The Influence of Hydrodynamic Bearing Configuration on Morton Effect

The Morton effect (ME) results from the synchronous, thermal excitation of a rotating shaft because of the uneven viscous shearing in hydrodynamic bearings and the asymmetric temperature distribution in shafts. The temperature difference bends the rotor, reducing the film thickness and increasing the thermal unbalance, which may cause excessive vibration level and unsteady phase angle. To predict the potential thermal instability from the ME, the finite element method is used to solve the transient rotordynamics and temperature distribution in the lubricant, bearing and shaft. The conventional thermal unbalance method is replaced by a more accurate thermal shaft bow model for rotordynamic analysis and the three-dimensional energy equation is utilized for the lubricant temperature prediction. Considering that the temperature change in the shaft and bearing occurs quite slowly relative to the shaft vibration deflection change, a staggered scheme is employed to assign a longer period to update the system temperature distribution and a shorter period to update the vibration orbits. Verified by a real overhung compressor model, the ME instability onset speed predicted by simulation coincides with the tested speed, at which large vibration level is observed. The hysteresis phenomenon, which is quite typical for thermal-induced vibration problems, can be caused by the ME and is demonstrated by the simulation. A stability recovery speed is confirmed, above which the vibration level and the rotor temperature difference will decrease to an acceptable level and the system will become stable. To investigate the influence of bearing configuration on ME, different bearing types including fixed pad bearings (FPBs) and tilting pad bearings (TPBs) with various pad numbers are analyzed. Meanwhile, the bearing clearance and preload intentionally remain unchanged in the comparison. Results show that despite similar critical speeds, the TPBs are better at suppressing the ME with lower average temperature and larger film thickness in the lubricant, especially at high speeds. This is due to the self-tilting ability for the TPBs to maintain a satisfactory bearing clearance. The four-pad TPBs outperform the five-pad TPBs with both lower average temperature and smaller temperature difference in the shaft considering that the effective load-carrying area is larger in the 4-pad bearings. Moreover, the asymmetric pivot offset of 0.6 is simulated to demonstrate its superiority in mitigating the ME compared with the common 0.5 offset. Considering that the ME instability occurs in the vicinity of the critical speeds in most cases, the bearing diameter-length ratio should be carefully designed to achieve a larger separation margin.