# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 5 Conclusions  
In this study, the influence of a nominal bearing clearance on the load-carrying capacity of gas polymer bearings (GPBs) was investigated. For this purpose, a bearing prototype with a polymer layer made of NBR with a thickness of 2 mm and hardness of 50 Shore A was investigated. The bearing gap of the prototype was varied by high-precision adjustment foils. The nominal clearances were measured and discussed. The determined nominal clearances were 25, 50, 75, 105, and 125 \(\upmu\)m. The accuracy of the method was estimated to be \(\sim\)10%.  
In order to determine the lift-off speed, which is used to consider the load-carrying capacity, transient run-ups were carried out and the resulting frictional torque was measured. Frictional torque curves that differ from the classical journal bearing theory were observed. For some test cases, a further drop in the frictional torque was noticed after the lift-off. The authors attribute this to the presence of the elastic structure. For this reason, a very practical method for the determination of the lift-off was presented, in which a 95% fallback of the friction torque time derivative was used as a criterion. A sensitivity analysis for other threshold levels (85, 90, 93, 97%) was carried out. It was shown that for all threshold levels, the characteristic of the dependency was similar, but the calculated load capacity coefficients (LCCs) were highly dependent on the chosen threshold level. Nevertheless, the use of a 95% threshold level, which is one of the more conservative values leading to lower LCCs, showed good results for the load capacity for all GBP configurations. LCC values above 3.070 \(\times\) 10\({}^{-4}\) N/(mm\({}^{3}\).k\(\mathrm{rpm}\)) (1.131 lbs/(in\({}^{3}\).k\(\mathrm{rpm}\))) could be reached. The achieved LCC values are on the same level as those for third-generation bump-type bearings or even higher. From a load capacity perspective, this makes them suitable for practical applications in high-speed regimes (other aspects, such as dynamic stability and temperature resistance, especially of the polymers, must be considered as well).  
Overall, the study was able to show that the influence of the bearing gap as a design parameter has a great impact on the load-carrying capacity of gas polymer bearings, with significant differences to the characteristics of bump foil bearings (minimum in the load capacity for clearances around 75 up to 105 \(\upmu\)m) instead of an optimal value in the load capacity. Furthermore, the data show that the use of gas polymer bearings with larger bearing gaps may have a positive impact on the cost of machines due to a possible reduction in tolerance requirements. Additional benefits can also be gained from the simplicity of the bearing design. Unlike bump foils, the large scale production of polymer layers is already being carried out in other industries, such as in the manufacturing of seals. This allows existing scaling effects to be exploited, thus saving further costs.  
Conceptualization, G.S.; methodology, G.S.; software, G.S.; validation, G.S.; formal analysis, G.S.; investigation, G.S.; resources, R.L.; data curation, G.S. and R.L.; writing--original draft preparation, G.S.; writing--review and editing, R.L.; visualization, G.S.; supervision, R.L.; project administration, G.S.; funding acquisition, R.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors declare no conflict of interest.