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Sticky Piston

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: jrrdw » Mon May 19, 2008 5:41 am

Your equilization hole can be small and does not have to be under the sealing face. If the hole is small, the stored air will take the path of least resistance 1st (out the barrel). If the hole is small 1/16th, not enough air from storage will escape back through it to make a difference.
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Unread postAuthor: daberno123 » Mon May 19, 2008 3:06 pm

So I could really put the hole somewhere else on the piston like on the cap? That makes sense that the air would go through the barrel, since after all that has the least path of resistance, just like jrrdw said.

I'll try it on the washer first, and if that doesn't work I'll try the cap, and if for some reason they both don't work, I've got a check coming from Mcmaster. :D
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Unread postAuthor: jrrdw » Mon May 19, 2008 6:29 pm

I would just put it in the side up close to the sealing face. Then you should be golden.
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Unread postAuthor: Gippeto » Mon May 19, 2008 11:23 pm

I think psyix has the right idea as far as increasing frontal area is concerned.

Your new piston looks promising, if I might offer a suggestion though;

If you make the o-ring grooves a little "sloppy", (deeper and wider) and then drill a hole half into the o-ring groove (on the chamber side), it'll work for equalization, and double as a check valve. :D

(simple floating o-ring)

I did this with the piston on my copper gun, it works very well.

I'll put up a pic, look close, the hole looks almost like a "chip" in the groove.
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Unread postAuthor: Biopyro » Tue May 20, 2008 1:12 pm

I had the same problem with my last piston housing. I tried so much to try and solve the problem, but the sealing face just slipped out the barrel under pressure.
In the end I just got a 63mm tee (instead of 50mm) so that the sealing face is about 55mm dia and the barrel is about 40mm dia. If I were you, I'd get a new housing.
Unfortuneatly I have the same problem as you with my new but the cause must be different!
:evil: it makes me so mad to keep getting it wrong! :cry:
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Unread postAuthor: daberno123 » Tue May 20, 2008 9:20 pm

Gippeto wrote:If you make the o-ring grooves a little "sloppy", (deeper and wider) and then drill a hole half into the o-ring groove (on the chamber side), it'll work for equalization, and double as a check valve.


Thanks alot Gippeto, I think that's the way I'm going to do it. I'm definitely going to need a check valve in this piston, I just got the o-rings today and it is a very tight fit.

It took less than 30 hours to ship from McMaster. :shock:
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Unread postAuthor: n00b » Tue May 20, 2008 9:45 pm

I agree, there appears to be too much piston clearance. That thing should be so tight you need to really shove it in there. Also, I think you might be jamming due to the weight of the iron pipe cap in front. As the piston disengages, it will force the front end down and the back up making it jam. See if you can't replace the cap with a 1/4 in. bolt, washer and nut, for plenty of holding power and much less weight. I've had similar problems with piston valves in the past.
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