## 3 Results and Discussion
### Measurements of Mechanical Properties  
To clarify the reason for the greater temperature rise during the operation of the foil bearing with a larger sleeve diameter, additional experimental research was carried out. The purpose of this research was to determine the actual clearance or assembly preload of the bearings tested. The foil bearing test method is often used in practice [22; 46; 47], as it allows the clearance/preload to be determined directly and the different foil bearings to be compared objectively without having any information on the manufacturing accuracy of their components. Structural stiffness tests were carried out on the test rig shown in Figure 13 and measured the displacement of a bearing sleeve subjected to a static load. As sleeve displacements were expected to be very small, a precision optical displacement sensor (Keyence LK-H050, Keyence, Itasca, IL, USA) with a 10 mm measurement range and 0.02% measurement linearity was used. This sensor was connected to a central unit (Keyence LK-G5001P, Keyence, Itasca, IL, USA). The bearing static load consisted of a stack of steel weights of different weights. The loading system was connected to the foil bearing sleeve via a force sensor (KMM30-200N, Wobit, Pinewy, Poland) with a measuring range of 200 N and a linearity tolerance of 0.2%. A data acquisition and real-time control system (Dewesoft IO-LITE, Dewesoft, Whitehouse, OH, USA) with a six-channel analogue amplifier (6 xSTG) was used for data acquisition and processing. The actual bearing load and bearing sleeve displacement were recorded simultaneously and presented on the display in real time. Structural stiffness tests of foil bearings were performed with a maximum static load of approximately 10 N. This load value was chosen due to the small diameter and load capacity of the bearing. It is worth mentioning that during the tests discussed in the earlier section, the foil bearing was statically loaded with only the weight of the sleeve, which is only 96 g. In addition to the loading system with appropriately selected weights, the bearing was continuously loaded with a force sensor weighing 78 g during static stiffness tests (Figure 14). With this sensor, it was possible to record the actual load acting on the bearing. Due to the complementary nature of the research in question, the structural stiffness was only determined in one direction, allowing different material and geometrical configurations of the foil bearing to be compared. Figure 13: Test rig used to determine the structural stiffness of foil bearings. The measurement results of the displacement versus the acting static load are shown in Figures 14 and 15. Figure 14 shows a comparison of the structural stiffness of the bearing with foils made from the material combination 2.0321/CuAl7. From the obtained characteristics, the bearing with a larger sleeve diameter (26 mm) had higher stiffness. The measured bearing clearance was about 6 \(\upmu\)m, while the clearance of the bearing with a sleeve diameter of 25.9 mm, which was estimated in the same way, was about 13 \(\upmu\)m. The further course of the structural stiffness of the two bearings was similar. Still, the characteristic obtained for the bearing with a smaller sleeve diameter was shifted to the right by a value close to the difference in the initial clearance. Different static stiffness waveforms were obtained for the bearing with foils made of CuAl7/Inconel materials (Figure 15). For the bearing with the smaller sleeve diameter (25.9 mm), an initial clearance of about 0.4 \(\upmu\)m was recorded, and the maximum displacement at the highest load of approximately 10 N was only 3.4 \(\upmu\)m. The bearing with a sleeve diameter of 26 mm featured even higher stiffness. In this case, no initial clearance was recorded because the initial load did not cause the sleeve to move. This means that in this bearing variant, there is the initial clamping of the foil on the journal, and a load had to be applied to achieve even a small displacement.