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Spool Valve Diameter Ratios

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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Spool Valve Diameter Ratios

Unread postAuthor: PVC Arsenal 17 » Mon Mar 11, 2013 2:33 pm

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.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 11, 2013 3:04 pm

I would match the port size. A bigger spool is heavier and has more friction.
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Unread postAuthor: Technician1002 » Mon Mar 11, 2013 3:07 pm

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.
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Unread postAuthor: PVC Arsenal 17 » Tue Mar 12, 2013 3:46 pm

Ah, that's what I was after. Thanks Tech
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 12, 2013 5:31 pm

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.
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