Need help understanding if this will work or not.

r84shi37 · Sun Sep 16, 2012 7:12 pm

I am building my first pneumatic air cannon. I have a 4ft long, 4 inch, 40 gauge pvc pipe capped at one end and connected to a 4"x3" reducing 90 fitting. It loops with another 90 into a U shape ending at 3". I figured I would add a sprinkler valve next but unfortunately, the largest size I could find was a 1" valve. If I reduce the size from the 3" 90 to a 1" coupler so I can connect to the valve, and then "reduce" it so that it's back to 2 inches or so (for the barrel) will the sudden drop in size negatively affect the performance of the gun?
<pre>
valve
_\/______
3"pipe->__=___ |
_________________| |
|___tank_____________|
4"pipe^^
</pre>

Basically, the size will go from 3" to 1" back to 3"... will this really matter? Do I need a bigger sprinkler valve? I don't know how to make my own piston valve btw. Tyvm.

Zeus · Sun Sep 16, 2012 9:12 pm

Yes, a lot. Use GGDT to model your launcher with a 1 inch valve, and a 3 inch valve. Look up a triggered burst disc, it's incredibly easy to make, and gets far higher perfomance than any piston or sprinkler valve.

r84shi37 · Mon Sep 17, 2012 10:03 am

Ah yes, I know that burst disk valve are the best... I just don't want to have to replace the disk for every shot- even if it is inexpensive. Is there anyway I could make a piston valve with the input and the output aligned with each other? like this: --->valve--->

not a 90 like:
............. ^
..............|
---->valve

I can't seem to find a bigger sprinkler valve. Should I just buy the biggest ball valve I can find (3") and lube it for easier turning? This is my first pneumatic spud gun so I'm not extremely worried about performance... I just need something that will fire a potato 200-300 feet or so. Preferably more but I plan to build better ones as I progress. Thank you for the help.

Technician1002 · Mon Sep 17, 2012 10:33 am

A coaxial piston valve design with the piston inside the air chamber would work as a piston valve inline with the tank and barrel. Most of my current designs are of this configuration.

r84shi37 · Mon Sep 17, 2012 12:05 pm

Perfect! Thank you guys. This seems to be an awesome community so far. I'll build a coaxial piston valve and stick it in the upper curve of my air tank. I probably should have planned more before I purchased my fittings but I think I can still make it work. Found a nice animation so it shouldn't be too hard to build. This was very helpful. Thanks =D

http://gbcannon.com/concepts/coaxial.html

Technician1002 · Mon Sep 17, 2012 3:01 pm

Just a quick note on the original design and the flow restriction. When you cut a diameter in half, the area is cut to 1/4. 4 inch to 2 inch is a 3/4 drop in area. Cutting it in half again to 1 inch cuts the area to 1/4 that or 1/16th the original area. Go as large in the valve as possible for best performance.