The structure of transmission chain is composed of inner and outer chains. It is composed of five small parts: inner chain plate, outer chain plate, pin shaft, sleeve and roller. The quality of chain depends on pin shaft and sleeve. In the machine tool transmission device, the commonly used transmission parts are pulley, gear, turbine, rack and nut. The connection between the power supply and the actuator or between two actuators is called a drive connection. A series of sequential transmission parts that constitute a transmission link are called transmission chains.

According to the nature of transmission links, transmission links can be divided into two types

1. Inline transmission chain: according to the motion analysis, the motion (rotation and linear motion) of the actuator itself associated with it belongs to independent row motion, so the relative motion relationship between actuator components should be strictly required. The transmission chain with such linkage characteristics is called internal linkage chain. Therefore, the transmission mechanism with uncertain or instantaneous transmission ratio changes in the internal link of the transmission chain, such as belt drive, chain drive, friction drive, etc. For example, when processing thread on horizontal lathe, the thread transmission between spindle and tool holder is an internal chain transmission chain with strict transmission ratio, which can ensure and obtain the required pitch of thread.

2. External linkage drive chain: it is the transmission chain connecting the motion source and the machine tool actuator. It allows the actuator to obtain a predetermined speed and drive a certain power, but does not require a strict transmission ratio between the motion source and the actuator. The change of outer chain transmission ratio only affects productivity and surface roughness, but does not affect the formation of workpiece surface shape.