## 2 Bearing Failure Modes
### Electromagnetic Detection  
Electromagnetic detection, as a non-contact detection method suitable for complex environments, achieves bearing fault detection by inducing the response in the electromagnetic field. A diagram depicting the principle of electromagnetic detection is shown in Figure 6.  
Figure 5: Principle of current signal detection: The detection process generally consists of current signal acquisition, signal preprocessing, feature extraction, and fault diagnosis. Typical electromagnetic-based bearing detection methods include eddy current detection, magnetic flux detection, and electromagnetic acoustic transducers (EMATs), each with unique advantages in different application scenarios. Eddy current testing is based on [PERSON]'s principle of electromagnetic induction. When a conductive material is excited by an alternating magnetic field, eddy currents are generated inside the material to detect surface or near-surface defects. When bearings experience wear, cracks, or fatigue damage, these defects can cause abnormal distribution of eddy currents, resulting in measurable changes in the detection coil. For example, [PERSON] et al. developed a method using eddy current detection. Rolling contact fatigue tests were conducted using several types of rolling bearings, and it was found that the eddy current detection measurements of the raceway surface exhibited behavior that was determined as fatigue progressed [51]. Magnetic flux detection identifies fault states by monitoring the changes in magnetic flux generated by bearings during operation. It establishes a connection between the magnetic field changes in bearings and their wear status and captures real-time magnetic flux fluctuations during bearing operation through induction coils or Hall sensors. Insufficient wear and lubrication can cause significant changes in the frequency and amplitude of magnetic flux. Therefore, by analyzing the characteristics of magnetic flux signals, the health status of bearings can be determined. For example, [PERSON] et al. proposed a micromagnetic bridge method for high-sensitivity magnetic leakage detection to detect microcracks and developed a new type of micromagnetic bridge probe (MMBP) with higher spatial resolution, which improves the magnetic leakage detection capability to the micrometer level and can be widely used to detect grinding microcracks in bearing rings [52]. The electromagnetic acoustic transducer (EMAT) is a non-contact detection technology that generates sound waves through the interaction between electromagnetic fields and material surfaces. It can detect defects on metal surfaces and sub surfaces without physical contact and is widely used for fault detection of bearings and other mechanical components. Compared with traditional ultrasonic testing, EMAT has stronger adaptability to surface  
Figure 6: Principle of electromagnetic detection. contamination and complex shapes, especially suitable for high-temperature environments and hard-to-reach places. [PERSON] designed a new type of electromagnetic-acoustic transducer and pulsed eddy current composite sensor. The amplitude of the bottom echo recorded by EMAT is used to identify tiny bottom cracks, as well as the amplitude of the pulsed eddy current signal picked up by the integrated symmetry coil. The signal fusion method can accurately correct the amplitude of the defective signal with a relative error of less than \(\pm\)8% [53]. Electromagnetic-based detection methods have the advantages of being non-contact, highly sensitive, and suitable for online monitoring. They can quickly detect structural defects caused by wear, cracks, etc., are insensitive to temperature changes, and are especially suitable for detection in complex environments. Additionally, electromagnetic detection has low requirements for the bearing surface and is not affected by oil or dust, making it ideal for harsh environments. However, its detection ability for non-magnetic materials is weak, the equipment cost is high, and it may impact the properties of the bearing itself.