Constant Force Spring Loading

[cad321]

We are looking at using constant force springs to deploy a hook this year for our climber (similar to 2056 in 2016). I have been browsing the Vulcan Springs website (specifically their design guide) and am uncertain how to determine what size we need. The way I understand it is, a constant force spring applies a relatively constant force, regardless of how far extended it is. So lets say I have a 10lb object I’m looking to lift; If I then attach a 10lb constant force spring to that, I’d be able to move the object up and down by hand, and it would stay in place wherever I leave it (forces cancel out).

In terms of making the spring actually move the part (release a switch and the springs lift/pull the object up) how much headroom do I need in the spring. Is it just as simple as putting an 11lb spring on, or do I need more than that (and how would I determine how much more force).

Any help would be greatly appreciated.

[GeeTwo]

You’d need to account for the friction in the system as well.

Beyond that, how fast do you want the object to move? If you apply one extra pound to that 10# load, it will begin accelerating at 3.2 ft/s/s, which is rather stately.

If unsure, it is easiest to try to overestimate the force by 20% or so when selecting the spring, then add ballast to the load to get a desirable speed. Then, once you have the thing working properly, if you have the time and money, you can then figure out the “proper” spring and scale back.

[amesmich]

You’d need to account for the friction in the system as well.

Beyond that, how fast do you want the object to move? If you apply one extra pound to that 10# load, it will begin accelerating at 3.2 ft/s/s, which is rather stately.

If unsure, it is easiest to try to overestimate the force by 20% or so when selecting the spring, then add ballast to the load to get a desirable speed. Then, once you have the thing working properly, if you have the time and money, you can then figure out the “proper” spring and scale back.

QFT, which I had to look up… when this acronym was used on one of my posts. “Quoted for truth”

[pfreivald]

If you don’t know, the situation where it takes basically zero effort to move and maintain the position of an arm/lift/etc is called “neutral buoyancy”.

As GeeTwo said, it can be difficult to peg exactly because of friction (as well as angle shifts due to moving parts, etc). In my experience if you’re a bit too high or a bit too low the friction in most gearboxes will keep the mechanism from moving (for certain definitions of “a bit.”)