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QEV triggering help

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 triggering help

Unread postAuthor: HoppinS10 » Wed Nov 17, 2010 6:13 pm

Hello all I have been viewing this site for a while now and finally decided to build a QEV cannon. I purchased a 3/4 inch valve from mcmaster and assembled a cannon without a barrel to test it out. I first tried to use a blowgun as a pilot valve with no luck. When i depressed the handle air just vented out of the blowgun. I then stepped up to a ball valve which did indeed trigger the QEV however if i leave the ball valve closed and remove the airline, the valve fires much harder than if i were to simply open the ball valve. I guess my question is do I need a bigger pilot valve or is the air rushing back into the valve from my air supply too fast for the valve to work to its full potential?
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Re: QEV triggering help

Unread postAuthor: Gun Freak » Wed Nov 17, 2010 6:30 pm

HoppinS10 wrote:if i leave the ball valve closed and remove the airline, the valve fires much harder than if i were to simply open the ball valve.

You mean open right?
I think this same discussion has already gone on somewhere, buy I think a blowgun should be piloting it fine...

Ahh found it, check it out.
http://www.spudfiles.com/forums/3-4-qev-wont-actuate-t21640.html
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Unread postAuthor: HoppinS10 » Wed Nov 17, 2010 6:59 pm

I did mean closed. So basically i am piloting the valve by removing the air line from my compressor. I did read the entire thread that you linked to before I posted but it seems like his valve is not fireing at all where as mine is fireing just not well. I will try to post a picture or diagram in the next day or so

Is it possible that my compressor is flowing too much air into the pilot area? I may try to restrict the air flow from my compressor into the pilot area and see if that improves the performance of the QEV.

Edit by jrrdw, double post.
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Unread postAuthor: Lockednloaded » Wed Nov 17, 2010 7:44 pm

If you put a check valve between the airline and the pilot you will cut down on dead space, allowing you to use a smaller pilot valve. everything that flow freely behind the piston is pilot volume
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Unread postAuthor: ThornsofTime » Thu Nov 18, 2010 12:27 am

Is it possible that my compressor is flowing too much air into the pilot area? I may try to restrict the air flow from my compressor into the pilot area and see if that improves the performance of the QEV.


No, the reason you fill a QEV from behind the piston is because it pushes the piston forward, sealing off the barrel. The AMOUNT of air flowing into the pilot area isn't a factor. There are 'equalization' holes in the piston that allow air to flow past it and into the chamber once the piston has moved to the fully closed position. Adding flow control to your compressor line will have zero effect on your QEV.

A few questions.
Have you modified your blowgun for greater airflow? or is it mounted on strait out of the package? What pressure are you trying to fire at? also a general picture of your setup might help with diagnosis.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 12:35 am

If your airline is filling the pilot area at a faster rate than the blowgun can empty it, the QEV will never fire. The whole point of piloting is to empty the pilot area, if you make a small hole in a bucket while still filling it with a garden hose it will never empty.

Either reduce the flow from your airline, or get yourself a three way valve that shuts off the airline when you open the pilot, this is the best option to be economical with air.

Another alternative is to connect the airline to the chamber side, and modifiy the QEV with a spring and small equalisation hole.
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Unread postAuthor: HoppinS10 » Thu Nov 18, 2010 7:46 am

If your airline is filling the pilot area at a faster rate than the blowgun can empty it, the QEV will never fire.



Thats what i think is going on here. The blowgun can not flow more air then being supplied by the compressor so it never fires. The ball valve can flow more air than being supplied but the QEV is not working to its potential.

Either reduce the flow from your airline, or get yourself a three way valve that shuts off the airline when you open the pilot, this is the best option to be economical with air.


Are there 3 way valves that I can buy that would work in this situation or is it something that has to be built?
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 18, 2010 8:10 am

HoppinS10 wrote:Are there 3 way valves that I can buy that would work in this situation or is it something that has to be built?


Thanks to our Polish comrade's almost religious dedication to the subject, all the information you need should be found in this thread :D if you have any specific queries it might be worth dropping POLAND_SPUD a Image
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Unread postAuthor: HoppinS10 » Thu Nov 18, 2010 11:58 am

Thanks. That thread helps out tremendously!!
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Unread postAuthor: HoppinS10 » Sun Nov 21, 2010 5:40 pm

so before buying a 3-way valve, i experimented with an UNMODIFIED 3/4 inch sprinkler valve i had laying around. It was discouraging that even with a sprinkler valve on the pilot side it still didn't actuate the QEV as well as if i just pull the airline off and let it dump out the air fitting. I am toying with the idea of putting a spring in the QEV and drilling a small hole in the diaphram so I can fill from the chamber side. Once i figure out how to post pictures on this site I will do so.
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Unread postAuthor: Technician1002 » Sun Nov 21, 2010 6:34 pm

Another option is to add a 1/4 ball valve in your air line. Charge the chamber and shut it off.
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Unread postAuthor: HoppinS10 » Sun Nov 21, 2010 7:15 pm

Another option is to add a 1/4 ball valve in your air line. Charge the chamber and shut it off.


I have done that and it works well. I guess im going to end up buying a 3-way valve just so the gun will fire easily. My goal is to make it semi auto but I wanted to get all the kinks worked out of the pilot venting first. Thanks for your suggestion though.
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Unread postAuthor: HoppinS10 » Sun Nov 21, 2010 8:47 pm

Finally got some pictures [img]<a%20href="http://s1106.photobucket.com/albums/h379/hoppins10/?action=view&amp;current=IMG_20101121_200036.jpg"%20target="_blank"><img%20src="http://i1106.photobucket.com/albums/h379/hoppins10/IMG_20101121_200036.jpg"%20border="0"%20alt="Photobucket"></a>[/img]
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Unread postAuthor: HoppinS10 » Sun Nov 21, 2010 8:47 pm

[img]<a%20href="http://s1106.photobucket.com/albums/h379/hoppins10/?action=view&amp;current=IMG_20101121_200042.jpg"%20target="_blank"><img%20src="http://i1106.photobucket.com/albums/h379/hoppins10/IMG_20101121_200042.jpg"%20border="0"%20alt="Photobucket"></a>[/img]
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Unread postAuthor: jackssmirkingrevenge » Sun Nov 21, 2010 11:39 pm

Looking good, were you planning on adding a magazine?

Regards the equalistation hole, as long as your intended pilot valve has more flow that the hole you make it should be enough. A 1/32" hole should suffice in this case. If you're cutting a new one out of suitable material as siggested elsewhere, a rough cut means you might not even need to make an extra hole.
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