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

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.