Digital High-Speed Photography of Cavitation in Journal Bearings

This paper presents current research comparing gaseous and vaporous cavitation in lubricant flows obtained by means of digital high-speed photography in un-precedented detail. Hydrodynamic journal bearings are compact and guarantee a nearly wear- resistant operation. These features make journal bearings the first choice for many applications. However, under particular operational conditions, e.g. a highly dynamic load, cavitation can occur which can lead to bearing failures. For the selected case of suction cavitation these conditions are characterized by high eccentricity combined with a rapid variation of the lubricating film thickness. The work at hand presents a new experimental approach to study suction cavitation in a scaled bearing model. Moreover, mechanical and fluid dynamic similarity laws are described which enable the transfer of bearing operation conditions into the model experiment and vice versa. An extensive literature research yields the parameters of operating conditions that are critical towards suction cavitation and puts the definition of cavitation into the particular perspective of the lubricating flow in journal bearings. The new experimental approach includes a specially designed fluid, which fulfills Reynolds and cavitation similarity and a scaled bearing model including a mechanism that re-produces the specific phases of shaft displacement which are necessary for the inception of cavitation. The experimental results include high-speed photography that captures the formation of bubbles at a rate of 10.000 frames per second (fps) which enables a detailed analysis of bubble growth yielding precise input data for an evaluation and comparison with simulation results. Numerical simulations are carried out by means of an unsteady and three-dimensional model utilizing a 2- phase code and an elasto-hydrodynamic journal bearing model that is state-of-the-art for the bearing design process. In summary, the work at hand provides a deeper understanding of the process of suction cavitation in dynamically loaded journal bearings.