Piloted piston valves explained

In my review of the Nerf Hornet AS-6, I briefly explained how the valves in the internals of the blaster work to fire the darts. I felt it might have still left some people wondering more about how piloted valves work, so I've decided to visually explain how a piston valve works in order to clear a few things up and go a little deeper in to this subject. I'm still not yet an expert with these, but I do understand the basic function quite well.

The 2D images used in this are drafted as cut-away views from the top, as if you cut the whole setup in half along the horizontal plane. The type of valve used is called a coaxial piston valve, which means the setup is all operating along the same axis. There are a few parts, the chamber (white), the piston (red) and the exhaust port (yellow). In the case of a Nerf blaster, the exhaust port is the barrel. The area on the left of the piston is called the pilot volume, and the whole system is filled via the hole in the left side of the chamber called an inlet port. The other space on the right of the piston, surrounding the exhaust port is the chamber volume. the black bit is an o-ring, to seal off the pilot and chamber from each other. Below, is a 3D image of the whole lot, but with just the chamber cut in half.

I'm using Autodesk AutoCAD 2011 to draft all this. Though, I'm not putting in much detail at all, partly because I'm feeling a bit lazy, but also because I don't want to clutter it up too much and confuse you guys. Anyway, If you are studying drafting or graphics then I highly suggest you get a student licence for AutoCAD 2011, or 2008. Or if you aren't a student but have oodles of money, go for a full licence. Both aren't bad but 2011 is a little more student friendly. In my personal opinion 2010 is rubbish, but I've not tried 2009. 

OK! right, so, in the first 2D picture here, we have the valve setup. priming the valve is done by filling with air or CO2 or whatever stable gas (green) through the fill port. In this case, the same port doubles as the pilot port. The air fills up the pilot chamber and pushes the piston (red) forward until it contacts the back of the exhaust port (yellow), making a seal and preventing gas from flowing through. The part of the piston that contacts the back of the exhaust port is called the sealing face, and the back of the exhaust port that contacts the piston is called the seat. This is a common problem area in home made piston valves, because if the sealing face doesn't make a good seal with the seat, air will leak through the exhaust port and the valve simply won't work.

Once the piston can't go any further, pressure starts to build in the pilot area. (dark green). The air then needs to fill the chamber area. This usually happens by air leaking past the piston, or through pipe leading from the pilot to the chamber with a check valve (one way valve) in the middle (not shown). For the valve to work, more are must be let out of the pilot than what can leak back into the pilot from the chamber side of the piston. Ideally, an o-ring makes a perfect seal and air can only enter on way through a check valve, and can't come back at all. Anyway the chambers keep building pressure until the maximum desired pressure is achieved and filling is stopped. the chambers equalize the pressure between themselves holding the piston still.

Finally, the system is triggered by piloting, or letting the air flow back out of the pilot side of the valve. This means that there will suddenly be less pressure on the pilot side of the piston as there is on the chamber side. The high pressure on the chamber side forces the piston back away from the exhaust port, which means the air in the chamber can then flow out the exhaust port, pushing out whatever is in the barrel along with it. All the air pressure drops, and it's all over very quickly. The cycle can then repeat.

There are a few factors that effect the performance of a piloted valve. first, piston weight. the lighter it is, the faster it can be accelerated, shortening the opening time of the valve and dumping more air at once. next, the seals and friction. Perfect seals makes it more efficient, but excessive friction on the piston slows opening time. Finally, pilot volume and pilot valve flow. the smaller the pilot volume, the better, because there is less air to be let out to drop the pressure, but not too small so that it doesn't allow the valve to fully open. Also, the higher flow the pilot valve has, the more air can be dumped at once, speeding up opening time.

So that's it. This knowledge can be applied to all sorts of things, Nerf has been using this sort of tech for years (though I think the valves Nerf uses are more like some sort of piloted spool valve) you could your own small piston valve and mod it in to a Nerf blaster, or even build your own vortex launcher, provided you live in a place where that sort of thing is legal...

...just food for thought. -Rolley