Rotor may physically contacts with stationary elements of a rotating machine, and the subsequent rubbing at the contact area is a serious malfunction in rotating machinery that may lead to the machine’s catastrophic failure. Usually, it is deemed as a secondary phenomenon resulting from a primary cause which perturbs the machine during normal operating conditions. Generally, there are two types of rubs, i.e., radial rub and axial rub. In this paper, the dynamic response of a rotor system with two types of rubs and unbalances is investigated numerically. Then, characteristics of dynamic behavior for both types of rubs could be achieved. It indicates that symptoms of axial rub are similar with that of unbalance, where 1X vibration is the main component in FFT results. While, radial rub will result 0.5X and 1X vibration in FFT result. Combing a troubleshooting process of a steam turbine in an ammonia plant and field test data, the numerical results are confirmed furthermore although there are some differences in vibration characteristics between numerical results and field test results. Under axial rub impact, the fault force emerges even at low speed. Its spectrum characteristics are more like those of radial rub impact at low speed and more like those of unbalance at high speed. On these bases, methods of preventing rub-impact faults as the machine operating are presented and investigated theoretically focusing on how to exert external forces to counteract those forces resulting from rubbing. Experimental investigations are conducted and their results indicate that the method presented in this paper is useful and feasible.

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