Whirl Instability of Externally Pressurized Gas Journal Bearing

Abstract

Whirl instability of an externally pressurized gas journal bearing which has a peripheral row of feeding holes is theoretically investigated and the analytical results about the natural frequency of the shaft-bearings systems, whirl onset speed and whirl frequency are compared with the experimental ones presented in a previous paper by the authors in relation to the bearing factors such as radial clearance, supply pressure and number of feeding holes. Based on the assumptions that the pressurized gas flows adiabatically into a bearing clearance space through an annular feeding slot and that the fluid motion in the bearing is completely viscous and isothermal, the reacting force of gas film for a small vibration of the shaft is obtained by making use of the perturbation method with respect to eccentricity ratio. The analytical natural frequency of non-rotating shaft is slightly larger than the experimental one. The analytical whirl frequency shows good agreement with the experimental one for the clearance larger than about 0.03 mm, but becomes considerably larger than that when the radial clearance is as small as 0.02 mm. On the whirl onset speed, the analytical results are found in good agreement with the experimental ones for the clearance of about 0.05 mm. The calculated onset speed becomes smaller than the experimental one for the clearance larger than about 0.05mm, whereas the former becomes larger than the latter inversely for the clearance smaller than about 0.05 mm