## 3 Results and Discussion
### Wear Measurements  
Since the shaft journal is in direct contact with the top foil during low-speed operation of the foil bearing, wear occurs under these conditions. To accurately assess the wear of materials in the tested bearings, in addition to the visual assessment of the sliding surfaces, precise weight measurements of the wearing parts were performed. An analytical balance RADWAG AS 60/220.R2 was used for precise mass measurement, enabling measurement with an accuracy of 0.00001 g. The results of these measurements are discussed in this section. Based on the measurements presented in Tables 2 and 3, it can be concluded that the loss of foil weight depended on bearing sleeve diameter, surface roughness (which collectively determine assembly preload and structural stiffness) and the cooling conditions. For the first pair of materials (2.0321/CuAI7), the weight loss of the top foil was much smaller (from several to several dozen times). For bump foils, it depended on the specific conditions. In the case of the first pair of materials (2.0321/CuAI7), the cooling influence on the top foil's wear was also much smaller than for the second pair of materials (CuAI7/Inconel). In the case of a bearing with a smaller clearance (bearing housing diameter of 25.9 mm) for the first variant of materials, the effect of cooling on the wear of this foil was not noticed. The cooling effect was more visible for the second variant of materials (CuAI7/Inconel). The use of external cooling reduced the weight loss of the top foil by about two times. Bump foil wear was random, with no clear trend to draw more general conclusions. This could be because the bump foil wears in many places simultaneously, both on the top foil side and on the side of the housing. The wear of these foils can largely depend on local operating conditions. Comparing the values of the mass consumption of individual foils, one can see the increased mass consumption of the CuAI7/Inconel material pair. The reduction in bearing clearance due to the reduction in the diameter of the bearing housing and the use of thicker Inconel foil results in both increased frictional heat generation and increased mass wear. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Foil Name**} & \multicolumn{2}{c}{**Weight Loss without Cooling [g]**} & \multicolumn{2}{c}{**Weight Loss with Cooling [g]**} \\ \cline{2-5}  & **2.0321/CuAI7** & **CuAI7/Inconel** & **2.0321/CuAI7** & **CuAI7/Inconel** \\ \hline Top & 0.00003 & 0.00088 & 0.00011 & 0.00032 \\ Bump & 0.00018 & 0.00011 & 0.00037 & 0.00006 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mass consumption of individual foils after 200 measurement cycles in various cooling conditions for a bearing housing diameter of 26 mm. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Foil Name**} & \multicolumn{2}{c}{**Weight Loss without Cooling [g]**} & \multicolumn{2}{c}{**Weight Loss with Cooling [g]**} \\ \cline{2-5}  & **2.0321/CuAI7** & **CuAI7/Inconel** & **2.0321/CuAI7** & **CuAI7/Inconel** \\ \hline Top & 0.00010 & 0.00226 & 0.00010 & 0.00118 \\ Bump & 0.00007 & 0.00000 & 0.00002 & 0.00007 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mass consumption of individual foils after 200 measurement cycles in various cooling conditions for a bearing housing diameter of 25.9 mm. The maximum consumption of the CuAl7 top foil (most exposed to friction of the rotating journal) is 0.00226 g for the CuAl7/Inconel material pair (top/bump foil). Figure 17 shows a view of the foils used in the tests for the CuAl7/Inconel material pair. The mating surfaces of the top foil and the shaft journal after 200 cycles of sequential start/stop work are visible. Figure 17a shows the surface condition after testing without additional cooling, and Figure 17b after applying external cooling. The consumption is not uniformly distributed. After the predetermined number of test cycles, wear of the shaft journal was not observed.