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Almost as speechless that such an incredible cannon should lack the perfect trigger...(quote]
Excuse me? Isn't that the same thing as me saying about your cannon.....
"I'm speechless. Almost as speechless that such an incredible trigger should lack the perfect cannon..."?
Every valve type has a certain amount of inefficiency in their speed of actuation and flow. Due to the diaphram actuator in a QEV (aka Sprinkler Valve"), they're slow to open in a situation where milliseconds are the difference between go or no go. If you build a cannon with .005" clearance between the sealing piston and cylinder walls, you'll find forces coming into the equation that you didn't think of.
Your statement :"recursion, sweet recursion" is great for computer code and 100% efficient but, in the physical world where gas can be a somewhat illusive substance it has it's flaws. As all valves are inefficient, you are suggesting that I actuate a valve / to actuate a valve / to actuate a valve. With that design, the inefficiency stacks up. To fully actuate, the piston in my cannon requires almost explosive force in order to overcome the high speed drag created by escaping air around pistons through tolerances and into the barrel. The pilot valve must open near instantly to free the piston from it's seat. This can not be accomplished with a slow, inefficient valve opening another slow, inefficient valve, to vent the pilot gas. I explained in my original post that my first choice was a solenoid pilot valve but, for reasons also explained, I used the ball valve.
Actually in this case, this is not true. Only the delays stack up. By definition a QEV is quick regardless of the stack in front of it. His stacked valves are like tumbling dominoes. 1 or 500, the last one falls at the same speed, only delayed by each delay before it. The valve speed is reasonably a fixed value in a QEV.
I personally think his valve choice is spot on, especially on this cannon...
Sometimes a little mechanicality(is that a real word) adds a nice touch on pneumatic artillery, it's something tangible, like a trigger and hammer on a six shooter, or the bolt of a rifle....
Is it the best choice on all pneumo's? No... But on a massive piece of themed artillery like this, it's perfect...
Technician..."Actually in this case, this is not true. Only the delays stack up."
Irrespective of the QEV's fast action, it is that delay that prevents my piston from retracting.
With the addition of each valve and necessary plumbing connections comes the addition of more pilot air volume to vent= more time required to retract the piston
The QEV was invented to quickly relieve the pressure in air cylinders when combined with a long small diameter control line. They are designed to release quickly when they pop open. Pilot area doesn't slow them down. Much like the dominoes mentioned, the when is delayed, not the how much.
I suspect you may have some other issues. Pilot volume and an undersized pilot may be contributing to a QEV re closing before the piston opens fully. It is true the stacked QEV's may present a higher pilot pressure to the final QEV than a larger valve would and contribute to early closing.
When I first switched to the ball valve design, I used a 3/8 valve. 4 out of 5 times, the piston wouldn't open fully. I moved up to a 1/2" ball valve and found a power increase and more firing reliability (about %80 reliable). Settling on the 3/4 ball valve, the piston ALWAYS %100 fully retracts and the cannon power became huge. I'm not saying QEV's are bad...they just don't work on this cannon.
Tech's just upset cause you haven't shot MARSHMALLOWS out of it yet...Have You??? roflmao
Seriously though, I'm not a big fan of diaghram qevs because of what you're describing, a shuttle/piston qev may have worked fine, but I also use Zero Leakback pistons on my cannons, if you have clearance built in for filling a qev might not have worked as well...
But I still think the one you have is perfect for the gun its on(unless you want this 1.5" butterfly valve i have on my desk)...
No interest in marshmallows......just going fast with hard projectiles!
Do you know if GGDT is a reliable modeling program. I saw it mentioned on a post and downloaded it. I'm starting a new cannon but, I'm sceptical of the performance GGDT predicts.
It's pretty accurate, but it is a modeler, remember gigo.... It has some deficiencies but it's also free, I'm sure if soeone wanted to pay for time and effort D-hall could improve it...
once you start modeling with it and share your model results we can help get it dialed in for differant parameters...
From what you are describing as symptoms indicate the valve seat is relatively small in relation to the OD of your piston. For example a 4 inch piston sealing a 2 inch barrel. What happens with these is due to the area of the front of the piston blocked by the barrel (small area) does not provide a large force increase as the piston opens as it is only 1/4 the area of the entire face of the piston. (boost 25%).
Because of this and the remaining 75% of the piston face is exposed to chamber pressure, the pilot area when vented down only 25% lets the piston begin to open. Two very important things happen.
One, the chamber pressure starts to drop rapidly
Two, The pilot at 75% of chamber pressure quickly feels the squeeze of the piston coming back. If it moves into the pilot area 1/2 way, the pilot pressure (assuming no continued venting for clarity) the pilot is now doubled in pressure or 1 and a half times the original chamber pressure, which is now rapidly diminishing.
The result is a valve that burps instead of popping fully open. If you can, re-make the valve seat area at least 3/4 the diameter of the piston diameter, or continue to use a really big pilot to keep ahead of the aforementioned problem.
When I build piston valves, I always make sure barrel sealer valves have a large seat in relation to the OD of the piston so the valve remains fully closed until the pilot area pressure is at least under 25% of the chamber pressure.
Watch how it is different.
Pilot at 25 % of chamber pressure will increase to only 50% of initial chamber pressure when the piston moves back and takes 50% of the pilot area. Hmmm. It's still being forced open, not closing..
The face of the piston with 75% of the face covered by the barrel sees a force spike that is 3X more than was originaly holding it closed.
These POP open, a true QEV as a main valve.
Oh by the way, what is the diameter of the sealing area of the piston and what is the OD? Is this ratio any where near 2:1 for an area ratio of 4:1?
EDIT, I just looked at your cannon on page 1.. Not a ratio problem, but may be a flow problem from the tank to the valve. As the valve opens the distance, plumbing and such may create a very rapid pressure drop at the valve on the chamber side of things. High piston blow-by will also cause it by keeping the pilot area pressure high. This will need a large pilot to vent. Is there any way to use larger plumbing from the SCUBA tank to the valve and maybe reduce the piston blow-by to the pilot area?
Restrictions between the chamber and piston will cause that type of problem. It is why I prefer coaxial designs.
Last edited by Technician1002 on Thu Jun 11, 2009 5:57 pm, edited 3 times in total.
And now, welcome to your first viewing of a spudfiles flame war !
Grab a bear and some popcorn and watch the excitement!
(It's a good thing most people on the forum live long distances from each other )
I don't know...QDV... What you're talking... Marshmallows... About there jimmy.... All in good fun...
Get it right.. It's QEV Piston. Umm. War.. I didn't know.. Let me grab some spuds.. Sorry tried to be informative. I didn't mean to start a war over it.. Wait a minute.. Don't want to seriously hurt anyone.. Where are my Marshmallows?
With cannons like the one above, im not sure if thats an issue any more. Atleast im over the pond. surely it cant reach -that- far?
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