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Will this design work?

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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Will this design work?

Unread postAuthor: grumpyoldman » Tue Jan 31, 2012 9:28 pm

First of all, I would like to say Hi to everyone. I've learned a lot from reading these forums.

I'm retired and thinking about building and selling t-shirt launchers. Safety needs to be number 1. I've designed a launcher and would appreciate any feedback on the design.

The t-shirts need to go 300 ft. I didn't want to go with a large chamber or use super high pressure. I decided to use a medium size chamber (2" X 10") and medium pressure, around 300 psi. The barrel will be 3" diameter and 10" to 15" length. I want this to be as compact as possible. Below is kind of how I want it to look.

The piston is housed in a 2" X 1" Reducing Coupling. There is a 1" X 1" smooth, round space that is large enough for a piston. The diagram from McMaster is below. All parts are from McMaster, total of about $85.

I was thinking about lubricating the threads and using o-rings as the sealer so it would be easy to assemble and disassemble for maintenance. I also wanted to use as many off the shelf parts as possible to minimize the amount of machining.

I wanted to check with you guys to see if this design would work and how safe it is before I spent money on all the parts. I tried to scale everything as well as I could. Thanks!
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T-Shirt-Launcher.jpg
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Cannon-Design-1.jpg
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 12:56 am

Welcome to spudfiles and as another grumpy old man in a community already infiltrated by them :)

As it is, your design wouldn't work though... the diameter of the piston on the pilot side needs to be bigger than the diameter on the barrel sealing part for it to work as an exhaust valve.

What you want is the yellow piston to be the full diameter of the blue chamber.

If you're thinking of turning this into a commercial proposition I would say you have your work cut out, don't forget to cover your posterior in terms of liability.
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Unread postAuthor: grumpyoldman » Wed Feb 01, 2012 1:27 am

Ha Ha. Yep, I don't want to lose the house. I've grown accustomed to a roof over my head. They probably won't sell anyway.

Was that inside or outside diameter? Can the piston can be the same size as the inside diameter of the chamber? So you could put a piston inside the chamber if it touches ( or almost touches) the chamber's walls?

Thanks. I guess it's back to the ol' drawing board.

BTW I've read a lot of your posts around here. Nice to meet you.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 1:44 am

grumpyoldman wrote:I think I see what you're talking about. The piston needs to be a larger diameter so the air pressure would have more area to push on to seal the barrel? The way it is now, the piston wouldn't seal. Is that correct?


Since the sealing part of the piston has a bigger diameter, it would seal too well, and therefore never open.

I made a little diagram to clarify, in a nutshell A (your piston diameter on the pilot side) needs to be bigger than B (the sealed diameter on the barrel side). You can also learn more about piston valve principles here: http://www.spudfiles.com/forums/piston- ... t8157.html
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Unread postAuthor: grumpyoldman » Wed Feb 01, 2012 2:21 am

Stupid me! :oops: I don't know why I didn't see that. Thanks for explaining it so even I could understand. I've read what was posted at the link before but I guess it didn't soak in. I'll re-read it several more times.

I guess I should make the bumper larger too so it hits the back of the coupling. Maybe even extend the bumper further inside the coupling to reduce air volume in the pilot. Thanks again.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 5:07 am

We were all pneubs once :)

Yep, the bumper should be the same size as the piston. Also, with regards to the green supports, I would think two is enough.
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Unread postAuthor: al-xg » Wed Feb 01, 2012 8:04 am

with regards to the green supports


With the large OD piston, one would be enough, although a finned design instead of the washers would improve the flow through the chamber, and avoid a lot of turbulence.


Conical seats on piston valves can have a tendency to get stuck, you might want to make sure the pistons can't deform too much.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 8:53 am

al-xg wrote:With the large OD piston, one would be enough


That's what I meant :oops:
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Unread postAuthor: grumpyoldman » Wed Feb 01, 2012 9:45 am

Great recommendations. Thanks. Now, what's a finned design? Sounds intriguing. I tried a some searches and couldn't find it.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 9:49 am

He means that instead of using a washer with holes in it to centre the piston rod, you should use pieces of sheet material in line with the flow, like the fins of a rocket.
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Unread postAuthor: grumpyoldman » Wed Feb 01, 2012 10:13 am

That sounds really good. I'll redesign the gun to incorporate the changes and post it later. I'm guessing one set of fins would be enough since one washer would have been sufficient.

I hope to start putting it together in a couple of weeks. I need to order the parts. I think I'll do that right now. I'll keep updating as I progress. Thanks for helping!
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Unread postAuthor: dewey-1 » Wed Feb 01, 2012 10:48 am

I think to give you better flow, use a 2 inch to 1.5 inch coupling on the barrel side with a 1.5 inch pipe nipple with reducer to 3 inch barrel.
This will give you much more power due to surface area on projectile.

1.5 inch pipe has an ID of about 1.59 inch versus 1.03 for 1 inch.

1.98 sq in versus .83 sq in or 238% more surface area.
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Last edited by dewey-1 on Wed Feb 01, 2012 11:07 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 10:59 am

When I saw you commented I was expecting a link to an ASCO based design ;)
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Unread postAuthor: grumpyoldman » Wed Feb 01, 2012 11:06 am

Thanks Dewy-1. Will do. I'm getting excited about building this thing. I hope my heart can take it :)
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 01, 2012 11:10 am

This is the link dewey subconsciously wanted to post ;)

http://www.spudfiles.com/forums/asco-ne ... 17764.html

It would work great for T-shirts, especially if used at 150-250 psi.
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