# Evaluation of a Novel Condition Indicator with Comparative Application to Hypoid Gear Failures
## 6 Conclusions  
It has been shown for the analyzed samples that the failures are visible in the trend (\(RSD_{abs}\)) and colorplot (\(S_{diff}^{rel}(i)\)) of all measured signals. With slight time differences in the possibility to determine when the initial failure occurs, all signals considered can be used for this purpose. In addition to demonstrating the strength of the proposed \(S_{diff}^{rel}(i)\) CI, it has been shown that the torque signals can also be used for condition monitoring, in addition to the well known acceleration and TE signals, offering possibilities for wide application of the CI and faster processing.  
In comparison of the proposed CI to other well known indicators, the only indicator which was similar to the \(RSD_{abs}\) and could indicate the damage in all cases was the SBI. The SBR showed a different trend from the \(RSD_{abs}\), but it could be used for damage detection. The crest factor and the FM0 did not produce comparable results to the \(RSD_{abs}\) in any case, and could not consistently indicate the damage. It can be concluded that to determine the gear damage in the shown case, the evaluation of the complete spectrum with the \(RSD_{abs}\) does not offer a benefit over the SBI, which only monitors gear rotational and meshing harmonics.  
It has been shown that with the usage of SBR as post-processing of the \(S_{diff}^{rel}(i)\) TE, the determination of the damaged gear is also possible in the case of pinion damage. This indicator is also capable of showing which one of the gears was damaged. This approach should be assessed in the future with more samples and varied test conditions to see if it can be consistently used to determine the damaged gear.  
In the testing considered here, the failure modes observed did not allow enough variation to make a significant conclusion on the proposed CI ability to identify between the route causes of the failures. By tracking the variation in the spectrum, it would be possible to identify which frequencies display a disparity and from this determine the likely cause of failure.  
It was observed that in the case of TE in the presented cases it is sufficient to monitor and evaluate the spectrum until approx. the 33 rd order of the pinion gear (slightly below the third harmonic of the meshing frequency). All other signals showed activity in the complete spectrum, so the recommendation to monitor the frequencies until the fourth harmonic of the meshing frequency, as discussed in the Section 3.3, was useful and this approach delivers acceptable results. However, approaches evaluating more or less spectral lines were not assessed in this study.  
Additionally, as presented in Table 4 the \(RSD_{abs}\) is the only indicator with which it is possible to detect the damage of the pinion gear and which can be potentially used for the monitoring of other components than gears. Furthermore, as shown in the comparison with other indicators in [23], the \(RSD_{abs}\) can also be used for the ultra-low -speed monitoring of planetary gears, where the other algorithms fail to detect the damage.  
Conceptualization, M.S. and M.L.; methodology, M.S., M.L. and C.R.; formal analysis, M.S.; investigation, M.S., M.L. and M.B.; data curation, M.S.; writing--original draft preparation, M.S., M.L. and C.R.; writing--review and editing, M.B. All authors have read and agreed to the published version of the manuscript.  
Open Access Funding by the Graz University of Technology.  
Not applicable.  
This research was supported by AVL List GmbH and the authors would like to thank them for their technical support in testing.  
The authors declare no conflict of interest.