An Investigation of Acoustic Emission Responses of a Self Aligning Spherical Journal Bearing

High power industrial machinery such as steam turbines and large pumps use journal bearings as rotor supports because this type of bearing is a high load carrying capacity. However, abnormal operating conditions in the journal bearings will degrade machine performance, shorten life time and increase the risk of operation. Bearing condition monitoring can detect faults at early stage and hence minimise the occurrence of catastrophic failures. Vibration measurement from an accelerometer is an effective method for monitoring journal bearing. However, due to frequency limitation, it cannot give accurate monitoring results when the bearing produces high frequency excitations such as asperity contacts. For overcoming the limits, acoustic emission (AE) is required for detecting early faults in self aligning journal bearings in high frequency ranges. AE source in journal bearings may originate from sliding friction. The friction in sliding bearings depends on lubricated regions which are influenced by load, speed, lubricant viscosity and bearing conditions. When bearing is operated in boundary lubricated region more asperity contact occurs and generates large AE responses. Hydrodynamic lubrication means that the bearing surfaces are completely separated by oil film, almost there is not severity contact and hence creates very small AE signal. Mixed lubrication occurs between boundary and hydrodynamic lubrication range and creates medium of AE value. The result of AE experiment for self-aligning spherical journal bearing indicates that AE can detect bearing fault in high frequency range till 15000 Hz. For addition there is a positive correlation between speed, load and AE RMS value. For lubricant, if the lubricant has higher viscosity, it generates smaller AE amplitude. The AE characteristic also shows that AE RMS value relates to the friction curve.