# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 2 Materials and Methods
### Nominal Clearance Test Rig  
To determine the nominal clearance of the tested GPB configurations, a commonly used force-displacement-based test setup was designed. Most test setups found in the literature use a lathe or a fixed shaft and horizontal bearing motion [14; 15; 16]. Figure 2 shows a picture of the test rig that is used in this paper. The bearing is placed on the pedestal, with  
Figure 1: Assembly of the gas polymer bearing with adjustment foils; **left**: test bearing, **right**: schematic cross section of the GPB.  
six angular positions for force application and displacement measurement. The additional horizontal movement of the bearing pedestal is generated by a positioning table with two axes. The second axis of the table is used for height adjustment. Unlike other published test facilities, the connection between the movable component (here: the positioning table) and the bearing pedestal is not realized directly via the force sensor. Instead, an elastic stinger is used to reduce the influence of misalignments or manufacturing deviations to the measurement. The pedestal is supported by linear roller bearings to reduce friction in its bottom plane. The shaft is oriented vertically. This avoids bending moments caused by gravity, which would affect the force measurement. The same shaft is later used to measure the friction torque to ensure that the determined clearances are valid and do not vary due to manufacturing tolerances. The measuring procedure starts with the movement of the positioning table, which also moves the bearing pedestal via the stinger. When the movement exceeds the bearing gap, the elastic structure inside the bearing is pressed against the fixed shaft, resulting in a rapidly increasing reaction force. This resistance is detected by the force sensor. This procedure has to be executed in both directions to determine the low force displacement range without physical contact between the compliant structure and the fixed rigid shaft. It should be noted that the displacement sensor is located on the opposite side of the force application point, instead of the positioning table, to ensure that only the stiffness characteristics of the compliant bearing structure are measured. The force-displacement curves generated in this way can then be used to determine the radial bearing clearance by applying different methods to the measured data.  
The most commonly used method for determining the nominal clearance uses the rapid increase in the reaction force after the air gap is exceeded. Here, either the force-displacement curve is used directly or its derivative (the stiffness-displacement curve) is used. Regarding the rapid but not instantaneously increasing force, a threshold value for the force or the stiffness has to be defined. Both variants have been used in the past by other research groups, e.g., the force-based version in [3; 11] and the stiffness curve version in [10; 14; 15]). The distance between the intersection points of the measured data and the threshold level corresponds to twice the bearing gap. It should be noted that the definition of the threshold stiffness used in this paper, as well as in the referenced papers, is subjective and based on a visual evaluation of curves in Section 3.1. Of course, the choice of the level has an influence on the determined bearing gap. How strong this influence is depending on the force or stiffness gradient. However, the threshold established in this study is the same for all bearing configurations tested, allowing for comparability between results.  
Figure 2: Illustration of the test rig for clearance measurements.