## 5 Conclusions  
Within the framework of the research presented herein, we conducted a visual analysis of displacements of the foil bearing's subassemblies during stable operation of the rotating system and at two different dynamic loads. This method of examining the characteristics of foil bearings has not been used by other researchers so far. A video recording made using a set including only an ultra-high-speed camera and a macro lens did not allow a high enough magnification to be achieved to see details on the foils. Therefore, it was not possible to use it for analysing the motion of components of the foil bearing during its operation. Nevertheless, we also used two sets of extension tubes during our research. They allowed increasing the distance between the lens and the CMOS sensor, thus decreasing the minimum focusing distance and increasing the reproduction ratio. The distance between the lens and the CMOS sensor was increased by 128 mm. As a result of using these accessories, two or three bumps of the bump foil were visible in each display frame. The magnification was high enough to analyse their motion and observe portions of the bearing.  
We observed different sections of the bearing (i.e., its top part, side part, and lock). Recordings presented various regimes such as the stable operation of the bearing at the nominal speed, the beginning of the operation, and run-up (during which the rotational speed was increased gradually). We also carried out several tests that aimed at checking the behaviour of the bearing after it was subjected to different impulse excitations. Depending on the magnitude of the excitation force, the bearing was able to function normally again (if the impact was not substantial) or plastic deformation of its structural supporting layer occurred (after a strong impact). It is worth noting that the bearing still operated after plastic deformation occurred, but its operating characteristics changed.  
During the research, we noticed something interesting, namely the vibration of the bumps and portion of the top foil, which was located between bumps of the bump foil. This phenomenon has already been observed in numerical studies, and this is the first time it has been observed in experimental investigations. During operation at the maximum rotational speed (24,000 rpm), the vibration frequency of the top foil was equal to the frequency corresponding to the current rotational speed. By using the visual method in the analysis of the foil bearing's supporting layer, we were able to assess the quality of the foil (and check if the bearing operated properly) and also to estimate the size of the area where the lubricating film formed itself. It was also possible to estimate the thickness of the lubricating film, which could have been very difficult or even impossible using other methods.  
Conceptualization, G.Z.; methodology, G.Z. and L.B.; validation, P.B.; formal analysis, L.B.; investigation, L.B. and P.B.; resources, G.Z.; writing--original draft preparation, L.B.; writing--review and editing, G.Z. and P.B.; supervision, G.Z.; project administration, G.Z.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript.  
The research presented in this article was carried out within the framework of project No. 2016/21/D/ST8/01711, entitled \"Examination and modelling of anti-vibration processes occurring in high-speed bearings with variable geometry\", which is financed by the National Science Centre (NCN) in Poland. The research was conducted using the equipment of KEZO research centre in Jablonna.  
Not applicable.  
The authors declare no conflict of interest.