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HELP FOR IDEA on multi-shot QEV gun.

This area is for the discussion of BB or airsoft sub machine guns, and or other small arms such as pellets.
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HELP FOR IDEA on multi-shot QEV gun.

Unread postAuthor: jmez1996 » Thu Dec 09, 2010 9:13 am

Hey all
I need help on the powerplant of my new gun im gonna start soon.
I want to make a multi-shot airgun using a QEV (see below for link) which is either semi-auto or having some system to some how allow multi-shot (i'd be happy with more than 5 shots).
I also want to have the main air tank on board, as apposed to being attached to a external tank etc.

I will probably end up using a blow gun as a pilot and if anyone has any ideas or plans about how this could be achieved, it would be much appreciated.

I am able to get additional valves etc. of ebay but this isnt preferable.

thanks again.
James.

(this is the QEV i planned on using http://cgi.ebay.com/Pneumatic-Quick-Exh ... 1312wt_775)
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Unread postAuthor: saefroch » Thu Dec 09, 2010 3:45 pm

Jmez... LOTRO?

Connect a fairly large reservoir tank to a valve or other flow constrictor, then that to the normal chamber side of the QEV, with a secondary chamber in between. Then simply attach the pilot valve (which must have greater flow when piloting than the flow constrictor, if you go that way) and voilà! When piloted, the secondary chamber acts as a chamber, and is re-filled by either opening the valve or the equalization through the flow constrictor. Dig around on the pneumatic showcase and you can probably find a few examples.

You could also go this route.
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Unread postAuthor: jmez1996 » Thu Dec 09, 2010 6:16 pm

@saefroch

yeh that sounds good.
so could i use a regulator as the flow restrictor?
and what other types of pilots can i use? or is the blow gun the best?
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Unread postAuthor: saefroch » Thu Dec 09, 2010 6:40 pm

You could use a regulator as a flow restrictor, but you'd have to evaluate flow through the regulator, as compared to the flow out the blowgun valve.

Blowgun valves are not the best pilot. They are however the most comfortable pilot, IMO and thus are often used and recommended if piloting the valve does not require a valve with high flow. If you want to use a blowgun to pilot, I'd install a ball valve just before or after the regulator, then shut that when firing and open it to refill the secondary chamber, so there is no concern about the QEV failing to pilot because of flow in being greater than flow out the pilot. I don't have any personal experience with QEVs, but I think you may need to "prime" from the pilot side to make the diaphragm or piston thing seat.

Alternatively you could use a ball valve, a modded sprinkler valve, or even another QEV or a piston valve to pilot the main QEV. The better your pilot valve, the faster and more efficient the main valve will be (to a point).
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Unread postAuthor: Hotwired » Thu Dec 09, 2010 10:50 pm

Discovered one method a while back:

http://www.spudfiles.com/forums/copperh ... t9916.html

Essentially the idea is to partially vent the pilot side so the chamber is partly dumped in response to the reduced pilot pressure.

The method I sketched and I've used myself uses a spool/sleeve valve or a blowgun and a ball valve to allow the pilot volume to expand into a small but sealed expansion area and so drop its pressure, causing the chamber to dump slightly through the barrel. The forces equalise as pressures drop and the QEV reseals with pressure still in chamber and pilot. Ready to fire again.

BrianTheBrain did a different variant which involves a hammer valve instead, hit the hammer valve on the pilot, it lets out a blast of air, lowering the pressure of the pilot side, dropping its pressure and again, the chamber vents slightly through the barrel. Valves reseal as forces equalise again and it can be fired again.

You only get a few shots as it's not all that precise. I was getting about 5-6 shots from my cannon. Think Brian was getting similar.

Still, multi shot capability is entirely there.
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Unread postAuthor: irisher » Thu Dec 09, 2010 10:59 pm

@hotwired Do you have an estimate for the volume of your expansion chamber?
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Unread postAuthor: Hotwired » Thu Dec 09, 2010 11:11 pm

Short answer, no.

Long answer, perhaps later.

I did a while back but the original design was accidental and ONLY happened because of a very unusual safety I put on my cannon design.

This is the control section. There's a T with fill port, blowgun and ballvalve from left to right.

To fire the ball valve safety was opened and then the blowgun was pulled. That gave a clear path to vent the pilot.

However if you accidentally pulled the blowgun when the ballvalve was closed the cannon would not fire. You'd just hear a small *PSH* as a small burst of air escaped through the blowgun. That was the safety.

HOWEVER if you then let go of the blowgun, and opened the ballvalve to the fire position the small volume between ball valve and blowgun acted as an expansion chamber for the pilot and dropped the pressure of the pilot volume slightly.

Was a surprise for me the first time. Especially since the cannon was still loaded with a solid plastic slug at that point... Fortunately pointing at the ground mind you.

Image

Here you can see the 30cm of the 8mm copper tube which as well as a small volume within the QEV was acting as the pilot.

Image
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Unread postAuthor: Lockednloaded » Thu Dec 09, 2010 11:28 pm

so basically what your saying is that all you need for a repeater without constant air is a plugged off slide valve for your QEV?
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Unread postAuthor: Hotwired » Thu Dec 09, 2010 11:35 pm

Yup.

Alternatively two manual valves but using a slide valve means you only need to operate one valve albeit forward and backwards (unless you use a spring return).

You need some fiddling with the size of your pilot volume and the expansion chamber to fine tune it but the theory works and that's just detail.
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Last edited by Hotwired on Thu Dec 09, 2010 11:39 pm, edited 1 time in total.
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Unread postAuthor: Lockednloaded » Thu Dec 09, 2010 11:39 pm

I have a cannon with a huge chamber volume and a 3/4" QEV piloted by a slide valve, would I need to make a small expansion chamber behind the slide valve? and how does your design propose to vent the air behind the slide valve?

EDIT
Wait are you putting the slide valve backwards?
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Unread postAuthor: Hotwired » Thu Dec 09, 2010 11:48 pm

You reverse the slide valve.

In normal use it provides air to a system when pushed forwards.

Then when you pull it back it vents air from the system through revealed holes as you pull the slide back.


Now if you turn it round you do the opposite to fire but you plug the end not attached to the pilot to create an expansion chamber.

In this mode, in one position, the slide will connect the expansion chamber to the pilot, allowing the pressure to drop.

And in the other it simultaneously disconnects the pilot and expansion chamber, seals the pilot again and vents the expansion chamber through its venting holes in the sides.

A diagram would help I'm sure but I have put a simplified diagram in the thread I linked to.

Have stuff to do in ~7h however and it's time to get this mornings sleep.
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Unread postAuthor: Lockednloaded » Thu Dec 09, 2010 11:53 pm

I understand now, so say a pilot volume was 100cc's what size would an expansion chamber have to be? double? triple? or does it depend solely on pressure?
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Unread postAuthor: Hotwired » Fri Dec 10, 2010 12:04 am

Purely on guesstimation:

The 8mm copper tube I was using as pilot and the ID of the fittings in the control section are much the same. Ignoring volume then and calling it length of tube:

Lets say 35cm for the pilot giving a bit for volume within the fittings at the QEV end.

And 5cm for the expansion chamber.

That works out to an expansion chamber 1/7 the size of the pressurised pilot volume.

So every time it's connected up it's dropping pilot pressure by increasing the volume by 1/7.

That's been giving me about 5-6 reasonable shots - take into account pressure and chamber volume, details in link in my sig. After that pressure gets too low to really use.
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Unread postAuthor: Lockednloaded » Fri Dec 10, 2010 12:11 am

It would seem that once you get the expansion chamber to minimum volume, increasing it would use more air, but give more power. I believe I have all the necessary part to test this tomorrow
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