Measurements Versus Predictions for the Rotordynamic Characteristics of a Five-Pad Rocker-Pivot Tilting-Pad Bearing in Load-Between-Pad Configuration

Abstract

Rotordynamic data are presented for a rocker-pivot tilting-pad bearing in the load-between-pad configuration for unit loads over the range 345–3101kPa and speeds over the range 4000–13,000rpm. The bearing was directly lubricated through a leading-edge groove with the following specifications: Five pads, 0.282 preload, 60% offset, 57.87deg pad arc angle, 101.587mm(3.9995in.) rotor diameter, 0.1575mm(0.0062in.) diametral clearance, and 60.325mm(2.375in.) pad length. Dynamic tests were performed over a range of frequencies to investigate frequency effects on the dynamic stiffness coefficients. Under most test conditions, the direct real parts of the dynamic stiffnesses could be approximated as quadratic functions of the excitation frequency and accounted for with the addition of an added-mass matrix to the conventional [K][C] matrix model to produce a frequency-independent [K][C][M] model. Measured added-mass terms in the loaded direction approached 60kg. At low speeds, “hardening” direct dynamic stiffness coefficients that increased with increasing frequency were obtained, which produced negative added-mass terms. No frequency dependency was obtained for the direct damping coefficients. The dynamic experimental results were compared to predictions from a bulk-flow computational fluid dynamics analysis. The static load direction in the tests was y. The direct stiffness coefficients Kxx and Kyy were slightly overpredicted. Measured direct damping coefficients Cxx and Cyy were insensitive to changes in either the load or speed in contrast to predictions of marked Cyy sensitivity for changes in the load. Only at the highest test speed of 13,000rpm were the direct damping coefficients adequately predicted. Measurable cross-coupled stiffness coefficients were obtained for the bearings with Kxy and Kyx being approximately equal in magnitude but opposite in sign—clearly destabilizing. However, the whirl frequency ratio was found to be zero at all test conditions indicating infinite stability for the bearing.