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Mini QDV pilot

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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Unread postAuthor: Ragnarok » Sat Oct 31, 2009 11:28 am

MRR wrote:I didn't say that it doesn't work, I just said (in other words) that the opening time suffers when there is no counter mass -> projectile.

Any pneumatically operated valve does suffer somewhat, due to the fact the pressure can't build up as much when there's no projectile.

Don't pretend that it's slow though.

Building a valve in tech's QDV style would be over complicated if you doesn't benefit from its original design purpose.

Not necessarily. As I have said in the past, pneumatically opened valves are sufficiently fast that a little increase (or decrease) in opening time is essentially inconsequential.

Secondly, I'm building one for 3vo's trigger valve (specifically, as a pilot for the QEV that pilots the main piston valve*). The combination as whole may be overkill, but some of the details of how it works that make it somewhat ideal for what I'm doing.

*And I would note, the fact the QEV essentially vents straight to atmosphere hasn't hampered it.
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Unread postAuthor: Technician1002 » Sat Oct 31, 2009 11:28 am

MRR wrote:
Technician1002 wrote:Mass of air seem sufficient. I have a video showing this as well as the sound of the 2 inch blowing off with no barrel. My o ring video has a fairly low pressure demonstration at only 12 PSI showing this. Look up my o ring youtube video and watch the last 20 seconds.
:)


The question is if the mass of air is still sufficient applied on a small pilot volume.

If you want it to open fast, give it a projectile that moves say 1/2 inch and then opens up a vent. QDV pops, projectile valve and QDV finishes opening quickly. No problems. For reliability a short length of pipe should work fine for a little back pressure using air mass.
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Last edited by Technician1002 on Mon Nov 02, 2009 10:15 am, edited 1 time in total.
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Unread postAuthor: Ragnarok » Sat Oct 31, 2009 3:45 pm

Yes, but still, as long as your pilot is fully open before the main valve has started to open, as far as power is concerned it doesn't matter if it was relatively slow to open.

As far as lock time is concerned - and consequently, making really precise shooting require more care - it is another matter. But, a matter beyond (almost) every spudgun out there.
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Unread postAuthor: MRR » Sun Nov 01, 2009 8:33 am

I was thinking about the general design of the QDV and how to improve the opening time without having a projectile in the barrel. It's just an idea but it should work quite well.

Image

There are two pipes with a tight fit in a tee. The green pipe has got two exhaust holes (1-2) and one side is sealed with epoxy. The red pipe is also blocked with epoxy at one side and keeps the exhaust hole #1 (EH#1) sealed by adding two o-rings.

When the red pipe is moved backwards and the first o-ring passes EH #1 the pipe is pressurized through EH #2 (EH #3 is still covered). The building up pressure makes the red pipe hit back and EH #3 is fully exposed.

Some may say that this would be to over complicated (including me) but why make things easy when you can have it complicated? :)
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Unread postAuthor: john bunsenburner » Sun Nov 01, 2009 8:52 am

My best bet, as of yet is a push button/ blowgun type valve...I am afraid tiny, pilot QDV aren't too practical...
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Unread postAuthor: psycix » Sun Nov 01, 2009 10:01 am

I think that an easier approach is to build it this way:
Sectional area of EH1 (all holes combined) > sectional area of ID of pipe > Area of EH3.
This makes sure the pipe will flood with pressure faster then it can vent, and thus the piston will be pushed back.

You COULD also do it with a pipe attached to the piston.
Just make sure EH3 is small enough, possibly enlarging over time (multiple holes in a row, from which more get exposed as the red pipe slides back.)
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Unread postAuthor: Technician1002 » Sun Nov 01, 2009 12:13 pm

psycix wrote:I think that an easier approach is to build it this way:
Sectional area of EH1 (all holes combined) > sectional area of ID of pipe > Area of EH3.
This makes sure the pipe will flood with pressure faster then it can vent, and thus the piston will be pushed back.


This is the normal way I build my QDV cannons. The total port area is greater than the area of the pipe ID.

Overlooked is if you use a sliding rod, the piston can be opened with an impact to get it started. It could be a variant of a hammer type valve.
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Unread postAuthor: Ragnarok » Mon Nov 02, 2009 8:02 am

Technician1002 wrote:The piston can be opened with an impact to get it started. It could be a variant of a hammer type valve.

Actually, that's loosely what I'm doing, building a spring loaded QDV.

It's part of a project to give 3vo a consistent lock time (although fast is good, consistent is also important), as well as trying to bring the muzzle velocity's standard deviation down.
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Unread postAuthor: Technician1002 » Mon Nov 02, 2009 10:20 am

Ragnarok wrote:
Technician1002 wrote:The piston can be opened with an impact to get it started. It could be a variant of a hammer type valve.

Actually, that's loosely what I'm doing, building a spring loaded QDV.

It's part of a project to give 3vo a consistent lock time (although fast is good, consistent is also important), as well as trying to bring the muzzle velocity's standard deviation down.


You are on the right track. I've been playing with the precison golf ball challange (I'm not in it, but testing stuff to lean) and standard deviation is getting low enough I can drop low pressure golf ball shots in a 2 ft square on the ground at 10 yards. I need to finish a solid mount to see if it is any tighter. This is with a standard hand pulled QDV. How close of a deviation do you need?
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Unread postAuthor: Ragnarok » Mon Nov 02, 2009 10:42 pm

Technician1002 wrote:How close of a deviation do you need?

In an ideal world, fractions of a metre per second.
As far as difference in projectile weight is concerned, I can compensate for that - I've got a balance that's accurate enough for that job.

The issue is going to be variation in chamber pressure and temperature. It doesn't take much to knock it enough out of whack to make what I'm trying to do pretty much impossible.
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