# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 1 Introduction
The measured drive torque of the investigated metal mesh bearing also shows a minimum after lift-off. This minimum was used as a criterion for the lift-off determination. So far, only a few researchers have published studies on gas foil bearings with a polymeric elastic structure. In 2004, [PERSON] et al. [7] compared two kinds of elastic foil structures for a small cryogenic turbo-expander. The application of rubber as a compliant wall component showed preferable damping and stability characteristics. Later, in 2004, [PERSON] et al. [8] published another investigation on a compliant foil bearing with a polymeric structure. The authors mentioned that the bearings also showed a superior dynamic behaviour and stability performance in the tested speed range up to 150 k\(\mathrm{rpm}\) (\(15.7\cdot 10^{3}\)\(\mathrm{rad}/\mathrm{s}\)). A hybrid gas foil bearing was the subject of an investigation by [PERSON] et al. [9]. Their rotordynamic test rig consisted of a flexible rotor in gas foil bearings with a bump foil and an acrylic polymer layer between the bump and top foils. The study compared the stiffness and damping characteristics with and without the polymer. It was shown that the hybrid bearing had a similar or slightly higher stiffness, but a significantly increased damping capacity. The rotor amplitudes could be considerably reduced using a polymer layer, especially in the range of the natural frequencies and in supercritical operation. Another performance study was conducted in 2017 by [PERSON] and [PERSON] [10], comparing the structural and rotordynamic behaviour of a conventional bump foil bearing, a hybrid version with a bump foil and polymer, and a gas polymer bearing. The experimental dynamic analysis for all three types of compliant structures also demonstrated the higher damping potential of the polymeric foil bearings. In addition, the appearance of subharmonic vibrations during the rotordynamic tests was also shifted to higher rotational speeds, and the reached amplitude level was significantly lower. In 2022, a further rotordynamic study from [PERSON] et al. [11] confirmed the beneficial damping behaviour of foil bearings with viscoelastic material (in this case: nitrile butadiene rubber--NBR) as part of the compliant bearing wall. In addition to experimental investigations, several numerical studies have already been carried out on the structural modelling of the polymers used (e.g., [12; 13]). The studies on gas polymer bearings presented so far have investigated the bearing behaviour in terms of subharmonic vibrations and stability, or structural modelling. The lift-off behaviour and load-carrying capacity of gas foil bearings are also important performance criteria for the application of this bearing technology. This paper experimentally investigates the load-carrying capacity of gas foil bearings with a polymeric, compliant structure. For this purpose, a GPB prototype was manufactured and its design is illustrated and described. The results of the bearing clearance measurements are then presented. Finally, the results of the lift-off speed and load capacity investigations are presented and discussed.