hard for me to make a piston- that one alone took me a couple hours because i had to cut it down on a bandsaw- does anyone have some better tips for making a better piston easier?
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Unread postAuthor: Ragnarok » Fri Sep 26, 2008 8:54 am

ilovetoblowthingsup wrote:um... we're talking about a ring of the piston about MAYBE 3/16" wide

It's not a lot of use just that information - I need to know the overall diameter of the piston to compare it to.
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Unread postAuthor: JDP12 » Fri Sep 26, 2008 8:56 am

oh sorry. overall diameter is 4 inches. only about 1/8"-3/16" wide ring is exposed for the air to push back against when its sealed against the barrel
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Unread postAuthor: Ragnarok » Fri Sep 26, 2008 9:04 am

Yeah, that will be a problem, that's only about 8% of the piston area.

How big is the equalisation hole(s) between the pilot volume and chamber? If you can cut them back, or ideally install some o-rings and check valves, then it might be possible to get it to pilot - if it worked, the valve's power would be very decent.

Really, that's the trade off. A bigger seat area gives more valve flow, and indirectly, better opening time, but it becomes increasingly hard to get it to pilot.
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Unread postAuthor: JDP12 » Fri Sep 26, 2008 9:07 am

well right now, my piston is really a piece of crap i'm going to have to make a new one. i'm hoping to make it out of a flat 3" end cap that is turned down a lathe so it slides smoothly, and rely on the small gap between the pipe and piston to fill it.

the equalization hole is big- and there is a lot of gaps around the piston.

so i think my plan is to make a new piston that fits good, and reduce the barrel size inside the tee itself, so there is a smaller seat area and larger area for the air to push back against.
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Unread postAuthor: psycix » Mon Sep 29, 2008 9:56 am

How do you get 3/16th inch difference in diameter when the piston is 4 and the seat area is 3 inch? 13/16th inch of pipe wall?!

It should be possible to pilot this with a good o-ring seal and a BIG pilot valve.

Instead of making your seat diameter 2", I'd recommend 2,5".

Pistons can actually work better with a restricted surface area - and indeed, HEAL uses the trick, with only about 15% of the piston area to push on, which actually improves opening time.

Interesting. Any more information or a link to some topic to enlighten this?

EDIT:
Ah it has to be based on the fact that the pressure starts working on surface area inside the seat diameter just after the valve starts opening and thus the valve slams open because the pilot is almost empty before it started moving.
I think Im going to make some graphs and do some mathematical approaches to check this out.
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Unread postAuthor: JDP12 » Mon Sep 29, 2008 4:27 pm

no psycix- the amount of piston exposed... and that was just a guess

i'm making a new piston anyway
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Unread postAuthor: psycix » Tue Sep 30, 2008 4:00 pm

The amount of piston area exposed IS the piston diameter surface minus seat diameter surface.
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