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First Airgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 5:46 am

Alster370 wrote:I think this is because my pilot is a 15mm ball valve and that its too big?


There is no such thing as a pilot valve that's too big ;) If you open your ball valve fast enough it should work even at lower pressures.
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Unread postAuthor: Alster370 » Sat May 07, 2011 6:54 am

wouldn't it allow more air to flow out of the pilot rather than the barrel? :?
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 7:02 am

Most QEVs have cup pistons that effectively act as one-way valves, allowing air to flow from the pilot to the chamber but not vice versa.

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This means that the air that can flow out of the pilot cannot flow out of the barrel.
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Unread postAuthor: Alster370 » Sat May 07, 2011 8:38 am

From visual inspection mine seems to be just a flat face piston, not a cup one. Which would explain some air coming from the pilot when firing. :roll:
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 8:45 am

Alster370 wrote:From visual inspection mine seems to be just a flat face piston, not a cup one.


Photo?

It could be a diaphragm with a equalisation hole, like in a piston valve.
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Unread postAuthor: Alster370 » Sat May 07, 2011 8:57 am

sure, but I can only take one from the barrel side as the pilot side is epoxied already.

I could give you the site I bought it off in the meantime if that helps

http://www.hosefit.co.uk/catalog/produc ... cts_id=357
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 9:06 am

That should look like the diagram I posted earlier, have you ever taken it apart?
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Unread postAuthor: Alster370 » Sat May 07, 2011 9:27 am

Nope, I didn't think I needed too. It works unlike my piston valve so I was pretty chuffed.
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 9:29 am

A good test would have been to plug the chamber end and blow through the barrel end to see if air comes out of the pilot end, is this possible?
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Unread postAuthor: Alster370 » Sat May 07, 2011 9:51 am

yeah I could give that a try, ive got the chamber epoxied shut. Il will do it in an hour or so as my fittings have been redone with epoxy and are drying.

So if I blow into it and no air comes out the pilot , do I have a cup piston after all?
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 9:53 am

Alster370 wrote:So if I blow into it and no air comes out the pilot , do I have a cup piston after all?


Technically yes, though what might happen if tolerances are not too good is that your breath alone will not provide sufficient pressure for the cup to seal against the sides of the valve body.
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Unread postAuthor: Crna Legija » Sat May 07, 2011 10:52 am

jackssmirkingrevenge wrote:
Alster370 wrote:So if I blow into it and no air comes out the pilot , do I have a cup piston after all?


Technically yes, though what might happen if tolerances are not too good is that your breath alone will not provide sufficient pressure for the cup to seal against the sides of the valve body.



thats exactly what happens with my qevs.
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Unread postAuthor: Alster370 » Sat May 07, 2011 10:59 am

no air came out of the pilot at all when I tested it. :D I have a new problem though, when pressurising I get a tiny bit of air leaking from the qev down the barrel. This is rather annoying because it just pushes the ammo out of the barrel before ive pressurised it fully. I think its because the 1/2 to 15mm fitting is not sealing at the bottom of the threads. would an o ring fix this?
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Unread postAuthor: Crna Legija » Sat May 07, 2011 11:04 am

umm no fittings on the out side should make the qev leak, id say you have a bit of dirt on the piston or seat.
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Unread postAuthor: Alster370 » Sun May 15, 2011 1:29 pm

Well its been a productive last week and the gun is nearly finished (mechanically) just one tiny leak to get rid of :evil: and then I can clean it up and move onto the stock. I managed to get the qev to seal at 6psi which is pretty good and stop the air leaking past it.

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Not bad to say that a fridge freezer is my workbench and I have to use an adjustable spanner that doesn't lock in place properly :D


Let me know what you think so far
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