Tilting-Pad Bearings: Measured Frequency Characteristics Of Their Rotordynamic Coefficients

Abstract

This paper reviews a long standing issue related to the stiffness and damping coefficients of tilting-pad (TP) bearings; namely, What is the nature of their frequency dependency? A research project was implemented at the Turbomachinery Laboratory (TL) at Texas A&M University (TAMU) around 2003 to examine the issue, applying procedures that had been developed and used to investigate the rotordynamic characteristics of annular gas seals. Those seals, using a smooth rotor and a honeycomb or hole-pattern stator were predicted to have strongly frequency-dependent reaction forces that could not be modeled by a combination of stiffness, damping, and inertia coefficients. Measurements confirmed the strongly frequency dependent nature of their stiffness and damping coefficients. Subsequent test have examined the following bearing types: (i) Two-axial-groove bearing, (ii) pressure dam bearings, (iii) Flexure-pivot-pad tilting-pad bearing (FPTP) in load-on-pad (LOP) and load-between-pad (LBP), (iv) Rocker-pivot-pad TP bearing in LOP and LBP configurations at two different preloads and 50 and 60% offsets, and (v) a spherical seat bearing in LOP and LBP configurations. Representative test results are presented for some of these bearings. In addition, this paper includes experimental results for 5-pad and 4-pad tilting pad bearings (with similar features to TAMU configuration iv) tested at the GE Global Research Facility (GRC) as part of an independent research initiative from GE Oil and Gas. Frequency effects on the dynamic-stiffness coefficients were investigated by applying dynamic-force excitation over a range of excitation frequencies. Generally, for all bearings tested at TAMU and GRC, the direct real parts of the dynamic-stiffness coefficients could be modeled as quadratic functions of the excitation frequency and accounted for by adding a mass matrix to the conventional [C][K] model to produce a frequency-independent [M][C][K] model. Additionally, the direct damping could be modeled by a constant, frequency-independent coefficient. Consequently, these experimental findings from two independent sources support the use of synchronously reduced force coefficients for characterizing the dynamic performance of tilting pad bearings in Oil and Gas applications, as prescribed by API 617 7th edition (Process Centrifugal Compressors) and more generally by API684 Rotordynamic Tutorial.