## 5 Conclusions  
The aim of this article was to provide information on the method of experimental identification of the efficiency of gearboxes at the place of their final installation and to teach the reader about important steps and pitfalls that need to be taken and overcome in order to obtain useful and reliable data using this method. An important aspect of the presented method is the direct application of strain gauges to the shafts of the measured gearbox. In this way, the final installation of the gearbox is not affected; thus, the measurement provides the most relevant data as possible. Because the direct use of strain gauges outside the laboratory is usually associated with a number of technical complications, the article provides suggestions on how to work with them during installation so that they can provide the most accurate signal. The article also informs about the possibility to reliably record the signal from strain gauges in given conditions without its amplification by means of 24-bit A/D converters. The signal thus obtained is not filtered as part of its processing, so as not to lose or distort the information contained therein. Instead, it is statistically processed, and, based on the knowledge of its distribution functions, correct mean values and corresponding uncertainties of the measured torques and the searched mechanical efficiency of the tested gearbox are obtained. In addition, the records contain information about torsional vibrations of the measured shafts, about geometrical inaccuracies of cooperating gearbox components, about condition of the gears and bearings, and about other mechanical stresses the measured shafts are exposed to during the gearbox operation. Confronting these data with the outputs of other specialized measuring systems, it is possible to identify the causes of reduced efficiency or even the causes of gearbox failure. Additionally, as the main output of the presented method, the efficiency map of the tested gearboxes is presented. Using these maps, the areas of advantageous and, on the contrary, disadvantageous use of the tested gearboxes can be reliably identified. The biggest challenge for the near future in this context is to further increase the sensitivity of used equipment, so that it is possible to reliably measure the gearboxes for rail vehicles during run-up, when frequent changes in the direction of power flow occur. To achieve this aim, the development of our apparatus is still ongoing. Alternatives to the foil strain gauges are being searched, more efficient ways to store data and to communicate with the surrounding are being developed, and various ways of operating data-loggers even more economically and recharging their batteries without stopping the tested gearbox are being tried. If the last-mentioned task is successfully solved, the equipment can become a permanent part of big gearbox installations. In connection with the already common telemetry, which most often includes measurements of vibrations, noise, and temperatures, the entire system can provide truly comprehensive information about the current condition of the gearbox and its development over time. The method can thus provide useful data not only for the users of the tested gearboxes about their actual efficiency at the place of their final installation, but also for their manufacturers in development of new types and type series. Conceptualization, F.L. and M.D. ; methodology of measurement, F.L. and M.D. ; methodology of data processing, D.H., J.J. and F.L. ; measurement, F.L., M.D., M.H., O.S. and J.F. ; preparation of measuring apparatus, T.V. and L.K. ; FEM analysis, M.D., F.L. and J.F. ; writing--original draft preparation, M.D., J.F., F.L. and D.H.; supervision, F.L. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The data presented in this study are available on request from the corresponding author. The data are not publicly available because they are a trade secret of WIKOV MGI company.