Generally, the gear whose parameters, size and weight are large enough to exceed the conventional conditions is called large gear. Large gears are mainly used in some key areas and important industries affecting the overall situation, such as water conservancy and power generation, mining industry, large engineering machinery, large ships and missile launch. It plays a very important role in the national economy and national defense.

Large gears can be divided into two categories:

(1) Large module gear is characterized by large module, small number of teeth, deep tooth slot and long elongation. It is mostly shaft gear, and generally herringbone or helical gear. It is widely used in steel rolling machinery.

(2) Large gear. Mainly refers to its outer diameter. The module of this kind of gear is not very large, but there are many teeth, and the outer diameter and weight are very large. Its shape is mostly disk-shaped. Strictly speaking, this paper mainly studies the latter.

The parameters, size and weight of large gears are so large that their accuracy measurement methods have changed from quantity to quality. It is difficult to realize the measurement with conventional accuracy measurement methods, which also seriously affects the manufacturing accuracy, performance and quality of large gears. Starting from the urgent problems to be solved, this chapter studies the characteristics and measurement difficulties of large gears, and gives the principle framework and idea of realizing the line efficiency measurement of large gears.