Review of Experimental Sub-Synchronous Vibrations on Large Size Tilting Pad Journal Bearings and Comparison with Analytical Predictions

Abstract

Tilting pads journal bearing (TPJB) force coefficients are usually evaluated by the Lund approach where pad degrees of freedom are reduced to those of the shaft (8 Synchronous Reduced Coefficients), therefore for each shaft perturbation pads are assumed to simultaneously response. This assumption might not remain valid in the presence of a particular combination of rotor-bearings system and operating condition. In such systems pad inertia leads to a phase delay between the same pad and shaft movement. Depending on the rotor-bearing system the synchronous reduced assumption may not be accurate to identify the instability threshold speed and an explicit modeling of pad dynamic (degree of freedom) has to be incorporated into the rotordynamic analysis. The synchronous reduced method is still accurate in the determination of threshold speed for current turbomachine technology. For future advanced challenging applications the understanding and definition of the applicability of the reduced method is necessary. The predicted sub-synchronous instability (negative logarithmic decrement) presented in this paper are calculated by introducing in the finite element model (beam elements) additional nodes (respect to shaft generalized coordinates) at bearing locations to represent the pads degrees of freedom. The calculations were repeated to predict the instability threshold speed using the classical Lund approach and it has been verified that using this method the instability threshold speed is over predicted with respect to those obtained in the above mentioned analysis. The comparison of the calculations highlights the important difference in the speed threshold values given by the two methods for the system considered. A development activity has been performed to test a configuration of rotor-bearings system of high peripheral speed (> 75 m/s) and large bearing diameter (> 180 mm) verifying the tools prediction capability. The scope of this work is to compare experimental observed sub-synchronous vibration (instability threshold speed) with the predictions in order to consolidate the reliability of the method. To enforce the validation the same test are repeated with different TPJB designs in order to test different bearing configurations in terms of micro-geometry, pad arc length, pivot stiffness etc.