Measurements Versus Predictions for the Static and Dynamic Characteristics of a Four-Pad, Rocker-Pivot, Tilting-Pad Journal Bearing

Abstract

Measured and predicted static and dynamic characteristics are provided for a four-pad, rocker-pivot, tilting-pad journal bearing (TPJB) in the load-on-pad (LOP) and load-between-pad (LBP) orientations. The bearing has the following characteristics: pad-pivot offset = 0.57, L/D = 0.6, pad length = 60.33 mm. Unit loads ranged from 0 to 2903 kPa, and speeds ranged from 6.8 to 13.2 krpm. Nonrotating tests were carried out using a small rotating load to precess the test-bearing stator around the rotor shaft while measuring the clearances. These tests produced “clearance rectangles” for the LOP case and “clearance rhombuses” for the LBP cases. These tests defined the bearing clearances for facing bearing pads that were significantly different with a ratio between the larger and smaller clearances at approximately 1.6. Clearances were measured at room temperatures and immediately following tests to obtain room temperature and “hot” clearances. Hot-clearance measurements showed a 16%–25% decrease as compared to room-temperature clearances. Static load-deflection tests were carried out to determine the pad's flexibility characteristics with respect to the housing (pad-pivot flexibility). Detailed circumferential temperature measurements were made on the loaded pad(s) with only leading and trailing temperatures for the unloaded pads. The radial thermal gradient was examined in the loaded pad via embedded thermocouples on the rotor and outside of the pads. Results showed a 5–25 °C decrease from the rotor side of the pad to housing side. An FEM analysis predicted that the radial and circumferential temperature gradients caused an uneven thermal deflection in the pad, changing the pads' radii of curvature. (However, the changes made scant differences in predictions.) Dynamic-excitation tests were performed over a range of excitation frequencies Ω to obtain 2 × 2 complex dynamic-stiffness matrices [Hij] as a function of Ω. The Re(Hij) coefficients were readily fitted as a linear function of Ω2, producing frequency-independent stiffness and virtual-mass coefficients. The Im(Hij) coefficients were readily fitted as a linear function of Ω, producing frequency-independent damping coefficients and supporting the adequacy of a constant-frequency MCK model for bearings out to running speed. Measured (separate) pad clearances, pad-contact flexibility characteristics, and input temperatures were used as input for a recently-developed code to predict the static and dynamic characteristics of the bearing. The code used a Reynolds equation model plus an adiabatic energy equation. It also accounts for pad-contact flexibility. Measurements versus predictions were made for the temperature distributions, the dynamic-stiffness coefficients, and the direct rotordynamic coefficients (stiffness, damping, and virtual-mass). The measured cross-coupled stiffness and damping coefficients were insignificant, and are not presented. Generally, the code predicts the trends of the circumferential temperature distributions well; however, it predicted a continuing increase in temperature from leading to trailing edge, while the tests show an increase through the next-to-last temperature probe and then a drop to the last probe nearest the trailing edge. Generally speaking, the code does an adequate job of predicting rotordynamic coefficients for both LOP and LBP conditions. The input data (clearances, pad-flexibility, etc.) and output results (temperatures, dynamic stiffness coefficients, rotordynamic coefficients) presented allow other researchers to directly make predictions for these bearings using alternate models and codes.