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First Piston Question

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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Unread postAuthor: jackssmirkingrevenge » Sat Sep 08, 2007 1:46 pm

PinHead wrote:By the way, what's the history on this measurement, and who came up with it?


It's explained in detail in this thread
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Unread postAuthor: Dornep » Sat Sep 08, 2007 1:52 pm

Yea apparently thats the route I'm going to have to go because my pass through barrel setup isn't keep the barrel perfectly straight which means it's not setting in the silicone groove.

I've been to the hardware store three times today. The cashier told me that when I get done building my still that I'd better bring her some moonshine. I'll take her a golf ball instead. 8)
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Unread postAuthor: MrCrowley » Sat Sep 08, 2007 4:35 pm

jackssmirkingrevenge wrote:The piston flies back with considerable force, since yours is on the heavy side even at low speeds it would carry substantial momentum.


Me and my friends calculated the force pushing my piston back was 2000kg per square Inch, or 4000PSI. I forgot what forumla we used but we managed to get that rough estimate.
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Unread postAuthor: Dornep » Sat Sep 08, 2007 4:58 pm

Thanks for the help guys. The new piston is almost done, it weighs 6.9 ounces. I will duct tape it to create a better seal around the piston which may add a little more weight but it should still be less then the other piston (10.6 oz.)

It has 3 layers of rubber tire tube. I used a smaller washer so I could get a seal with smaller diameter barrels.

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Unread postAuthor: mopherman » Sat Sep 08, 2007 5:03 pm

plug the other end of that coupling for reduced pilot volume.
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Unread postAuthor: Dornep » Sat Sep 08, 2007 6:17 pm

If I add another plug thats gonna put it well over 10 oz. How big will the boost in performance be?
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Unread postAuthor: Hubb » Sat Sep 08, 2007 7:07 pm

You could fill the piston with expanding foam, such as Great Stuff. That will reduce the pilot volume as well as not raise the weight considerably.
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Unread postAuthor: Dornep » Sat Sep 08, 2007 11:35 pm

Ok I put it together in a hurry before the race came on and just got back home, so I do a low pressure test with propane to see if it seals and it doesnt. I also realized I have a flaw in my design, the fitting where I add the air is forward of where the rear of the piston is. So I will have to use a ball valve exhaust so I can press the piston against the barrel before I add pressure.

I will try to pressurize it again tomorrow with a compressor and see what it does. If it doesn't work I will cut the cap off and redo it and think about it before I start tapping holes.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 09, 2007 4:30 am

Ah, looking much better. You can afford to cut it down a but though to cut doen on mass and pilot volume, it doesn't need to be longer than its diameter.
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Unread postAuthor: Dornep » Sun Sep 09, 2007 4:22 pm

I cut the piston down about a 1/4" of it's total length and made a light weight end cap for it out of a dixie cup. The dixie cup will probably implode over 5 PSI but we'll see. I've measured the travel of the piston with the bumper installed and it's about 3/4 of an inch. I'm going to do a test to see if the piston seals the breech after everything has cured. If it doesn't seal, I'm gonna make a coaxial combustion and say to heck with pneumatics. :wink:


This is the shortened piston with crap tube lube (A.K.A. Vaseline) and the dixie cup end cap super glued in.
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Unread postAuthor: Modderxtrordanare » Sun Sep 09, 2007 4:42 pm

You really should have just filled the piston with expanding foam, it would have been easier than gluing a dixie cup in to the back and far more efficient. (Since it won't implode :lol: )
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Unread postAuthor: Dornep » Sun Sep 09, 2007 4:46 pm

I can still fill it with foam, but the only can of Great Stuff foam I have seems to be to old to expand properly. I will try it anyway and see what happens.
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Unread postAuthor: dongfang » Sun Sep 09, 2007 4:55 pm

Hi,

Fill it with foam, and then (after the foam has hardened and has been ground down) fit the cup. That´s the best way of them all. Without some sort of cover over it, you will have little fragments of form everywhere (guess how I know....)

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Unread postAuthor: Dornep » Mon Sep 10, 2007 7:31 pm

I did a pressure test and the piston seals air almost as good as a pasta strainer. I first tried it with a small "automotive" compressor and figured it didn't have the burst of air pressure to fill the pilot and press the piston against the breech. So I used the compressor at work (125PSI) and it did just as bad, I used foam blocks to hold the barrel centered in the chamber so the piston's seal is no good or the pilot volume is too big. If I have to cut the end cap off a second time this thing is going to end up as a sprinkler valved cannon.

I did as you said Dongfang and covered the foam with a plastic cup after it had expanded. Maybe I just suck at pneumatics...

I used a 1/2" steel Tee with a schrader in one fitting and a short section of hose connected to a blowgun on the other fitting. The only leak that I found was the air escaping from barrel... I can get pic's if you want to see the Tee setup.
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