## 3 Results and Discussion
### Large Impulse Excitation Force  
Figure 10 shows a portion of the tested bearing at two different points in time after an excitation force was applied using a hammer. The value of the excitation force was not measured but its value was about 200 N. This is the value of the force that can destroy the bearing. On the left-hand side, we can see the maximum deflection of the foils due to the impact, and it was the moment when the distance between the bearing sleeve and the rotating rotor was the shortest. The most significant deflection was observed for the bump foil, whose height was approximately 400 m before the excitation and decreased by around 50% after it. The camera recorded displacement of the bearing (which was about 200 m) by registering movement of 27 pixels; by dividing these values we were able to estimate the value of bearing displacement. One pixel on the photograph corresponds to a movement of the bearing of 7.4 m. On the right-hand side of Figure 10, we presented a photo of the bearing, which was taken soon after the time when its operation became stable again. The bearing was overloaded only for a short period, and the journal took its previous position shortly after that. In Figure 11, we showed the displacement of the bearing sleeve versus time, registered after impulse excitation was applied during operation at a rotational speed of 24,000 rpm (i.e., the maximum speed). This time, the magnitude of the excitation force was higher than  
Figure 10: Maximum deflection of the foils caused by an impulse excitation (**left**). Stable operation of the bearing (**right**). Figure 9: Displacement of the bearing to which a small excitation force was applied. that of the force used in the test shown in Figure 10. It turns out that when the magnitude of the excitation force was too large, the plastic deformation of the bump foil took place. Even though the bearing was still able to operate, its operational characteristics changed. In a foil bearing, the pre-clamp is an essential parameter. If the pre-clamp is not firm enough, the entire space between the top foil and the bearing sleeve will not be adequately filled by the bump foil. In such a case, during operation at the nominal speed, the rotor (due to the unbalance) keeps pressing against the top foil that can deflect in places in which the bump foil is not tightened firmly. The vibration amplitude of the rotor is then significantly increased. We observed such behaviour only in several cases when we used the bump foil, whose bumps were not high enough. A screenshot from a recording made using a high-speed camera is shown in Figure 12 (on the left side). This is a recording of the top part of the bearing. At the bottom of the screenshot, there is the rotating shaft whose speed was 24,000 rpm. On the right image shown in Figure 12 the differences between the two extreme shaft positions are marked with rectangles. The pixels that differ in these positions are marked in red. Two green boxes were used to label two bumps of the bump foil. The whole bump located inside the right rectangle is marked in red. On the left-hand side, the bump situated inside the frame is almost white (i.e., the number of red pixels is small). It means that on the right-hand side, the entire segment bends, and thus it takes part in the bearing operation. The differences between the sections are due to the non-homogeneous manufacture or assembly of the foils. These differences affect the dynamic properties of whole foil bearings. Figure 11: Displacement of the bearing to which a high excitation force was applied. Figure 12: Picture recorded using a high-speed camera (**left**). The changes between the two extreme journal positions are marked in red (**right**). During operation at a constant speed, we noticed vibrations of the portion of the top foil, which was situated between the bump foil's bumps (it is visible in Figure 12 (right)). The red dots were created based on the differences between the two frames recorded at different times.