Purpose—The flexspline of a harmonic drive constitutes a thin-walled structure with discontinuous gear rim and cylinder configuration, where cyclic stresses induce stress concentration, followed by crack initiation, propagation, and ultimately fatigue failure. This paper reviews advancements in understanding its mechanical properties and fatigue failure mechanisms, aiming to establish a foundation for enhancing operational longevity and guiding future research. Design/Methodology/Approach—The study integrates meshing theory, tooth shape parameters, cylinder stress influencers, and assembly/meshing stress considerations. Theoretical analysis, finite element simulations, and experimental methods are employed to examine stress patterns and fatigue dynamics. Structural parameters and tooth profiles are systematically analyzed for their impact on stress distribution and fatigue life. Findings—Flexspline fatigue failure arises from tooth root stress concentration and cylinder bending stress accumulation. The double-circular-arc tooth profile boosts load capacity by 35% relative to the involute profile, yet demands high-precision machining to preserve meshing performance. Increasing cylinder length mitigates stress concentration but reduces torsional stiffness, while optimized root fillet radii can lower the stress concentration coefficient by 28%. Assembly interference and meshing contact stress accelerate crack initiation, as validated by transient dynamics simulations. Surface strengthening processes (e.g., shot peening) enhance fatigue life by up to 66% through residual compressive stress regulation. Originality/Value—This paper synthesizes multi-scale research on flexspline design, structural optimization, and fatigue mechanisms, proposing novel approaches such as “manufacturability-oriented optimization” and digital twin-driven monitoring. By linking dynamic loads, material properties, and geometric parameters, it bridges theoretical gaps and provides actionable insights for high-precision harmonic drives in robotics and aerospace, advancing reliability in precision transmission systems.

To address the limitations of existing studies on harmonic drive (HD) systems, such as the lack of systematic exploration of the influence of flexspline (FS) structural parameters on service lifetime and insufficient parameter optimization strategies, this study establishes a comprehensive research framework integrating finite element (FE) simulation, response surface optimization, and experimental validation. Three key structural parameters of FS, including cup length (CL), cup wall thickness (CT), and tooth width (TW), which were selected as design variables to investigate their effects on the displacement, von Mises stress, and fatigue lifetime of the HD system. A reliable FE model of the HD system was constructed, with mesh independence verified and experimental validation conducted using a servo motor-driven test platform. The results showed that the relative errors between simulated and measured displacement (4.7%) and stress (4.0%) were within the acceptable engineering range (<10%), confirming the model’s reliability. Systematic analysis revealed that the FS lifetime is jointly determined by global stress distribution and structural rigidity, rather than local tooth root stress alone. Increasing cup length reduces tooth root stress but increases cup wall bending stress, which ultimately dominates the fatigue failure process. Based on Box–Behnken Design (BBD) response surface analysis, the optimal parameter combination was determined as CL = 48 mm, TW = 6 mm, and CT = 0.64 mm, achieving the dual optimization of low system stress (77.7 MPa) and long FS lifetime (2950 h). Relative to the baseline control group (CL = 48 mm, CT = 0.46 mm, TW = 6 mm), the optimized configuration reduces system stress by 10.1% and extends fatigue lifetime by 1.7%. This study clarifies the multi-factor coupling mechanism of structural parameters regulating HD system performance, providing a robust theoretical basis and engineering reference for the structural design and performance improvement of HD systems.