Estimating the Severity of Sub-Synchronous Shaft Vibrations Within Fluid Film Journal Bearings

Frequency analyzers sometimes show that shaft proximity probe signals have both a subsynchronous and synchronous component. For shafts supported on fluid-film bearings, a widely-held concern exists over any subsynchronous vibrations, even when the levels are limited in magnitude. Little information has existed for judging the impact of such vibrations on the bearing. In the present paper, a rational method is developed for estimating the dynamic load imposed on a bearing by combinations of sub-synchronous and synchronous vibration. Curves are provided for several bearing geometries which can be used to estimate the maximum dynamic load produced by such vibrations. The curves may also be used to estimate the combination of sub-synchronous and one-per-rev shaft vibrations permissible for some limiting dynamic bearing load.