## 3 Results and Discussion
### Gear Bulk Temperature
#### 3.1.2 Hybrid Polymer Gears
As the gears rotate, additional air flow and heat exchange is generated by convection through the metal inserts exposed outside of the thin-rim polymer gear hub. Due to the fan-like shape of the hybrid polymer gear, convection is not exclusively free or natural, but also forced. When measuring the bulk temperature of the hybrid polymer gear with this design, a significant contribution was obtained, so this new design is the only one considered. In the following, this design is referred to as the hybrid polymer gear. The improvement in the thermal behavior of the new hybrid polymer gear design was already visible in the thermographic images, as a significant difference in temperature and heating could be seen compared to the polymer gear (Figure 12). The image of the hybrid polymer gear captured with the thermal imaging camera shows an area with the metal inserts where the temperature was significantly lower due to greater heat exchange by convection (Figure 11a), while the thermal image of the polymer gear indicates higher temperatures and a smaller temperature difference between the tooth and the rest of the polymer gear hub, indicating greater heating (Figure 11b). The temperature of the metal insert area (orange) of the hybrid polymer gear was approximately 40 \({}^{\circ}\)C, while the temperature of the polymer gear hub (red) was around 50 \({}^{\circ}\)C. The highest  
Figure 11: Tooth flank melting of the hybrid polymer gear with (**a**) aluminum and (**b**) steel inserts. Figure 12: Thermographic image of (**a**) the hybrid polymer gear and (**b**) the polymer gear at 2 Nm after 25 min of the step load test. temperatures appeared in the teeth area (white), where most heat is generated, as expected. The same observations were confirmed by measuring the bulk temperature of the hybrid polymer gear with metal or aluminum inserts (Figure 13). Both hybrid polymer gears with aluminum or steel inserts achieved a lower gear bulk temperature compared to the polymer gear; however, the lowest gear bulk temperature across all four stages of the conducted step load test was observed for the hybrid polymer gears with aluminum inserts. The highest bulk temperature difference between the hybrid polymer gear with aluminum inserts and the hybrid polymer gear with steel inserts was noticed in the first stage at the lower loads, where the bulk temperature of the hybrid polymer gear with steel inserts was closer to that of the polymer gear. Due to the lower thermal conductivity of steel compared to aluminum, more time was required for the steel insert to absorb the heat and warm up. A smaller temperature difference between the steel insert and the environment resulted in a reduced heat flow and slower heat transfer by convection. Once the heat flow was established, the bulk temperature difference between the hybrid polymer gear with aluminum inserts and the hybrid polymer gear with steel inserts was almost constant in the range of 1 \({}^{\circ}\)C and 2 \({}^{\circ}\)C. For all of the gears, an increase in the bulk temperature was observed at the beginning of each stage as a result of the increase in the transmitted torque. After the initial increase in the bulk temperature in the first 5 min of each stage, the temperature increase gradient during the rest of the stage was lower, or the bulk temperature remained almost constant, with fluctuations of less than 1 \({}^{\circ}\)C, as in the case of the hybrid polymer gear with aluminum inserts in the second and third stages. The temperature differential between the hybrid polymer gears and the polymer gears increased with the increasing load in each stage, and was the greatest in the fourth stage of the load test. At the end of the last stage of the step load test at 2.9 Nm torque, the measured bulk temperature of the hybrid polymer gear with aluminum inserts was found to be 9 \({}^{\circ}\)C (17%) lower than that of the polymer gear, while it varied in the range of 4 \({}^{\circ}\)C to 8 \({}^{\circ}\)C in the previous stages.