Piston Q's

[Juggernaut12121]

I feel like I come here and ask too many questions...
Anyhow, I've been making a golf ball cannon for a while with a 2" piston valve but it won't work. My piston sucks so I figured I'd rebuild that. I've got metal shop next year in high school so I wanted to machine one, but then I took a look at my cannon and thought my piston area might be too short. I now plan to rebuild the entire piston area and piston but before I do I really want to know wha sort of dimensions would actually work and work efficiently. I'd hate to get the lengths wrong again and have to rebuild it AGAIN so I would love some input on it. Thanks in advance!
(Sorry I don't have pictures or dimensions or my current setup, if it's wanted I could post some when I get the chance)

[noname]

Well, you want your piston to be a very close fit with the walls of the pipe it's traveling in. It should also be able to slide back and forth without much resistance.
The rule of thumb for piston travel is that it should travel about 1/4 the length of the valve port. If you're shooting golf balls, about 1.6", your piston only has to travel about 0.4" to achieve maximum flow. I'd let it travel about half an inch personally.

Got any pictures of the valve for us?

[Juggernaut12121]

It's at my mom's, I'll be able to post pictures by later today

Piston-about 4 3/4''
2'' pipe the piston rides in-about 4 3/4'' (with current crappy back stop)
Space between sealing face and pipe-about 1''

[Juggernaut12121]

It works! it was a combination of needing an air compressor and one of the parts mysteriously becoming very leaky that didn't allow the chamber to pressurize and seal the piston.

[Technician1002]

The rule of thumb for piston travel is that it should travel about 1/4 the length of the valve port. If you're shooting golf balls, about 1.6", your piston only has to travel about 0.4" to achieve maximum flow. I'd let it travel about half an inch personally.

This rule is based on the area inside the valve seat for flow being equal to the area opened between the piston and valve seat. This assumes two things.

1 flow into the gap between the piston and valve seat is not impeded in any direction. For a valve in a T this is not true. Flow enters from one side.

2 Water does not expand with pressure changes. Air does expand with a drop in pressure so as air flows past the gap between the piston and valve seat, the air expands with lowered pressure. Reducing or eliminating the first pressure drop will result in higher pressure at the valve set for higher flow through the valve.

For these reasons, the 1/4 diameter rule is the barely minimum specification. For a T, the valve should open at least 3/4 the valve seat diameter. For a coaxial design the valve should open at least 1/3 to 1/2 the valve seat diameter.

In my valves, I have the porting into the valve at least double the size of the cross section of the orifice the air exits for good flow.

[Juggernaut12121]

Forgive me for being incompetent, but let's say I had a coaxial with a barrel O.D. of .5" and and I.D. of .36", would I make the dimensions so the piston would be be fully open at .25" or so?

[Technician1002]

Piston movement minimum would be 1/4 the orifice, or opening out the valve. This would be 1/4 of .36 or 0.09 using the quarter rule. At .25 inch it would be wide open. At double the minimum it would be only 0.18 inches.

[Juggernaut12121]

Alright I understand now, thank you!

[jackssmirkingrevenge]

What is this, a dumbbell for ants?!?!