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My Pneumatic Pen Gun -- piston doesn't activate

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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My Pneumatic Pen Gun -- piston doesn't activate

Unread postAuthor: plinkin » Sun Jun 12, 2011 5:25 pm

Hi, newbie here, but been lurking for forever.
The question is self explanatory, really...

The sizes of the chamber and barrel are identical to JSR's, same schrader valve pilot/filling setup.
BUT the piston is different -- it consists of two segments of metal tube, 2 and 3 sizes smaller than the chamber inside of each other (that part is the same as JSR's), and is filled with hot glue to provide support for the actual sealing face, which is a circle cut from a neoprene washer. The barrel itself is bare -- the small barrel presses against the washer/piston to seal.

However, filling it up to 200 psi with a shock pump, then pressing (both ways -- gently and with force) on the valve stem with a BB, THE PISTON WILL NOT ACTUATE!!!


WHY?
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Unread postAuthor: Gun Freak » Sun Jun 12, 2011 5:59 pm

Your piston may be too loose and the chamber air could be coming out the pilot... Is that happening?
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Unread postAuthor: plinkin » Sun Jun 12, 2011 6:13 pm

Well, yeah all the air is dumping out the pilot. I can't see how my piston is too loose, though. It's two sizes down from the chamber's diameter, or two sizes apart from what they sell at the hardware store (I'm guessing a 1/32" gap around the piston). Maybe sanding the leftover epoxy (from the rubber washer gluing job) from the sides of the piston removed too much material off?
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Unread postAuthor: clemsonguy1125 » Sun Jun 12, 2011 6:30 pm

The gap is too big on this small of piston valve, I would use a solid rubber piston like JSR did, this worked well for me.
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Unread postAuthor: plinkin » Sun Jun 12, 2011 7:25 pm

Darn it. I don't have any 1/4" thick solid rubber!!! Where did you get yours?

Otherwise I may just make a new piston, smaller gap.
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Unread postAuthor: warhead052 » Sun Jun 12, 2011 7:53 pm

Plinkin, it may be too small, but try a eraser from the end of a pencil.
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Unread postAuthor: saefroch » Sun Jun 12, 2011 9:52 pm

You can also try to stamp a gasket from gasket material, by sharpening the end of a pipe fitting of the same ID as the piston housing.
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Unread postAuthor: Crna Legija » Sun Jun 12, 2011 9:53 pm

warhead052 wrote:Plinkin, it may be too small, but try a eraser from the end of a pencil.


it would be too week rubber at 200psi you get a donut, buy a rubber door stopper they are real hard type of rubber.
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Unread postAuthor: clemsonguy1125 » Sun Jun 12, 2011 10:44 pm

I actually use a keychain.
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 13, 2011 12:06 am

I agree with the piston-too-loose diagnosis. A schrader is a marginal pilot at best with tiny flow and is not tolerant of loose fitting pistons.

An interim solution would be to wrap the piston with masking or scotch tape until it sealed well yet was loose enough to be moved with one finger.

All my recent pen guns have used solid rubber pistons very effectively.

You might want to post photos or diagrams, it will help us help you :)
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Unread postAuthor: plinkin » Mon Jun 13, 2011 1:29 am

OK! Thanks for all the help.

I just took apart the gun, and after lots and lots of scraping away old epoxy :x I got to take out the piston.

ID of chamber was 0.282", while OD of piston was 0.250". That's a 0.016" gap all around the piston. (o boy i finally found a good use for my caliper!!!)

So I went ahead and epoxied a 0.281" OD aluminum tube that fit perfectly around the piston, just to decrease the gap a bit. Of course, when it's dry, I'm going to reduce the diameter down just a bit to make it slide freely (poor man's lathe -- drill press) with some sandpaper. What's a good diameter to shoot for? Maximum/minimum OD, perhaps?
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 13, 2011 1:34 am

You have a caliper, I presume you have a calculator, do some math ;)

There is a magic piston diameter where if it is any smaller, it will not pilot, with a bit of mental effort you can figure it out.
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Unread postAuthor: plinkin » Mon Jun 13, 2011 1:45 am

doing the math... sorry if it hurts anyone's head by posting it here :) just thought I might.

ID of chamber = .282"
so area = ((0.282 / 2)^2) * pi = 0.0624580035
OD of piston = ???
What in the world is the cross section of the flow of a typical schrader valve? Can't seem to figure out how to measure the flow of the valve core.
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 13, 2011 2:11 am

A schrader has a 2mm hole with a 1mm stem through it, that's 0.02357 cm<sup>2</sup> cross section for the air to flow through.

If the area of the gap (chamber internal cross section area - piston cross sectional area) is bigger than this value, the piston will never actuate.
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Unread postAuthor: plinkin » Mon Jun 13, 2011 8:05 pm

All right. So the glue job dried up, and I miscalculated the diameter of the piston, then turned it down to size. Only after that did I realize I'd gotten it all wrong. GAHH!

So I made a whole new piston, new rubber seal, and turned it down to the right diameter this time (0.273628216 in -- I got it down to .277 so that's good enough).

The whole thing is drying up right now... Oh my goodness waiting is the hardest part of ANY build!!! (besides hand-sanding and filing etc etc)
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