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QEV mod

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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QEV mod

Unread postAuthor: skyjive » Fri Jun 26, 2009 11:11 am

So I'm sure I've seen this around somewhere but couldn't find it with a search, so: How does one mod a QEV to get it to fill from the chamber side? I am trying to make an automatic using a QEV and popoff valve and the constant air flow into the pilot area makes it difficult for the pilot valve to be effective if you have to fill the QEV from the pilot side, but if it were filling from the chamber side I imagine this problem would be eliminated.
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Unread postAuthor: Technician1002 » Fri Jun 26, 2009 11:37 am

Filling from the chamber side is a balancing act.

If the pilot eq port is too big, it won't fire or needs a really big pilot valve. If too small the rising pressure in the chamber will compress the air in the pilot area firring it. It's why sprinkler valves honk when the chamber is filled too fast.

To get an initial seal, the use of a spring is required to close the valve as pressure in the pilot won't blow it closed. The spring pressure is again a balancing act. Too stiff and the eq provided pressure in the pilot area and limited opening force may prevent firing.

As a best of both worlds, I built some of my piston valves with 2 fill valves. One on the pilot to close it and provide an initial holding pressure and one on the chamber for a rapid fill.

Working it that way for semi auto would be a PIA. If you are into robotics, a fill sequencer can be built with 2 solenoids. One to close the piston and one to fill. A third could be added for full electric sequence for full or semi auto.

Contest idea anyone? I won't be able to contest build this year. Idea up for grabs.
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Unread postAuthor: spudtyrrant » Fri Jun 26, 2009 11:43 am

i'm pretty sure you open it and add a small spring behind the piston to make it fill from the chamber but it works fine and has been used in at least 2 full auto valves i know of without making an modifications the pop-off will release air faster than your fill valve will fill it so you piston will actuate and then refill and repeat the cycle Edit: also agree with what tech says its a delicat balancing act
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Unread postAuthor: psycix » Fri Jun 26, 2009 2:10 pm

-Use a spring behind the piston
-Make a tiny hole with a needle
-Make sure you arent filling too fast

If it doesnt fill properly:
-stronger spring
-bigger hole
-fill slower

But, as the other said, try to keep the hole as small as possible.
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