TILTING-PAD BEARINGS: MEASURED FREQUENCY CHARACTERISTICS OF THEIRROTORDYNAMIC COEFFICIENTS

摘要

This paper reviews a long standing issue related to thestiffness and damping coefficients of tilting-pad (TP) bearings;namely, What is the nature of their frequency dependency? Aresearch project was implemented at the TurbomachineryLaboratory (TL) at Texas A&M University (TAMU) around2003 to examine the issue, applying procedures that had beendeveloped and used to investigate the rotordynamiccharacteristics of annular gas seals. Those seals, using asmooth rotor and a honeycomb or hole-pattern stator werepredicted to have strongly frequency-dependent reaction forcesthat could not be modeled by a combination of stiffness,damping, and inertia coefficients. Measurements confirmed thestrongly frequency dependent nature of their stiffness anddamping 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 TPbearing in LOP and LBP configurations at two different preloads and 50 and 60% offsets, and (ⅴ) a spherical seatbearing in LOP and LBP configurations. Representative testresults are presented for some of these bearings. In addition,this paper includes experimental results for 5-pad and 4-padtilting pad bearings (with similar features to TAMUconfiguration ⅳ) tested at the GE Global Research Facility(GRC) as part of an independent research initiative from GEOil and Gas.Frequency effects on the dynamic-stiffness coefficientswere investigated by applying dynamic-force excitation over arange of excitation frequencies. Generally, for all bearingstested at TAMU and GRC, the direct real parts of the dynamicstiffnesscoefficients could be modeled as quadratic functionsof the excitation frequency and accounted for by adding a massmatrix to the conventional [C][K] model to produce afrequency-independent [M][C][K] model. Additionally, thedirect damping could be modeled by a constant, frequencyindependentcoefficient. Consequently, these experimentalfindings from two independent sources support the use ofsynchronously reduced force coefficients for characterizing the dynamic performance of tilting pad bearings in Oil and Gasapplications, as prescribed by API 617 7th edition (ProcessCentrifugal Compressors) and more generally by API684 Rotordynamic Tutorial.