Experimental Measurements and Modeling of Tilting-Pad Bearing Performance and System Stability Under Reduced Oil Supply Flow Rates

Rotor instability is an ever present problem in the turbomachinery industry that continuously grows more important as the limits of operating speeds and pressures are pushed increasingly higher in industrial machines. Many of these high-speed, rotating machines depend on fluid-film, tilting-pad bearings to introduce stabilizing damping forces into the system through the development of a hydrodynamic film wedge. Tilting-pad bearings rely on a constant flow of fresh oil to keep bearing temperatures low while providing sufficient oil to support the shaft. Reduced oil supply flow rates in fluid-film bearings can cause cavitation, or lack of a fully-developed film layer, over one or more of the pads due to flow starvation. Partial bearing starvation has the well-documented effect of reduced power losses at the expense of increased operating temperatures. Minimizing the required oil supply flow rate provided to the bearings while safely supporting the rotor can lead to significant decreases in bearing power loss and is of great interest to the turbomachinery industry. While its effects on power loss and temperature are well known, few studies have been published which focus on the effects of oil supply flow rate on dynamic bearing performance and system stability.
A series of studies are conducted to provide a comprehensive look at the effects of oil supply flow rate on tilting-pad bearing performance through both experimental measurements and theoretical bearing model predictions. All measurements are performed on a test rig consisting of a flexible rotor supported by two tilting-pad bearings in flooded housing designs. The test rig is dynamically similar to modern, industrial machines with a high critical speed ratio and mode shapes analogous to those of a between-bearing machine. Experimental tests are performed under a number of operating speed and load conditions while systematically reducing the oil supply flow rate provided to the bearings. Steady-state bearing performance measures and subsynchronous vibration patterns are recorded for various operating conditions. Sine sweep perturbations of the rotor are performed via magnetic actuator under varying oil supply flow rates. The vibrational response is then used in a single-input, multiple-output (SIMO) system identification technique to obtain system frequency response functions (FRFs) and, in turn, modal parameters, specifically damped natural frequencies and damping ratios. The identified damping ratio provides a direct measure of system stability under varying speeds, loads, and oil supply flow rates.
All experimental results are compared to predicted results obtained from bearing models based on thermoelastohydrodynamic (TEHD) lubrication theory. These models include an oil flow starvation model meant to predict pad cavitation due to starvation in low oil flow conditions. For the conditions tested, the starvation model predicts partial pad cavitation of unloaded bearing pads under lightly loaded conditions that increases in severity with increasing speed and decreasing oil supply flow rate. This progressive decrease in film development correlates to higher journal operating positions, increased subsynchronous vibrations, and decreased system stability observed in the experimental data. This correlation suggests that the starved model relatively accurately predicts film cavitation and bearing stiffness under reduced oil flow conditions; however, higher predicted damping ratios indicate that the bearing models over predict bearing damping. These studies provide a detailed report on the effects of oil supply flow rate on system stability and help highlight improvements that can be made to the current theoretical models.