## 3 Results and Discussion
### Wear Evaluation  
The tooth flank wear was determined by measuring the base tangent length over four teeth and the weight loss of the gear. The measurements were carried out before and after the step load test for all of the gears, including the lower-quality (Fine) polymer gears. In this test rig configuration, the tooth meshing was unidirectional with respect to the direction of the rotation, so the tooth flank wear refers only to the right tooth flank of both the pinion and the wheel (where the pinion rotates clockwise). The gear base tangent length is an indirect method of measuring the tooth width, which has a significant impact on the backlash required for the thermal expansion of the tooth due to heating during operation. Although the teeth were designed with a prescribed thinning for the backlash (e28), an uneven distribution of deviations was recorded when measuring the base tangent length before the test both for the polymer gears and the hybrid polymer gears. A negative deviation implies an increase in the backlash, while a positive deviation results in gear teeth interference. Figure 14 presents the results of the measurement of the base tangent length of the pinion and the wheel conducted before (BT) and after (AT) the testing with the indicated minimum, maximum and mean measurement values. The deviation from the theoretical upper (W4,upper) and lower (W4,lower) limits of the base tangent length was caused by the inaccuracies of the FDM printing process and the non-uniform shrinkage of the PA polymer across the facewidth during cooling, which affected the dimensional instability of the polymer gear hub. Considering the largest positive deviation of the base tangent length of 0.15 mm compared to the theoretical upper limit, it was necessary to increase the center distance. Since involute gears are insensitive to small changes in the center distance and the standard center distance deviation of +0.023 mm was insufficient, the center distance was increased by the value of the tooth thinning for the backlash (0.120 mm) and the actual center distance was set at 78.12 mm. In accordance with ISO 1328 [66], and based only on the measurements of the base tangent length obtained before testing, it was determined that both the pinion and the wheel were excluded from all 12 quality grades. The initial wear caused by manufacturing inaccuracies was observed on all tested gears during the first 10 min of the step load test, and the tooth flank wear was manifested in the form of strip-like debris. The gear operation in the early test stage was associated with increased noise and vibration, which decreased after the initial wear-in phase. It is assumed that the tooth flank wear in the further course of the test was caused by the material softening and the degradation of the profile geometry. The tooth flank wear deteriorated the gear mesh, which increased the temperature of the gear and negatively affected the transmission efficiency. Also, for all of the gears tested, the wheel showed higher wear, which was related to their greater thermal exposure compared to the pinion. The differences in the maximum values of the measured base tangent length after the test between all of the gears was low, so it can be neglected. Both the lowest minimum and the mean value of the base tangent length were found for the polymer gears with the lower quality (Fine), indicating the greatest wear. The tooth flank wear of the pinion was very similar for the higher-quality polymer gears (Extra Fine), the hybrid polymer gears with the aluminum inserts and the hybrid polymer gears with the steel inserts, while the difference was distinct for the wheel. The higher-quality polymer wheel showed less wear compared to the lower-quality polymer gear, while the hybrid polymer wheel showed a significant improvement in wear resistance.