# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 4 Discussion
### Load Capacity of GPBs  
The presented results of the study show that the bearing gap has a significant influence on the lift-off speed and thus on the load-carrying capacity of GPBs. In order to obtain a better comparability to other load capacity measurements and estimations, the load capacity coefficients (_LCC_), with regards to the paper of [PERSON] et al. [1], were calculated for all investigated GPB configurations. The \"rule of thumb\" (ROT) formula presented in [1,3] was applied, where the variables correspond to the physical or geometrical parameters listed below. \[LCC=\frac{F}{(LD^{2}\Omega)} \tag{1}\]  
_LCC_--load capacity coefficient [N/(mm\({}^{3}\cdot\)krmp\(m\))];  
_F_-static load [N];  
_L_-axial bearing length [mm];  
_D_-bearing inner diameter [mm];  
_\(\Omega\)-lift-off speed [krmp\(m\)]. The calculated LCC values as well as their mean value are presented in Table 3. It should be mentioned that the values are presented in SI units as well as in the imperial unit system [lbs/(in\({}^{3}\cdot\)krmp\(m\))] to allow a direct comparison to the coefficients published in the already mentioned papers. In reference [1], the authors presented an overview for different types of gas foil bearings to show how the load capacities have increased over the time due to further development. The main difference between all these bearing generations can be found in the specific structure implementations. The authors would like to increase the readability of the following section; for this reason, all comparisons of the LCCs with results from the literature are in (lbs\(\cdot\)in\({}^{-3}\cdot\)krmp\(m\)\({}^{-1}\)). The published LCCs for air bearings with a rigid inner wall are in the range of 0.24 up to 0.3. The first-generation bump-type gas foil bearings could reach an LCC up to 0.43, which corresponds to an increase of around 25%. The second generation of compliant foil bearings, in which changes in the structural design were implemented (e.g., circumferential splits or multiple bump layers), could achieve even higher LCC values in the range up to 0.6. The third generation had an LCC range of 0.8 up to 1.4. In this generation, the elastic foils were split in the axial direction or staggered, which allows more adaptability of the structure to the pressure field. The range of LCCs in this study was 1.131 up to 3.257. As mentioned in Section 4.1, the sensitivity for the threshold level of 95% should be further investigated. For this reason, the LCC values for different threshold levels were calculated and are presented in Figure 10. The LCC values show a relatively high dependency on the threshold level. The scattering of the coefficients is higher for the levels of 95% and 97% as for the other values. \begin{table}
\begin{tabular}{c c c} \hline
**Nom. h\({}_{\text{0}}\)** & **LCC** & \\ \hline
**[\(\mu\)m]** & **[N/(mm\({}^{3}\cdot\)krmp\(m\))]** & **[lbs/(in\({}^{3}\cdot\)krmp\(m\))]** \\ \hline
25 & 8.844 \(\times\) 10\({}^{-4}\) & 3.257 \\
50 & 7.273 \(\times\) 10\({}^{-4}\) & 2.678 \\
75 & 3.070 \(\times\) 10\({}^{-4}\) & 1.131 \\
105 & 3.205 \(\times\) 10\({}^{-4}\) & 1.181 \\
125 & 3.849 \(\times\) 10\({}^{-4}\) & 1.417 \\ \hline \end{tabular}
\end{table} 
Table 3: Calculated load capacity coefficients (LCCs), for all bearing clearance configurations in SI-units as well as in the imperial unit system (for better literature comparability). Nevertheless, it is worth noting that for all the cases presented, the LCCs are not above 1, and it can therefore be assumed that gas polymer bearings have carrying capacities that are comparable to those of gas foil bearings of the third generation. According to the authors, there are two main reasons that can explain the high values.