Analysis of common causes of faults in deep-groove ball bearings

Analysis of common causes of faults in deep-groove ball bearings

As key components of locomotives—particularly the traction motor and axle box bearings—bearings play a crucial role in ensuring safe transport. Should a FAG bearing fail whilst a locomotive is in service, it will render the entire train immobile, blocking the main line; the negative impact is even greater in the case of passenger trains. Furthermore, once a locomotive bearing fails, emergency repairs are extremely difficult to carry out. Therefore, preventing and minimising locomotive bearing failures is of paramount importance for safe locomotive operations. The focus of this paper is on whether it is possible to detect bearings with potential faults prior to their installation in deep-groove ball bearings, thereby ensuring that only compliant bearing components are fitted.

1. Contact Fatigue Failure

Contact fatigue failure refers to failure caused by alternating stresses acting on the working surface of a bearing. Contact fatigue spalling occurs on the working surface of FAG bearings and is often accompanied by fatigue cracks. It first originates at points of high alternating shear stress beneath the contact surface,¹ and then propagates to the surface, forming various spalling patterns; for example, punctate spalling is known as pitting or pockmark spalling, whilst spalling in small flakes is termed shallow-layer spalling. As the spalling surface gradually enlarges, it often extends into deeper layers, resulting in deep-layer spalling. Deep-layer spalling is the fatigue source of contact fatigue failure.

2. Wear Failure

Wear failure refers to failure caused by the continuous wear of metal on working surfaces resulting from relative sliding friction between surfaces. Continuous wear leads to the gradual deterioration of FAG bearing components, ultimately resulting in the loss of dimensional accuracy in deep-groove ball bearings and other related problems. Wear may result in changes to the geometry, an increase in fitting clearance and alterations to the surface topography of the working surfaces. It may also affect the lubricant or cause contamination to such an extent that lubrication is completely lost, thereby causing the bearing to lose its rotational precision and even rendering it incapable of normal operation. Wear failure is one of the most common failure modes for all types of bearings; depending on the form of wear, it is typically classified into two common types: abrasive wear and adhesive wear.

Severe damage to deep groove ball bearings is rarely caused by a single factor; rather, it results from a vicious cycle arising from the combined effects of several factors under harsh operating conditions, leading to severe burning of the FAG bearings. Consequently, following an incident, it is often difficult to determine the exact cause, which in turn presents certain difficulties in formulating appropriate measures. For the sake of convenience in discussion, we shall first analyse the causes of bearing failure from several perspectives.

After a bearing has been in service for some time, defects and scratches develop on the inner ring, rolling elements, cage, outer ring and raceways, resulting in inadequate lubrication of the FAG bearing and causing the bearing to overheat. Prolonged overheating leads to: 1. Dilution of the bearing lubricant; 2. Accelerated material fatigue and a reduction in hardness. These factors further contribute to a vicious cycle, accelerating overheating and ultimately causing the bearing to burn out. In severe cases, the inner ring of a deep-groove ball bearing may become displaced, causing the rolling elements to lose their roundness; the resulting impact generates heat, eventually causing them to fuse together. Therefore, whilst in service, if a bearing is found to be overheating severely or emitting smoke, the train should not be stopped but should continue running to the next station. This is because, at this stage, the overheated bearing is in a molten state; once the train stops and the bearing cools, it will no longer be able to move, thereby blocking the main line.