Subharmonic vibrations of a roll tube supported by plain journal bearings

Abstract

This study presents a dynamic model for describing a journal bearing with circumferential waviness of the shaft journal. The journal bearing model is based on an analytical solution of the Reynolds equation and defines the non-linear hydrodynamic forces that act on the journal bearing. The waviness of the shaft journal is modelled through the use of a Fourier cosine series to describe the shape of the film thickness of the bearing. The waviness of the shaft journal can lead to subharmonic vibrations in which the natural vibration mode of the rotor is excited when the rotation speed is a fraction of the natural frequency of the rotor-bearing system. This phenomenon is investigated in this study utilizing the proposed bearing model in a rotor system simulation. The rotor system under investigation consists of a roll tube, two journal bearings and the supporting structure. The proposed journal bearing model can be used in a general multi-body or rotor dynamics computer code as the interface element between the rotor and the bearing housing. In this study, the model of the journal bearing is implemented in a commercial multi-body simulation software application. This approach enables a detailed analysis of the rotor-bearing system in such a way that the dynamic interaction of the rotor with the bearing is taken into consideration. The simulation result agrees with the experimental measurements obtained from a real roll tube system.