Bearing failure sometimes occurs in bearing rings, either the outer or inner ones, but it can also occur in the rolling elements. The failure of a bearing may be linked to its application and the inherent material properties.
Definition and Investigation of Bearing Failure
For a quality bearing, when the stress surpasses the tensile strength limit of the bearing material, cracks can form and propagate. Failure occurs when these cracks extend to a certain degree, causing a section of the component to separate completely.
Under normal application conditions, the tensile strength limit of a quality bearing’s material is sufficient to meet the application demands.
However, premature failure can occur when external working conditions change and stress exceeds the limit that the material can withstand. In such cases, it’s crucial to inspect the bearing’s application conditions to ensure its normal operation. Bearing users should self-inspect application situations to eliminate fault-inducing factors.
From the definition of bearing failure, factors leading to failure include stress and material tensile strength.
If external application conditions are normal, and the stress generated under these working conditions still exceeds the tensile strength limit of the bearing material, this may indicate an issue with the material itself. At this point, bearing users should seek assistance from suppliers to inspect the bearing material.
Typically, in the event of bearing failure, the broken bearing should be checked first for failure analysis. This helps to identify application-related causes and clues. Once application-induced factors are ruled out, a material inspection can be conducted.
Failure Analysis of Bearing Fracture
Bearing fractures primarily fall into three categories: overload fracture, fatigue fracture, and thermal cracking. Investigating these three types can uncover potential causes of bearing fractures in their application conditions.
An overload fracture occurs when concentrated stress surpasses the tensile strength of the material. It can also result from excessive local stress, such as the impact or excessive stress caused by overly tight interference fits.
Common bearing overload fractures may originate from impacts during use, hence they might be accompanied by local bearing defects, as illustrated in the image.
Under bending, tensile, and torsional conditions, continuous stress exceeding the fatigue strength limit can lead to fatigue fracture. Crack initiation occurs at areas of high stress and gradually extends to a certain section of the component interface, ultimately resulting in overload fracture. Fatigue fractures primarily occur in rings and retainers.
Within the fracture surface of a fatigue fracture, sometimes the initiation point of fatigue can be found, expanding in a shell-like pattern. As illustrated, the crack striations at the fatigue initiation point are clearly visible.
Hot cracking is caused by the high frictional heat generated by sliding, with cracks typically appearing perpendicular to the direction of sliding. Hardened steel is particularly susceptible to hot cracking.
From the definition of hot cracking, it is related to frictional heat. Therefore, during the diagnosis of bearing failures, color changes in the relevant parts and relative friction may be observed, with the cracks being perpendicular to the direction of friction.
The image below shows an inner ring fracture caused by heat due to friction from the inner ring spinning. It is evident in the image that the friction is circumferential, the fracture direction is axial, and there are traces of adhesive wear on the related contact surfaces due to friction.