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Basically, will this QEV set up work?

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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Basically, will this QEV set up work?

Unread postAuthor: ham_machine » Sun Sep 28, 2014 9:10 am

quick exhaust diagram converted.jpg
I know.. I misspelled Schrader, I hope I have the correct terminology.


qev1.JPG
Ebay brought Quick Exhaust Valve.
qev1.JPG (77.75 KiB) Viewed 423 times


This Quick Exhaust Valve in this diagram is a shop brought QEV. One of those ones from ebay.

I have spent a week or so researching into different ideas of Quick Exhaust Valves. I noticed that allot of people have placed the air compressor/ or large chamber of compressed air behind the pilot valve, and used a sliding valve as the pilot valve. (If pilot valve is the correct terminology). They can then easily refill the chamber of air (coming off port A) by sliding the "sliding valve" back.

However, this is not what I want to do.. Would it be possible to fill the pressure from port A, the same way that the people have from port P? I understand that this will only be a single shot fire, however, I plan to add a ball valve attached to a large storage of air.

So, as my diagram illustrates, the pressure is filled into a chamber connected to port A. My main concern is that this sudden burst of air (from filling the chamber) might activate the piston inside the QEV to move backwards and open up the outlet.(which will release the air through port R)! From the diagrams I have seen on the internet, this could be a possibility. But as I have never worked with Quick Exhaust Valves before, I have no idea.

Could someone tell me if this diagram will work or not? Before I commit myself to buying materials..
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Re: Basically, will this QEV set up work?

Unread postAuthor: Technician1002 » Sun Sep 28, 2014 9:54 am

Without a minor modification it would not work. The reason is the pilot would always be at 0 PSI so it will always be in fire mode when you try to fill it. A small port is needed to bleed some pressure into the pilot. This can be done using a small refrigeration type capilary tube salvaged from a bar fridge, or the piston modified in the valve so some leakage is allowed from the chamber to the pilot. This modification is most often done by gently filing a very small groove in the side of the piston so pressure leaks into the pilot while filling.
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Re: Basically, will this QEV set up work?

Unread postAuthor: ham_machine » Sun Sep 28, 2014 1:07 pm

"Technician1002"


Ah thank you, I think I understand now. So is there some kind of seal on the piston that allows the pressure to get from P to A but not the other way round (unless I modify it of course)

diagram, sliding valve converted.jpg


Another thing, What if I was to put a sliding hand valve as the pilot valve. One end is connected to port P, and the other connects up with port A and the chamber. My theory is that the pressure in both chamber P and A will be the same while the sliding valve is in position A. This is what will happen anyway if i were to fill the pressure from behind the pilot, or if I was to modify it like you said. So with the sliding valve in position A, the pressure will be the same in the pilot chamber, as it is in the main chamber. By sliding it into position B (as illustrated) the sliding valve will seal the two chambers, and instantaneously eject the pressure from the pilot chamber through the holes- and so activating the QEV.

Should this work?
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Re: Basically, will this QEV set up work?

Unread postAuthor: Technician1002 » Sun Sep 28, 2014 1:18 pm

Hmm Interesting configuration. Never thought of that configuration, but it looks promising. This would allow a very fast fill if you used a quick connect instead of a tire valve, and permit retaining the chamber charge while triggering the QEV. I like it. Great Idea. Other than impossible to disassemble if service is needed, looks great.
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Re: Basically, will this QEV set up work?

Unread postAuthor: ham_machine » Sun Sep 28, 2014 1:44 pm

"Technician1002"


Thanks, well I might give this a go then.

Just thought of a possible problem though.. As we all know - if the pressure is greater at port A than it is at Port P, the pressure difference will pull back the piston and activate the QEV. I am going to have a ball valve (in the location of the schrader valve) which will connect this system to a large storage vessel.

What I am worried about is - when I release the pressure from the large storage vessel (using the ball valve) into this system I have illustrated above. Lets say the ball valve is located very close to Port A - like it is in the diagram. The pressure released will reach Port A first before it gets to Port P, and the pressure difference may activate the QEV (because it takes longer for the pressure to reach Port P than it does Port A)- it has to go round a bend!

My design requires the ball valve to be closer to Port A..

Or perhaps Pressure doesn't work anything like I think it does.. What do you think?
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