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Ideas for why my QEV is leaking [new gun]

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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Ideas for why my QEV is leaking [new gun]

Unread postAuthor: andras23 » Thu Nov 17, 2011 9:37 am

Hi guys nice to meet you all, I'm a new member to spudfiles but have definitely been coming back to this site over and over again and watching DIY videos. Hope I dont break any forum rules!

But anyway I am constructing my first air gun with a QEV. I used a blow gun to pilot the piston however I'm not sure of all the little essential things necessary for it to work flawlessly. The chamber is a small scuba tank rated at 3000 psi but I only have been putting in 80 psi with a small hand pump.

The problem I am having is that air is seeping out of the exhaust when the ball valve is opened so I guess that means that the seal between the piston and the connection inside is not perfect. I tried a stronger spring to keep it closed but when I press the blow gun trigger all of the air rushes out of the blow gun and the piston does not move backwards. Even with a weak spring the piston does not move backwards.

I was wondering does the piston need to be perfectly round inside the QEV? I'm using a 3/4" Tee since I already made the piston for that, but perhaps I should change it to a 1/2" Tee to also decrease my pilot volume since there are so many connections needed to get my blow gun to connect to the 3/4" as you can see.

Also:
- Is a large pilot volume detrimental to the triggering of the QEV?
- Is there a minimum pressure required for triggering the piston?
- Should I switch to a 1/2" Tee QEV to eliminate some of those connections to my blow gun?

Note: my piston is not 100% cylindrical as there is a layer of thin plastic around the hot glue and at one point it overlaps by a few mm.

any help would be much appreciated! thanks :D
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air gun.jpg
various images, first attempt at air gun with homemade QEV

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Unread postAuthor: jhalek90 » Thu Nov 17, 2011 1:13 pm

you pilot valve needs to be faster.
Or, make your piston tighter in the housing.

I would recommend trying a ball valve as a replacement for the blow gun.

Also, the pilot volume seems a little bit massive.
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Unread postAuthor: Labtecpower » Thu Nov 17, 2011 1:18 pm

You have connected your blowgun the wrong way. If you put the other side on the pilot, it won't leak anymore.

If you have the piston in the housing, and you watch through the tube, can you see light around the piston? If you see light, it isn't tight enough.

Also, the pilot volume seems a little bit massive.


Agree :)
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Unread postAuthor: Crna Legija » Fri Nov 18, 2011 1:50 am

the piston does not need to be perfectly round but need to fit really really well less then 0.5 mm gap between the wall and the piston is a good start. take the spring out no need for it, and make sure you sealing faces are flat and square.

also try a bigger pilot valve 1/4'' ball valve would be great.
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Re: Ideas for why my QEV is leaking [new gun]

Unread postAuthor: jackssmirkingrevenge » Fri Nov 18, 2011 3:16 am

andras23 wrote:- Is a large pilot volume detrimental to the triggering of the QEV?


Yes, it means slower pressure reduction in the pilot area and consequently slower piston opening

- Is there a minimum pressure required for triggering the piston?


There is a minimum pressure difference between pilot and main chamber.

- Should I switch to a 1/2" Tee QEV to eliminate some of those connections to my blow gun?


Yes.

But your main problem as I see it is poor piston fit.

Cast a better one ;)
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