A worm gear drive is chosen mainly because it

Correct answer to question 30 (Mechanical Comprehension): gives a large speed reduction and usually cannot be driven backwards

Method

A single-start worm advances the wheel one tooth per turn, giving reductions of forty to one or more in a small space. It also self-locks, so the load cannot drive the input backwards, which is why hoists and gate openers use it. The sliding contact makes it relatively inefficient, and the shafts sit at right angles.