## 1 Introduction
[41; 42] introduced a thermo-mechanical model to determine the bulk temperature and validated the results experimentally, whereby the deviations between the results were small. However, analytical models and numerical simulations are subjected to limitations and assumptions that can lead to incorrect temperature predictions. To improve the poor thermal conductivity of polymer gears, the addition of metal inserts into the polymer matrix has been proposed and investigated. The hybrid technology of polymer and metal combines the advantages of both materials and improves the thermal behavior of gears. The addition of steel, copper or aluminum inserts increases the evacuation of heat from the tooth to the polymer gear body. When the mass and cost of the inserts are considered, aluminum represents the best choice [43]. The drawback of hybrid gears is an increase in the stress concentration in the insert region due to differences in thermal expansion between the metal insert and polymer matrix. It can be concluded that the optimal material of the insert would be a highly conductive material with similar properties to the polymer matrix [44]. The introduction of hybrid polymer gears represents a significant advance in terms of wear resistance and durability. However, a more complex polymer matrix design can significantly increase the manufacturing costs, especially when it comes to one-off production with conventional polymer gear manufacturing methods. Although originally used for prototyping, additive manufacturing is very effective and is now the best solution for the fast, simple and efficient production of single parts with a complex near-net shape. Compared to machine cutting, there is no waste material, and compared to injection molding, there is no high initial investment. In terms of the technology used, the most common additive manufacturing process is Fused Deposition Modeling (FDM). FDM polymer gears have a significantly higher load-bearing capacity compared to Stereolithographic (SLA) gears [45]. Polymers such as polylactide (PLA), ABS, PA or PEEK are generally used for the manufacturing of gears with FDM technology, and these polymers have been the most investigated [46; 47; 48; 49; 50]. FDM technology parameters such as the infill percentage, printing speed, nozzle temperature or bed temperature have a significant influence on the strength and service life of gears. By optimizing these parameters, the manufacturing time of gears can be shortened [51; 52]. In experimental tests of ABS, PLA and PA gears, ABS was found to have the highest wear rate, while PA achieved the lowest wear rate [53]. At lower loads, the PA 618 gears produced using FDM technology even achieved a longer service life than the injection-molded PA 66 gears [50]. An experimental investigation of the thermal behavior and wear of PA gears produced with FDM additive manufacturing technology is performed in this article. The main goal is to increase the heat transfer from the gear tooth to the gear hub or the environment. For this purpose, hybrid polymer gears have been investigated and their performance is compared to that of polymer gears. Hybrid polymer gears are a combination of a polymer matrix and a metal insert, which improves the thermal behavior, wear resistance and service life. The novelty of this research compared to the previously published articles is the experimental analysis of hybrid polymer gears operating under different conditions, and, for this purpose, a new design of hybrid polymer gear is introduced.