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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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Mini QDV pilot

Unread postAuthor: john bunsenburner » Sat Oct 31, 2009 7:03 am

Well I am making a fairly small QDV as a pilot. I have thought of two ways of doing so, i have no pictures as of yet, nor do I have diagrams. I tried two things both having failed i will describe these briefly:

Failure one: I drilled a hole into a 10mm threaded rod and then tapped M10 threads into two pieces of alli, that had an OD of 13mm. I tried brazing these to a piece of pipe with an ID of 13mm using special alli braze. One side came out nicely, the other too too much heating melting and deforming the pipe.
Failure two: I wanted to use a similar concept but with threaded parts all the way through. I tried using an 8mm threaded alli rod that could have two spacers on either end that were to be threaded with M10 threads and have inside threads of M8 so that the rod with the hole would be threaded into them and they in turn would be threaded into a pipe with M10 internal threads. This failed because i drilled a hole too large(i didn't want to make a piston under 5mm, thats too fiddly) and so the walls of the threaded pipe twisted of in the threading process.

Now I will try version two again but with larger pipe. The problem is that i only have threading apparatus up until M10 and so i though i could do the following:

Take a threaded rod, alli or steel, and turn down the middle until it is 7 or 8mm in diameter, then drill a hole of 5mm down the middle and cut ports in the part that was "hollowed out". Turn the hole thing into an internally threaded pipe(thick walled being preferable) and then putting some kind of fitting there where the hollowed out part with the ports is, to connect ti to the pilot area of my piston cannon. I then make a piston with a diameter of 7 or 8mm, or what ever the diameter of the hole is, put two O rings on it and tada, i have a perfectly working QDV pilot.

Now knowing the parameters of my apparatus i ask for advice and comments to improve on my method. I am sure is better ways to do it to have less fiddly work todo. However I do insist on a QDV as a pilot(perhaps, if it is impossible i will change that a a hammer/blowgun valve.

Thanks in advance, and happy spudding.
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Unread postAuthor: MRR » Sat Oct 31, 2009 7:16 am

I've built something with a QDV in mind and came up with a kind of sliding valve that opens easy at high pressure with high flow.
Image

The valve is pulled open mechanically (I will add a strong spring...).

The main problem when using a QDV as a pilot is, that there is no projectile (mass) that counter acts with the piston. The opening time would be pretty low I guess.
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Unread postAuthor: spudtyrrant » Sat Oct 31, 2009 7:41 am

uhhh mrr did you see that the piston itself it supposed to slide separate the pull rod. if that piston is part of your pull rod and it doesn't slide you are going to get a nasty surprise if not then nvm
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Unread postAuthor: MRR » Sat Oct 31, 2009 7:49 am

I know how techs qdv work and it is not that practical on a pilot. The weight of the projectile makes the piston slam backwards.
On my pilot there is nothing in the way of the piston and the exhaust hole, that's why the piston is effected very little.
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Unread postAuthor: c11man » Sat Oct 31, 2009 9:48 am

oh no it needs no projectile to function, i fire blanks quite often and it sure flys back! the air is enough to slam the piston back even when the pull rod is pulled very slowly, im sure tech will come in a bit and give a more indepth explination of the hole thing.

but qdv work VERY well as pilots
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Unread postAuthor: MRR » Sat Oct 31, 2009 10:05 am

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.

Building a valve in tech's QDV style would be over complicated if you doesn't benefit from its original design purpose.
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Unread postAuthor: john bunsenburner » Sat Oct 31, 2009 10:08 am

Alright, I very much like MRR's version, the problem is, mainly, that making the valve small, and from scratch, is very fiddely work. Perhaps I will make a hammer valve instead. What are the measurements of your QDV MRR?
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Unread postAuthor: c11man » Sat Oct 31, 2009 10:09 am

mrr, you are mistaken, a qdv opens extremly fast with no ammo. i have trouble finding a bumper that can take the impacts of BLANKS so that tells be that its moving damn fast even with my light piston. so opening time does not suffer at all
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Unread postAuthor: Gippeto » Sat Oct 31, 2009 10:30 am

c11; Do you have this posted? I must have missed it.

Link please. :)

MRR is absolutely correct by the way. A qdv with other variables fixed, will open quicker with heavier ammo. Quicker is a bit relative when we'
re talking about events that happen in ms.

Think of it like this...you place a ball on either end of a spring. One is heavier than the other. Compress the spring, and release both balls simultaneously.

Given that the force acting on each ball is the same, which one would you expect to accelerate the fastest?
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Unread postAuthor: c11man » Sat Oct 31, 2009 10:38 am

ok i know it will be slower but that much slower makes how much of a differce when used as a pilot? almost none
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Unread postAuthor: MRR » Sat Oct 31, 2009 10:47 am

@JB
I used 3/8" fittings and the length is 75mm.

@c11man
Well, I didn't try it myself so I believe you that it opens fast and I know that it works without a projectile but the performance "DOES" suffer without. My question: Did you try a QDV as a main valve or a pilot?

Edit:
Just wanted to know if the small pilot volume has enough flow to act properly on the piston.
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Last edited by MRR on Sat Oct 31, 2009 10:51 am, edited 1 time in total.

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Unread postAuthor: Gippeto » Sat Oct 31, 2009 10:49 am

Opening time...maybe no difference.

Flow is another thing though. The pilot valve must have enough flow (at whatever degree of "open" it is at), or it will cause piston bounce, and lost performance.

Of course, flow is generally closely related to how "open" a given valve is.

I'm really curious as to how you configured a qdv as a pilot. Waiting on that link. :)
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Unread postAuthor: MRR » Sat Oct 31, 2009 10:52 am

Damn, beaten again!!!
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Unread postAuthor: Technician1002 » Sat Oct 31, 2009 10:53 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.

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


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.
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Unread postAuthor: MRR » Sat Oct 31, 2009 11:03 am

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.
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