# Evaluation of a Novel Condition Indicator with Comparative Application to Hypoid Gear Failures
## 4 Evaluation of Condition Indicators
### Test Results
This phenomenon has been assessed in detail in [10]. It was found that is caused by continuous impacts from an unknown source, either related to the condition of the gears nor to the meshing. The impacts begin after starting the tests, appearing between 15% and 23% of the testing time. After the restart at 23% of lifetime, the impact reappears and fades out. The same trend can be observed after the restart at 40%, where the impacts fade out and completely disappear. After 50% of the testing time the trend increases linearly, similar to that of TE and \(M_{Dr}\), but with some local maximum and minimum until approx. 95%, where an rapid increase in the trend starts. Some of the dominant orders causing the rapid increase are 2.93 rd, 4.97 th, 5.85 th, 7.9 th, 13.46 th, and 24.86 th. On the colorplot between the 12 th and 17 th orders, the lines are smeared, so it is not possible to identify single orders. Furthermore, between the 36 th and the 53 rd orders, numerous lines of the colorplot feature increased activity. In Figure 13, the results of the accelerometer on the pinion gear are shown. The trend and the colorplot are very similar to that of the other accelerometer. The stop of the testing at 23%, 38% and 46% of lifetime is clearly visible in the spectrum. Nevertheless, the change in the trend after restarting the test is minimal. From around 25%, the following dominant orders appear on the colorplot: 11.99 th, 25.45 th, and 38.03 rd. Near breakage, from approx. 95%, the trend increases rapidly. With the use of the same scaling of the color axis as in other diagrams, it is not possible to identify single orders contributing to the change, as most of the orders show a change by at least 75-100%. By changing the scale the following orders are the most dominant: 2.93 rd, 7.9 th, 10.24 th, and 21.06 th. Figure 11: Colorplot and trend of M of Sample \(A_{H}\). The results calculated using the \(S_{diff}^{rel}(i)\) TE are depicted in Figure 14. The indicators SBR_TE_Pinion, SBR_TE_Ring and SBI_TE_Pinion are similar to the \(RSD_{abs}\) of TE (called CI_TE, on the very top of the figure). The SBR of the ring gear (called SBR_TE_Ring, fifth diagram) has a significantly different development. The absolute value of the SBR_TE_Ring is lower than the same indicator of the pinion gear. This points to the direction that the  
Figure 12: Colorplot and trend of the accelerometer IEPE1 (ring gear) of Sample \(A_{H}\). Figure 13: Colorplot and trend of the accelerometer IEPE2 (pinion gear) of Sample \(A_{H}\). SBR, based on the \(S_{diff}^{rel}(i)\) can be used to detect the damaged gear, as the SBR of the pinion gear shows the damage as clearly as the CL_TE, whereas the SBR of the ring gear does not exhibit a clear exponential increase with the same magnitude. On inspection, the damage to Sample \(B_{H}\) was found to be a fatigue tooth root bending crack, which resulted in the breakage of two teeth and the cracking of a further two. The metallurgical analysis found that the cause of the failure had been a fatigue breakage, with an unknown initiation site. No cracks were found on the ring gear through visible inspection or with magnetic particle inspection. The colorplot and trend calculated using TE, of Sample \(B_{H}\) are shown in Figure 15. Similar to Sample \(A_{H}\), after the restart and re-positioning of the samples at 16% of the testing time, slight changes could be seen in the spectrum, without significant influence in the trend. The trend stays stable on a low level till around 50% of the lifetime of the sample. After this point, harmonics next to the rotational frequency appear in the spectrum, such as 2.05 th, 2.93 rd, 4.97 th, 5.85 th, 7.9 th, and 14.92 nd, which contribute to a linear increase in the trend. This increase continues more or less linearly till 90%, where an exponential increase in the trend is to be observed until breakage.