# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 4 Discussion
### Load Capacity of GPBs
Due to the more uniform distribution of the stiffness, as a result of the use of a continuous polymer layer:  
* the adaptability of the structure to the shape of the pressure distribution in the gap is better than that with gas foil bearings of the first generation; and
* a thinner top foil can be used without a higher risk of sagging between the bumps, which also increases the adaptability of the structure. Despite the strong sensitivity of the values determined with regards to the threshold value, it can be observed that the general characteristics are very similar for all cases. A comparison of the clearance study results published by [PERSON] [3] shows a completely different behaviour. The LCC values of the investigated bump-type bearing (third generation) show an optimal value of the load-carrying capacity with regard to the bearing clearance. In contrast, the LCCs of the tested GPB configurations have a minimum but not a maximum that could be determined within the investigated clearance range. Unfortunately, there is no simple explanation for the observed differences. One possible reason could be the different measurement methods, but this cannot be confirmed with certainty. The influence of the temperature could also be a factor, as there was no thermal equilibrium on the test rig in this study. However, the results from [PERSON] et al. [1] contradict this. Here, the load capacities for a bearing were determined at 25 \({}^{\circ}\)C, 315 \({}^{\circ}\)C, and 650 \({}^{\circ}\)C. The LCC values were in the range of 0.8 to 1.0. This shows the dependency of the temperature, but from the authors' point of view, the differences in the LCC values, even at a temperature difference of \(\sim\)300 \({}^{\circ}\)C, are too small to explain the differences observed in the LCC characteristics. Additionally, a significant temperature rise during the test was avoided by the short test durations and long breaks between the tests. In order to find an explanation for this issue, further investigations are necessary. Another important result of this study is the further increase in the load capacity above the minimum. From the perspective of application design, this behaviour can offer an economic benefit. Due to the increasing tendency for higher clearances, the requirements for manufacturing tolerances can be reduced, which can reduce overall manufacturing efforts and the overall costs of an application. However, it should be noted that, due to the way aerodynamic bearings work, an indefinite increase in the bearing clearance is not possible. The actual value up to which the bearing gap can be increased depends on various factors. However, other publications (e.g., [PERSON] et al. [10], [PERSON] et al. [11]) show that polymer bearings with bearing gaps of \(\sim\)200 \(\upmu\)m can also be used in high-speed operation and show good performance results. Figure 10: Load capacity coefficients (LCC) for different fallback threshold levels used for lift-off detection (blue: threshold level 97%), (yellow: threshold level 95%), (green: threshold level 93%), (red: threshold level 90%), (purple: threshold level 85%); (1 k\(\mathrm{rpm}=(100/3)\upmu\cdot\mathrm{rad}/\mathrm{s}\)).