## 5 Experiments
### Experimental Setup  
The dual-rotor test bed was designed and built as shown in Figure 14.  
Figure 12: Kurtogram of noisy simulated signal.  
Figure 13: Envelope order spectrum of noisy simulated signal.  
The structure of the test bed includes the motorized spindle (outer rotor), coupling, bearing housing, #1 intershaft bearing, #2 bearing housing (including the bearing casing), bearing housing, coupling, and motorized spindle (inner rotor). To a large extent, the test bed reproduces the running status of an actual intershaft bearing and the sensor installation in the following three aspects.  
(1) In actual dual-rotor equipment, the intershaft bearing has no directly linked bearing housing. Therefore, the vibration response must be collected by indirect measurement. The vibration response of the intershaft bearing needs to be transferred to adjacent bearings via the shaft; then, via elastic supports and complicated thin-walled paths, the response is transferred to the sensor-equipped casing surface. As shown in Figure 15, the #2 bearing with a thin-walled casing as a support is the closest to the intershaft bearing on the test bed, which is similar to the structure of the actual dual-rotor equipment. The vibration of the #1 intershaft bearing is transferred to the #2 bearing via the rotor, which is in contact with the intershaft bearing. Then, the signal is transferred to the sensor position through the elastic support and the thin-walled struts of the #2 bearing. Finally, the signal can be detected. The transmission path in the test bed is almost the same as that in actual dual-rotor equipment. (2) In this test bed, the rotating speeds of the intershaft bearing's inner and outer rings are different and can vary frequently. The operating condition of the intershaft bearing in this test bed is similar to that in actual dual-rotor equipment. (3) The geometric parameters of the intershaft bearing used in the experiment are the same as those of the intershaft bearing in a certain dual-rotor gas turbine. The accelerometer is mounted on the #2 bearing housing, which is the closest to the intershaft bearing. As shown in Figures 14 and 15, the positions of the sensors (including the accelerometer and key phase sensors) are similar to those of the actual dual-rotor equipment.  
The faults of inner and outer rings of the intershaft bearing were simulated as follows. As for the outer ring fault, a 1 mm deep and 1 mm wide groove was machined across the axis on the inner surface of the outer ring. As for the inner ring fault, a 1 mm deep and 1 mm wide groove was machined across the axis on the outer surface of the inner ring.  
An LMS SCADAS data acquisition system was used [32], along with a BK4519 accelerometer as a vibration sensor, and a proximity switch as a key phase sensor. The accelerometer was installed in the vertical direction outside the thin-walled casing of the #2 bearing and controlled the rotational  
Figure 14: Dual-rotor vibration test bed.  
speeds of the inner and outer rings via two motorized spindles. Finally, the fault data of the intershaft bearing at the variable rotational speeds of the inner and outer rings could be obtained.  
The data used for analysis could be acquired from #2 bearing housing and consisted of two groups: (1) the fault data of the outer ring, where \(N_{2}\) (rotational speed of outer ring) increased from 919 rpm to 1190 rpm, \(N_{1}\) (rotational speed of inner ring) increased from 295 rpm to 395 rpm, the sampling frequency was 25.6 kHz, and the vibration waveform and speed trends are shown in Figure 16; (2) the fault data of the inner ring, where \(N_{2}\) increased from 1743 rpm to 2237 rpm, \(N_{1}\) increased from 418 rpm to 588 rpm, the sampling frequency was 102.4 kHz; the vibration waveform and speed trends are shown in Figure 17.  
Figure 16: Fault in outer ring: (**a**) waveform of vibration and (**b**) speed trends.  
Figure 15: Transmission path of vibration signal of intershaft bearing.