SpudFiles

Does anyone happen to know if there is an optimal ratio between chamber diameter and port diameter in a balanced spool valve?

With a standard QDV it is obvious that increasing the port size will increase performance quadratically as the air pressure has a larger area to act upon. In this case, though, there must be diminishing returns as the flow path around the piston becomes smaller with increasing port diameter.

I would match the port size. A bigger spool is heavier and has more friction.

Your answer lies in the math. You want the gap area from the chamber a minimum of equal to the cross sectional area of the valve seat opening. Beyond that you get diminishing returns. I generally have the area entering the valve about 2X the area of the valve sear orifice. For example my t shirt cannon has a two inch valve core and outlet port. Using Pi X Radius squared, you get 3.14 sq in. My ports from the chamber is 6 sq inches of area.

The math needed is the area of a circle.

This keeps the velocity down in the first gap to minimize pressure drop and turbulence in the first gap. This provides higher pressure and flow into the valve seat orifice for higher flow and pressure in the barrel behind the projectile. See my sig for my design notes.

^^^ a bigger spool has a larger cross section resulting in higher force to match the larger diameter. Keep it short to save mass.

Ah, that's what I was after. Thanks Tech

Larger diameter certainly allows for bigger flow for less movement, but the trade-off as I mentioned is that larger mass and friction means that it will move slower for a given actuator. I would go for smaller spool and consequent faster opening.