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Mechanical anomalies such as load troubles, great torque dynamic variations, and torsional oscillations result in the shaft fatigue of electrical machine and other mechanical parts such as bearings and gearboxes. Particularly, the torsional vibration may attain a significant level at resonant frequencies which damage or cause additional lifetime consumption of mechanical parts. In this way, this paper proposes a noninvasive technique through the electromagnetic torque estimation of driving induction machine as a mean of mechanical torsional stresses monitoring. The lubrication loss is considered as a gear failure to demonstrate its influence on the vibration and on the electromagnetic estimated torque signatures. Then, it is shown that the information in the electromagnetic torque can be decomposed into high- and low-frequency bandwidths which are associated to induction machine and gearbox mechanical-related frequencies, respectively. A setup based on a 5.5-kW three-phase squirrel-cage induction motor connected to a 4-kW wound-rotor induction generator via a one-stage gearbox has been used to validate the proposed method in both stationary and nonstationary conditions.