The friction coefficient between two meshing tooth surfaces has an important influence on the magnitude of tooth surface friction, so the friction coefficient also has a significant impact on the dynamic characteristics of helical gear transmission. In helical gear transmission, the friction coefficient of the tooth surface changes with the load distribution, the speed of the helical gear and the surface morphology of the tooth profile, and the lubrication state of the tooth changes constantly between fluid lubrication and boundary lubrication. The widely existing contact state in helical gear transmission is mixed elastohydrodynamic lubrication, including fluid elastohydrodynamic lubrication, boundary elastohydrodynamic lubrication, thin film elastohydrodynamic lubrication and so on. Zhu and Hu put forward the formula of point contact load bearing coefficient of full oil film lubrication under mixed elastohydrodynamic lubrication as follows:
Where:
σ – Tooth profile surface roughness Ra root mean square（ μ m) ;
σ P – tooth profile surface roughness Ra of driving helical gear（ μ m) ;
σ G – driven helical gear tooth profile surface roughness Ra（ μ m) ;
Tmin – minimum oil film thickness（ μ M)