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Will this piston 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 piston design work?

Unread postAuthor: ThatGuy » Sun Jan 22, 2012 8:42 pm

I have 2 ideas of how to make a shoulder fired piston cannon.

They both have a 1" Barrel surrounded by a 2" chamber.


The first, and the preferred design uses a piston made from a 1 1/4" PVC cap with a rubber seal for the barrel that is connected to a smaller piston by a screw. The smaller piston fits into a smaller pipe that goes right up to the back of the 2" PVC cap. Then it will lead into a pressurized hose that leads to a blowgun. The blowgun is the exhaust valve/ trigger.

This design is more complicated but is more compact and would allow me to fin the whole setup into a 3" PVC Pipe for extra protection, since I am planning to use it for airsoft.


Image

The second is design uses just the 1 1/4" PVC cap as a piston and uses a 1/2" PVC pipe and Ball valve to vent.


Image

Would the first design work? If not is there anything I could change to make it work?
Also is the 2" chamber and 1" barrel going to give me very much power at 70-90 PSI?
Thanks
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Last edited by ThatGuy on Thu Jan 26, 2012 11:19 pm, edited 1 time in total.

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Unread postAuthor: Gun Freak » Sun Jan 22, 2012 8:52 pm

As drawn, design 1 won't work.
Use design 2, but make the pilot volume small enough to use a blowgun pilot.
You'll have good power with those sizes.
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Unread postAuthor: frigaardj » Mon Jan 23, 2012 12:37 am

The first design is liable to break quite easily and seems overcomplicated. The second design should work, although the main concern is exhaust valve throughput (1/4 of an inch may not have the flowrate to vent the pilot almost instantly).

Also, think about how you will service your piston if it breaks? Finally, you don't want to lose any projectiles into the chamber when you muzzle-load so sticking a bolt through the internal end of the barrel could help.
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Unread postAuthor: Technician1002 » Mon Jan 23, 2012 11:30 am

A quick note on "venting the pilot almost instantly".

When the valve seat is quite small in relation to the piston diameter, then the valve tends to open early and not very fast. The valve opening causes the chamber pressure to drop (duh). If the pilot is vented slowly, this will cause the piston to close again before it fully opens.

A piston on the other hand that has a large diameter valve seat in relation to the piston OD will remain closed until after the pilot pressure has vented to a low value. When this type opens, it does so with a pop and slams all the way open.

An article in the wiki covers this some.

After building a few cannons, I am a fan of small diameter pistons in relation to the valve seat size.

http://www.spudfiles.com/spud_wiki/inde ... ston_valve

Large valve seat diameter is slightly less than the piston OD. Works very well.
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Unread postAuthor: ThatGuy » Mon Jan 23, 2012 4:50 pm

If I were to put a 1 1/4" Pipe behind the barrel and make a piston that size could I make #1 work?
Kind of like Technician's picture?

The reason I'm pushing for a hose is because between the 2" and 3" PVC there is not very much room and although I could probably get 1/2" PVC to fit next to the 2" It would be very difficult to install the trigger/ valve.

as for maintenance I was planning on having threaded pieces a few inches in front on the piston. (close to the middle of the cannon).

Thanks for all the suggestions.
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Unread postAuthor: Technician1002 » Mon Jan 23, 2012 6:26 pm

Yes. My picture is of my Mouse Musket. Construction details can be found in my sig. The barrel is 1 inch. The chamber is 2 inch. The piston is 1-1/2 inch. I used a trick to place a 1.5 inch female adapter and pipe inside the breech of the 2 inch chamber. I hope this helps.
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Unread postAuthor: ThatGuy » Mon Jan 23, 2012 6:41 pm

So if I used a 1" cap as a piston would a blowgun at the end of a 2ft hose be able to effectively vent it?

and finally would it be better to make the back cap on the chamber screw on or use the threaded coupling in the middle?

Thanks for all the help! I'm excited to finally get a piston cannon working! :D
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