In a rotating bearing, periodic stress changes occur beneath the contact surface between the raceway and rolling elements.
Consider the inner ring of a radial bearing that can withstand radial forces. As the inner ring of the bearing rotates, a certain point on the raceway will enter the load zone, and continuing to run will reach the maximum load (stress), and then run away from the load zone. During each rotation, as the point on the raceway enters and leaves the load zone, compressive and shear stresses are generated (see Figure 1).
Depending on the load, temperature, and number of stress cycles within a certain period of time, residual stresses will form inside the bearing Causing the material to transform from an unoriented grain structure to a fracture plane.
Within these planes, at the weakest point around the area of maximum shear stress under the surface, subsurface microcracks typically form, with a depth of 0.1-0.5 mm (see Figures 2 and 3). The specific depth depends on factors such as load, material, cleanliness, temperature, and microstructure of the steel. These cracks will eventually propagate to the surface, causing surface peeling (see Figure 4).
When the surface of the bearing peels off, the bearing is damaged. However, this does not mean that the bearings cannot continue to function. Over time, surface peeling will become increasingly severe, causing equipment to produce noise and vibration. In this case, the equipment should be shut down and repaired before the bearings completely fail.
The length of time between initial bearing peeling and complete failure depends on the type of equipment and operating conditions.
The peeling caused by fatigue is shown in Figure 5 (magnified at high magnification). The bearing can still operate for a long time before the crack extends to the surface. This type of damage typically occurs on bearings made of very pure steel and operating under clean and well lubricated conditions.
Pay attention to the flat bottom surface of the peeling area and the "neat" cracks around it. These cracks have already extended to the surface and can cause more severe material peeling.
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