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I was in Home Depot yesterday looking at copper fittings.

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: Technician1002 » Thu Jul 09, 2009 5:46 pm

MRR wrote:Keeping the pilot volume as small as possible is an practical issue for me.

I experienced that it is hard to make a really well fitting piston without professional equipment. If the piston doesn't fit properly or the equalization is to big a small pilot volume helps to reach the actuation point.

Your QDV uses a mechanical "trigger" and industrial QEV's are fabricated on a very high scale.

Another point is, if you try to go semi / auto, you fill from the chamber. A small pilot volume will make the piston seal faster because of the faster equalization. I think that's the main reason why I prefer small pilot volumes.


Good points, all of them. Well done.
:wav:
One more to throw in; With an EQ port, the larger the pilot volume, the Hissssssssssssss is longer, the more chamber air is lost through the eq port into the pilot. A small pilot volume is desirable. I try to keep it about 1/3 to 1/2 the volume the piston displaces when it moves. Best of both worlds when adjusted. Too small is bad. Too big is bad. Best is somewhere in the 1/4 to 1-1/2 the displacement volume depending on the trip point pressure. Often the bumper space, ball valve, and pipe in between is enough to meet these requirements. :wav:

Done properly, the result is sssSSSSsssPOW. :D
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Unread postAuthor: jeepkahn » Thu Jul 09, 2009 6:37 pm

Technician1002 wrote:
MRR wrote:Keeping the pilot volume as small as possible is an practical issue for me.

I experienced that it is hard to make a really well fitting piston without professional equipment. If the piston doesn't fit properly or the equalization is to big a small pilot volume helps to reach the actuation point.

Your QDV uses a mechanical "trigger" and industrial QEV's are fabricated on a very high scale.

Another point is, if you try to go semi / auto, you fill from the chamber. A small pilot volume will make the piston seal faster because of the faster equalization. I think that's the main reason why I prefer small pilot volumes.




Good points, all of them. Well done.
:wav:
One more to throw in; With an EQ port, the larger the pilot volume, the Hissssssssssssss is longer, the more chamber air is lost through the eq port into the pilot. A small pilot volume is desirable. I try to keep it about 1/3 to 1/2 the volume the piston displaces when it moves. Best of both worlds when adjusted. Too small is bad. Too big is bad. Best is somewhere in the 1/4 to 1-1/2 the displacement volume depending on the trip point pressure. Often the bumper space, ball valve, and pipe in between is enough to meet these requirements. :wav:

Done properly, the result is sssSSSSsssPOW. :D


or you could use a cup style piston seal and you won't have to worry about eq holes or pilot compression....
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Unread postAuthor: boyntonstu » Thu Jul 09, 2009 6:51 pm

jeepkahn wrote:
Technician1002 wrote:
MRR wrote:Keeping the pilot volume as small as possible is an practical issue for me.

I experienced that it is hard to make a really well fitting piston without professional equipment. If the piston doesn't fit properly or the equalization is to big a small pilot volume helps to reach the actuation point.

Your QDV uses a mechanical "trigger" and industrial QEV's are fabricated on a very high scale.

Another point is, if you try to go semi / auto, you fill from the chamber. A small pilot volume will make the piston seal faster because of the faster equalization. I think that's the main reason why I prefer small pilot volumes.




Good points, all of them. Well done.
:wav:
One more to throw in; With an EQ port, the larger the pilot volume, the Hissssssssssssss is longer, the more chamber air is lost through the eq port into the pilot. A small pilot volume is desirable. I try to keep it about 1/3 to 1/2 the volume the piston displaces when it moves. Best of both worlds when adjusted. Too small is bad. Too big is bad. Best is somewhere in the 1/4 to 1-1/2 the displacement volume depending on the trip point pressure. Often the bumper space, ball valve, and pipe in between is enough to meet these requirements. :wav:

Done properly, the result is sssSSSSsssPOW. :D


or you could use a cup style piston seal and you won't have to worry about eq holes or pilot compression....


A little elaboration please.
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Unread postAuthor: jeepkahn » Thu Jul 09, 2009 6:58 pm

a cup style piston seal allows air from the pilot to the chamber, but once piloted the chamber pressure seals the "cup" to the walls allowing 100% seal, but because it will allow pressure from the pilot it will not compress the pilot volume... Have you ever taken apart a store bought bike pump...the seal used in them is a cup style....

I use a design similar in all my cannons...

basically take mark.f's piston posted earlier and use smaller washers on the chamber side of the rubber sealing discs, when there is more pilot pressure than chamber pressure the disc will flex forward to allow air past, but when the pilot side has less pressure than the chamber, the chamber pressure forces the disc flat and out to seal the sides...
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