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Pilot valve help for piston guns

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: Engineererrrr » Tue Feb 28, 2012 8:34 pm

ohkay.... i kinda understand

Yeah 1" by 1" is (Pi x r square x length)

So lets say im building a coaxial piston gun where the chamber is a 1" pipe and barrel is 1/2" pipe, the barrel in the 3' chamber is 2' and 8" The piston is 3 inches and has 1/2 a mm equalizer spacing around. So there is a 1" by 1" pilot volume, will a blowgun pilot the piston back, venting all the air within?
If not what would you recommend?

Thanks for all the help so far!
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Unread postAuthor: Technician1002 » Wed Feb 29, 2012 12:14 am

The pilot will work as long as the piston does not leak too much air from the chamber into the pilot. The pilot pressure must be reduced enough below the chamber pressure so the forces on the piston from the chamber and pilot are reversed resulting in force to open the piston instead of holding it closed.

The piston will move only due to the forces on it. The pilot is to reduce the force from the back of the piston so it is less than the force from the chamber trying to open it.

A large air leak can prevent the required pressure drop in the chamber while the chamber pressure leaks out past the piston.

Learn of the force.. Use the force.
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Unread postAuthor: Crna Legija » Wed Feb 29, 2012 2:26 am

0.5mm if around the piston is to much for a blow gun for sure, 0.05 now your talking :lol:
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Unread postAuthor: sharpshooter11000 » Wed Feb 29, 2012 2:36 am

I agree with crna legija, you need to make the piton a better fit. I would reccommend using a QEV to pilot the piston and a blow gun to pilot the QEV.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 29, 2012 2:38 am

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Unread postAuthor: Engineererrrr » Thu Mar 01, 2012 4:49 pm

if i made a piston with hot glue poured into a 3" length of 1" pipe with dw40 on the side, then dried, pushed it out, is that a good piston for this scenario?

When you say the pilot volume should be less of the chamber's, do you mean before the pilot is open? or when i open the valve, the loss of air pressure in the pilot volume that the pressure in the chamber forces the piston back exhausting the chamber pressure out the barrel....right?
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Unread postAuthor: jackssmirkingrevenge » Fri Mar 02, 2012 1:28 am

if i made a piston with hot glue poured into a 3" length of 1" pipe with dw40 on the side, then dried, pushed it out, is that a good piston for this scenario?


http://www.spudfiles.com/forums/making- ... t8919.html

for lower pressure applications the techniques can be transferred to hot glue.

When you say the pilot volume should be less of the chamber's, do you mean before the pilot is open? or when i open the valve, the loss of air pressure in the pilot volume that the pressure in the chamber forces the piston back exhausting the chamber pressure out the barrel....right?


Image

Two things to consider:

1) piston leakage: some air will leak past the piston into the pilot chamber, unless you have o-rings fitted.

If there is more air leaking past the piston than is escaping from the pilot valve, the piston will remain shut.

Think of the pilot chamber as a bucket, the piston leakage is a hose you're filling it with and the pilot valve is a hole in the bucket.

If more water is coming into the bucket than what is coming out of the hole, it will never empty - so that flow from the hose must be reduced to a dribble.

2) pilot chamber size: continuing the bucket analogy from the previous point, if the bucket is our pilot chamber we want to empty it as quickly as possible. Two ways to help with this:

- make your bucket (pilot volume) as small as possible. For a given hole (pilot valve) size, a small bucket will empty much faster than a large bucket

- make your hole (pilot valve) as big as possible. For a given bucket size, a big hole (pilot valve) will empty the water out much quicker than a small hole
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Unread postAuthor: Engineererrrr » Fri Mar 02, 2012 11:37 pm

thanks for the help its really appreciated, from what i understand, ill build a piston with o rings fitted, and make my pilot volume relatively small compared to my chamber volume.

also, are you saying hotglue pistons should be used for low pressures, and should use epoxy pistons for higher psi s? If so could i mold my piston like above but filled with epoxy instead?

sorry to be this much of a bother...
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Unread postAuthor: whoa044 » Thu Jun 21, 2012 10:46 am

I use a 1/4 ball valve to pilot my 3/4 valve. In all honesty, I thought I had to go out and get a bigger pilot valve, but I ended up super gluing some teflon tape around the piston to make the seal better.

How well your piston prevents air flow around it will factor greatly with how you'll have to vent your pilot. Wrapping my piston *once* with teflon tape added .004" to its thickness.

Basically, I coated some teflon tape with super glue just like this, and wrapped it around my piston.

EDIT: Basically, if you piston looks like it's "loose" in the piston guide, you may want to glue some teflon tape around it. Oh and be sure to apply grease to the teflon after it has dried.
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