The present invention relates to a method and a device for detecting abnormal changes in play in a transmission unit configured to transfer rotational movements of a motor, having a speed of rotation that depends on a torque reference signal (Tref), to a movable mechanical unit, see industrial robot 1. The device (20) comprises a first storage (27) for storing torque reference signal values for the motor at least in a time interval during which the motor is running at a steady state rotation speed, a computing unit (22) configured to retrieve measured rotation speed values (ωmeasured) of the motor during said time interval, to identify the steady state rotation speed of the retrieved rotation speed values, to calculate a frequency spectrum of the torque reference signal, to calculate the energy generated around the second harmonic of the identified steady state rotation speed in a selected frequency band of the calculated frequency spectrum, to compare the calculated energy with at least one limit value, and based thereon, to detect abnormal changes in the play, and a second storage (28) unit for storing the energy calculated at a plurality of different points in time
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