Monitoring Mis-Operating Conditions of Journal Bearings based on Modulation Signal Bispectrum Analysis of Vibration Signals

Abstract

Journal bearings are widely used to undertake a wide range of operating conditions. Under high radial load, low rotating speed and thin lubrication films, asperity collisions between the journal and the bearing surfaces may occur. These self-excitations can create wideband random vibrations due to asperity contact and asperity churn. On the other hand, under low radial load and high rotating speed they can have instable operations due to oil whirls and shaft fluctuations. In addition, because of high lubricant viscosity, fluid shearing forces can be dominant, which may also result in wideband vibrations. These types of excitations together with structural responses can be coupled to produce nonlinear modulation phenomena. In this paper, modulation signal bispectrum (MSB) is used to analyses the vibration responses in order to identify the vibration signals values under different operating conditions and hence achieve condition monitoring of journal bearings. Furthermore, MSB coherences display clear regular modulating components in the high carrying frequency range due to coupling between shaft frequencies and the wideband compounds. In associating MSB-Coh. with normalising of MSB magnitude makes possibility to differentiate responses between asperity collision vibration and instability vibration. Thereby, it is possible to identify optimal operating conditions and detect abnormal operations caused by degraded lubrications or worn surfaces.