# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 2 Materials and Methods
### Rotor Lift-Off Test Rig  
The design of the friction torque and lift-off speed setup presented in Figure 3 and described in the following is similar to other published setups, e.g., [3, 4, 6]. As mentioned earlier, the shaft used is the same as for the clearance measurement setup. While the test bearing is placed on the front of the shaft, a 3D-printed impulse turbine is on the back side. The shaft is supported by eight ball bearings in two planes (four per plane) along the shaft. These planes are located between the turbine and the test bearing. The housing in which the ball bearings are located is open at the top and bottom, ensuring that the pressure can quickly decrease after the turbine housing, thus preventing air flow through the bearing gap.  
An infrared (IR) contrast sensor is used to determine the rotational speed. The static load is applied via a tear-resistant polymer thread (connected to weights) and measured continuously with a load cell. An elastic coupling connects the force sensor, which is used to measure the frictional torque, and the bearing mount. The use of elastic couplings between the bearing seat and surrounding components allows the bearing to self-align. This can help to reduce the influence of reaction forces as a result of misalignment. Nevertheless, it should be mentioned that the setup is not designed to measure the absolute value of the friction torque. A general problem for such designs is a resulting righting moment from the static load thread and preloads from assembly. The resulting offsets are nearly unpredictable and can result in different effects, e.g., negative friction torques after lift-off if the offset is subtracted from the force signal before run-up, as shown by [PERSON] et. al. [5]. To acknowledge these effects, the presented torques in this paper are normalised by their absolute maximum value.  
In this study, static loads of 10, 30, 50, and 70 N are applied to each bearing configuration to investigate the lift-off behaviour. In contrast to the work of [PERSON] [3], a transient procedure, similar to that of San Andres [4, 6], is used in this study. In [6], a temperature increase of 10-18 \({}^{\circ}\)C was measured by San Andres, after a time period of 15 min of constant operation at a specific load, an ambient temperature of 21 \({}^{\circ}\)C, and a rotational speed of 65 krpm (\(6.8\cdot 10^{3}\) rad/s). Due to the high temperature dependency of polymeric stiffness behaviour, the overall time per test has to be as short as possible to avoid significant temperature increases during the tests. The ambient temperature during the measurements presented in Section 3.2 was between 20 and 21 \({}^{\circ}\)C. The amount of time for each test at a specific load was in the range of 6 to 12 s between the beginning of the rotation and its  
Figure 3: Illustration of the test setup for lift-off measurements.  
end. A time at zero speed of at least 60-90 s for weight change and data processing passed between two measurements. Due to the extremely short time periods per test and the much longer periods out of operation between two measurements (5 to 15 times longer), the polymer as well as the whole test rig showed no significant temperature increase during the tests. Nevertheless, the temperature aspect has to be addressed and will be discussed in Section 4.