Model Based Analysis of Coupled Vibrations Due to the Combi-Bearing in Vertical Hydroturbogenerator Rotors

The studies presented in this paper focus on analyzing how the combined thrust-journal bearing (commonly called combi-bearing) influences the dynamics of hydropower rotors. Thrust bearing is a component used in vertical rotating machinery and shafts designed to transmit thrust. The total axial load is carried by the single thrust bearing. Any design, manufacture, or assembly error in this component (thrust bearing) would certainly influence the functionality of the entire machine. The analyzed combi-bearing is an existing machine component used in the hydropower unit Porjus U9 situated in northern Sweden. This combi-bearing is a fluid-film lubricated tilting-pad thrust and journal bearings combined together. Only linear fluid-film stiffness was taken into account in the model while fluid-film damping and pads inertia effects were not taken into account. The linearized model shows that the combi-bearing couples the rotor’s lateral and angular motions. However, if the thrust bearing’s pads arrangement is not symmetrical or if all the pads are not angularly equidistant the rotor axial and angular motions are also coupled. This last case of coupling will also occur if the thrust bearing equivalent total stiffness is not evenly distributed over the thrust bearing. A defective pad or unequal hydrodynamic pressure distribution on the pads’ surfaces may be the cause. The Porjus U9’s simulation results show that the combi-bearing influences the dynamic behavior of the machine. The rotor motions’ coupling due to combi-bearing changes the system’s natural frequencies and vibration modes.