In mechanical transmission, compared with other transmission devices, gear transmission has the highest efficiency. However, with the increase of transmission power and rotating speed of gear drive, the power loss caused by gear drive is still considerable (especially in multi-stage gear reducer or planetary drive). The loss of power will not only be uneconomical, but also cause the heat of the machine, increase the wear and vibration of the machine, and reduce the working life and reliability of the machine. Therefore, it is of great theoretical and practical significance to improve the efficiency of gear transmission.
Efficiency and friction are inseparable. The efficiency of gear transmission is determined by the power of friction loss. Friction always exists in the mechanical transmission, which can not be eliminated, and can only be reduced. Human beings have known and analyzed the friction phenomenon for a long time, and applied it to production practice, such as “drilling wood to make fire”. According to the records of the book of songs, Jiafeng and spring water, lubricants have been used to reduce the loss of friction power as early as the spring and Autumn period. But in the long history, the development of tribology has been very slow. This is mainly because the research on the mechanism of friction has not put forward convincing and practical theory. In this paper, the study of gear transmission efficiency mainly lies in the calculation formula of gear efficiency and the calculation of friction coefficient in gear transmission.
Previous researches on the friction coefficient of gear transmission have done a lot of work. However, they all made too many idealized assumptions, and did not consider the actual surface roughness, lubrication state and center distance error of the gear on the meshing efficiency, which makes the results far away from the use of engineering practice. Lu Yunzhao  of Hangzhou shipbuilding industry school gives the available calculation formula of gear meshing efficiency, but the influence of basic parameters of gear on meshing efficiency is ignored due to over strength variable speed transmission, and only a hypothetical friction coefficient is given mechanically; Wang Cheng et al  deduces the calculation formula of meshing efficiency based on basic parameters of gear wheel, and also only the friction coefficient is given mechanically, and the calculation formula of meshing efficiency is public There is a mistake in the formula, which leads to the conclusion that the meshing efficiency of gears increases with the increase of transmission ratio, which is inconsistent with the actual situation.