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m72 LAW Design questions

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 » Wed Aug 01, 2012 3:39 am

Say you have a 2 inch piston and a 1 inch barrel in a launcher pressurised to 100 psi. The force keeping the piston shut is 235 lbs. If you manage to slam the launcher down on the ground hard enough to shift that, it will shatter to smithereens long before the piston moves.

A lighter piston is also a lot less likely to move because of the reduced momentum, another argument against heavy pistons ;)
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Unread postAuthor: Zorlord » Thu Aug 02, 2012 1:51 pm

So, assembled everything today, and it seals the barrel!. Only problem now is when i trigger the pilot valve, all the air goes out of it, and not the barrel..suggestions?
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Unread postAuthor: jackssmirkingrevenge » Thu Aug 02, 2012 10:12 pm

Zorlord wrote:So, assembled everything today, and it seals the barrel!. Only problem now is when i trigger the pilot valve, all the air goes out of it, and not the barrel..suggestions?


Piston not tight enough. Try wrapping it with tape until it's a snug fit.

Pilot valve too small. Pictures of your setup?
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Unread postAuthor: Zorlord » Fri Aug 03, 2012 12:55 pm

jackssmirkingrevenge wrote:
Zorlord wrote:So, assembled everything today, and it seals the barrel!. Only problem now is when i trigger the pilot valve, all the air goes out of it, and not the barrel..suggestions?


Piston not tight enough. Try wrapping it with tape until it's a snug fit.

Pilot valve too small. Pictures of your setup?


well i would, but yesterday it fell and the back nipple snapped in half! woohoo.. Im starting to regret trying to build this and shouldve just went with the airsoft 203 version.. oh well. And the piston is a resin mold of the pipe.. is that still going be too loose?

EDIT: Pics..

It was being piloted by a 1/4 inch ball valve on the back.. but as you can see the nipple broke..

Image
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As you can see, the ass end of the piston fits pretty well inside the pipe.. So i hope that isnt causing the piloting failure.. i dont think i'd be able to create anything with closer tolerances
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Unread postAuthor: jackssmirkingrevenge » Sat Aug 04, 2012 1:45 am

Hmmm... I think what is happening is that it is twisting under pressure and locking in the chamber.

I would make the wooden part out of resin too, then drill holes in it.

Also, your current resin part is far too thick, you can make it half the size and save a lot of weight. That's a pretty heavy piston you got there, which will punish your launcher when it works well.
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Unread postAuthor: dewey-1 » Sat Aug 04, 2012 6:30 am

jackssmirkingrevenge wrote:
I would make the wooden part out of resin too, then drill holes in it.

Also, your current resin part is far too thick, you can make it half the size and save a lot of weight.


Carefully cut your current "resin" piston in half and use that for your wooden portion.

On barrel end of your piston, why are there 2 small rubber washers and one large?
They should be all the same if actually sealing to a pipe.
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Unread postAuthor: Zorlord » Sat Aug 04, 2012 10:54 am

dewey-1 wrote:
jackssmirkingrevenge wrote:
I would make the wooden part out of resin too, then drill holes in it.

Also, your current resin part is far too thick, you can make it half the size and save a lot of weight.


Carefully cut your current "resin" piston in half and use that for your wooden portion.

On barrel end of your piston, why are there 2 small rubber washers and one large?
They should be all the same if actually sealing to a pipe.


It isnt sealing against the face of the 1 inch pipe.. rather the face of the reducing bushing. It seems to seal fine.. if not ill replace the smaller ones with another large one and see what happens
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