Recent comprehensive experimental data showcasing the force coefficients of commercial size tilting pad journal bearings has brought to rest the long standing issue on the adequacy of the [K,C,M] physical model to represent frequency independent bearing force coefficients, in particular viscous damping. Most experimental works test tilting pad journal bearings (TPJBs) with large preloads, operating over a large wide range of rotor speeds, and with null to beyond normal specific loads. Predictions from apparently simple fluid film bearing models stand poor against the test data which invariably signals to theory missing pivot and pad flexibility effects, and most importantly, ignoring significant differences in bearing and pad clearances due to actual operation, poor installation and test procedures, or simply errors in manufacturing and assembly. Presently, a conventional thermo hydrodynamic bulk flow model for prediction of the pressure and temperature fields in TPJBs is detailed. The model accounts for various pivot stiffness types, all load dependent and best when known empirically, and allows for dissimilar pad and bearing clearances. The algorithm, reliable even for very soft pad-pivots, predicts frequency reduced bearing impedance coefficients and over a certain frequency range delivers the bearing stiffness, damping and virtual mass force coefficients. Good correlation of predictions against a number of experimental results available in the literature bridges the gap between a theoretical model and the applications. Predicted pad reaction loads reveal large pivot deflections, in particular for a bearing with large preloaded pads, with significant differences in pivot stiffness as a function of specific load and operating speed. The question on how pivot stiffness acts to increase (or decrease) the bearing force coefficients, in particular the dynamic stiffness versus frequency, remains since the various experimental data show contradictory results. A predictive study with one of the test bearings varies its pivot stiffness from 10% of the fluid film stiffness to an almost rigid one, 100 times larger. With certainty, hearings with nearly rigid pivot stiffness show frequency independent force coefficients. However, for a range of pad pivot stiffness, 1110 to ten times the fluid film stiffness, TPJB impedances vary dramatically with frequency, in particular as the excitation frequency grows above synchronous speed. The bearing virtual mass coefficients become negative, thus stiffening the bearing for most excitation frequencies.

The accurate prediction of the forced performance of tilting pad journal bearings (TPJBs) relies on coupling a fluid film model that includes thermal energy transport, and on occasion fluid inertia, to the structural stiffness of the pads' pivots and the thermome chanical deformation of the pads' surfaces. Often enough, the flexibility of both pads and pivots is ignored prior to the bearing actual operation; practice dictating that force coefficients, damping in particular, decrease dramatically due to pivot flexibility. Even in carefully conducted experiments, components' flexibilities are invoked to explain dra matic differences between measurements and predictions. A multiple-year test program at TAMU has demonstrated the dynamic forced response of TPJBs can be modeled accu rately with matrices of constant stiffness K, damping C, and added mass M coefficients. The K-C-M model, representing frequency independent force coefficients, is satisfactory for excitation frequencies less or equal to the shaft synchronous speed. However, as shown by San Andres and Tao (2013, "The Role of Pivot Stiffness on the Dynamic Force Coefficients of Tilting Pad Journal Bearings,'' ASME J. Eng. Gas Turbines Power, 135, p. 112505), pivot flexibility reduces the applicability of the simple constant parameter model to much lower excitation frequencies. Presently, a fluid film flow model predicts the journal eccentricity and force coefficients of a five-pad rocker-back TPJB tested at TAMU under a load-between-pad (LBP) configuration. The predictions agree well with the test results provided the model uses actual hot bearing clearances and an empirical characterization of the pivot stiffness. A study follows to determine the effects o f pad pre load, Tp=0.0, 0.27 (test article), and 0.50, as well as the load orientation, LBP, and load-on-pad (LOP), on bearing performance with an emphasis on ascertaining the con figuration with most damping and stiffness, largest film thickness, and the least drag fric tion. In the study, a rigid pivot and two flexible pivots are considered throughout. Further examples present the effective contribution of the pads' mass and mass moment of inertia and film fluid inertia on the bearing force coefficients. To advance results of general character, predictions are shown versus Sommerfeld number (S),a design parameter pro portional to shaft speed and decreasing with applied load. Both LBP and LOP configura tions show similar performance characteristics; the journal eccentricity increasing with pivot flexibility. For LBP and LOP bearings with 0.27 preload, pivot flexibility decreases dramatically the bearing damping coefficients, in particular, at the low end of S, i.e., large loads. The model and predictions aid to better design TPJBs supporting large spe cific loads.In tro d u ctio n Accurately predicting the performance of TPJBs operating over a wide range of rotor speeds and static loads requires of a comprehensive analytical model, experimentally benchmarked to measured performance characteristics in actu...

Recent comprehensive experimental data showcasing the force coefficients of commercial size tilting pad journal bearings has brought to rest the long standing issue on the adequacy of the [K,C,M] physical model to represent frequency independent bearing force coefficients, in particular viscous damping. Most experimental works test TPJBs with large preloads, operating over a large wide range of rotor speeds, and with null to beyond normal specific loads. Predictions from apparently simple fluid film bearing models stand poor against the test data which invariably signals to theory missing pivot and pad flexibility effects, and most importantly, ignoring significant differences in bearing and pad clearances due to actual operation, poor installation and test procedures, or simply errors in manufacturing and assembly. Presently, a conventional thermo hydrodynamic bulk flow model for prediction of the pressure and temperature fields in TPJBs is detailed. The model accounts for various pivot stiffness types, all load dependent and best when known empirically, and allows for dissimilar pad and bearing clearances. The algorithm, reliable even for very soft pad-pivots, predicts frequency reduced bearing impedance coefficients and, over a certain frequency range, delivers the bearing stiffness, damping and virtual mass force coefficients. Good correlation of predictions against a number of experimental results available in the literature bridges the gap between a theoretical model and the applications. Predicted pad reaction loads reveal large pivot deflections, in particular for a bearing with large preloaded pads, with significant differences in pivot stiffness as a function of specific load and operating speed. The question on how pivot stiffness acts to increase (or decrease) the bearing force coefficients, in particular the dynamic stiffness vs. frequency, remains since the various experimental data show contradictory results. A predictive study with one of the test bearings varies its pivot stiffness from 10% of the fluid film stiffness to an almost rigid one, 100 times larger. With certainty, bearings with nearly rigid pivot stiffness show frequency independent force coefficients. However, for a range of pad pivot stiffness, 1/10 to ten times the fluid film stiffness, TPJB impedances vary dramatically with frequency, in particular as the excitation frequency grows above synchronous speed. The bearing virtual mass coefficients become negative, thus stiffening the bearing for most excitation frequencies.

The accurate prediction of the forced performance of tilting pad journal bearings (TPJB) relies on coupling a fluid film model that includes thermal energy transport, and on occasion fluid inertia, to the structural stiffness of the pads’ pivots and the thermomechanical deformation of the pads’ surfaces. Often enough, the flexibility of both pads and pivots is ignored prior to the bearing actual operation; practice dictating that force coefficients, damping in particular, decrease dramatically due to pivot flexibility. Even in carefully conducted experiments, components’ flexibilities are invoked to explain dramatic differences between measurements and predictions. A multiple-year test program at TAMU has demonstrated the dynamic forced response of TPJBs can be modeled accurately with matrices of constant stiffness K, damping C, and added mass M coefficients. The K-C-M model, representing frequency independent force coefficients, is satisfactory for excitation frequencies less or equal to the shaft synchronous speed. However, as shown by the authors in Ref. [1], pivot flexibility reduces the applicability of the simple constant parameter model to much lower excitation frequencies. Presently, a fluid film flow model predicts the journal eccentricity and force coefficients of a five-pad rocker-back TPJB tested at TAMU under a load-between-pad (LBP) configuration. The predictions agree well with the test results provided the model uses actual hot bearing clearances and an empirical characterization of the pivot stiffness. A study follows to determine the effects of pad preload, r¯p = 0.0, 0.27 (test article) and 0.50, as well as the load orientation, LBP and load-on-pad (LOP), on bearing performance with an emphasis on ascertaining the configuration with most damping and stiffness, largest film thickness, and the least drag friction. In the study, a rigid pivot and two flexible pivots are considered throughout. Further examples present the effective contribution of the pads’ mass and mass moment of inertia and film fluid inertia on the bearing force coefficients. To advance results of general character, predictions are shown versus Sommerfeld number (S), a design parameter proportional to shaft speed and decreasing with applied load. Both LBP and LOP configurations show similar performance characteristics; the journal eccentricity increasing with pivot flexibility. For LBP and LOP bearings with 0.27 preload, pivot flexibility decreases dramatically the bearing damping coefficients, in particular at the low end of S, i.e., large loads. The model and predictions aid to better design TPJBs supporting large specific loads.

The scope of this paper is to define the effect of pad materials on the limits of operation of tilting pad thrust bearings. In this paper, the Authors extend the work done by Martin on the limits of operation for steel/whitemetal bearings to other pad backing and lining materials widely used and accepted in the turbomachinery industry, such us, AlSn and PEEK for the lining and CuCr for the backing of the pad. This work has been achieved following a comprehensive experimental characterization of several different tilting pad thrust bearing sizes, with multiple pad backing/lining material options, over a wide range of speeds and loads. The Authors developed and validated a state-of-the-art thermo-elasto-hydrodynamic numerical model to predict the performance of tilting pad thrust bearings and define the limiting envelope. The numerical model couples the generalized Reynolds equation with a 3D heat transfer and structural-mechanics model of the pad, which allows to consider the effect of the pad material properties on the bearing performance. Experimental data along with numerical results reveal the effect that materials have on the limits of operation for a tilting pad thrust bearings for safe operating load and speed in hydrodynamic lubrication.