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Piston vs. Piston [Update: video comparison]

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: MRR » Sat Feb 21, 2009 5:36 pm

I always use ball bearing grease before I insert the piston.
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Unread postAuthor: sputnick » Sat Feb 21, 2009 5:58 pm

IMO, If you made it, and it works better, then who cares what anyone else thinks, run with it! I think your theory as to how it works makes sense, then again I am no engineer... :D :wink:

Hell I think just the fact that it works proves you right! :twisted: Take that science :!:
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Unread postAuthor: Brian the brain » Sat Feb 21, 2009 5:58 pm

Does the solid piston slide all the way into the opposite side of the Tee?

In other words..does it block almost all flow from the reservoir to the front of the piston?

Than might explain it.
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Unread postAuthor: MRR » Sat Feb 21, 2009 6:12 pm

The piston and the hose barb that I'm using meet in the middle of the tee and both pistons have exactly the same length.
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Unread postAuthor: Brian the brain » Sat Feb 21, 2009 6:17 pm

Time to contact Mythbusters then...
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Unread postAuthor: maggotman » Sat Feb 21, 2009 6:27 pm

the piston in my getto cannon "the one with the gas tank as the chamber"
im using 50mm washes but thay always bend and jam up so i have to replace them every 10 or so shots
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Unread postAuthor: kablooie » Sat Feb 21, 2009 6:39 pm

It seems to me like the only thing that could by making the solid hot glue piston harder to actuate would be the friction the hot glue makes against its housing. Even with grease, I've noticed that hot glue is much stickier than other materials. Also, the pressure on the hot glue could be causing it to deform and press harder into the housing, creating more friction.
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Unread postAuthor: sputnick » Sat Feb 21, 2009 6:42 pm

kablooie wrote: the pressure on the hot glue could be causing it to deform and press harder into the housing, creating more friction.


I very much doubt this is the case, why would pressure pushing down on something on all sides cause it to expand?
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Unread postAuthor: kablooie » Sat Feb 21, 2009 9:21 pm

What I was thinking was that the force (from the incoming air) behind the piston is pushing the piston forwards onto the barrel, and because the barrel limits the surface area on that side of the piston, the air pressure from behind is greater than from the other side, causing the piston to be squashed up against the barrel. That pressure could be causing the piston to widen as it is squeezed. I've had sealing faces be punctured from the force of the piston, so I know that the power is there, but for this gun, that force could be negligible, so I could be wrong. It's a possibility though.
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Unread postAuthor: psycix » Sun Feb 22, 2009 11:36 am

I've got two theories.
1-The hot glue compresses a little under pressure, making a larger gap.
2-At the hot glue piston, the air flows straight past it. At the other one, the air goes turbulent in the space between the washers. This causes extra resistance for the air and thus less air leaks out of the pilot.
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Unread postAuthor: MRR » Sun Feb 22, 2009 11:43 am

to #1: how i mentioned in the video, I already used different materials on solid pistons, all results were the same.

#2: that's one more theory to think about. Thanks for the comment.
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Unread postAuthor: FishBoy » Sun Feb 22, 2009 12:37 pm

Well the video results sertainly speak for themselves. I can't wait to see that piston used for a more constructive (destructive) purpose :D .
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Unread postAuthor: Copperboy » Sun Feb 22, 2009 1:32 pm

I was just about to say the same thing as FishBoy, also from personal experience, and I think that is the single most important factor here. The one with the washers are more slippery (against the inside of the tee). It looks like when you made the hot glue piston you've greased the mold up so to get the finished thing out more easily (at least that's what I did), the remaining grease on the piston combined with the somewhat squishy consistence om hot glue makes it stick like an angry biiiiaaatch! :wink: At least that's what I think! I really like your rig by the way, looks sturdy and clean!
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Unread postAuthor: Brian the brain » Sun Feb 22, 2009 3:12 pm

After checking the video i'm convinced the first piston is slow, the second one works like it should.The second does what all my pistonvalves do, so nothing " special" , just...a piston valve.

So it's not a question of it being faster than a solid piston, it's a question of what's wrong with the first one.And frankly, it's not worth investigating.
I'd scrap it, and keep the other one.

A piston that doesn't give you vapor is just plain faulty.Especially without a barrel.
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Unread postAuthor: MRR » Sun Feb 22, 2009 3:31 pm

B.t.B., maybe your right, let's burry this topic. The piston design is good, now I can start to finish something.
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