Analysis of the Behavior of Kurtosis by Simplified Model and its Applica- tion to Machine Diagnosis

Among many dimensional and dimensionless amplitude parameters, Kurtosis (4-th normalized moment of probability density function) is recognized to be the sensitive good parameter for machine diagnosis. Kurtosis has a value of 3.0 under normal condition and the value generally goes up as the deterioration proceeds. But there are cases that kurtosis value goes up and then goes down when damages increase as time passes. In this paper, a simplified calculation method of kurtosis is introduced for the analysis of impact vibration with one sided affiliated impact vibration which occurs towards the progress of time. That phenomenon is often watched in the failure of such as bearings’ outer race. One sided affiliated impact vibration is approximated by one sided triangle towards the progress of time and simplified calculation method is introduced. By varying the shape of triangle, various models are examined and above phenomenon is traced and its reason is clarified by the analysis. As peak value grows up, Kurtosis increases, then after the damage spread to other bearings, the width of the signal shape spread and Kurtosis falls. When the peak value arises after that, Kurtosis rises up again. Such movement can be confirmed. Utilizing this method, the behavior of kurtosis is forecasted and analyzed while watching machine condition, and correct diagnosis is executed.