Yan Jiang, Qinyuan Yang, Qiang Guo, Bo Zhao, Tian Zi Li, Ying Niu
Tooth surface properties are critical for gear longevity, efficiency, and accuracy. Traditional strengthening methods often fall short in high-precision, long-life applications. Fortunately, with its precise energy control, ultrasonic machining offers a novel approach to overcoming these drawbacks and achieving superior surfaces quality. However, there are few studies on the effect elucidation of longitudinal-torsional ultrasonic machining for gears. Thus, this paper introduces a new gear surface strengthening method by integrating longitudinal-torsional ultrasonic machining with gear meshing theory, focusing on surface formation mechanisms and roughness evolution. The underlying micro-forming mechanism, based on Boussinesq-Flamant theory, models the process in three stages: 1) Triangular indentation, where initial grinding peaks yield and subside under combined static and ultrasonic loading; 2) Crushing collapse, involving material work hardening, fracture, and flow into valleys under cyclic load, dissolving the original topography; 3) New surface formation, where stabilized plastic flow fills valleys to create a smooth, dense surface with beneficial residual compressive stress. This model systematically describes the evolution from the initial to the hardened surface. For validation, a dedicated experimental platform was used alongside white-light interferometry. Results show high consistency between experimental and theoretical surface morphology trends along the tooth profile, with an average quantitative error of 15%. These findings validate the three-stage model, providing a theoretical foundation for understanding and optimizing ultrasonic gear strengthening processes.