**6. Conclusions**

A new configuration of the test bed is presented in order to evaluate the dynamic behavior of a two-stage planetary gearbox (TSPG). The conceptual design of the test-bed enables the development of experiments on rotating machinery with similar operating requirements. In this work, experimental tests at constant operating speed and varying load are carried out in order to test the operation of the TSPG designed at LARM2.

The TSPG mechanism consists of a two-stage planetary gearbox in parallel with floating sun gears. The mechanism operates di fferentially as a function of the load torque and the internal friction forces that are caused by the contact between gears. The first operation condition (FOS) is self-actuated under low load conditions. The second operation stage (SOS) is self-actuated under high load conditions. The characteristics of the gearbox enable the self-exchanging of the output link under variable load conditions. The SOS enables the ring gear as the output link when the load in the output shaft increases suddenly. As a consequence, the stress in the internal gearbox elements is reduced under high load operation conditions.

The experimental results demonstrate the usefulness of the gearbox design since the SOS is self-actuated when the load in the output shaft increases suddenly. The high load condition of the gearbox produces a sudden increment in the amplitude of the torques. However, the torques decrease as the SOS achieves the nominal operation conditions. Thus, the actuation of the SOS reduces the stress on the gearbox elements. The results show that the actuation of the SOS reduces the torque 57% in the output shaft and 65% in the input shaft during the test at 120 rpm and 63% and 76%, respectively, during the test at 210 rpm. The e fficiency, inertial mass of the coupled system gearbox-load and the friction torque in the internal gearbox components are estimated from the torque signals. It is found that the e fficiency of the gearbox is η ≈ 40%. This value is expected to be low since the lubrication of the gears is absent due to the missing housing of the gearbox. The inertial mass of the coupled system gearbox-load is estimated as *MI* = 0.89 Nm and the friction torque generated by in the internal gearbox elements is *Tf* = 1.23 Nm.

Future work is planned for the modelling of the TSPG mechanism and the identification of parameters from vibration signals, such as the design modifications in order to improve the e fficiency and to use the torque transmitted by the ring gear during the SOS.

**Author Contributions:** Conceptualization, C.A.G.-C. and M.C.; methodology, C.A.G.-C. and M.C.; software, C.A.G.-C.; validation, C.A.G.-C. and M.C.; formal analysis, C.A.G.-C.; investigation, C.A.G.-C. and M.C; resources, M.C.; data curation, C.A.G.-C.; writing—original draft preparation, C.A.G.-C.; writing—review and editing, M.C.; visualization, C.A.G.-C.; supervision, M.C.; project administration, C.A.G.-C. and M.C.; funding acquisition, M.C.

**Funding:** This research received no external funding.

**Acknowledgments:** The first author would like to gratefully acknowledge the Mexican Government Foundation CONACYT for the fellowship to pursue her postdoctoral studies at LARM2 in the academic year 2018-2019. The authors wish to thank the technical assistance of B.Sc. Mario Alimehemeti in the development of the experiments.

**Conflicts of Interest:** The authors declare no conflict of interest.
