A method and a device for estimating the winding clamping force on a winding package of a transformer or a reactor, the winding package comprising at least one winding with connections and an iron core. The device includes a current pulse generator configured to generate a current pulse sufficient to excite the winding, means for supplying the current pulse to the winding, a sensor for measuring vibrations of the transformer or the reactor generated by the current pulse, and a computing unit configured to calculate a mechanical resonant frequency based on the measured vibrations and to estimate the clamping forces on the winding packages based on the calculated mechanical resonant frequency and a predetermined relation between the expected mechanical resonant frequency and the clamping force.