gearbox
A gearbox is a sealed set of meshing gears that sits between a motor and the thing it moves, trading away speed to buy more turning force. Most electric motors spin fast but feebly — like a kid spinning a bicycle wheel by flicking it with one finger: fast, but easy to stop. A gearbox is the bike's low gear: it makes the output turn much slower while pushing far harder, so the wheel can now haul a rider up a hill. Inside, a small gear drives a big gear; the big gear turns slower but with more muscle.
The trade is captured by the gear ratio — how many times the input must spin for the output to turn once. A 10:1 gearbox makes the output turn one-tenth as fast but with roughly ten times the torque (turning force). To reach a big ratio without a single enormous gear, designers chain several reduction stages in a row, each stage multiplying the one before, so two 10:1 stages give 100:1. Nothing is free, though: friction between the teeth wastes a slice of the power as heat, so a real gearbox might pass on, say, 80 to 95 percent of what goes in — that fraction is its efficiency.
A small robot's motor spins at 6,000 rpm but can barely turn its own shaft. Add a 50:1 gearbox and the output drops to 120 rpm while the torque rises about fiftyfold — now strong enough to drive a heavy wheel.
A 50:1 reduction: one-fiftieth the speed, about fifty times the force.
Speed and torque always move in opposite directions through gears: whatever you gain in force, you give up in speed, and vice versa. The motor's raw power (speed times torque) sets the ceiling — a gearbox only repackages that power into the blend of speed and force a joint actually needs.