Using a QEV with the chamber and the supply as one ?

[evilvet]

I am now the proud owner of a basic eBay QEV, furthering my knowledge of pneumatic launchers for high speed photography here in wonderful land of Oz.

This is probably one for Poland or Wyz to shoot me down , so to speak.

Assume "P" supply side is connected via a slide valve to a shop compressor at 10 bar, about 50 litres. The "A" chamber is currently 150mm of 50mm steel pipe. "R" is feeding a 0.177" brass tube 300mm long.

Leaving aside issues such as C:B ratio etc (I used what I had available in the shed), what would be the result if any if I ran "A" back to to the shop compressor tank ?, using it as both supply and chamber.

Can this work, is it some sort of pneumatic Mobius strip, an Escher drawing of air or am I feeling the effects of a warm afternoon consuming Heineken ?

Cheers

[jackssmirkingrevenge]

Leaving aside issues such as C:B ratio etc (I used what I had available in the shed), what would be the result if any if I ran "A" back to to the shop compressor tank ?, using it as both supply and chamber.

Do you mean feeding through A and not through P?

In that case, it will never seal, because the piston is also a cup seal which will not allow air back into the pilot area.

If you feed through both A and P, it will fire once but never reseal.

In order for it to reseal, you have to replace or modify the piston, essentually poking a small hole in its rim in order to make sure it's not completely airtight, and a small light spring behind it also helps with resealing.

[wyz2285]

Jack can you find a link to that thread where BTB and somebody talked about the alternative way of using a 3-way/slide valve as a pilot by blocking one of.the port? I can't find it. In that way he may not need to poke a hole if the chamber is big enough and the flow it's just right to suck the piston back( it can happen, I did it once). However, a little hole will make sure it work flawlessly.

[POLAND_SPUD]

jack I think he meants attaching the A to the compressor as well as having it connected to port P... at least that's my understanding

What will happen ? I think it will work as the piston area is larger at the pilot side so if pressure in on both sides is the same the piston will seal


evilvet care to explain why you want to set it up like that ?? it's not that you gain anything

[jackssmirkingrevenge]

If the compressor is attached to a slide valve pilot and to the chamber, then shooting will continue to empty the compressor tank until the slide valve is reclosed...

evilvet care to explain why you want to set it up like that ?

Yup, that would help.

[wyz2285]

Or connect directly the tank o the valve, mounting the slide valve backwards and block the end of the slide valve. Got my idea?

[evilvet]

Poland, JSR et al.

I think he meants attaching the A to the compressor as well as having it connected to port P

The idea popped in while I was searching for something to use as a temporary chamber for testing. I looked at the compressor and saw the two outlets, one regulated and one raw. It occurred to me that I could use the supply reservoir tank as the chamber as well. Obviously as Jack says it will continue to vent A-R until either the valve is reset or the tank empties.

I appeals from a neatness point of view, only one chamber/cylinder. Of course at some point the overall supply to the system needs to be refreshed via a separate fill port to the tank.

[jackssmirkingrevenge]

Or connect directly the tank o the valve, mounting the slide valve backwards and block the end of the slide valve. Got my idea?

I don't remember the thread you had mentioned

If you mount the slide valve upside down and block the end...

- slide valve with ring moved away from the pilot, air can flow to the blocked end.

- side valve with ring moved towards the pilot, blocked end empties and nothing else happens.

That's what I understood, diagram maybe?

[wyz2285]

By filling up the blocked end, short pressure drop trigger a small burst. Putting it back to position, exhaust the air trapped in the blocked end, preparing for another shot. I'm on my phone, can't make a diagram.

[jackssmirkingrevenge]

Ahhhhh ok got it.

By trapping the pilot volume it would be great for a silent system

You would still need to mod the piston for chamber filling though.

[wyz2285]

The muzzle blast it's probably much more noticeable
The piston should be mod, but these rectangular QEV's diagram doesn't provide 100% airtight seal, if the pilot closes fast enough it won't dump all the air.

[jackssmirkingrevenge]

The muzzle blast it's probably much more noticeable

No point in having a great silencer if you're dumping HPA out of the pilot though, which is why QDV/valveless systems are so attractive...

The piston should be mod, but these rectangular QEV's diagram does provide 100% airtight seal, if the pilot closes fast enough it won't dump all the air.

... but the air in the pilot will be at lower pressure, so as the air in the chamber increases it will act as a pop-off valve and either fizzle air out or go full auto

[wyz2285]

First, change the word does to doesn't, my mistake.
When the pilot side is full, the diagram should remain shut. But let's just say put a hole in there to end this

[Hotwired]

By filling up the blocked end, short pressure drop trigger a small burst. Putting it back to position, exhaust the air trapped in the blocked end, preparing for another shot. I'm on my phone, can't make a diagram.

http://www.spudfiles.com/forums/viewtop ... tml#110053

[jackssmirkingrevenge]

Yup, pretty much what Hotwired had proposed... it should work well but in practice I don't see the advantage compared a conventional 3 way valve setup, aside from the fact that you don't need a separate firing chamber.