There are usually two types of transmission mechanisms in the transmission chain: one is the transmission mechanism with fixed transmission ratio and transmission direction, such as fixed ratio gear pair, worm and turbine pair, which is called fixed ratio transmission mechanism; The other is the transmission mechanism that can change the transmission ratio and transmission direction according to the processing requirements, such as the change gear transmission mechanism, sliding gear transmission mechanism, etc., which is called the displacement mechanism.

According to the structure, it can be divided into roller chain, toothed chain and other types, of which roller chain is the most widely used. The structure of the roller chain is shown in Figure 1, which is composed of the inner chain plate 1, the outer chain plate 2, the pin shaft 3, the sleeve 4 and the roller 5. The inner chain plate and the sleeve, the outer chain plate and the pin shaft are respectively connected by interference fit, called the inner and outer chain links; The gap between roller and sleeve, sleeve and pin is fit. When the inner and outer chain plates are relatively bent, the sleeve can rotate freely around the pin shaft.