## 4 Discussion  
Section 3 works as an illustrative example of the interpretation of compiled efficiency maps. Each sudden efficiency change announces the need for more thorough examination of surroundings of the relevant operating points in order to identify the reason for this change using one of other specialized methods (vibrometry, thermometry, acoustic measurement, etc.). Very often, a decrease in efficiency is associated with an increase in vibration. Some gearbox components have high resonance and cause a significant outflow of drive power. Another relatively frequent example is imperfect teeth mesh of the meshing wheels due to too low or too high deformations of shafts, gears, or the case. The crucial moment for a reliable interpretation of measurement outcomes is the correct identification of the absolute values of measured deformations, resp. torque. This step is important, because the searched efficiency is calculated according to common definition formula by the ratio of the absolute values of output and input torque. Thus, it is always necessary to reliably identify artefacts that could disrupt the described approach and cause an increase of uncertainty accompanied by a loss of data credibility. This step is not yet a part of the set of mathematical definitions of individual data processing steps. The main reason here is a significant increase in the necessary mathematical apparatus and, above all, the loss of direct control over the outputs of the data processing process. Therefore, so far, all data evaluated within the input task of data processing phase are checked visually. Based on this check, all data containing step changes in the measured steady signal from strain gauges after the gearbox has started operation are excluded from further processing. This symptom is a typical manifestation of the presence of the initial, resp. residual stress in the measured shafts. If we skip this step, we could obtain false absolute values of the actual torque on the measured shafts. However, since the results of FEA are at disposal, these artefacts are also reliably identified during the virtual calibration process described in Section 2.2. The text above shows why the described method is such a robust tool, but also that there is still room for its further improvement to provide even more reliable and accurate outcomes. Therefore, it must be highlighted here that the above method is designed mainly for practical use to verify engineers' expectations and to monitor the actual efficiency of gearboxes directly in their operational installations. Application of strain gauges directly to the gearbox input and output shafts eliminates the need to create space for torque sensors installation, which can affect the measured data, because the gearbox installation is altered. Therefore, we believe that the data obtained by the described method provide, in principle, as accurate as possible information about the operation of the gearbox in its working conditions and environment. Often, if there is enough space, we also install bridges to detect bending stresses in the measured shafts. This allows us to determine the quality of the gearbox installation or to assess the effectiveness of used couplings in this terms. In addition to the fact that bending usually reduces the efficiency of gearboxes, its presence always increases the risk of fatigue damages in their components. However, even in the torque recordings, much important information is hidden, from torsional vibrations, through manifestations of misalignments and ovalities, to projection of various defects mainly in gearings and bearings. Another interesting and, nowadays, more and more important issue is the monitoring of gearboxes efficiency in relation to the direction of power flow. This is especially important in gearboxes for rail vehicles where, in the run-up, the gearbox is loaded very lightly, and the direction of power flow changes repeatedly.