Classification of Gears
Gears can be classified according to tooth shape, gear shape, tooth line shape, tooth surface and manufacturing method.
Gear profile includes tooth profile curve, pressure angle, tooth height and displacement. Involute gears are relatively easy to manufacture, so in the modern use of gears, involute gears account for the absolute majority, while cycloidal gears and circular arc gears are less used.
In terms of pressure angle, the bearing capacity of small pressure angular gear is smaller, while that of large pressure angular gear is higher, but bearing load increases under the same transmission torque, so it is only used in special cases. The tooth height of gears has been standardized, and the standard tooth height is generally used. Modified gear has many advantages and has been used in various mechanical equipment.
In addition, gears can be divided into cylindrical gears, bevel gears, non-circular gears, racks and worm gears according to their shape; spur gears, helical gears, herringbone gears and curved gears according to the shape of the tooth line; external gears and internal gears according to the surface of the gear teeth; casting gears, cutting gears, rolling gears and sintered gears according to the manufacturing method.
The manufacturing materials and heat treatment process of gears have a great influence on the bearing capacity and size and weight of gears. Before 1950s, gear was made of carbon steel, alloy steel was used in 1960s, and surface hardening steel was used in 1970s. According to hardness, the tooth surface can be divided into soft tooth surface and hard tooth surface.
The gear with soft tooth surface has low load-bearing capacity, but it is easy to manufacture and has good running-in performance. It is mostly used in general machinery with no strict restriction on transmission size and weight and small production. Because of the heavy burden of small wheels in paired gears, the hardness of small wheels is generally higher than that of large wheels in order to make the working life of big and small gears roughly equal.
Hard face gear has high bearing capacity. It is quenched, surface quenched or carburized after finishing cutting to improve its hardness. But in heat treatment, the gear will inevitably deform, so after heat treatment, grinding, grinding or cutting must be carried out to eliminate the errors caused by deformation and improve the accuracy of the gear.