Identification of nonlinear dynamic parameters of thrust bearings in rotating machines: Modelling of axial Sub-Synchronous vibration and experimental verification

Axial Sub-Synchronous Vibration (SSV) is an uncommon phenomenon that occurs in turbomachines. Specifically in compressors and large gas turbines equipped with double-sided, oil-lubricated thrust bearings. This phenomenon manifests as high-amplitude vibration which extends throughout the entire machine axial clearance. Consequently, the structural integrity of the machinery can be compromised, as the SSV can lead to detrimental effects such as fatigue cracks, seal abrasion, and fretting on bearings. This vibration develops unpredictably, with sudden amplitude jumps related to operational parameters such as active power, axial clearance adjustments, and lubricating oil temperature. While the literature extensively discusses potential causes, often linking vibration levels to fluid film instabilities on the reverse side of thrust bearings, this theory does not account for all observed cases. To understand the problem, the nonlinear behaviour of the oil film in the thrust bearing is analysed by solving the well-known Reynolds equation. The thrust force on the bearing collar is calculated and used to create a simplified axial model of the machine. Time integration and Harmonic balance methods are used to study the nonlinear dynamics, to identify the bifurcation region, and to perform sensitivity analyses by varying operational parameters. This phenomenon usually amplifies the vibration induced by the coupling between axial-lateral vibration, related to the thrust bearing collar. The proposed model, applied to a well-known example in the literature, offers insights into mitigating the adverse effects of axial SSV and improving the reliability of turbomachines.