Abstract: The stability and the stability margin of an elastic rotor supported by short journal bearings is discussed, focusing particular attention on its nonlinear aspects. A more reasonable nonlinear unsteaby oil-film force model, which is determined only by three functions, is adopted and the explicit analytical formula of oil-film force for short journal bearing is used for bifurcation and chaos stuby. The extended phase area stability criterion by trajectory-reduction-coupling in high dimensional nonlinear systems, which has got an extensive application in power system stability, is creatively introduced to nonlinear rotor system, A quantitative criterion of nonlinear rotor system stability is presented, which is called CPAC (Comparative positive-area criterion), The definition of stability margin is introduced, A quantitative example on rotor system stability is given by CPAC.

Key words: Unbalance elastic rotor Stability margin Comparative positive-area criterion(CPAC) Bifurcation and chaos