Abstract: This paper discusses for the first time the stability margin of rotor bearing systems. It is emphasized in this paper that the linear model of the rotor systems supported by hydrodynamic lubricated bearings should be viewed as the first approximation or linearization of the original non-linear system because of the non-linear characteristics of the hydrodynamic forces of bearings. The definition of stability margin was introduced and the criterion was deduced and proved. Finally, some examples are given to show the validity and uniqueness of the criterion.

Key words: Liapunov function, Rotor-bearing System, Stability domain, Stability margin