Decoupling the Role of Thermal Preload Relaxation in Reshaping the Intrinsic Axial Nonlinear Stiffness Profiles of Machine Tool Spindles

The axial nonlinear stiffness of machine tool spindles critically governs machining stability, yet isolating its thermal-driven evolution from intertwined, antagonistic rotational dynamic interferences remains a long-standing challenge. The main contribution of this study is the “Double Decoupling” framework, which isolates the thermal-driven evolution of spindle axial stiffness from dynamic interferences and distinguishes internal preload relaxation from localized component expansion. Experimentally, a “thermal-equilibrium and rapid-shutdown” methodology is developed to physically isolate quasi-static thermal boundaries, successfully capturing pristine intrinsic stiffness profiles within a critical 5 s post-shutdown window. Analytically, a thermomechanical model coupling micro-geometric thermal distortions with dynamic preload evolution is established. Our findings quantitatively elucidate that temperature escalation (26 °C to 43 °C) prompts a monotonic global stiffness degradation exceeding 12% in the central stiffness plateau. Thermomechanical inversions reveal that localized bearing thermal expansion plays merely a secondary role; instead, the non-uniform internal temperature field drives a macroscopic axial thermal mismatch between the spacers, triggering “thermal preload relaxation” as the primary contributor to global thermal stiffness loss. Consequently, practical thermal stiffness regulation should preferentially target macro-preload and spacer management rather than complex component dimensional compensation.