Barrel same size as piston

[bugsingelyn]

I'm trying to build a small "plinker" out of copper without having to buy anything, and I came across a Tee with 1/2" ports on 2 sides and a 3/4" port on the third, and I was planning on using it for my cannon, but somewhere, I forget where, I read that the piston's diameter has to be larger than the barrel's diameter. If I understand correctly (and if I am wrong, please correct me), if they are exactly the same, there isn't anything to push the piston back, so nothing would happen. If the barrel is bigger than the piston, well it wouldn't be possible I guess, but the piston would be forced forward instead of back. Right?

Soooo, what if I made the I.D. of the barrel smaller? like fill it with epoxy and drill a hole in the center of it? I know it would reduce flow but I don't care, it's only for fun not shear power.

BTW, I've already made a piston-valved launcher complete with O-rings on the [EDIT] piston and a sprinkler valve pilot, someday soon if I find a camera I will post it.

[DYI]

You obviously don't understand how piston valves work. The barrel is not pressurised before firing, so the only thing that pushes the piston back is the air around its edges.

The surface area being pushed backwards on a piston must be greater than the area being pushed forward after the pilot has been vented. In your design, there would be no force pushing either way, and all the air in the chamber would flow out the pilot.

[bugsingelyn]

I do understand what you are saying. But I am trying to understand this more, and I think the piston has about ~14.7 PSIA pushing it back, and if the pilot is vented completely, it will also have ~14.7 PSIA pushing it forward. So I am correct in that if the barrel ID is the same as the piston OD, nothing would happen. I would like to make the piston larger, but I can't, so the only other option, if I really wanted to use this, is to make the barrel ID smaller. Right?

[Hawkeye]

The best way to make it work is to file out the stop on the barrel side and put a reducer(1/2-3/8) on the barrel sealing end. You will have to make the reducer a bit shorter to get it to fit up through the 3/4 inch opening. You will have to assemble it and then heat the T a bit longer to allow solder to flow down the barrel and reach the reducer as well.
I would attach the 3/4 side after so that you can direct the torch into the 3/4 port to ensure the reducer is heated sufficiently to draw the solder into both joints.
Be careful when attaching the last two pieces that you don't melt the barrel side. A damp cloth over the finished joint can help.

[Hotwired]

so the only other option, if I really wanted to use this, is to make the barrel ID smaller. Right?

Or of course you could make the piston wider.

You don't need it to be kept away from the walls of the tee, it could be physically touching the tee walls and there wouldn't be a problem.

[bugsingelyn]

EDIT I get it, that was supposed to be a 3/8". I like that idea, thanks a bunch!
EDIT2 There is less room than in the diagram I think, and there would be no easy way of getting the piston in.

[Killjoy]

Just a side note, but I have made a piston valve in which the piston diameter was the same as the barrel (barrel was 3" and piston was a 3" peice of pipe), and it work perfectly fine. It did eventually fail, but that is because the sealing face got sucked off because I secured it inproperly.

[jackssmirkingrevenge]

Just a side note, but I have made a piston valve in which the piston diameter was the same as the barrel (barrel was 3" and piston was a 3" peice of pipe), and it work perfectly fine.

Do you mean the inner diameter of the barrel was 3" and the outer diameter of the piston was 3"? If so, it sounds physically impossible for that to work.

[Killjoy]

Ah no, I mean they were the same, but I just got what he was talking about, I read what was going on wrong.

[Hotwired]

Do you mean the inner diameter of the barrel was 3" and the outer diameter of the piston was 3"? If so, it sounds physically impossible for that to work.

Impossible is a dangerous word

Add a piston return spring to crack open the valve and it could work

[jackssmirkingrevenge]

Add a piston return spring to crack open the valve and it could work

ah, but now you've changed the dynamics of the system ;p yeah, in that case it could work, but you'd need a quick filling valve from the pilot area to seal the piston in the first place.