A gearbox consists of two main components: the variable-speed transmission mechanism and the shifting control mechanism. The primary function of the transmission mechanism is to alter the magnitude and direction of torque and rotational speed; the primary function of the control mechanism is to govern the transmission mechanism, thereby enabling changes in the gearbox's transmission ratio-that is, executing gear shifts-to achieve the desired variations in speed and torque.

Simple-type gearboxes offer the advantages of high efficiency, simple construction, and ease of use; however, they feature a limited number of gears and a narrow range of transmission ratios (resulting in a limited range of tractive force and vehicle speed). Consequently, they are suitable only for use in certain vehicles where a limited number of gears is acceptable. If one were to attempt to expand the range of transmission ratios within a simple-type gearbox, it would result in an increase in the overall dimensions of the gearbox and an enlargement of the shaft span. To increase the number of gears without causing the shaft span to become excessively large, a compound-type gearbox may be employed. A compound-type gearbox typically consists of a combination of two simple-type gearboxes; of these, the unit featuring the greater number of gears is referred to as the main gearbox, while the unit with fewer gears is termed the auxiliary gearbox.

Mechanical gearboxes primarily utilize the principle of speed reduction via gear transmission. Simply put, a gearbox contains multiple sets of gear pairs, each possessing a distinct transmission ratio. The act of shifting gears while a vehicle is in motion involves using the control mechanism to engage specific gear pairs within the gearbox. For instance, at low vehicle speeds, a gear pair with a high transmission ratio is engaged, whereas at high vehicle speeds, a gear pair with a low transmission ratio is engaged.