# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 1 Introduction  
One of the main objectives in the development of future-proof machinery is to enhance efficiency, both from an economic and ecological standpoint. Simplifying systems in general, while simultaneously reducing operating resources and maintenance during operation, can offer significant benefits. In the field of fast-rotating rotors, improvements in bearing technologies can serve as a potential starting point for increasing efficiency. The implementation of aerodynamic bearings represents a particularly promising approach to tackling these challenges, as they possess a straightforward design and do not require lubricants. As a result, auxiliary components such as pumps and supply systems can be eliminated. However, due to the lower viscosity of gases, these bearings are associated with reduced load capacities when compared to those of hydrodynamic oil bearings. To address these drawbacks and enhance the performance of aerodynamic bearings, gas foil bearings have been developed. Another advantage of the elastic structure is that additional damping is provided to the rotor through energy dissipation, which occurs through friction within the elastic structure. Unlike conventional air bearings, gas foil bearings have a compliant support structure instead between the bearing bore and their housing. Due to the interaction between the supporting pressure field and the elastic bearing wall, a self-optimized gap geometry can be reached. As a result, higher load capacities and an improved stability behaviour can be achieved [1; 2]. There are several different parameters that have an impact on the load-carrying capacity of these bearings. In addition to the stiffness properties of the elastic support structure, both the shape and the size of the bearing gap are important design parameters, with respect to the achievable load capacity. The most investigated variant of gas foil bearings is the bump foil or bump-type bearing. In 2002, [PERSON] et al. [3] published their performance results of a clearance study for a bump-type bearing with an axially split bump foil. For this purpose, they defined a bearing coefficient in the form of a measure for the maximum load that is bearable, depending on the bearing's length and diameter as well as the shaft speed. The presented test method used a shaft that ran at a constant speed. A load was then applied and increased until the measured frictional torque increased significantly. The results showed that an optimal range for clearance in terms of the load capacity does exist. The data also showed a rapid decrease in the capacity when the air gap becomes too small. Another measurement of the lift-off speed as a performance measure was presented by [PERSON] et al. [4] in 2012. In this work, the authors compared the performance of a metal mesh foil bearing and that of a common bump-type bearing. For this comparison, among other aspects, the lift-off speed and the maximum starting torque were used. In contrast to the work of [PERSON] et al. [3], a different method was used in this paper. [PERSON] did not investigate the load capacity by increasing the load at a constant speed. Instead, several transient rotor run-ups were performed, and the frictional torque was measured. The lifting point was then determined based on a subjective limit for the frictional torque. Further lift-off and drive torque measurements were presented by [PERSON] et al. [5]. The presented test rig uses an elastic bearing mount (squirrel cage) instead of a free-floating housing. The bump-type bearing was placed in a rolling bearing, while the torque was transmitted via a lever connected to a force gauge. The presented friction torque characteristic is like that published by [PERSON]. When the shaft is accelerated beyond the lift-off speed, the torque drops rapidly until only air friction remains. It is notable that the transient signal of the torque displays a minimum after the lift-off event has occurred. A very similar behaviour is presented by [PERSON] et al. [6].