piston questions

[cps zer0]

would a piston design like super cannon 2 work with a spring instead of air pressure to push the piston cups????

[aEx155]

Yes, a spring would work. Someone else made a gun with a spring from a motorcycle suspension system, so I'm guessing it would work; I don't know how powerful it would be, though, seeing as how hard it would be to pump the gun up with the spring in place (I don't see any other way.) unless you use a weaker spring.

[cantab]

It could certainly work.
As for how to pressurise it, I'd suggest some sort of crank/winch arrangement to mechanically pull the piston back. Sort of like a crossbow.

[cps zer0]

a wire could be connected to the piston cups so you could put the wire under your foot and you would hold the gun and pull up and open the ball valve at end another person could fill it well you are holding the spring down once it is full you close the ball valve and release the spring to fire you open the ball valve

[Ben]

I've given spring water guns a lot of thought and they don't seem practical. If you want something small that will have more power than simple air pressure, spring water guns are very good. The main disadvantage is that helical springs are definitely not CPS. You could design the system to work like an LPD design but unless you use a complicated system of pulleys or something similar it would get very long.

Constant force springs (the ones like measuring tapes) are possibilities. Months ago I bought two 40.9 pound springs for $25 each and I intended to use them in a CPS water gun. The problem is to get any reasonable PC capacity at reasonable pressure you need to use a pulley system, which gets complicated to build. It's not impossible, but there's more reasonable options.

Though, given that I haven't built a spring gun and barely anyone has from what I know, it's worth experimenting with in my opinion. All you really need to know is how to figure out the pressure from the applied force, which is just an application of the simple equation pressure = force / area.

For example, if I had a spring that applied 200 pounds of force onto a 2 inch piston, the pressure = 200 pounds / pi*1^2 = 63.7 PSI. That should be more than adequate to make a powerful water gun.

Now you see one of the problems. Stronger springs are expensive. Air on the other hand is free or nearly free. Generally it's more economical to make an air pressure gun than a spring gun.

One advantage spring guns would have over air pressure though would be safety. Air expands a lot, so when a pipe fails it fails explosively. If you use a spring on the other hand, the water barely expands at all so there will be no explosion. Springs are safer in that respect.

[Silence]

I personally wouldn't use a spring in a PVC water gun. The pressure at the end of the spring is uneven, and endcaps (or whatever holds the spring in place) aren't very strong.

After seeing how well limited pressure drop and CPS soakers work without truly constant pressure, I would just get a very long spring with a low force constant (which is measured as force per distance compressed). Compress it into your space and it should already be at 80% or more of the final pressure, which is sufficient.

But powerful springs are expensive and heavy. They *do* limit pressure drop with much less extra space than air pressure offers, though.

[cantab]

Springs will of course have the major drawback that metal rusts, and if it's in a water gun it stands a good chance of getting wet.

[Silence]

Well, I'd assume you'd use stainless steel for this purpose. Maybe stainless loses strength more quickly, but its lifespan is probably still much longer than what you'd expect from a store-bought water gun.

[cantab]

Good point. Though I'm not sure I've ever come across stainless springs. They probably exist. But not all steels are equal, and stainless steels often have poor mechanical properties - e.g. stainless steel swords can shatter.

[Silence]

McMaster-Carr carries them. And they might be used in water guns, although given manufacturers' track records of using rustable steel for screws, I doubt they would use stainless for springs. I've never seen rusted springs in water guns, though.

[cantab]

Well...the screws rusting doesn't matter, since water guns aren't meant to be opened up.

[Silence]

That's true. These guys have a track record of gluing on pieces and generally changing the design of water guns between marks in apparent efforts to deter opening the cases. Don't ask me why. But you're right. As long as the screws remain strong enough, even after rusting, they probably care more about the springs.