Estimation formula of friction coefficient of rolling-sliding contact surface under mixed lubrication condition

The new estimation formula of friction coefficient of rolling-sliding contact surface for gears and other machine elements was devised. To decrease the power loss of power transmission gears, it is necessary to estimate friction coefficient at tooth meshing surface. And to prevent the failures like scuffing and pitting on tooth surface, it is necessary to decrease the flash temperature and tangential force that are proportional to friction coefficient. But there was no formula with high accuracy for estimation of friction coefficient under mixed lubrication condition considered a ratio of surface roughness and oil film thickness. In this paper, the new estimation formula of friction coefficient was presented. In the contact zone under mixed lubrication, a combination of fluid friction and boundary friction exists. The new function of load ratio expressed as a simple function of the ratio of surface roughness and elastohydrodynamic lubrication minimum oil film thickness was introduced. To evaluate the formula, two discs testing machine, four discs testing machine and power circulating type gear testing machine were conducted. The estimated values agreed well with the experimental results. The friction coefficient difference between estimated value and measured was less than 0.01. To improve estimation accuracy, it is necessary to collect the boundary friction data by additional experiment.