Measurements Versus Predictions for the Rotordynamic Characteristics of a 5-Pad, Rocker-Pivot, Tilting-Pad Bearing in Load Between Pad Configuration

Rotordynamic data are presented for a rocker-pivot tilting-pad bearing in a load-between-pad (LBP) configuration for unit loads over the range [345, 3101 kPa] and speeds over the range [4k to 13k rpm]. The bearing was direct lubricated through a leading-edge groove with the following specifications: 5 pads, .282 preload, 60% offset, 57.87° pad arc angle, 101.587 mm (3.9995 in) rotor diameter, .1575 mm (.0062 in) diametral clearance, 60.325 mm (2.375 in) 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 60 kg. At low speeds, “hardening” direct dynamic stiffness coefficients that increased with increasing frequency were obtained that 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 CFD analysis. The static load direction in the tests was y. The direct stiffness coefficients K_xx and K_yy were slightly over predicted. Measured direct damping coefficients C_xx and C_yy were insensitive to changes in either load or speed in contrast to predictions of marked C_yy sensitivity for changes in the load. Only at the highest test speed of 13000 rpm were the direct damping coefficients adequately predicted. Measurable cross-coupled stiffness coefficients were obtained for the bearings with K_xy and K_yx 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.