## 3 Generating Synthetic Bearing Fault Data via cycleGAN
### Experimental Activity  
The experimental study relates to the test rig designed for medium-sized industrial bearings; the test rig is located in the laboratories of the Politecnico di Torino [78]. The experimental campaign associated with this test rig represents a pioneering effort to investigate vibration data for large industrial bearings with localized damages [50]. The test rig shown in Figure 3a can accommodate a maximum of four bearings, each having an outer diameter ranging from 280 mm to 420 mm. The main shaft is driven by a three-phase  
Figure 2: CycleGAN working diagram and role of the cycle-consistency loss function in the coupling of two generators and two discriminators.  
motor with a power output of 30 kW. A PERIFLEX(r) (Unna, Germany) elastic coupling connects the shaft to the electric motor. Hydraulic actuators are employed to apply radial and axial loads independently, with a maximum magnitude of 200 kN, by means of adapters (Figure 3b) that enclose the bearings being tested. The hydraulic actuators are fed by air-oil conversion pumps that are coupled to the pneumatic system present within the laboratory. The bearings being examined are lubricated by an external recirculation system, which involves the injection of ISO VG 150 oil at a flow rate of 2.5 L/min under a pressure of 6 bar. The architectural design of the test rig is characterized by the so-called \"self-contained box\" enclosure. The present architecture ensures that loads are balanced by the elastic deformation of the box, thereby dissipating the load circuit within the box. One notable benefit of such a design is in its capability to effectively manage substantial loads without necessitating the use of oversized main bearings supporting the whole system. Each adapter contains a SKF CMS 2200T sensor (Gothenburg, Sweden) for measuring acceleration and temperature. The sensors are coupled to a Scadas III LMS acquisition system. Further details regarding the experimental setup are provided in references [50; 78].  
The experimental activity involved the analysis of vibration samples extracted from the test rig being investigated. Namely, the spherical roller bearing SKF 22240 CCK/W33 underwent testing. The bearing has an internal diameter of 200 mm, with a taper ratio of 1:12, and an external diameter of 360 mm. The disassembled bearing is shown in Figure 3c. The bearing was tested under both normal operating conditions and a damaged condition specifically focused on the inner race (IR). The localized defect was induced by machining, and it has a diameter of 2 mm with a depth of 0.5 mm, as shown in Figure 3d. The test settings and parameters of accelerometer signal extraction are presented in Table 1. The generative model was trained using exclusively data related to the nominal speed of 877 rpm. By converse, the data obtained from the working conditions at 607 rpm  
Figure 3: Experimental activity: (**a**) test rig for industrial bearings [78]; (**b**) interior of the “self-contained box” [78]; (**c**) SKF 22240 CCK/W33 bearing disassembly; (**d**) inner race damage (IR) [50; 79].  
and 997 rpm were excluded from the training process of the cycleGAN model. Thus, the generative model was unaware of the existence of images representing frequency distributions at speeds other than the nominal 877 rpm.