Usign a 2-1/2 bore, 24" stroke pneumatic cylinder.

[pfreivald]

Originally Posted by hrench

This constant-force spring thing seems like a good idea, but it means that it takes more pressure (equals force) to extend your rod initially. Which means it uses lots of your stored air reaching up for the bar.

Unless you add in the additional pressure regulator, you're extending at 120 psi anyway...so add ~75 lbs (or however much) of constant force springs, extend at full pressure to extend them, then let them help you on the way down. The pneumatics will be smaller, so you'll be saving air on the up and down strokes (which might obviate the need for the additional regulator to bring down the extension pressure.)

Some math (maybe some graphs with an eye toward that 150-lb requirement) should help you determine an optimal* amount of spring vs. bore.

*optimal, of course, depending on which resources you're trying to preserve.

As another idea, consider constant force springs that are already extended along the body of the cylinder, but don't lock in to the rod until it's fully extended--so you don't have to push against them at all, but they'll help you pull it back. Might require some cams, spring-pins, or whatnot.