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First Cannon Problems

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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First Cannon Problems

Unread postAuthor: Alucard » Tue Aug 11, 2009 9:10 pm

Well I built a cannon the other day, its basically a 1"x1' Pressurized PVC Pipe with a slip to thread addapter, thread wraped in teflon, then screwed into a end cap with a shrader valve put in, then theirs a 1" Ball valve on the other side as well as a 4' barrel also 1" I filled the gun with a friends Air compressor and it would shoot 2" long 1" thick dowel rods like no other, I bought a really high quality hand pump and have been trying it and when I hook it up, all the air leaks out from every pump, If I hold it completly straight It pumps air, but if I disconnect it all the air bursts out of the Bike valve, I JB welded the Valve on, then electrical taped it all the way down too the chamber, I then lubed the ball valve and painted it flat black, the problem still persists, How do I fix it? Is it the Bike pump?
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Unread postAuthor: geardog32 » Tue Aug 11, 2009 11:04 pm

so its leaking from the scrader valve? if it is i would say its a crappy fill nozzle on the pump they are usually kinda screwy. or maybe the schrader valve stem is getting stuck? can you describe the leak a little better where is it coming from? spray some soapy water on it and watch the bubbles.
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Unread postAuthor: Technician1002 » Wed Aug 12, 2009 12:11 am

It is possible the valve core is either bad or loose. A good bike shop has a core tool to unscrew the valve core and replace it.
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Unread postAuthor: jackssmirkingrevenge » Wed Aug 12, 2009 1:34 am

Technician1002 wrote:A good bike shop has a core tool to unscrew the valve core and replace it.


I use tweezers...
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Unread postAuthor: jimmy101 » Wed Aug 12, 2009 12:47 pm

Technician1002 wrote:It is possible the valve core is either bad or loose. A good bike shop has a core tool to unscrew the valve core and replace it.

You can also buy one from most car parts supplies. Should cost less than a buck. Heck, you might already have one and not know it.
Image
near the center is "valve stem tool". It doubles as a stem cap since the top end is basically a valve stem cap.

Alucard: Sounds like the schrader adaptor on the pump isn't very good. It works when held straight but doesn't seal otherwise. In addition, the clamp on type schrader fill valves tend to come off pretty slowly when you remove them. The schrader valve stays open for a while while the adpter is being removed. Since your chamber is so small you are depressurizing it with just the leakage as the adaptor is removed.

Take a close look at the clamp on adaptor, I assume it's clamp on and not one that is just held in place by hand. Sometime the outer bushing on the adapter has an ID that is really a bit too small. That keeps the adaptor from fitting far enough down the schrader to give a good seal that'll tolerate the weight of the hose trying to pull it out of alignment.

Your adapter might also be a threaded one. In that case, as it is unscrewed you'll loose to much air from the small chamber.

Sounds like you need to either replace the adaptor on the pump hose or come up with a different connection scheme. You can install the schrader valve on a small ball valce then screww that into the chamber. That will let you attach the pump, open the ball valve, pump up the chamber, close the ball valve (isolating the schrader, adapter and pump) and remove the pump's adapter.

Edit: Fixed the image link
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Unread postAuthor: Alucard » Thu Aug 13, 2009 2:14 pm

Fixed the problem, I replaced the Bike valve and now I can pump it up :D, ive been shooting paper darts so hard they crumple, I have a technique to making them, ill make a how to eventually.
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Unread postAuthor: maverik94 » Thu Aug 13, 2009 2:35 pm

Cool, I'm glad you got it to work, one thing you might want to look into for a future design is, try to have a chamber that is a bit larger size than the barrel, so you could have a C:B ratio (chamber:barrel ratio) of say...2:1, meaning your chamber is 2 times the size of your barrel, that will give you better performance. Just something to look into!
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Unread postAuthor: Technician1002 » Thu Aug 13, 2009 5:43 pm

maverik94 wrote:Cool, I'm glad you got it to work, one thing you might want to look into for a future design is, try to have a chamber that is a bit larger size than the barrel, so you could have a C:B ratio (chamber:barrel ratio) of say...2:1, meaning your chamber is 2 times the size of your barrel, that will give you better performance. Just something to look into!


At higher pressure with a hand pump, I have had pretty good luck with a small chamber due to the expansion ratio. It eliminates extra pumping.

Long barrel tennis ball test launch is here.
http://www.youtube.com/watch?v=ZUTXomfdLTE

The short barrel used with t shirts has a 200 foot range at 80 PSI. Perfect for concerts with fast reload times.

This was used to shoot t shirts at this concert.
http://www.youtube.com/watch?v=nYNZqwBtg88
I am getting photos of it in action at the concert next week.
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Attachments
Tennis ball barrel1.jpg
Short tennis ball barrel.
Tennis ball barrel1.jpg (36.58 KiB) Viewed 280 times
Tennis ball barrel2.jpg
Long tennis ball barrel. Only works at high pressure. Low pressure provides too little volume.
stadium1.jpg
This cannon shoots t shirts 200 feet with the small barrel. Range from the stage at the Hillsborro stadium.
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