An investigation of surface vibration, airbourne sound and acoustic emission characteristics of a journal bearing for early fault detection and diagnosis

High power machinery such as steam turbines, large pumps and motors often use journal bearings as rotor supports. This type of bearing is simple, low cost and with high load carrying capacity. However, abnormal operating conditions in the journal bearings will degrade machine performance, increase operating cost and may cause unexpected sudden failure which is dangerous in both engineering and safety terms. Bearing condition monitoring can detect faults at an early stage and prevent the occurrence of such failures which can be catastrophic. Monitoring techniques that have been used for monitoring of journal bearing are lubricant analysis, vibration analysis, noise and acoustic emission analysis. Lubricant analysis has been used effectively for condition monitoring for a long time but cannot be implemented in real time. Many researchers have studied the use of the vibration and sound signals and acoustic emissions generated by the hydrodynamic journal bearing for detecting and diagnosing faults. The studies give relatively little information regarding surface vibration and airborne sound characteristics for self-aligning spherical journal bearings, nor has comprehensive condition monitoring been implemented for a particular self-aligning spherical bearing journal. Surface vibration, airborne sound analysis and acoustic emission monitoring can be used simultaneously to detect any signal emitted from the bearing at very wide frequency range. Sound vibration occurs in solid structure, liquid and gases transmitted to air surrounding create airborne sound. This study has conducted a thorough review of theoretical and experimental studies. The research began with designing and building a test rig consisting of a drive system, radial loading system, torsion loading system, the bearing testing system itself and control, data acquisition and measurement instrumentation systems include encoder, pressure transducers, thermocouples, load cells, vibration transducer, acoustic and acoustic emission sensors. Preliminary experiments were conducted to ensure all equipment and instrumentation worked well and also to test measurement repeatability. Preliminary experiment results showed that all the equipment either driving, loading, data acquisition and measurement system works well. Experimental analysis of the surface vibration, airborne sound and acoustic emission analysis responses in time domain and frequency domain analysis include RMS value, Kurtosis and mean value showed good repeatability. The AE measurement response showed the best repeatability, followed by surface vibration and airborne response. Theoretical study shows that the self-aligning spherical journal bearing system under radial load generated surface vibration, airborne sound and acoustic emission responses that originated from external force excitation such as fluctuating loads due to system misalignment or unbalance and internal excitation such as asperity in boundary or mixed operation. These excitations generate structure-borne vibration and acoustic emission. The structure-borne vibration dynamic responses then radiated airborne sound. Airborne sound also originated from oil pressure fluctuation and flow turbulence. The surface vibration and airborne sound frequency responses occur at frequencies 100kHz. The amplitude and frequency of surface vibration, airborne sound and acoustic emission is influence by radial load, shaft speed and surface quality of journal and bearing components themselves. The quality of asperity contact between journal and bearing may be due to manufacturing defect, lubricant and surface deterioration over time during operation. The experiments and analysis of the surface vibration, airborne sound and acoustic emission characteristics of the self-aligning spherical journal bearing indicate that there is a positive correlation between the spectrum mean value of surface vibration, airborne sound and acoustic emission with radial load and speed. Meanwhile, when use higher lubricant viscosity creates lower surface vibration, airborne sound and acoustic emission mean amplitude. Investigation of lubricant deterioration due to water contaminant indicated that when use higher concentration contaminant in the lubricant generates higher spectrum mean value of surface vibration, airborne sound and acoustic emission responses. The surface deterioration experiment showed that there is a clear significant different in the frequency domain of surface vibration, airborne sound and acoustic emission between a scratched surface and a normal surface journal bearing. The surface vibration, airborne sound and acoustic emission frequency characteristic for scratches and lubricant deterioration creates different peak amplitudes and different frequency. The larger the scratch generate the greater the amplitude and higher frequency. From of the three measurement systems used, acoustic emission is the most sensitive and a better detect of the bearing fault than followed by vibration and air-born sound measurement system. Therefore the acoustic emission measurement technique can be integrated with surface vibration, airborne sound for rotating machinery/engine condition monitoring. Using surface vibration, airborne sound and acoustic emission monitoring the symptoms of early damage at low, medium or high frequency can be detected and more severe and catastrophic failure can be prevented, and finally very high maintenance costs can be eliminated.