This sprocket assembly is located at the top of the motor unit, the gear and sprocket assembly drives the chain that connects to the trolley and moves the trolley along the rail to raise and lower the garage door.

A broken gear and sprocket assembly won't move the chain when the motor runs. This is used either to transmit rotary motion between two shafts where gears are unsuitable or to impart linear motion to a track, tape etc.

Sprockets and chains are also used for power transmission from one shaft to another where slippage is not admissible, sprocket chains being used instead of belts or ropes and sprocket-wheels instead of pulleys.