Abstract <p>Asymmetric meshing behavior between the right- and left-side teeth is an inherent characteristic of helical gears, yet this spatial phase difference is frequently simplified in conventional dynamic modeling, which limits the prediction accuracy of system stability. An improved nonlinear torsion–vibration dynamics model for high-speed EMU traction gears is established, in which the difference between two flanks of a tooth is explicitly incorporated. A precise phase angle model is constructed to describe the asynchronous meshing states. The time-varying parameters, including load distribution rate (TVLDR), time-varying mesh stiffness (TVMS), and time-varying friction coefficient (TVFC) are determined by explicitly accounting for the contact line length variation derived from the teeth surface equations, distinct from traditional symmetric calculation methods. The system dynamics are investigated by defining three specific Poincaré sections to capture the right-side teeth collision and left-side teeth contact. It is revealed that the bifurcation structure is significantly altered and unique boundary impact phenomena are induced by the flank difference, which are unobservable in symmetric models. It is demonstrated that the traction gears operate in a stable period-1 motion at 350 km/h under full load, for both new and worn wheel diameters, with sufficient safety thresholds. This work provides a more accurate theoretical framework for the design of high-speed train transmissions by clarifying the mechanism of asymmetric meshing excitations.</p> Graphical abstract <p>The difference between two flanks of a tooth is a critical factor in calculating TVLDR, TVMS, and TVFC for helical gear. The contact line on the right-side teeth is very short, close to 0 when the gear is in the mesh-in position and there is no backlash. However, there is already a certain contact line length on the left-side teeth. A certain gear rotation angle is needed by the pinion to achieve the current contact line length on the left-side teeth. It is referred to as the difference angle between two flanks of a tooth. It must add a gear rotation angle corresponding to the backlash when there is backlash. TVLDR, TVMS, and TVFC of the right- and left-side teeth exhibit different values at the same moment due to the difference between two flanks of a tooth. The system is more prone to chaos and becomes more complex considering the difference between two flanks of a tooth.</p>