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New Piston Design (Around the Barrel)

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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New Piston Design (Around the Barrel)

Unread postAuthor: Xerxys » Sun May 18, 2008 4:11 am

I am in to pneumatic spudding (I have produced three cannons) and recently had an idea for a new kind of piston.

As opposed to sealing against the end of the barrel the end of the barrel is capped off and a piston (The piston is shown in grey) slides around the outside.

When the piston is forward it covers a few large holes ( A bit like a breech) put when it is piloted it slides back opening them

Two (or three or four etc.) home-grown one way valves ( The valves are similar to the one in Ant's simple airgun) are installed in the Piston itself.

They are simply mildly strong springs pushing a ball bearing against a bit of rubber to seal them.These might be unnecessary ( You could get away with just small holes) but will improve effiancy

The second diagram shows a gun using the piston, and is relatively simple once the above is understood. The only things that need to be noted are
- The barrel passes through the tee which only supports it.
- The the tee and the pipe leading to the valve has holes in it so when the valve is opened air is released

I am posting to share the idea and to ask if the extra expense (This requires O-rings and a lathe) counters any improvement in power.

P.S. If anyone spots any problems with the design let me know.

P.P.S. I apologize for my appalling art-work (The computer was giving me trouble) and voracious use of brackets.

-Sorry, Just noticed, there should be a tee were the piece with the valve meets the barrel but the atachment keeps deleting it
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Attachments
new piston design.JPG
This is the Piston
new piston design.JPG (21.82 KiB) Viewed 646 times
co-ax using new piston.JPG
And this is the Cannon using it.
co-ax using new piston.JPG (17.8 KiB) Viewed 645 times

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Unread postAuthor: ghostman01 » Sun May 18, 2008 4:23 am

maybe if you could make a animation it might just clear up a few things, looks pretty resonable from what i can work out, good work keep it up. Welcome to spudfiles.
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Unread postAuthor: Hydra » Sun May 18, 2008 5:49 am

Wow i think you might have just created an entirely different type of valve.
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Unread postAuthor: Brian the brain » Sun May 18, 2008 6:04 am

It's a slide valve inside a coax.
Hard to get it right and make it seal but it will work.
Not more efficient than a regular piston valve, but it opens up possibillities.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 6:13 am

that will work but has some extreme amounts of pilot volume
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Unread postAuthor: jrrdw » Sun May 18, 2008 6:50 am

With holes in the barrel large enough to not restrict flow the sealing end of the barrel would be to weak to with stand a decent amount of pressure. Thats if the slide didn't pull the end of the barrel off inside the tee.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 7:02 am

he just need 4 1/4" holes for a 1" barrel and 4 1" holes for a 4" barrel
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Unread postAuthor: psycix » Sun May 18, 2008 7:07 am

Well it may work, but a normal piston will probably more effective.
Your pilot volume is HUGE. And as far as I can see there are no flow improvents. It has no benefits above a normal piston valve.

It wont do any good, but atleast you tried coming something innovative.
And thats always good. :)


Btw, one check valve in the piston will be enough.
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Unread postAuthor: jrrdw » Sun May 18, 2008 7:11 am

spudfarm wrote:he just need 4 1/4" holes for a 1" barrel and 4 1" holes for a 4" barrel


You just cut the end of the barrel off.
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Unread postAuthor: SpudFarm » Sun May 18, 2008 7:52 am

what do you mean jrrdw?
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Unread postAuthor: jrrdw » Sun May 18, 2008 8:19 am

4 quarters equals 1 whole. 4 one's equal 4. The 4D rule applies to the end of a pipe diameter, not the circumference of the pipe. Also it's not 4:1, it's the piston only needs to move 1/4 of the barrel diameter to get optimun flow (for lack of better description of the 4D rule).
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Unread postAuthor: bluerussetboy » Sun May 18, 2008 9:14 am

The only problem I see is this piston will probably need o-rings. Either on the barrel(easier, but weakens barrel) or on the interior of the piston(harder). Pilot volume is easy enough to limit by blocking off the end of the chamber. The strength of the barrel, if everything is made out of PVC than the mostly accepted maximum PSI is 150psi, as long as the ports are small and alternated it should hold up to 150psi.

I can see this as a good start to a breech loaded RPG or Vogt pattern.
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Unread postAuthor: Velocity » Sun May 18, 2008 10:51 am

spudfarm wrote:he just need 4 1/4" holes for a 1" barrel and 4 1" holes for a 4" barrel


No...

If you want the flow into the barrel to equal the flow out of the barrel, then (pi)(R^2) = (x)(pi)(r^2), where R is the barrel diameter, r is the diameter of the drilled holes, and x is the number of drilled holes. So say you had a 1" barrel, and you wanted to drill 1/4" holes in it. Using the equation:

(pi)(1/2)^2 = x(pi)(1/16)^2
x = 64 holes
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Unread postAuthor: D_Hall » Sun May 18, 2008 11:03 am

jrrdw wrote:
spudfarm wrote:he just need 4 1/4" holes for a 1" barrel and 4 1" holes for a 4" barrel


You just cut the end of the barrel off.


Holes can be staggered.
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Unread postAuthor: Velocity » Sun May 18, 2008 12:04 pm

D_Hall wrote:
jrrdw wrote:
spudfarm wrote:he just need 4 1/4" holes for a 1" barrel and 4 1" holes for a 4" barrel


You just cut the end of the barrel off.


Holes can be staggered.


And anyways, the circumference of the outside of the barrel is not equal to the diameter; it is equal to the diameter times pi. Therefore, it would really take more like 12-13 1/4" holes to cut off the barrel.
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