Large Axial Vibrations in Turbomachines: Non-Linear Behaviour of Double Sided Lubricated Thrust Bearing

Even if not a common phenomenon, large axial vibrations are a critical concern in turbomachinery, particularly when dealing with double-sided, oil-lubricated thrust bearings in compressors and gas turbines. This phenomenon, known as axial Sub-Synchronous Vibration (SSV), manifests as high-amplitude vibrations that can extend throughout the entire machine clearance. The high level of vibrations is a main concern for the structural integrity of the machines, and it can lead to the onset of a trip condition. The development of the vibration is often unpredictable, marked by sudden jumps in vibration amplitude due to various operational parameters of the machine, such as active power, clearance adjustments, oil temperature, and flow rate. To understand the problem, the non-linear behaviour of the oil film of the thrust bearing is analysed by solving the wellknown Reynolds equation. The integral force on the bearing collar is calculated and then used to create a simplified axial model of the machine. Harmonic Balance is used to study the non-linear dynamics of the system, the bifurcation region is found, and a sensitivity analysis is performed by changing the most important operational parameters of the bearing. This phenomenon usually amplifies the vibrations induced by the coupling between axial-lateral vibrations, related to the thrust collar. Numerical results are compared to experimental data available in the literature.