Gear rotation is the most widely used and common transmission form in mechanical transmission. The gear uses its teeth to transmit torque and motion, change the direction of motion, indicate the reading and change the position of the mechanism.
The manufacturing materials and heat treatment process of gears have a great influence on the bearing capacity, size and weight of gears. Before the 1950s, gears were made of carbon steel. In the 1960s, alloy steel was used, while in the 1970s, surface hardened steel was used. According to the hardness, the tooth surface can be divided into soft tooth surface and hard tooth surface.