When the stress of the bearing parts exceeds the fracture limit stress of the material, fractures and local fractures occur inside or on the surface. This phenomenon of discontinuity or fracture of the material is called a crack. There is a kind of hair-like cracks on the surface or under the surface of the material called hairline. When the hairline expands to a certain extent, the phenomenon that part of the material is completely separated from the part matrix is called fracture.

The cracks are generally linear, with variable directions, a certain length and depth (width), and sharp roots and edges. Cracks are divided into internal cracks and surface cracks, as well as visible and invisible to the naked eye. For invisible cracks to the naked eye, non-destructive testing methods are required for observation. The hairline is generally in the shape of a thin line, and the direction is intermittently distributed along the rolling direction of the steel, with a certain length and depth, sometimes single or several.

The causes of cracks are more complicated, and there are many influencing factors, such as raw materials, forging, stamping and folding, heat treatment, grinding, and local excessive stress. The reason for the formation of hairline is the bubbles or inclusions generated in the smelting process of steel, which exist on the surface of the material after being deformed by rolling. For cracks that are not visible to the naked eye, non-destructive testing methods are required for observation.

The preventive measures for cracks mainly include the control of raw material defects such as non-metallic inclusions, surface slag inclusions, folds, microscopic pores, shrinkage holes, bubbles, etc. in manufacturing. Control processing stress, such as internal stress (thermal stress and structural stress), grinding stress, stamping stress, etc. generated during heat treatment and quenching. In use, pay attention to abnormal knocks (bumps) during the bearing installation process and excessive local stress caused by poor installation. In addition, it is necessary to ensure lubrication, enhance the sealing effect, control the inflow of external impurities, and avoid contact between the bearing and corrosive substances.