A necessary condition for chaos in a rotor system with two-degrees-of-
freedom is developed. The procedure is based on the well-known Melniko
v method. The conditions for existence of the homoclinic orbit are obt
ained. The energy description of the Melnikov function is given. The t
heoretical results are compared with a numerical one. An example of th
e rotor on which the damping and hydrodynamic forces act is considered
. (C) 1997 Elsevier Science Ltd.