Biopyro wrote:I don't think I'm understanding it correctly then?
I'm pretty sure the barrel has to extend all the way through to the "pilot" port, and I really don't get the snooker cue analogy
I thought that when the guide rod is pulled, the piston's face is exposed, pushing it all the way back through the barrel, and allowing air to escape in front of it out of the barrel.
Edit: Wow your diagram looks better haha! Yeah so it does really have to be coaxial to some extent because the barrel must go all the way throught the air chamber.
The diagram is close. One important thing missed. The ports are right smack up against the very front of the tank. The piston does not go all the way through the tank when this is fired. It is limited to about 1.5 inch travel in the 2 inch cannon and 3/4 inch in the 1 inch cannon. It doesn't move very far. Since the rod and knob weigh more than the piston, getting this up to speed before contacting the valve, helps the initial opening.
The snooker cue analogy is simply mentioning since the piston is not attached to the rod, the rod can have a couple inch stroke before contacting the piston, knocking it open instead of just pulling it along.
It does not have to be coaxal at all. This can be built inside a T which is then screwed onto a tank like a supah valve.
I have started a thread on this valve design. I hope to answer all your questions. Look for a thread on a Quick Dump Valve. I found out that is what this design is. It is most often used to quickly dump large liquid holding tanks and is air operated instead of manual.
The only partial triggering means that the gas assistance only really kicks in bigtime if the projectile has enough mass to it, giving the resistance to force back the piston the other way. If the projectile gives too little resistance it could be out before the valve had finished opening itself.
I have found the mass of the air in the barrel is plenty. Check out my youtube videos with the same user name. Look for the one with the blue plastic bucket. The back pressure is enough to do the job. With some tricks with the o rings to reduce friction, it works very well.
Appreciated, but just a small note on forum etiquette, try and avoid making consecutive replies to a thread when you could easily condense them into one
Tried to fix it, but can't seem to delete the prior post.
Surely the valve can be more efficient than a normal coaxial, because the piston is the size of the barrel, not the chamber, which cancels out the surface area loss by weighing less?
Overall I'd be inclined to say this would perform better, and if not then certainly you don't have the wasted pilot air.
Most important is the lack of pressure behind the piston, so when the chamber pressure opens it, it can open faster instead of pushing against a pressurized area.
The surface area is not important if there were the same pressure in the pilot area. A three inch piston 2 inches thick pushed by 100PSI will fly the same speed as a 1/2 inch piston 2 inches thick. The 1/2 inch piston does not need to be as thick as the 3 inch.
In summary, no back pressure, possibly thinner = faster. I used a low mass material. It floats. It is less than 1/2 the weight of aluminum.
The design that's the subject of this thread on the other hand has all the limitations of a piston valve. Granted there's no pilot pressure and the piston is smaller and therefore lighter, however it does have a smaller surface area and doubtless more friction since it needs to be an airtight seal.
This valve was designed to eliminate some of the limitations of a piston valve. The three items this fixes is;
Operating pressure range
Difficult to get to seal
Friction with high blowby causing sticking in either open or closed positions.
Friction is a biggie and was addressed in the design. I'll be covering the friction issue on the thread i started on the Quick Dump Valve. I built the marshmallow cannon as a second generation of the t shirt launcher to test some o ring use. The new cannon uses a 1 inch valve. The friction is so low, that when not under pressure, simply pointing it up and giving a light shake will open the valve by gravity. I'll cover how this works on the other thread.
This valve seals easily and fills with a bicycle pump. No high volume compressor needed to get the piston to seal.
It has low leakage. No high volume compressor needed to keep ahead of leaks.
It triggers easily at all pressures all the way down to zero. I have barrel acceleration graphs where the expansion volume was less than the volume of the barrel causing vacuum on projectile exit.
http://www.spudfiles.com/forums/quick-d ... 17858.html
I am working on a video to show the valve construction and details on low friction use of o rings. I hope to have something up online in the next week or two.
Biopyro wrote:Surely the valve can be more efficient than a normal coaxial, because the piston is the size of the barrel, not the chamber, which cancels out the surface area loss by weighing less?
Overall I'd be inclined to say this would perform better, and if not then certainly you don't have the wasted pilot air.
One of the big reasons to play with this design was building for speed. Turbulence in the valve area limits the flow when near supersonic. In a traditional coaxial, the valve is near or at the very rear of the chamber. The air flow has to make a high speed U-turn in an area full of square corners and edges.
On the down side, this design has 2 area needing a good seal, unlike a piston which needs only one, so designing to eliminate friction is important to keep it fast. In theory a regular piston would have less moving friction. Individual results may vary considerably.
When not under pressure, this valve can open by gravity and a mild shake. More on the low friction design later.
Moving the valve to the other end of the tank eliminates a 180 degree turn and takes the high velocity flow and does a lane change as the air moves from the rear of the tank and takes a wiggle into the barrel. Not perfect, but in theory it might be better.
The other design consideration is the chamber shape. The less the air has to pick up speed before reaching the valve, the less Turbulence it will have when reaching the valve. This in theory should help get air from the tank into the valve.
I think a combination of these helped the apple in the 2 inch valve version reach 882 FPS. I'm not sure traditional piston valves reach this speed at below 100 PSI. Anyone have good chronograph readings of coaxial spud and apple launches for comparison?
Good fix on separating the piston from the manual trigger when the air pressure engages it.
Thanks. This was inspired by our desire to have a hand left. The math on the 2 inch valve came out to an opening force of 314 Lbs if it was coupled in a 100 PSI shot. We could see the rod stuck through someone's hand. When it was decided to decouple, we realised the lowered moving mass would help the piston speed.
**Edit**
For safety, I used the big round ball in the big cannon and a big trigger knob on the marshmallow cannon. If the stop assy failed, it would protect against a nasty spear through the hand.
The second safety item in the design is the use of a 1/16th inch diameter wire pin to close the valve. If the stop failed, the pin is designed to shear off. This happed in the little cannon when the pin was too far forward and the piston hit it.
In the In using the quick couplers with no check valve prevents accidents from leaving it loaded. Unhook the hose and on the large launcher leave the ball valve open and the cannon is safe.
In both launchers, the original tank safety device was left fully functional. The Freon tank has the original rupture disk and the little one has the relief valve.
Think Safety
