## 3 Results and Discussion
### Gear Bulk Temperature
#### 3.1.1 Polymer Gears  
The objective of the experimental tests carried out in this study was to determine the bulk temperature of the wheel during operation under specific loads. Compared to the pinion, the wheel is thermally more exposed due to the reduced facewidth and, therefore, the reduced surface area available for heat exchange. Also, the smaller volume leads to a lower heat capacity, which results in the quicker heating of the gear. The polymer gear heating has a dominant effect on the performance, so it was necessary to conduct an investigation for its use in real applications. The measured temperature is the bulk temperature, which was measured close to the tooth root and determined by the amount of heat generated and dissipated, as opposed to the short-lived and difficult-to-measure flash temperature. Preliminarily, a comparison of the polymer gears produced with two different print qualities was performed. Polymer gears of a higher quality (Extra Fine), with a layer height of 0.06 mm, and polymer gears of a lower quality (Fine), with a layer height of 0.1 mm, were investigated. The average bulk temperature measurements of the gears produced with two distinct print qualities is shown in Figure 9.  
Figure 8: (a) Profilm3D profilometer and (b) Mettler Toledo JSI603C weight-measuring device. The conducted experimental tests confirmed the differences in the thermal behavior of the polymer gears produced with two specific print qualities. For both print qualities, the bulk temperature was increased with an increasing load, but the polymer gears produced with a lower quality heated up faster than the higher-quality gears. The gear bulk temperature was compared throughout the entire test cycle, and it was recorded that the temperature of the lower-quality gear was 1.4 \({}^{\circ}\)C to 4.4 \({}^{\circ}\)C higher than that of the higher-quality gears. The exception was the measurement at the end of the cycle, where the bulk temperatures were almost the same between the two print qualities. The bulk temperature at the end of the fourth stage stabilized at 61 \({}^{\circ}\)C as a result of tooth flank surface smoothening during operation. Regardless of the greater roughness and higher deviations of the lower-quality gear, the low surface hardness of the tooth flank resulted in smoothening as the testing went on. Low material stiffness and tooth flank wear allowed for the progress of the test; otherwise, plastic deformation, tooth root fracture or melting would occur. The quality of the polymer gear manufactured with the FDM method was determined by the height of the layer that extends in the direction of the gear facewidth, which has a direct influence on the surface roughness of the tooth flanks. For this purpose, tooth flank surface roughness measurements were performed by means of the Profilm3D optical profilometer (Boston, MA, USA), and the results are presented in Figure 10.  
The surface roughness of the tooth flanks was measured before the test, and the arithmetic mean of the height S\({}_{\text{a}}\), and thus the surface roughness, was compared for both print qualities. By measuring the surface roughness, a greater number of layers was visible for the higher-quality gears (Figure 10a) compared to the lower-quality gears (Figure 10b). It was observed that the lower-quality polymer gears have a rougher surface compared to the higher-quality polymer gears. The arithmetic mean height of the lower-quality polymer gear was S\({}_{\text{a}}\) = 19.64 \(\upmu\)m compared to the higher-quality polymer gear, at S\({}_{\text{a}}\) = 18.72 \(\upmu\)m. The rougher tooth flank surface of the lower-quality gear caused more friction and heat generation due to sliding, which explains the higher bulk temperature and faster heating. The print quality has a significant influence on the heating of polymer gears and, therefore, a polymer matrix for all of the hybrid polymer gear designs was produced with a higher print quality (Extra Fine). Lower-quality polymer gears were excluded from further bulk temperature analysis.