Theoretical Stability Threshold Speed and Whirl Frequency Ratio of Bump Foil Journal Bearings (Part 1): Effect of Excessive Static Friction and Assembly Preload

摘要

In our previous report, two models were presented to bump foil journal bearings. The assembly preload was shown to influence the maximum load capacity of the bearing. One model corresponds to the bearing with an excessive static friction between the top and bump foils, and the other corresponds to the bearing without the friction. This study aims to predict the stability threshold speed and whirl frequency ratio of a horizontal rigid rotor supported in the bearings using the frequency response method. The following results were obtained. Relatively small deformation of the top foil with the excessive friction contributed to little change in the stability threshold speed of the rotor-bearing system when compared with that of the circular bearing. On the contrary, compliance of the top foil with no friction can change the speed because the foil forms a relatively steep wedge in the steady-state equilibrium while it smoothens the wedge when the air film pressure is fluctuated by the small oscillation of the journal. Stability charts standardized by the mean radial clearance in the assembled bearing clearly show that the larger assembly preload results in a decrease in stability threshold speed of the bearing with no friction.