# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 3 Results  
### Nominal Clearance Measurement  
To determine the radial clearance, the stiffness curve over the bearing displacement is used. Note that the presented stiffness is calculated using force signals normalized to their absolute maximum value, and the offset is adjusted. This pre-processing improves the comparability within a series of measurements. The determination of the threshold value in this publication, as in the works referenced above, is subjective and in this case defined visually with respect to the graph. Figure 4 shows the measured stiffness (black crosses) for the bearing configuration without the adjustment foil. The graph shows a low stiffness region and a rapid increase at both ends. Due to the small residual ripple in the lower stiffness region, a threshold of 1000 \(\upmu\)m\({}^{-1}\) is defined (red dashed line) and used for all bearing configurations in this study. The total distance between the two intersecting points is approximately 250 \(\upmu\)m, which is equal to twice the radial bearing clearance. The measurement results for all the configurations with an adjustment foil are shown in Figure 5. For the thinner foils (20 and 50 \(\upmu\)m) shown in Figure 5a,b, the value of the stiffness is nearly constant between the intersecting points. In contrast, the data from the thicker foil configurations (75 \(\upmu\)m and 100 \(\upmu\)m) do not show a constant course anymore. This effect can be attributed to the fact that the foils and the elastic material are not completely attached to the rigid inner wall of the housing. The main reason for this issue is the pre-processing of the adjustment foils. Due to the higher stiffness, the sheets with a thickness of 75 and 100 \(\upmu\)m were pre-curved before installation to prevent lifting at the foils' ends. However, due to the pre-curving, the foils no longer laid as smoothly against the inner wall of the bearing. Comparable behaviour has also been measured in other publications, e.g., [14; 15; 17]. Figure 4: Example of a stiffness curve for the bearing configuration without adjustment foil at 60\({}^{\circ}\)-240\({}^{\circ}\); position-dependent normalized stiffness (black crosses); and threshold level (red dashed line). Below the threshold level, the stiffness curve does not show a flat range, instead it shows a slope and a rise, as well as the minimum in between. Nevertheless, the gradient (inline/decline per \(\upmu\)m displacement) of the stiffness curve also increases rapidly above the threshold. For the 100 \(\upmu\)m adjustment foil configuration in case (d), the difference between the curves' gradients below and above the threshold disappears completely. However, the minimum of the curve remains below the threshold. Table 1 shows the mean values of all the measured angular positions for all the bearing configurations, as well the mean of the maximum and minimum measured clearances. The nominal values given correspond approximately to the mean value between the maximum and minimum measured gap. At this point, the subjective choice of the threshold value should be emphasized once again since it has a significant influence on the results of the gap measurement. Due to the rapidly increasing stiffness, the absolute deviations of the results can also vary depending on the threshold value. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Adjustment Foil** & **Mean Clearance** & **(h\({}_{\text{max}}\) + h\({}_{\text{min}}\))/2** & **Nominal Radial** \\
**Thickness [\(\upmu\)m]** & **[\(\upmu\)m]** & **Clearance** & **Clearance h\({}_{\text{0}}\)** \\ \hline
0 & 122.83 & 124.5 & 125 \\
20 & 102.667 & 105.5 & 105 \\
50 & 78.83 & 76.5 & 75 \\
75 & 53.92 & 53 & 50 \\
100 & 26.92 & 25.5 & 25 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the results of the bearing gap measurements.