Metal gear refers to a metal mechanical element with teeth on the rim that can continuously engage and transmit motion and power. The metal gear relies on the meshing of the teeth to drive with other toothed mechanical parts (such as another gear, rack, worm), and can realize the functions of changing the speed and torque, changing the direction of movement and changing the form of movement.

Metal gears are widely used and there are many types. According to the tooth profile curve, they can be divided into involute gears, cycloidal gears, arc gears, etc. According to the shape, it can be divided into cylindrical gears, bevel gears, non-circular gears, racks, worms, worm gears, etc.; according to the surface of the gear teeth, it can be divided into external gears and internal gears. According to the shape of the tooth line, it can be divided into spur gears, helical gears, herringbone gears, curved gears, etc. According to the manufacturing method, it can be divided into casting gears, cutting gears, rolling gears, sintering gears, etc. Gears used in life are: automobiles, clocks, elevators, mechanical watches, automobiles, variable speed bicycles, noodle machines, juicers, food processors, egg beaters, water meters, gas meters, sewing machines, CD players, wine bottle openers appliances, clocks, automobile gearboxes, swing heads of electric fans, washing machines, alarm clocks, etc.

The tooth surface of the metal gear should have high resistance to wear, pitting corrosion, gluing and plastic deformation, and the tooth root should have high resistance to breaking. Therefore, metal gear teeth are usually heat treated by surface quenching, carburizing, nitriding, soft nitriding and cyanidation. The basic requirements of material properties are: the tooth surface should be hard and the tooth core should be tough. Commonly used gear materials are low carbon alloy steel, high carbon steel, manganese steel, carbon steel, cast iron, brass, stainless steel, titanium, aluminum alloy, and some rare metal alloys such as tungsten, molybdenum, etc.