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QEV Pilot question

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 Pilot question

Unread postAuthor: vng21092 » Fri Apr 17, 2015 5:08 pm

Well shooting season is coming back, and after all these years I FINALLY bought me a PCP pump :D
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Got me a fill valve and cross section off Amazon, and a gauge and ball valve off co2reg:
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Hello high pressure (and it could even go higher :wink: )
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So now I've got a question, it turns out those little block QEV's can handle 600 psi, BUT my pilot valve is very hard to push now. I'm still using the same one from my last project
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So, what would be the best way to pilot it now, what design would allow for the least amount of force to activate it?
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Re: QEV Pilot question

Unread postAuthor: jrrdw » Sat Apr 18, 2015 6:27 am

So, what would be the best way to pilot it now, what design would allow for the least amount of force to activate it?


Spring loaded hammer setup? Long trigger for leverage hinged just below the button?
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Re: QEV Pilot question

Unread postAuthor: Tep » Sun Apr 19, 2015 9:47 am

I was having the exact same problem recently and happened upon a solution you might be interested in.

I basically created an 8mm cross between a balanced spool valve and a QDV valve from a store bought (or ebay bought) three way valve. I'll do a quick and very rudimentary paint drawing of it for you and I can post some pictures tomorrow if you need.

I found that I can pilot the QEV up to 600 psi now no problems at all, if I keep the valve I made lubed, and can get some crazy power out of it now.

I don't trust it above 600 for obvious reasons but at that pressure I can fire an 8mm steel ball through 30+ sheets of cardboard stacked together using a similar set up to what you have. Compared to piloting with a Schrader valve (even with literally <0.5ml or pilot volume) I get more than double the performance from my QEV, my cardboard sheets chrory was only about 15 - 17 sheets previously.

valve.png
valve.png (17.51 KiB) Viewed 876 times


The grey thing is the aluminium rod i replaced the previous valve parts with.

green dots are o'rings

the black thing is the previous valve service screw

blue is the valve body.

This kind of set up could be made easily from a block of aluminium with a few holes drilled in it or just use your imagination.

http://www.ebay.com.au/itm/S3R-M5-2-Pos ... 4d2ec2c6ac

the valve I modded was essentially like the one in the link but without all the garbage hanging off the front.

After about 200psi the little button was almost impossible to push by hand and the flow was too low to effectively pilot the QEV, it did fire but the power was low compared to using a slide valve, which I use as a base line comparison.
Now that the valve is counter balanced I can actuate it with one finger.

Good luck with it.
I'm sure you will find a way of making a counterbalanced valve that works for you.

Tep
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Re: QEV Pilot question

Unread postAuthor: vng21092 » Sun Apr 19, 2015 1:51 pm

hmm that seems really interesting, please post some pictures if its not too much a hassle, much appreciated. I've never understood how piloting with a schrader valver worked, I tried it once, but it seems the flow rate is too low, am I missing something?
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Re: QEV Pilot question

Unread postAuthor: jrrdw » Sun Apr 19, 2015 3:40 pm

vng21092 wrote:hmm that seems really interesting, please post some pictures if its not too much a hassle, much appreciated. I've never understood how piloting with a schrader valver worked, I tried it once, but it seems the flow rate is too low, am I missing something?


Your pilot volume was to big or the piston wasn't fitted correctly.
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Re: QEV Pilot question

Unread postAuthor: Tep » Mon Apr 20, 2015 3:52 am

Ok, here go's.

IMG_2044.jpg


This is the balanced valve I use to pilot now, it's awesome, literally the best thing I have made.

IMG_2045.jpg


This demonstrates the hole I drilled through the end cap of the valve, I also expanded the in and out ports of the valve. That tube is 8mm like I showed in the diagram above.

IMG_2039.jpg


I have trimmed down the head of the schrader to allow some of the pin to be exposed for striking.

IMG_2040.jpg


This is the fitting I use for schrader piloting, Notice that the body of the fitting is fully taken up by the rubber of the schrader, I also glue them in place with some crazy rubber metal glue that I have (it's from locktite, can't remember the number). This has the effect of reducing the pilot volume significantly but I also put in a bunch of plastic washers that compress against the body of the QEV and the schrader rubber to further reduce the volume to basically just the area of the piston times the travel distance between open and closed (about 0.2 - 0.5ml is what worked for me).

In order for this to work in a chamber filled QEV you need a hole in the piston - HOWEVER- in my experience when people tell you to make a 1mm hole in the piston this is WAY to big at high pressures with a small pilot like a schrader. The hole I made was make with a 32gauge needle, it's so small I can't see through it but can suck some air through with my mouth. That is only a 0.2mm hole, but I found that it didn't work without it or with a hole much bigger.

That being said remember that you will loose heaps of performance using a schrader and a much better way is to use a fast flowing valve. If you are feeling brave you could try hooking the pilot up to a pressure washer handle?? might be a bit fiddly but it would really help the ergonomics of your gun.

Finally, I know you didn't ask about it but, a very small change to your chamber set up would provide some advantages; by replacing the current male "T" fitting with a female one and turning it 90 degrees to where you currently have it you could further improve ergonomics and improve the flow from your chamber to barrel. (i'll attach a sketch)...

T chamber.jpg
T chamber.jpg (13.78 KiB) Viewed 816 times


I guess this would also be more sturdy but let me know what you think.


Regards

Tep
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