Influence of axial bearing dynamics on rotor vibrations: Transient analysis considering cavitation and coupling of axial and radial fluid films

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In addition to the elastic and inertia properties of the rotor as well as the external loads, the vibration behaviour of rotor systems is mainly determined by the bearings. Due to the advantageous mechanical and thermal characteristics as well as the low cost, hydrodynamic bearing constructions are often used. On the one hand, this applies to the journal bearings, which dominantly influence the bending vibrations, and at the same time thrust bearings are used to account for axial loads. Due to their respective stiffness and damping parameters, both bearing types significantly contribute to the system behaviour, which has a drastic effect on the resulting vibrations and stability. Continuous advancements and increasing demands require an improved simulation quality of transient system behaviour, which is why detailed modelling of the system properties of the rotor system, bearings and their interactions are necessary.For transient studies, it is not adequately possible to use established steady-state solutions in the field of coupled rotor bearing simulation. While the solution of a non-linear equation system is sufficient for quasi-stationary considerations of the bearing situation, time integrations must be carried out for transient questions with respect to rotor dynamics. A correspondingly detailed consideration of the bearing makes the evaluation of the Reynolds differential equation necessary at each step of the time integration, taking into account the time-variant cavitation state. In the previous project, a modified cavitation algorithm was developed for journal and floating ring bearings based on Elrod’s approach, which is now to be transferred to thrust bearings. In this context, in addition to the simple thrust bearing, various other forms of construction occur, which can be divided into combi bearings (combined journal and thrust bearings), floating disc bearings and tilting pad bearings. Hydraulic couplings between axial and radial lubricating films as well as mechanical couplings to the components of the rotor bearing system under the influence of cavitation must be taken into account. The extended simulation model is intended to validly predict relevant vibration phenomena (subharmonic vibrations, backward whirl excitation by external loads, tilting pad flutter) concerning both frequency and amplitude.