## 3 Results and Discussion
### Gear Bulk Temperature
#### 3.1.2 Hybrid Polymer Gears  
In order to reduce the bulk temperature and improve the thermal behavior, four specific hybrid polymer gear designs were introduced. The first hybrid polymer gear design (Figure 4a), in which the heat is mainly transferred by conduction, is similar to the concept proposed by [PERSON] et al. [43]. In their study, FEM analyses was performed on POM gears with module m = 4.5 mm, and thus different geometrical characteristics of the metal inserts. The first hybrid polymer gear design failed to complete the experimental test cycle because of the shearing of the polymer gear hub in the shaft-hub connection. This occurred in both cases of hybrid polymer gears with aluminum and a steel inserts. The metal inserts accelerated the heat transfer from the gear teeth to the shaft, which led to material softening, resulting in excessive surface pressure and the shearing of the hub. The test was aborted in the middle of the first stage (after 7 min) in the case of the hybrid polymer gear with an aluminum insert and at the beginning of the second stage (after 20 min) in the case of the hybrid polymer gear with a steel insert, respectively. This could be explained by the higher thermal conductivity coefficient of aluminum compared to steel, and, consequently, the faster heating and softening of the PA polymer gear hub. The length of the metal insert in the second design of the hybrid polymer gear (Figure 4b) was halved to slow down the heat transfer to the shaft, and thus prevent the failure of the shaft-hub connection due to material softening. The hybrid polymer gears were tested, and the melting of the gear tooth flanks was reported due to increased tooth stiffness. The incorporation of the metal inserts into the polymer gear hub increased the tooth stiffness, and thus disabled the tooth deformation. The inability of the tooth to deflect and the lack of thermal resistance of the PA led to excessive heating and an increase in the bulk temperature, as well as the melting of the gear tooth due to the tooth flank overheating (Figure 11a,b). An approximately three times higher modulus of elasticity and an almost six times lower thermal conductivity coefficient of steel compared to aluminum resulted in the earlier failure of the hybrid polymer gear with the steel insert (after 15 min) in relation to the hybrid polymer gear with the aluminum insert (after 50 min). In order to avoid both the gear tooth melting and the shaft-hub connection failure, a third design of the hybrid polymer gear was presented (Figure 4c). In this design, the metal insert was removed from the tooth area and positioned below the gear root diameter to prevent the reduction in the tooth root cross-section and to avoid an additional tooth root stress concentration, which can lead to tooth root fracture. At the same time, the distance between the metal inserts and the shaft was large enough to maintain the shaft-hub connection intact. However, in terms of the bulk temperature improvement, the third design of the hybrid polymer gear showed no contribution. Almost identical measurement results were observed as with the polymer gears. The only difference between the hybrid polymer gear and the polymer gear was a faster temperature stabilization across the polymer gear hub. The experimental analysis of the first three designs for the hybrid polymer gear, where the heat transfer was primarily carried out by conduction, revealed disadvantages, and none of the proposed designs improved the thermal behavior of the  
Figure 10: Tooth flank surface roughness of (**a**) higher-(Extra Fine) and (**b**) lower (Fine)-quality gear. gears; on the contrary, the first two designs failed to complete all stages of the step load testing. The given designs were found not to be applicable for the hybrid polymer gears when the gear hub was made of PA polymer. In comparison to the three previous designs, the fourth hybrid polymer gear design (Figure 4d) enabled greater heat exchange through convection and conduction.