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QEV Help (firing out pilot)

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 Help (firing out pilot)

Unread postAuthor: svdesigns » Sun Dec 18, 2011 12:45 pm

Hey People been researching a lot in the past few months and need a bit of help. So heres my problem, my qev isnt working, its firing out the pilot exit. Heres the thing i made a mock up piston out of wood and glued a rubber washer on the end, it was working fine (about 200 shots) untill the wood started to wear out ( running at 130psi from an air compressor) i sanded the piston to smooth it out and its never fired since.

I figured the piston was too small and the air was equalizing too fast. I have since made a mold out of epoxy its a snug fit with about .2mm play in the shaft but slides freely in the T piece, however its still only firing out the pilot end (feels like 20psi comming out, same as when it was working)

I have tried to
1. make another piston with more play in the shaft( aswell as changing the spring rates and removing the spring all together, normally it would shoot the piston out of place after 1 shot but atleast i knew it worked)
2. Reduce pilot volume by removing some fittings.
Im thinking maybe im not expelling enough air for the pressure to drop, but then again it worked before. Any Ideas guys? sorry about the ranting just trying to be informative. heres some pictures of my setup
length between the piston and washer is about 25mm(tried with and without springs)

my scrappy mock up gun :)
I got abit annoyed trying to fix this so ive orded and QEV and slide valve pilot from ebay along with some nice 8mmODx6mmID and 6mmODx4mmID barrels got an idea to make a nice pump action reload barrel by sleeving the two.
I still want to work out whats wrong with this QEV, any help will be much appreciated thanks guys.
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Unread postAuthor: Brian the brain » Sun Dec 18, 2011 1:02 pm

Step 1:How big is the sealing surface?

It needs to be smaller than the diameter of the piston.
(The smaller the difference between the two the harder it is to actuate.)
If that is the case move on to step 2
Step 2:
A spring is not necessay for a piston valve to work but if you want multishot capacity it helps.
It should be very weak.Just strong enough to move the piston, no more.
Also, it shouldn't touch the sidewalls or it might snag.
Check it and move on to step 3

Step 3:
A piston should fit snugly but still be able to slide in forward and backwards without a problem.
The amount of blowby should be as small as possible..

Step 4:
Chamber volume should be significantly larger than the pilot volume.
In any case, pilot volume should be as small as possible.

Step 5:
The pilot valve should dump the pilot volume effectively.
Bigger is better, but overkill doesn;t improve performance.
make sure the pilot valve you select is able to dump the pilot volume instantly.
If needed, modify it or replace it by a better one.

Remember that some materials, like rubber can be squashed by the pressure changing it's dimensions.
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Gun Freak wrote:
Oh my friggin god stop being so awesome, that thing is pure kick ass. Most innovative and creative pneumatic that the files have ever come by!

Can't ask for a better compliment!!
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Brian the brain
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Unread postAuthor: svdesigns » Sun Dec 18, 2011 1:31 pm

ok so i guess its the sealing surface. its almost the exact same size as the piston, nothing for the pressure to push on? i guess i need a bigger T piece because i cant make the seal surface any smaller.
Thanks for your help mate.
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