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marble gun help

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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marble gun help

Unread postAuthor: koolaidman » Mon Feb 18, 2008 2:35 pm

I built this gun yesterday just off the top off my head. Ive already gone through about 3 bike pumps and dont want to mess with compressors of co2, so i built this gun with a built in homemade pump. In the pic below you can see how it works. Its got a copper tube (sleeved in 1" pvc) that seats a piston. There is one check valve to allow air into the chamber and another to allow air back into the tube as the piston retracts. Its got a psi guage and a ball valve for the trigger. It works perfectly, its just that... its kinda weak. Somehow it only pumps up to probably like 5 psi, not even registering on the guage. Im positive that the valves are opening right. It leaks a little bit, but i dont think it would account for it. Please let me know if there is a better piston i could make. My initial concern was that there would be too much dead space in the piston. My guess though is that the piston compresses to about 6 times less the volume and goes from both ends of the copper tube. Im not sure what pressure this equates too because i dont know how to use the ideal gas law. Advice, help anyone?
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Unread postAuthor: SpudFarm » Mon Feb 18, 2008 3:03 pm

i think the problem is the dead space. your piston dosent get all the way to the check valve and just compresses the air in the dead space between the piston on the deepest point and the check valve.

it is acctualy a really strange thing that it don't load air :?:
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Unread postAuthor: koolaidman » Mon Feb 18, 2008 3:43 pm

Alright, well, the piston i made is crap. It would be great for a valve, but for a pump, its gotta be much beefier and no air can slip past it. I need to find a way to make an extremely airtight 1/2" copper piston, please help!
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Unread postAuthor: SpudFarm » Mon Feb 18, 2008 3:49 pm

mabe if you make a aluminium mould in a 1/2" pipe and cut multiple o-ring groves in the aluminium you get? should make a REALLY tight fit and no air to pass between also use a pipe with no inside damage like microscopic (?) dents and things like that
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Unread postAuthor: koolaidman » Mon Feb 18, 2008 7:24 pm

That could work, its just i have absolutely no means of doing that. Im really pissed cause the whole gun works beautifully its just the damn piston. C'mon guys there must be a way to make a perfect seal for this. At least tell me its possible and i didnt build this gun in vain.
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Unread postAuthor: Gippeto » Tue Feb 19, 2008 12:33 am

Are you really sure it's the piston? Those look like water checks to me. Do they have a soft (rubber) sealing face? When I have a pump that won't pump, it's usually the suction check leaking.(discharge checks are always spring loaded)
Add sealing faces if there are none. Then try again. With 6:1 compression ratio you should be able to get around 80-90psi.(ideally)

That being said, you do have an awful lot of dead space.

Where:

P=initial pressure (100psi)
V=initial volume (20 cu.in.)
T=initial temperature (20c)

p=final pressure (unknown)
v=final volume (30 cu.in.)
t=final temperature (20c)

P*V*T=p*v*t

for your purposes P*V=p*v

So 100psi * 20cu.in. = 2000

therefore p*v=2000

you know that v = 30cu.in.

so

p=2000/v or 2000/30 or 66.67psi

Therefore 20 cu.in of air @100 psi will have a pressure of 66.67psi when expanded to 30cu.in. (assuming a constant temperature)

Also, atmospheric pressure tends to be about 14.7psi (this varys with altitude and weather systems)

You have a 6:1 compression ratio so 6* 14.7psi=88.2psi
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Unread postAuthor: koolaidman » Tue Feb 19, 2008 11:08 am

Wow, thank you gippeto, thats so helpful, i thought i was far more complicated. I was opting for more, but close to 80 psi is still quite effective. The checks are for rated for wog, they seal quite well with air. Im sure its the piston thats leaking. Ive taken it off the gun and pumped it with my hand on the other end. It pushes alot of air, but the pressure is completely gone after a second. Could someone please give me some help on an idea for a piston.
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Unread postAuthor: Gippeto » Tue Feb 19, 2008 7:35 pm

I'll try to help you, but you must list the tools you have access to.

Could you access a lathe anywhere? Drill press? Hand drill? Metal files? Soldering torch?

If you don't have access to tools like these, a viable piston might be cast from epoxy. Never give up. :)

Are the checks leaking at all? Use soapy water in a squirt bottle to verify.
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