**5. Discussion**

An estimation of parameters such as inertial mass, friction torque and efficiency of the gearbox can be made from experimental data. In this work, these parameters are estimated from the torque signals as follows: (1) the inertial mass *MI* of the coupled system gearbox-load is estimated from the input torque data as the difference between the maximum value during the start-up process of the system and the nominal value at low load condition; (b) the friction torque of the internal elements *Tf* is estimated as the difference between the peak value when the load increases and the nominal value at high load condition; (c) the efficiency η is estimated as the rate between the input and output torques. These parameters are estimated from the experimental data at ω*n* =120 rpm. The estimated values are *MI* = 0.89 Nm, *Tf* = 1.23 Nm and η ≈ 40%.

The low transmission e fficiency is expected since the gearbox housing is missing in the prototype. This fact generates a low lubrication condition for the internal gearbox elements and, consequently, it produces an increment in the internal friction forces. The amplitude of the torques is higher during the SOS than during the FOS because the higher load in the output. Nevertheless, the experimental results prove the usefulness of the SOS in the gearbox design, since it is self-actuated when there is a sudden increment in the load conditions. It can be seen in Figures 6a and 7a that the torque increases suddenly when the load torque increases, but the torque amplitude decreases as the SOS achieves the nominal operation conditions. Thus, the actuation of the SOS reduces stress on the gearbox elements.
