Planetary Gears (Epicyclic Gears) is a gear system consisting of one or more outer gears (planet gears), revolving about a central (sun gear).
A worm drive is a gear arrangement in which a worm (screw) meshes with a worm gear (aspur gear).
Reduce rotational speed or transmithigher torque.
Transfer motion in 90 degrees.
The gear ratio is: 1 to teeth of the worm wheel
The direction of transmission is not reversible due to the greater friction involved between the worm and worm-wheel.