## 3 Results and Discussion
### Measurements of Mechanical Properties
It is worth noting that the assembly of the bearing with a bump foil made of Inconel was very difficult (the considerable force was required to move this foil in the sleeve hole during assembly), which may be indicative of the high friction coefficient for the Inconel/steel 1.0562 material pair. It was also noted that the bump foil made of Inconel had a higher stiffness than the bump foil made  
Figure 14: Comparison of structural stiffness curves obtained for two foil bearings with sleeves of different diameters and top and bump foil made from the material combination 2.0321/CuAl7. Figure 15: Comparison of structural stiffness curves obtained for two foil bearings with sleeves of different diameters and top and bump foil made from the material combination CuAl7/Inconel. of a copper-aluminium alloy (CuAl7), which made it difficult to match the shape of the Inconel foil to the sleeve hole during bearing assembly. These factors may have been the reason that the clearance values obtained for the second material variant of the foil bearing (CuAl7/Inconel) were significantly lower than those obtained for the first material variant of the bearing (2.0321/CuAl7), despite using the same sleeves. The results of the static stiffness tests discussed above were, on the one hand, surprising. On the other hand, they allowed us to explain the reasons for obtaining the unexpected thermal characteristics discussed in Section 3.1. Without external cooling, foil bearings with sleeves whose inner hole diameters were larger heated up to a higher temperature than foil bearings with sleeves of a smaller diameter. Foil bearings with sleeves of a smaller diameter should feature a tighter fit, so more heat should be generated when the journal rotates. However, as the studies discussed in this chapter have shown, smaller diameter bearings provided less support stiffness and a looser fit, which explains the generation of less heat in the bearing node. To unambiguously explain the reason for the higher structural stiffness of foil bearings with larger diameter sleeves, an attempt was made to indicate the source of this anomaly. A detailed visual inspection of the used bearing sleeves revealed that the differences in the characteristics of the tested bearings could be attributed to the manufacturing quality of the inner surface of the sleeves. This surface is crucial for the correct operation of the foil bearing, as it mates with the bump foil, which in turn supports the top foil. To objectively assess the quality of these surfaces, roughness measurements were carried out using a device (FTA H4D4000-D, Mitutoyo Corporation, Aurora, CO, USA) produced by Mitutoyo Corporation. The method of carrying out roughness measurements is shown in Figure 16.  
Roughness measurements were conducted three times at different circumferential locations within the hole of each sleeve. The measurements showed that the inner surface roughness (Ra) of the sleeve with a diameter of 25.9 mm was 0.526, 0.530 and 0.541 \(\upmu\)m. The average value of the roughness (Ra) was, therefore, 0.532 \(\upmu\)m. The following roughness (Ra) values were obtained for the sleeve with an orifice diameter of 26 mm: 0.644, 0.744 and 0.709 \(\upmu\)m. The average value of the roughness (Ra) was, therefore, 0.699 \(\upmu\)m and was 31% higher than that obtained for the sleeve with a smaller diameter. The differences in surface roughness were due to the treatment method used. The holes in both sleeves were initially made to have a diameter of 25.9 mm. Only in the second stage, the hole in one sleeve was enlarged to a diameter of 26 mm by mechanical working. A material allowance of 0.1 mm (around the circumference) proved insufficient to achieve good machining parameters and maintain the original surface roughness. Based on the additional tests performed, it was concluded that the quality of the inner surface of the bearing sleeves could be of great importance for the static and operational characteristics of the tested foil bearings.