Worm gears are used where very large speed reductions are required. This is a shaft with a thread like a screw. This connects at 90° to a large gear (the thread shaft points along the outside edge of the larger gear). Each time the shaft spins one revolution, the gear turns forward by only one tooth. If the gear has 50 teeth, this creates a gear ratio of 50:1. The worm can drive the worm gear round, but the worm gear cannot drive the worm.