Most industrial plants and machinery have rotational parts. Among them, bearings are very important since the failure of critical bearing may cause a system shutdown. The role of early detection of damages is especially important for journal bearings since they are often used under high speed, high temperature and heavy load conditions. The current methods of bearing diagnosis generally utilize traditional tools such as the measurement of shaft vibration or displacement, the measurement of oil or bearing temperature, and the monitoring of oil contamination, which are conducted as either on-line or periodic inspection. However, they are practically useful only after the abnormal condition of structures developed. In other words, they are incapable of detecting the incipient failure.
It has been known that acoustic emission is an effective tool for early detection of damages due to metallic contact or wear action. The purpose of this study is, first, to verify the possibility of early detection of abnormal conditions in the journal bearings, second, to obtain the relationship between AE results and damage characteristics obtained from experiments, third, to suggest the diagnosis techniques for the detection of damages in journal bearing based on the experimental results.
In this experimental approach, acoustic emission technique is employed to detect and characterize the damages caused by the insufficient lubrication and the intervention of foreign particles most probable in journal bearing. Most of wear tests or simulated bearing tests have been conducted on specialized test rigs which enable simple testpieces to be loaded and rubbed with mating surface. In this study, however, a journal bearing is designed to run in the condition of typical hydrodynamic lubrication in order to control oil film thickness between the bearing and the journal. Experiments are carried out under several abnormal conditions by adding foreign particles to lubricant layer or by making the condition of lubricant shortage. Various tools such as AE rms and AE parameter, waveform, bearing temperature, displacement measurement by gap-sensor, observation of damage surfaces etc. are used for better analysis.
The results show that the progress of damage is well correlated with the changes of AE rms voltage. The type of AE sources can be verified using other informations such as waveforms and distribution of events by signal energy. Also, the type of damage sources is characterized by using AE rms voltage, waveform, spectrum analysis, AE parameters etc.. By scanning elctron microscope observation, the relationship between damage patterns and AE results can be confirmed. Especially the change of AE rms voltage is very well correlated with the change of bearing temperature in case of insufficient lubrication test. However, in case of interventing of particles bearing temperature hardly changes in contrast with sensitive AE rms changes. The type of signal which resulted from the serious damage of bearing is characterized as the increase of AE energy by longer event duration with very little change in peak amplitude. The change of AE activity with the size of particles is much affected by MOFT value.
Finally, based on the experimental results the diagnosis techniques for the early detection of damage in journal bearings is suggested.
