Gearbox is a gear system that changes the speed-torque ratio of a rotational movement with the help of gears. It is an element of parallel gear trains of gear wheel assemblies with gearboxes. Structurally, gearboxes, gear wheels, shafts, bearings, etc. systems made up of such parts. Academic definition: Closed gear assemblies designed to reduce the high rotational speeds of electric motors to those required for machines.