I’m trying to pick a spring to tension the rigging on our continuous elevator, like I’ve seen many teams do before (254 and 973, at least). If anyone knows how I should select a spring, just write it and skip the rest of the post, where I explain what I’ve done so far and why I’m lost.

The factors I’m examining are primarily min load, max load, and rate, as McMaster calls them. When designing for how much the rope will “slack” (and safety factor), I need to know the max load the rigging (and thus the spring) will experience.

The load is dependent on the elevator’s acceleration. 0 acceleration and the load is the weight of the carriage. 9.81 m/s^2 of acceleration and the load is twice the weight of the carriage. So I need to determine the max acceleration the elevator will experience. When I use popular FRC calculators, they return two numbers that are “like” load.

The first is “Load Current Draw” or “Current Draw Per Motor”, depending on which one you use. When I run this back through motor constants (K_\tau), the number of motors, gearbox, and spool, I arrive at the input “Load” I gave the calculator, plus some extra which comes from efficiency factors, I believe. So I guess this number has been calculated the same way, but in reverse, which doesn’t help me. Per my above intuition about load on the spring, the load will be larger than the weight of the carriage.

The second is “Stall Drag Load” or “Stall Load”, but this number is far too high, and I believe it to simply be the load the motors will create at stall torque, accounting for the gearbox and spool. However, I don’t believe the elevator will ever stall. Also, this number is 200+ lbs, and springs that can withstand 100+ lbs are hard to find and expensive.

Thus, I am left with two questions:

- How do I decide which spring to use for inline cable tensioning? Do I need to account for the weight of the second stage at all? Should I make sure the motors physically cannot break the springs (I was planning on solving this in code)? How much should the spring extend at the accelerations we want or at 0 acceleration?
- How do can I calculate how fast a motorized mechanism will accelerate? Is the only answer to use a differential equation motor model (like in Austin Schuh’s System Modelling talk)?

Information about the elevator:

3x 775pro, 14.12 : 1, 2" dia. spool, continuous rigging.,15-25ish lb carriage.