- A gear train is a combination of gears arranged in a chain fashion where the follower of one gear is the driver of the next. Gear trains are used to change the speed or torque of rotating members. - The angular velocity ratio of a gear train is the ratio of the output angular velocity to the input angular velocity. It is determined by the number of teeth on the driving and following gears. - Gear trains are used to change speeds in many mechanical applications, such as changing the rotational speed between an engine and wheels. The velocity ratio of compound gear trains is calculated as the product of the velocity ratios of each section.