Pneumatic Design

[LucyInTheSky]

I have decided to make a larger gun with the money I get from christmas. I is a basic PVC piston cannon, so I thought I would put my design up.

This is what the piston will look like, with a built in check valve and the pilot are being totally air tight I should have no back flow, I have decided to drill the middle out to reduce weight but this also increases the volume of air to be vented but I thought it would be a good trade off.


And this is the design of it, Im probly going to go with the 50mm chamber to save money and depending on how much money I get may put the chamber closer to the barrel with 2 45* elbows.
With the sealing face being right far forward I hope to in crease flow to about 50% maybe.
The C:B ratio should be about 1:1 including the area in the elbow and piston area.
I was thinking I could fill the piston with expanding foam to reduce the pilot volume, think that would work?


Tell me what you think, and Im open to suggestions becuase nows the time to change the design.

[jrrdw]

The chamber isn't big enough, and it will be a pain to fill with the shreader valve in the blow gun. Over all a workable plan.

[LucyInTheSky]

How big should the chamber be? would it make a noticable difference, just my compressor isnt very good so it takes a long time to fill?

I might use some 90mm instead of the 50 if its going to make that much difference.

[djt]

I think he is saying it will be a pain to have to open and close the blogun at the same time you are trying to fill it. Why not just use a ball valve with a male quick connect on it?

[noname]

510 grams? Jeez that thing will be heavy. Definitely reduce the pilot volume with something, expandable foam or a plug over the back.

[jrrdw]

I think at least 1 inch bigger in diameter then your barrel. Seems to me the size that your useing now wont give you enough volume to make the projectile clear the muzzle, because the barrel has more volume then the chamber. I like to think a good rule of thumb is "Have twice the chamber volume as you do barrel volume". Use the GGDT with what you have designed above and see what it comes out with. If you inter the infomation correctly it should be accurit.

Yes, i ment that it will be a pain to have to hold the valve open just to fill. I would also think the less fulling around with the trigger when filled the better. Someone should come up with a safty for these blow gun triggers. I'll see what i can do about that.

You think 510 grams is heavy? I bet my cannon now weights aprox. -- 30lbs, and thats without the chassis! (chassis not built yet so i cant guess the total weight yet)