A gearbox (also called a transmission or speed reducer) is a mechanical device that houses a set of gears designed to transmit power, adjust torque, and control rotational speed between a prime mover (like an engine or motor) and the driven equipment.By changing gear ratios, a gearbox can increase torque while reducing speed or increase speed while reducing torque.Gearboxes are used in everything from automobiles and industrial machinery to wind turbines and marine vessels.