# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 4 Discussion  
### Methodology  
In this paper, two measurement concepts were described and used to investigate gas polymer bearings. The determination of the nominal gap via force-displacement or stiffness-displacement curves is common, as already mentioned in Section 2.2, and has been used by various research groups for several years. Although it is the standard, its validity should be discussed again at this point. In contrast to bearings with a rigid bearing wall, whose stiffness is high enough that contact-based measurement methods can be used without causing significant deformations, a different approach must be taken due to the elastic bearing wall. The use of non-contact measuring methods is also only suitable to a limited extent, as it would have to be ensured that all components of the elastic structure are in full contact. Since this is not guaranteed for bearings without preloading, the explained measuring method is used in the area of gas foil bearings. Due to the compliance of the elastic properties of the structure, the force does not increase infinitely faster after contact is reached. This means that for stiff structures, a change in the threshold value used has a smaller effect on the determined bearing gap than that for more compliant structures. It should also be taken into account that influences such as frictional forces in the contact surfaces of the bearing seats or bending moments due to weight forces, as well as alignment errors, have an effect on the measured curves. Table 2 shows the results of the measurement for all angular positions, as well as the mean value and the average value (\(\text{h}_{\text{max}}+\text{h}_{\text{min}}\))/2 of the measured interval of the results.  
Figure 9: Results of a linear regression for each gap configuration; (**a**) lift-off data and linear regression (blue: 125 \(\upmu\)m), (green: 105 \(\upmu\)m) (red: 75 \(\upmu\)m), (turquoise: 50 \(\upmu\)m), (purple: 25 \(\upmu\)m), (1 k\(\text{r}\text{r}\text{p}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m} \text{m}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m}\text{m }\text{m}\text{m}\text{m}\text{  
The bold values in Table 2 are measured clearances with the greatest deviation from the measured interval average. The maximum deviation for all configurations is around 10%. Due to the fact that all the bearing configurations consist of the same components, except for the high precision adjustment foils, a deviation of ~10% can be interpreted as a kind of accuracy of the measurement method in this specific case.  
The second method that was used in this work was the 95% fallback method. This method was introduced due to the absence of a minimum after the lift-off in some test cases. This minimum has been used by other researchers to determine the lift-off speed, e.g., [PERSON] [6]. Other groups of researchers, such as [PERSON] et al. [18], have used the deceleration phase to determine the lift-off speeds. A rapid change in the deceleration rate was the determination criterion in their paper. With the exception of the minimum criterion in the friction torque, the determination of the lift-off speed in the literature is generally based on the definition of a changing rate or threshold value in the measured friction torque or rotational speed. The choice of this value is subjective and has an influence on the results. The method of 95% fallback used in this work has been proven to be practicable according to a visual check across all the measured test cases in this study. However, the authors would like to point out that the results of this publication as well as the results of other lift-off measurements only reflect the range of the lift-off, and possible deviations cannot be excluded. The sensitivity of the results with regards to the threshold level will be further discussed in Section 4.3. The absence of a minimum in the friction torque is addressed in more detail in Section 4.2.