A CFD-Radial Motion Coupled Model for the Evaluation of the Features of Journal Bearings in External Gear Machines

This paper presents an approach for the evaluation of the effects related to the behavior of the journal bearings on the operation of external gear machines (EGMs). In EGMs journal bearings usually support shafts of both gears. The prediction of their instantaneous axis of rotation is a key feature of EGMs, affecting the displacing action of the inter-teeth volumes and the leakages at the tip of each tooth. Past approaches presented in the literature for the evaluation of the radial micro-motions of the gears are based on classic simplified models for the hydrodynamic journal bearings based on steady-state solutions of the fluid film neglecting important hydrodynamic effects due to film squeeze — which can be relevant in case of significant radial loads variations — or they concentrate the focus of the study on the journal bearing itself regardless of the impact on the global machine operation. This work presents a CFD model coupled with a rigid model for the evaluation of the radial motions of the journal that permits to overcome the above-mentioned limitations of the existing approaches of study. The paper details the proposed model, in particular as concerns the CFD approach to solve the film flow and its coupling with a previously developed model (HYGESim) developed by the authors’ team for the evaluation of the overall operation of the EGM, used also to find the instantaneous loads on the gears. The potentials of the proposed approach are shown for the case of a pump for high pressure applications. For this case the results show how it is necessary to have a detailed approach for the evaluation of the journal bearings for a correct evaluation of the overall EGM behavior. In particular, the importance of hydrodynamic squeeze effects are shown, by comparing the model prediction with those given by the commonly used simplified steady-state approach. The comparison of simulation results with experimental data is also shown as pertain to pump case wear associated to the break in process.