## 2 Methodology
### Test Rig Description  
To explore the bearing performance of a compressor in a real working environment and verify the correctness of the model and the accuracy of the simulation results, a test rig to model the working performance of HTBs of the star wheel was developed in this research, as shown in Figure 8.  
Figure 6: Geometric model: (**a**) water fields in the bearing clearance; (**b**) water flow in the HTB clearance. Figure 7: Mesh model of the water film. The injecting pressure of the water injecting pipe was adjustable and the outlet pressure of water film in the sliding clearance was atmospheric. Moreover, water injection pressure could be measured by a pressure sensor. A non-contact displacement sensor was installed on the upper base of the thrust surface of the bearing to measure the axial displacement of the star wheel shaft under the action of the pulsating force. A vortex flowmeter was also installed on the water injecting pipe to measure the water injection flowrate. According to the force analysis of the star wheel, the axial gas force on the star wheel is an alternating load that varies with the rotation of the star wheel following a certain periodic law. In particular, pulsating load was generated by the combination of an electromagnet and control circuit to simulate the actual load of the hydrostatic thrust bearing in this research [27]. To exert a pulsating axial load (see Figure 5) on the bearing rotating ring of a star wheel, an electromagnet (5) was equipped in the test rig and placed on one side of the test rig. The electromagnetic would generate a suction force on the rotating circle plane after energizing. Under appropriate assembly conditions, it was necessary to ensure that there was not only a certain clearance between the electromagnet and the rotating circle plane, but also that the electromagnet could produce enough suction force to the rotating circle plane under this clearance. Given that the rotating circle plane was connected with the spindle through a fixed base, the pulsating electromagnetic force was converted to the force exerted on the spindle. The rated working voltage of the electromagnet was 110 V DC voltage, and the maximum suction force that could be generated on the rotating circle plane under rated working conditions was 5000 N. The suction force of the electromagnet would increase with the increase in DC voltage. To simulate the actual force acting on the HTB of the star wheel shaft in WSSCs, electromagnetic force was calibrated with a ring pressure sensor to determine the specific relationship between the axial gas force acting on the bearing and change of input voltage. During calibration, the hydrostatic thrust bearing was replaced by an annular pressure sensor, and the clearance between the rotating circle plane and the electromagnet was adjusted to 0.3 mm and 0.4 mm, respectively. The input DC voltage was adjusted to gradually increase from 0 to 110 V, and the angle of the rotating circle plane was changed to measure the pressure corresponding to each voltage value at different rotation angles. The average value was obtained by multiple measurements to obtain the corresponding pressure relationship. Figure 8: Test rig of water-lubricated HTB of star wheel shaft (developed by author): (**a**) layout of test rig arrangement; (**b**) test rig photo (1 joined cylinder, 2 coupling, 3 HTB, 4 cylinder, 5 electromagnet, 6 rotating disc, 7 injecting hole, 8 shaft, 9 disc for displacement measurement, 10 installation frame of displacement sensor,11 displacement sensor). In the experiment, it was necessary for the input voltage to be processed using the control circuit to make the voltage changes periodically according to the pulsating force. Hence, the electromagnet produced periodic suction, and the corresponding gas force was eventually simulated by adjusting the voltage. The rotation speed of the HTB shaft driven by a motor was adjustable to test the influences of sliding speed on the lubricating state.