Problems with wire-based recharging#

Repeatedly recharging a high-force spring by pulling it with a motor struck me as neither stable nor efficient.
The biggest reasons: the more torque required, the slower recharging can get, and I simply wasn’t given the environment or the time to test with a pile of different motors and gears.
On a smaller note, there was also the problem that the wire pulling the spring could get tangled during recharge.

Let’s build a loop that adds force#

There is another way. With the spring we’re using, in theory — absent any resistance — it could keep bouncing forever. So what if we give the spring a slight push at the moment it launches, so that the bounce force grows over time? With each repeated bounce it could go higher and higher. Even accounting for real-world air resistance and the energy lost as the spring compresses, I figured this approach could be more instantaneous and more efficient than continuously recharging the spring.

How to do the pushing#

The first idea was solenoid-like: attach a rod carrying a metal core on a non-metallic shaft to the bottom of the spring, wind a coil around the non-metallic section, and push a strong current through it to add force. It has the advantages of variable force control and being relatively instantaneous, but producing high force requires a shorter stroke, which makes the design difficult.

So after more thought, what came out was this: give the motor a high gear reduction ratio and have it press down firmly once the spring reaches the maximum point it can naturally charge to. Since the motor can move and push along during the window in which the spring is pushing against the ground, I figured a high gear ratio could still be covered well enough.

The spring#

For the spring I’m considering the SF60X500.