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3 inch piston help

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: Gaderelguitarist » Fri Apr 15, 2011 11:33 am

I've already said to make one from wood. Any scrap will do. Glue it to size and then turn it on a lathe or drill press. you could even drill out the back side of it and fill it with foam. Wood is very cheap, and holds up to plenty of abuse if done right.
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Unread postAuthor: cannon monkey » Fri Apr 15, 2011 4:33 pm

I hot money but not alot a I can buy stuff but can't pay like 35+ for the piston
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Unread postAuthor: Technician1002 » Fri Apr 15, 2011 5:29 pm

Some of my projects simply wait for funds. It's something the US government will learn to do soon when it can't extend credit (deficit) indefinitely. Spending money faster than it comes in is a bad idea and is not sustainable.

It's called living within your means. My latest build is both waiting for warmer weather, a refill on an Oxygen tank, and buying fittings for the barrel and a metal 2.5 inch pipe cap for the breech. It doesn't mean I don't build, but it does mean there are serious delays once in a while.
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Unread postAuthor: Tobin » Sun Apr 17, 2011 12:05 pm

Im building a piston with an internal check valve to my metal gun . The problem is, the piston weights 180 grams. According to ggdt that doesnt affect the performance, but with all this talks about light weight pistons, it confuses me? Will this heavy piston affect my performance after all?
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Unread postAuthor: Gaderelguitarist » Sun Apr 17, 2011 12:09 pm

Yes. Which is easier to move; a feather or a brick?
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Unread postAuthor: Labtecpower » Sun Apr 17, 2011 12:23 pm

And, more important: if you use PVC, it will get smashed to pieces by the heavy piston.

Even my QDV shows deformation at the end of the valve, wich is 3 mm solid steel.
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Unread postAuthor: Tobin » Sun Apr 17, 2011 12:33 pm

Gaderelguitarist wrote:Yes. Which is easier to move; a feather or a brick?

Of course it will affect, but how much? If i could ask for a simple answer on a complicated question?
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Unread postAuthor: jackssmirkingrevenge » Sun Apr 17, 2011 12:44 pm

Simple physics tells us Force = Mass x Acceleration.

Acceleration therefore = Force / Mass

Force is fixed by the exposed area of the piston coupled with the pressure difference provided by the pilot valve.

For Force X and Mass Y, Acceleration will be A.

If I double Mass to 2Y, Acceleration will be halved to 0.5A and so on.

Valve opening time has a massive effect on launcher performance, try modelling your launcher on GGDT with a "generic valve" and play with the times.
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Unread postAuthor: Technician1002 » Sun Apr 17, 2011 1:08 pm

For short barrels you want a valve faster than 3ms. For long barrels you want faster than 10ms as a general rule.

If you shoot very lite projectiles out very short barrels, you want an insane valve speed.

lite projectile and slow launch
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lite projectile and fast launch
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Empty pop cans vs steel plate
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Unread postAuthor: Tobin » Sun Apr 17, 2011 2:51 pm

Okay. The best option then is that I continue with my heavy brass piston and make some test shoots with it. Then I'll do one of the aluminum drill it out and fill it with foam and compare the guns performance with the different pistons :)
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Unread postAuthor: Gaderelguitarist » Sun Apr 17, 2011 4:37 pm

Make sure to also take note of the wear the piston creates on the piston housing. Labtecpower brings up an extremely good point concerning the impact resistance of the housing material.
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