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Help troubleshooting - tennis ball attachment not working

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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Help troubleshooting - tennis ball attachment not working

Unread postAuthor: Guaps » Tue Nov 16, 2010 2:56 pm

I made this exact piston valve (MrRedRooster)
http://www.youtube.com/watch?v=2bvXiswEDSo
I put an adapter on there so it has a removable barrel, and removable canister. I made a 3/4" marble/acorn barrel that works beautifully with a 1L metal cylinder. Then I made a 2"x36" PVC canister for a 2.5" x 60" tennis ball barrel. When I put 100 psi in it, the tennis ball barely comes out of the barrel. It flies maybe 10 yards.
I used GGDT to try and figure out what I'm doing wrong, but it is asking for a lot of info I'm just guessing at. I estimated friction at 12psi per some other thread I found here, put in all the other values and it says a 6" barrel is optimal. Something is wrong. Any ideas? What info should I post here?
Thanks!
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Unread postAuthor: Fnord » Tue Nov 16, 2010 3:29 pm

Your valve is restricting flow. You really should have a valve that is 50% or more of the barrel diameter. 75% and up works great.

Also, your chamber : barrel ratio is on the low side; around 0.3:1.

At some point adding more barrel length will decrease power, which may also be happening here. A ratio of 1:1 is fairly common, though not super-efficient for 100psi.
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Unread postAuthor: Guaps » Tue Nov 16, 2010 5:39 pm

Thanks - I guess this just isn't going to be shooting tennis balls if the valve is the main problem.

Like I said in my original post, GGDT said my barrel was too long, but I thought maybe that had something to do with my guesstimates.

How did you calc that chamber:barrel ratio?
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Unread postAuthor: jackssmirkingrevenge » Tue Nov 16, 2010 6:02 pm

How did you calc that chamber:barrel ratio?


2"x36" chamber -> 113 cubic inches

2.5"x60" barrel -> 295 cubic inches

So around 0.38:1, as Fnord pointed out, that kind of ratio coupled with your limited valve flow vis a vis the barrel diameter is not going to make for good performance.

One thing to get around poor valve flow/opening time is to use a detent or burst disk, here's something I proposed recently elsewhere:

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Unread postAuthor: Guaps » Tue Nov 16, 2010 6:37 pm

I'm brand new to all of this - I saw the video on how to make a piston valve and decided I needed to try it. And it was tons of fun. That said, I'm totally lost looking at your diagram. I need to go do some reading about detent and burst disks. Looks like another fun project. Thanks!
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Unread postAuthor: Technician1002 » Tue Nov 16, 2010 6:52 pm

The rate of flow into the large diameter tennis ball barrel is more important than just a large chamber. I tested a similar small chamber/large barrel setup going to an extreme. The valve is much larger with the minimum diameter orifice between the chamber and barrel of 1 inch.

Here is the launcher;
Image

The ability to flow enough air in a short time is very important. Use a large valve on a large barrel.

And here is the performance at about 110 PSI;
[youtube]http://www.youtube.com/watch?v=ZUTXomfdLTE[/youtube]

A larger valve with a larger chamber on the same barrel has some serious raw power.
3gallon chamber and a 2 inch porting valve;
[youtube]http://www.youtube.com/watch?v=s0l5t7D0FDM[/youtube]
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