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Considerable studies have been carried out on fault diagnosis of gears,with most of them concentrated on conventional vibration analysis.However,besides the complexity of gear dynamics,the diagnosis results in terms of vibration signal are easily misjudged owing to the interference of sensor position or other components.In this paper,an altative gearbox fault detection method based on the instantaneous rotational speed is proposed because of its advantages over vibration analysis.Depending on the timer/counter-based method for the pulse signal of the optical encoder,the varying rotational speed can be obtained effectively.Owing to the coupling and meshing of gears in transmission,the excitations are the same for the instantaneous rotational speed of the input and output shafts.Thus,the differential signal of instantaneous rotational speeds can be adopted to eliminate the effect of the interference excitations and extract the associated feature of the localized fault effectively.With the experiments on multistage gearbox test system,the differential signal of instantaneous speeds is compared with other signals.It is proved that localized faults in the gearbox generate small angular speed fluctuations,which are measurable with an optical encoder.Using the differential signal of instantaneous speeds,the fault characteristics are extracted in the spectrum where the deterministic frequency component and its harmonics corresponding to crack fault characteristics are displayed clearly.