On the Pneumatic Hammer Instability of Cryogenic Hydrostatic Journal Bearings: Test Results with Liquid Nitrogen

Abstract

The present research aims to investigate the onset condition of pneumatic hammer instability, which is responsible for the failure of cryogenic hydrostatic journal bearings (HJBs) during the test with liquid nitrogen. To maintain cryogenic conditions, the rotor is supported by two HJBs, i.e., free end bearing and drive end bearing, and is covered by insulation housing in the test rig. The temperature differential between the supplied fluid and the bearing surface results in liquid nitrogen vaporization and phase changes, which induces compressibility of gaseous nitrogen and leads to pneumatic hammer instability. As a consequence, self-excited vibration produces severe adhesive wear only on the drive end bearing during the extended pre-cooling time. To identify the onset conditions of pneumatic hammer instability of the two HJBs, theoretical predictions are presented [1,2]. Measurements are conducted to validate the theoretical model under various supply pressure, rotor speeds, and bearing loads. The measurements show good agreement with the predictions in general. The results reveal that the clearance and bearing load differences of the two HJBs are the main reason for the different onset conditions. Predictable pneumatic hammer characteristics of HJBs are helpful in identifying the onset condition of pneumatic hammer instability and avoiding dangerous self-excited vibration.