## 3 Results and Discussion  
### Temperature Measurements  
Figures 8-12 show the results of temperature measurements using thermocouples placed in the measuring chamber. The temperature rise curves in the test chamber for one bearing housing diameter (25.9 mm), with cooling taken into account, are presented in Figure 8. In the case of the CuAl7/Inconel material pair (top/bump foils), the most significant differences can be seen at the 200 th cycle. The use of cooling slows down the temperature rise in the chamber, so it is possible to obtain quasi-stable conditions in the test chamber without boiling the working medium used as a lubricating liquid. When analysing the temperature rise curves for the material pair 2.0321/CuAl7, in the absence of cooling, the characteristic takes a steadily increasing course (Figure 8). Since two test modes were carried out directly, one after another, the thermal inertia of the system after the first test mode increased the temperature in the chamber. However, the subsequent course of the characteristics already proves the effectiveness of the cooling system. This was manifested by stabilizing the temperature in subsequent cycles. More cycles would reveal the resulting temperature differences. Figure 8: Graph of temperature changes in the test chamber for 200 cycles for a housing diameter of 25.9 mm for material pairs 2.0321/CuAl7 and CuAl7/Inconel with the use of cooling. Figure 7: Course of rotational speed changes during the test cycle: Tc—cycle duration, Tr—run-up time, Tp—work time, Tw—run-down time, Tn—pause time. Figure 11: Temperature rise curve for CuAl7/Inconel material pair (top/bump foils) without a cooling system for two bearing housing diameters. Figure 10: Temperature rise curve for material pair 2.0321/CuAl7 (top/bump foils) when using a cooling system for two bearing housing diameters. Figure 9: Temperature rise curve for material pair 2.0321/CuAl7 (top/bump foils) without cooling system for two bearing housing diameters. By comparing the temperature rise curves in the test chamber for two different bearing housing diameters (25.9 mm and 26 mm), which were also characterised by different bearing clearance (h\({}_{0}\)), for two selected material pairs and the cooling capacity of the chamber, visible differences can be observed. Figure 9 clearly shows that the temperature in the bearing with the larger diameter sleeve increased more. However, attention should be paid to the fact that the bearing clearance was lower in the case of the bearing with a larger diameter, which is discussed in detail in the next section of the article. After 200 start/stop cycles, the temperature was 32.9 \({}^{\circ}\)C for the bearing with the 26 mm inner diameter sleeve and 26.1 \({}^{\circ}\)C for the bearing with the 25.9 mm inner diameter sleeve. The higher temperature rise for the bearing with the larger diameter sleeve can be attributed to different assembly preloads and structural stiffness due not only to the inner diameter of the sleeve but also to the geometry of the top and bump foils and surface roughness. The technology manufacturing bump foils are typically characterised by poor repeatability [44,45]. It is affected, among other things, by elastic and plastic deformation of thin metal sheets. As a result, there may be certain deviations in the dimensions and shape of the foils. To clarify this, additional tests were conducted to determine the actual clearance or assembly preload of the tested bearings. The course of this research and its outcomes will be discussed in the following section of the article. After using an additional external cooling system, the temperatures of the two bearings with foils made from the material combination 2.0321/CuAI7 stabilised at a similar level (Figure 10).