Harmonic drive

A harmonic drive uses a flexible toothed ring that deforms as it turns, which gives a high reduction ratio in a small, light package. That is why it is common in robot arms and the wrists of humanoids where precision matters.

The trade-off is cost and some fragility under shock loads. Some newer robots use planetary gearboxes or direct-drive motors instead, trading precision for toughness and lower price. Neither is better in general, it depends on the job.