Enhancing gear surface integrity: A study of longitudinal-torsional ultrasonic strengthening effect on surface roughness

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.