Machine condition monitoring and fault diagnosis of rotating machinery are very important because of the wide use of rotating machinery in industry. Couplings and gears are used in many mechanical sys tems to connect elements and transmit power. The system is usually modeled as a single degree of free dom system with a piecewise linear spring property, where the mass of main machine is only considered. In the present study, the dynamic behavior of a system with an unsymmetrical nonlinearity and a signif icant mass of the connecting part was investigated both experimentally and by numerical simulation. In the experiment, a 1/3 sub harmonic oscillation was observed, but this oscillation was not found in the simulation using a single degree of freedom system, in which the mass of the connecting part was ignored. However, when a two degrees of freedom system was used that considered both the mass of the connecting part and the impact property, the 1/3 sub harmonic oscillation was observed. Thus it is recognized that an adequate mathematical model for diagnosis in the early stage of abnormality must be selected on the basis of the mass ratio between the connecting part and the main body.
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