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 fourpad, rockerpivot, tiltingpad journal bearing (TPJB) in the loadonpad (LOP) and loadbetweenpad (LBP) orientations. The bearing has the following characteristics: padpivot 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 testbearing stator around the rotor shaft while measuring the clearances. These tests produced “clearance rectanglesâ€‌ for the LOP case and “clearance rhombusesâ€‌ 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 “hotâ€‌ clearances. Hotclearance measurements showed a 16%–25% decrease as compared to roomtemperature clearances. Static loaddeflection tests were carried out to determine the pad's flexibility characteristics with respect to the housing (padpivot 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 آ°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.) Dynamicexcitation tests were performed over a range of excitation frequencies خ© to obtain 2 أ— 2 complex dynamicstiffness matrices [Hij] as a function of خ©. The Re(Hij) coefficients were readily fitted as a linear function of خ©2, producing frequencyindependent stiffness and virtualmass coefficients. The Im(Hij) coefficients were readily fitted as a linear function of خ©, producing frequencyindependent damping coefficients and supporting the adequacy of a constantfrequency MCK model for bearings out to running speed. Measured (separate) pad clearances, padcontact flexibility characteristics, and input temperatures were used as input for a recentlydeveloped 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 padcontact flexibility. Measurements versus predictions were made for the temperature distributions, the dynamicstiffness coefficients, and the direct rotordynamic coefficients (stiffness, damping, and virtualmass). The measured crosscoupled 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 nexttolast 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, padflexibility, 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.