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Help with QEV valve, first time builder.

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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Help with QEV valve, first time builder.

Unread postAuthor: loaferz » Mon Dec 24, 2007 1:35 am

So Im really excited about building my first pneumatic cannon, got all the parts together and thought I had everything working, but I'm running into some problems.

First off, let me describe my design, I am going for a portable design, so I am deciding to use a bicycle pump connected to a schrader valve in order to charge up the system.

how it should work:

1. Air is pumped in via a bicycle pump, cylinders start to charge and because the ball valve is closed, pressure on the inlet should still keep the qev closed
2. ball valve is opened, thus inlet pressure drops and all air from cylinders goes to the outlet barrel.

The problem is, that when I pump up the system, i get one pump,it charges in the tanks, then the qev discharges. I think this is because pressure is getting lost through the inlet port. Is this because of the bicycle pump? Do I need a different design? Or are QEV's not possible to run without an air compressor.

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Unread postAuthor: clide » Mon Dec 24, 2007 1:50 am

It should be possible to do with a bike pump, but you need to make sure everything is sealing well. Check all your plumbing and make sure that isn't leaking and check the fitting that hooks up to the schrader, if you can't get that fitting to seal well against the schrader you may have to cut it off and try a different kind or maybe even add a check valve between the schrader and the pilot area.
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Unread postAuthor: MrCrowley » Mon Dec 24, 2007 1:58 am

<A HREF="http://www.spudfiles.com/forums/viewtopic,p,123928.html#123928">My QEV cannon</a> has a similar setup to yours and works fine with a cheap shock pump I got at Wal*Mart.

The only thing that may be a problem is if your chamber is quite large you may have to pump faster to get it to seal. Maybe try a pump with more volume, that should fill the chamber up faster.
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Unread postAuthor: MaxuS » Mon Dec 24, 2007 5:29 am

As clide said:
Put on a check valve, its most likely that when you draw air into the pump on the upstroke that its lowering the pressure slightly inside the pilot volume. A check valve will keep air from escaping whilst pumping.
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