Gears can be classified according to tooth shape, gear shape, tooth line shape, surface on which the gear teeth are located and manufacturing method.
       The tooth profile of the gear includes tooth profile curve, pressure angle, tooth height and displacement. Involute gears are relatively easy to manufacture, so among the gears used in modern times, involute gears account for the absolute majority, while cycloidal gears and arc gears are rarely used.
  The tooth height of gears has been standardized, and standard tooth heights are generally used. Variable gears have many advantages and have been used in various types of mechanical equipment.
       In addition, gears can also be divided into cylindrical gears, bevel gears, non-circular gears, racks, and worm gears according to their appearance.
       According to the shape of the tooth line, it is divided into spur gears, helical gears, herringbone gears and curved gears.
       According to the surface where the gear teeth are located, it is divided into external gears and internal gears; according to the manufacturing method, it can be divided into cast gears, cut gears, rolled gears, sintered gears, etc.
       The manufacturing materials and heat treatment process of the gear have a great impact on the load-bearing capacity and size and weight of the gear. Before the 1950s, carbon steel was mostly used for gears, alloy steel was used in the 1960s, and case-hardened steel was used in the 1970s. According to hardness, tooth surfaces can be divided into soft tooth surfaces and hard tooth surfaces.
  Because among the matched gears, the small wheel has a heavier load, so in order to make the working life of the large and small gears roughly equal, the tooth surface hardness of the small wheel is generally higher than that of the large wheel.
   However, during heat treatment, the gear will inevitably deform, so it must be ground, ground or precision cut after heat treatment to eliminate errors caused by deformation and improve the accuracy of the gear.