## 4 Conclusions  
This study presents an experimental analysis of the thermal behavior and wear resistance improvement of hybrid polymer gears produced with the FDM method. The performance improvement of hybrid polymer gears compared to polymer gears with respect to bulk temperature and wear is achieved. The correlation between the bulk temperature and wear was demonstrated for all of the gears subjected to the employed step load test. The outstanding potential of the hybrid polymer gears emphasizes the importance of their further development. The following conclusions were drawn from the conducted research:  
* The surface roughness of the tooth flank is directly correlated with the quality of the printing process. In this study, polymer gears were fabricated using the following two distinct printing qualities: lower (Fine) and higher (Extra Fine) quality. Notably, an increase in the bulk temperature was observed in the lower-quality polymer gears throughout all testing stages, with the exception of the final stage. By the conclusion  
Figure 15: Wear of (**a**) the lower-quality polymer gear, (**b**) higher-quality polymer gear and hybrid polymer gear with (**c**) aluminum and (**d**) steel inserts. of the experiment, the bulk temperatures for both gear types equilibrated, which was attributed to the smoothening of the tooth flank surface, which subsequently diminished the heat generation. * To reduce heating, four distinct hybrid polymer gear designs comprising a polymer matrix and a metal insert were developed. Hybrid polymer gear configurations, where heat was transferred via conduction, either did not contribute significantly or failed to complete the testing process. A novel hybrid polymer gear design featuring a thin rim facilitated enhanced heat exchange through convection, leading to a reduction in the bulk temperature. * The housing of metal inserts into the polymer gear hub is effective in decreasing the bulk temperature. A reduction in the hybrid polymer gear bulk temperature throughout the entire test was achieved. The bulk temperature increased with an increasing load, and the largest temperature difference was measured at the end of the last stage at the highest load (2.9 Nm). * Hybrid polymer gears with a steel insert achieved a reduction in the bulk temperature of 6.8 \({}^{\circ}\)C (12%) at the end of the test, while the difference in the previous stages was between 1 \({}^{\circ}\)C and 6 \({}^{\circ}\)C. Hybrid polymer gears with an aluminum insert achieved a bulk temperature reduction of 9 \({}^{\circ}\)C (17%) at the highest load compared to the polymer gears, while the difference at lower loads was in the range of 4 \({}^{\circ}\)C to 8 \({}^{\circ}\)C.
* The tooth flank wear was determined using the base tangent length and the weight measurement before and after the test. A correlation between the wear and the bulk temperature was recorded for all of the gears subjected to the step load test. Furthermore, the thermally more exposed wheels exhibited greater wear compared to the pinions. For hybrid polymer gears, a lower bulk temperature resulted in less wear, and the hybrid polymer gears with the aluminum inserts showed the highest wear resistance. In addition, the steel inserts increased the overall weight of the hybrid polymer gears, while the addition of aluminum inserts was negligible. * Considering the thermal behavior, wear and total weight, the greatest improvement was achieved with the new hybrid polymer gear design with aluminum inserts attached. Conceptualization, I.S., V.T. and M.P. ; methodology, I.S., V.T. and I.V. ; software, I.S. and I.V. ; formal analysis, I.S., V.T. and M.P. ; investigation, I.S., V.T. and I.V. ; writing--original draft preparation, I.S., V.T. and M.P. ; writing--review and editing, I.S. and V.T. ; visualization, I.S. and V.T. ; supervision, I.S. and V.T. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.