Gearboxes, normally referred to as transmissions, are mechanical or hydraulic devices used to transmit power from an engine or motor to different components within the same system. They typically consist of a series of gears and shafts that can be engaged and disengaged by an operator or automatic system. The term gearbox also refers to the lubrication filled casing that holds the transmission system and protects it from various contaminants.
The majority of gearboxes are used to increase torque and lower the output speed of the motor shaft; such transmissions, many of which also include the ability to choose from a number of gears, are regularly found in automobiles and other vehicles. Lower speed gears have increased torque and are therefore capable of moving certain objects from rest that would be impossible to move at higher speeds and lower torques; this accounts for the usefulness of low gears in towing and lifting operations. In some cases, gears are designed to provide higher speeds but less torque than the motor, allowing for rapid movement of light components or overdrives for certain vehicles. The most basic transmissions simply redirect the output of the engine/motor shaft.
Automotive transmissions fall under three main categories: automatic, semi-automatic, and manual. Manual transmissions tend to be the most fuel efficient, as less fuel is wasted during gear change; in these systems, the operator determines when to change gears and activates the clutch mechanism. Automatic transmissions perform gear changes based on fluid pressure in the gearbox, and the operator has limited control over the system. Semi-automatic transmissions now see wider use, and allow the user to engage a manual gear change system when necessary, while normal gear operations are controlled automatically.
Gearboxes utilize a wide range of gear types, including worm gears, bevel and spiral bevel gears, helical gears and spur gears. These mechanisms are each engineered to perform a specific task within the gearbox, from reducing speed to changing output shaft direction. However, each additional gear results in power lost due to friction, and efficiency is key to proper system design.