## 4 Discussion
### Vibration Analysis  
Section 3.3 described how the spectrum of the ISM is dominated by a lower frequency, such as the shaft frequency. In contrast to that, those frequencies are barely visible for the BBS. One aspect of the domination of the ISM by the shaft frequency is due to the rotating coordinate system of the ISM accelerometers within the Earth's gravitational field. The second aspect is the movement of the accelerometer relative to the mesh position of the gear teeth, which causes a change in the signal path of the vibration and therefore a change in the damping of the signal. Both of these movements occur at the shaft frequency, resulting in a high content of the shaft frequency in the ISM spectrum. As the BBS does not move in relation to the earth's gravity or the mesh position of the gears, the content of the shaft frequency in the spectrum is much lower. This phenomenon was also found by [PERSON] et al. The signal properties and the GCM do not show the same linear behavior as the wear. This discrepancy shows the complex relationship between the wear and the vibration signal. The results of the vibration analysis can be divided into three phases. These phases are visible in the spectrum shown in Section 3.3 and also in the GCMs. The first phase is accruing because of the changes in the GMF and their sidebands. In the second phase, the changes in the vibration signal are too small to have an impact on the GCMs. The last phase is based on an increase in the amplitudes of frequencies between the \(GMF_{i}\) values. Those are part of the differential and the residual signal, which is why the Energy Ratio (ER) and NA4* are increasing. The increase in GCMs of the ISM in the third phase started earlier and steeper than that of the BBS. This earlier increase emphasizes that the ISM is more sensitive to changes in the gear than the BBS. In the third phase, the sensitivity of the ISM to the measured wear was on average 81% higher for the ER and 56% higher for the NA4*. This higher sensitivity could be due to the shorter signal path resulting in a less damped vibration signal. In Section 3.3, it was shown that the amplitudes of the sidebands relative to the \(GMF_{i}\) of the BBS are much smaller than those of the ISM. These lower sidebands are another sign of the strong dampening effect of the signal path and better signal quality of the ISM. In addition, the longer signal path leads to more interference from other vibrations. These would mainly affect the differential signal. As a result, the NA4* of the BBS fluctuates more. This fluctuation makes the interpretation of the data more difficult. As a result, fault detection may be more robust with the GCMs based on the vibration data measured with the ISM. The observed higher amplitudes measured using the BBS contradict the idea that the ISM is more sensitive. However, the amplitudes measured are so high that they are not very plausible. As the characteristics of the sensors have been chosen to suit the described application and there is experience of these sensors in other applications, it is assumed that the fault is in the setup. With the BBS, this fault could be the amplifier or the supply voltage. With the ISM, this fault could be the layout of the components or the power consumption. Furthermore, the comparison between the ISM and the BBS was made on a relative basis, so the absolute values do not contradict the results. The reason for the higher amplitudes of the BBS could not be found. As shown, the phases identified in the vibration signals do not correlate to the wear rate. [PERSON] et al. mapped the different phases of the signal properties to different wear mechanisms [23]. Different wear mechanisms could also be the reason for the different phases found in this study. A hint gives the maximum temperature at each two-hour sequence. Since it is increasing along the first phase identified in the vibration signal, this could mean that in this phase, a more temperature-dependent wear mechanism is present.