Rotor Stability Effects of Tilting Pad Journal Bearings With Assembled Clearance Asymmetry

Abstract

This paper presents component and system-level analyses to understand the stiffness, damping, and rotor stability effects of tilting pad journal bearings (TPJB) with assembled clearance variation across pads. This variation results from a normal manufacturing process of the bearing components, i.e., shaft, bearing shell, pad radius, and pad thickness, whose machining tolerances add in a closed-loop producing a different assembled clearance per pad. The results are compared to the standard analysis practice of uniformly setting all pads assembled clearance to its maximum and minimum stack-up values to find the worst case for stability. The component-level analysis follows a rigid rotor approach. While in the system-level analysis, a full rotor-bearings-seals model of a lightweight, high-speed centrifugal compressor is employed. A four-pad TPJB with a load between pads (LBP) configuration is considered. Large differences in stiffness and log dec are encountered, while almost negligible differences are found in damping. Also, some interesting observations related to rotor-bearing planar mode shapes are reported when the bearing pads have dissimilar assembled clearances. These modes disappear once the labyrinth seal force coefficients are incorporated into the full model.