Bearings and other revolute pairs in gear drives will form clearances due to wear after working for a long time, which will aggravate the vibration and noise of the gear. Therefore, taking the gear drives with revolute pair clearance as the research object, the calculation formulas of the backlash and contact ratio is derived, and the time-varying mesh stiffness model considering the clearance is presented in this work. Then, a nonlinear dynamic model for the gear drives with revolute pair clearance is proposed by combining the backlash, contact ratio and the time-varying mesh stiffness. The effects of the clearance on the meshing stiffness, load sharing factor and nonlinear behavior are investigated. Results indicate that the revolute pair clearance weakens the meshing stiffness and increases working hours in single teeth contact area, thereby increasing the risk of premature failure of the gear drive. In addition, the revolute pair clearance has negligible effect on the nonlinear behavior of gear pair in low-speed running, but it can obviously change the bifurcation behavior and aggravate the vibration of the gear drive.

In this study, an adhesive wear model for a gear drive in mixed elastohydrodynamic lubrication (EHL) is proposed. The mixed-EHL model combines the average Reynolds equation with the ZMC rough-surface contact model to determine the asperity contact pressure. By incorporating the fractional film defect into the Archard wear equation, a wear rate model under mixed EHL is developed and verified against published experimental data. This wear-rate model is subsequently coupled with a transient line-contact mixed-EHL model for gears to establish a tooth-surface wear prediction model that iteratively updates the tooth geometry and contact pressure. The evolution of tooth-surface wear under mixed EHL is investigated, and the resulting wear characteristics are compared with those under dry-contact conditions. The influence of tooth-surface roughness is also systematically evaluated. The results indicate that tooth-surface wear is substantially reduced under mixed EHL and that the maximum wear occurs between the lowest point of single-tooth contact and the pitch point. Increasing surface roughness intensifies wear and shifts the maximum-wear location toward the tooth root. These findings demonstrate that appropriate lubricant selection and effective control of tooth-surface roughness are important for improving the wear resistance of gear drives.