Stiffness and Damping Characteristics of Tilting-Pad Journal Bearings

Abstract

Lateral vibrations of large turborotors of steamturbines or hydroelectric power plants are highly influenced by the stiffness and damping characteristics of the bearings. Large turborotors are usually guided in hydrodynamic journal bearings. Tilting-pad journal bearings are used particularly for horizontal or vertical shafts of high diameters. If the amplitude of the shaft vibration is small compared with the clearance of the bearing, the characteristics of the hydrodynamic oil film can be described, corresponding to a linear theory, by unsymmetrical stiffness and damping matrices of dimension (2×2), containing four stiffness and four damping coefficients, respectively. When assuming a rigid and immovable bearing house then the stiffness and damping coefficients of the oil film depend only upon the static bearing load, the clearance and speed of the shaft and the specifications of the lubricant, see Ref.1,2. They are not influenced by the frequency of lateral shaft oscillations. However, when the bearing house is flexible or movable — for example, when a tilting-pad bearing with flexibly pivoted pads is used — then an essential influence of lateral shaft vibrations upon the stiffness and damping coefficients could occur 3,4,5. This holds even for small amplitudes of vibrations where a linear theory can be applied.