# The Influence of Bearing Clearance on the Load Capacity of Gas Polymer Bearings
## 3 Results
### Lift-Off Results  
The explained criterion was used for all twenty bearing and load configurations. The determined load-dependent lift-off speeds are presented in Figure 7.  
Figure 6: Test results from the friction torque measurements. Frictional torque (black line); rotational speed (gray line); time derivative of the frictional torque (red dashed line); (a) nom. clearance 105 \(\upmu\)m static load: 10 N; (b) nom. clearance 25 \(\upmu\)m static load: 70 N; (1 k\(\text{rpm}=(100/3)\uppi\cdot\text{rad}/\text{s}\)).  
For the configurations with a nominal gap of 125 \(\upmu\)m (blue), 105 \(\upmu\)m (green), and 75 \(\upmu\)m (red), the lift-off speed increases almost linearly with the static load. In contrast, for the two configurations with a nominal gap of 50 and 25 \(\upmu\)m, the lift-off speed does not increase for all the measured loads. It is also notable that the lift-off event for the 50 \(\upmu\)m gap configuration with a static load of 10 N occurs at a higher rotational speed than for the other configurations. In this study, the 105 \(\upmu\)m configuration has the highest lift-off-speed values for all the loads. The only exception is the 10 N load case, in which the lift-off speed of the 50 \(\upmu\)m configuration is higher than that of the others. The 10 N load case shows a different lift-off order than for the other load cases and is discussed separately.  
For the 105, 75, 50, and 25 \(\upmu\)m configurations, the lift-off speed decreases with the clearance. A decrease is also detectable between the 105 and the 125 \(\upmu\)m configuration, implying that the load capacity has a minimum value in the investigated range.  
Another view of the data is shown in Figure 8. The graphs show the determined lift-off speeds for each static load with the nominal clearance as the abscissa. For each specific load, the lift-off speed is non-linear as a function of the clearance. This non-linear behaviour becomes more dominant as the load increases and indicates a maximum in the clearance range of around 105 \(\upmu\)m. In contrast to the other load cases, the lift-off speed with a 10 N static load does not change significantly, except for the 50 \(\upmu\)m variant.  
The results shown so far provide information on the load at which the rotor starts to move, which allows the load capacities to be compared. The results of a linear regression are shown in Figure 9a. The slope of the straight line indicates the amount of load that  
Figure 8: Illustration of the determined lift-off speeds (krpm) for each bearing configuration and the corresponding nominal clearance (\(\upmu\)m) depending on the applied static load (N); (blue: 10 N), (green: 30 N), (red: 50 N), and (purple: 70 N); (1 krpm = (100/3)\upmu\cdot\mathrm{rad/s}\)).  
Figure 7: Illustration of the determined lift-off speeds (krpm) depending on the applied static load (N) for the bearing configurations with the nominal clearances; (blue: 125 \(\upmu\)m), (green: 105 \(\upmu\)m), (red: 75 \(\upmu\)m), (turquoise: 50 \(\upmu\)m), and (purple: 25 \(\upmu\)m); (1 krpm = (100/3)\upmu\cdot\mathrm{rad/s}\)).  
can be carried by each type of gap per revolution. In Figure 9b, these gradients are shown above the gap. The load-carrying capacity of the bearings increases more with the rotational speed for small gaps than for the larger gap configurations. It is interesting to see that in contrast to the measurements of [PERSON] et al. [3], an optimal value for the bearing clearance does not exist. Instead, the gradients of the regression graphs show a minimum in the gap range of 75 \(\upmu\)m. The results will be further discussed in Section 4.3.