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1/2" QEV + 1/2" Slide-valve - first multi-shot build problem

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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1/2" QEV + 1/2" Slide-valve - first multi-shot build problem

Unread postAuthor: shrimp » Tue Jan 06, 2015 7:26 am

Greetings everyone, short time lurker, first time poster here.

In the past I have built a few combustion cannons and couple of sprinkler valve pneumatics but after finding this site decided to have a go at building a QEV multishot.

I have mashed together a 1/2" QEV, a 1/2" slide, a schrader valve, sodastream bottle and some fittings all together but it doesn't quite operate as I was expecting.

Image
http://s23.postimg.org/vv095ywiz/IMG_2496.jpg

Basically, as I'm trying to fill via the schrader it seems that the QEV's diaphragm isn't really making a a good seel, there is some faint hissing and it is "popping off" by itself on some instances, also, if I am fortunate enough to be able to fire via the slide valve, it doesn't reset and allow for a secondary firing.

As I understand, there are some modifications that can be made to a QEV but I cant seem to find any clear threads, if someone could point me in the right direction it would be mucho appreciado.

Or,

If I'm doing everything wrong, then, please let me know.
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Re: 1/2" QEV + 1/2" Slide-valve - first multi-shot build pro

Unread postAuthor: vng21092 » Wed Jan 07, 2015 11:51 am

If you want multiple shots you'd have to put a spring behind the piston in the QEV to help it reset. At this point when you pull back the slide valve all the air rushes out, as its suppose to.
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Re: 1/2" QEV + 1/2" Slide-valve - first multi-shot build pro

Unread postAuthor: shrimp » Fri Jan 09, 2015 8:36 pm

many thanks,

I have put a small spring behind the qev's diaphragm, upon a test at about 90 psi it sill vented almost all of the air, should i keep trying to place stronger springs or are my test pressures too low to allow the spring to do anything?
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Re: 1/2" QEV + 1/2" Slide-valve - first multi-shot build pro

Unread postAuthor: vng21092 » Sat Jan 10, 2015 12:10 am

From what I understand the spring doesn't need to be strong, it's only there to push the piston against the barrel. If you put a stronger spring you run into two problems. 1, you shorten the span which the valve stays open, not letting all the air go to use. 2, the valve might not open at all. What I suggest you do (I don't know how else it can be done) is control the volume inside the chamber, the way you have it set up in that picture, you'll only have one shot. You'll have to separate the chamber into two volumes. Like so:

Image

A is the amount of air you'll want to use to propel your projectile. Search around the forum for recommended C:B ratios, use what you like.

B is a valve which controls flows from C--> A

C is essentially your source, the big tank. So C feeds A, and B shuts off the flow so when you shoot, not all the air rushes out. After you shoot, you open up B so C can recharge A, and then you shut off B again and so on. Hopefully now you see why the spring is necessary. When recharging, the spring seals off the barrel which allows air to get behind the piston through an equalization hole.

This was my latest build, it uses the exact same configuration I'm explaining to you, in my case, look at the labels.

Image

Just read around the thread, I'm sure you'll find something useful http://www.spudfiles.com/pneumatic-cannon-database/topic26107.html
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Re: 1/2" QEV + 1/2" Slide-valve - first multi-shot build pro

Unread postAuthor: DrGPaw » Thu Aug 13, 2015 4:55 am

i have made a prototype of this but a bit different, i just wanted to know how you filled up the carbonator, did you make a hole and put the schrader valve in ? and fill it up from there?
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Re: 1/2" QEV + 1/2" Slide-valve - first multi-shot build pro

Unread postAuthor: voi9viper » Mon Nov 09, 2015 8:10 am

I made one of these from comercial parts for the QEV and slide valve. I don't know how long your barrel is, but I think about 6" of 2" PCV pipe, one end caapped and the other end reduced to go where currently you have the hose connected to the QEV. Connect the hose to the open end of your slide valve. In one position the slide valve allows air to flow through it, pushing the pistion in the QEV closed and air goes into the dump chamber you just made. When you open the slide valve pressure on the QEV is released and the piston slides back, sealing the path to the slide valve and opening a path to the barrel. Also, the slide valve closes flow from the tank, so each shot only uses the amount of air in the dump chamber, which you can change by resizing it. You want a little more than your barrel volume in it. There are examples everywhere.
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