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Need help understanding if this will work or not.

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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Need help understanding if this will work or not.

Unread postAuthor: 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.
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Unread postAuthor: 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.
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/sarcasm, /hyperbole
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Unread postAuthor: 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.
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Unread postAuthor: 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.
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Unread postAuthor: 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
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Unread postAuthor: 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.
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