Gears can be classified by tooth shape, gear shape, tooth line shape, tooth surface and manufacturing method.

The tooth profile of a gear includes profile curve, pressure Angle, tooth height and displacement. Involute gears are relatively easy to manufacture, so the absolute majority of gears in modern use are involute gears, while cycloidal and circular gears are less used.

In the aspect of pressure Angle, the bearing capacity of the gear with small pressure Angle is small. While the large pressure Angle gear, although the bearing capacity is higher, the bearing load increases when the transmission torque is the same, so it is only used in special cases. And gear tooth height has been standardized, generally used standard tooth height. Variable gear has many advantages and has been used in all kinds of mechanical equipment.

In addition, gear can also be divided into cylindrical gear, bevel gear, non-circular gear, rack, worm and worm gear according to its shape; According to the shape of the tooth line is divided into spur gear, helical gear, herringbone gear, curve gear; According to the tooth surface is divided into external gear, internal gear; According to the manufacturing method can be divided into casting gear, cutting gear, rolling gear, sintering gear, etc.

The commonly used steels for gear manufacturing are tempered steel, hardened steel, carburized hardened steel and nitriding steel. The strength of cast steel is slightly lower than that of forged steel, and it is often used for larger gears. Gray cast iron has poor mechanical properties and can be used in light load open gear drive. Ductile iron can partially replace steel in gear making. Plastic gear is used for light load and low noise requirements, and its matching gear with good thermal conductivity steel gear.

In the future, gear will be developed in the direction of heavy load, high speed, high precision and high efficiency, and strive for small size, light weight, long life and economic reliability.