The Dynamics of Multi-Rotor Systems Supported on Oil Film Bearings

Abstract

The self-excited transverse vibration of an elastic multirotor system due to vertical misalignment of the support bearings is investigated using initial value problem techniques. The equations of motion are expressed in terms of the free-free modes of the shaft and the modal coefficients propagated in time. The method was used to study a two-rotor four-bearing system subjected to misalignment. Previous authors have reported that for bearings with circular symmetry, some external damping acting on the shaft was necessary to obtain finite limit cycle motion. This is confirmed and it is found that, using noncircular lemon bore bearings with wide feed ports, realistic limit cycles were obtained which were in agreement with the amplitude and frequency of limit cycle motion observed on large turbogenerators. The method is general and may be used for any rotor geometry for which the free-free modes and natural frequencies are known and for any number of bearings. As a by-product, the correct alignment of the system is calculated and the linearized properties, such as stability and forced response, are produced.