The simple transmission has the advantages of high efficiency and simple construction and convenient use. However, the number of files is small and the range of change of i is small (the traction force and the speed range are small), and it is only suitable for some carpenters who do not have many files. If the range of i is increased, the size of the transmission is increased and the span of the shaft is increased. In order to increase the number of gears without making the shaft span too large, a composition type transmission may be used. The so-called compositional transmission is usually formed by a combination of two simple transmissions, in which a higher number of gears is referred to as a main transmission, and a smaller number is referred to as an auxiliary transmission.