I've always been fascinated by springs.
All sorts, in watches, screen doors, and the sort attached to garage doors. They help us tell time, automate our activities, or lessen the effort it takes to complete a task.
One of my earliest and favorite was a toy that came in a cardboard box, the Slinky. My first one was steel, with the ends attached to the main spring with a metal clip, so children didn't spear themselves on the sharp metal. Later, I had plastic varieties, often purchasing them for children's party favors, one to a child.
The original metal ones were the coolest, the sound they made as you rocked the spring from hand to hand, sort of like a metallic waterfall, as it flowed from one palm to the next.
Here's the thing about Slinkys, though. They wear out. Through repeated use, the metal in the spring loses its rigidity, the spring stretches out, and it deforms. It's no longer good for its intended purpose, and it must be discarded and replaced.
The same thing occurs in screen doors. The springs stretch out and no longer close the door properly. The massive springs atop garage doors will eventually break, and the door can hardly be lifted without it.
Who's to say that a spring that runs an entire starship wouldn't eventually need to be replaced, even if that starship is a tree? It probably wouldn't happen all at once; it would just slowly loose its effectiveness, much like that treasured Slinky that eventually can't maintain the factory tension that allows it to slither smoothly from one hand to another.
In Chapter 22, a near disaster is averted when the Godders attempt to destroy the replacement spring. Good going, Slate! Once again, you saved the day!