Causes and Meanings of Rolling Tracks of Failed Bearings
Regardless of whether the rolling bearing fails, every bearing type that has been worked will show traces on the contact surfaces of the rings and rolling elements. These marks are produced when the surface structure produced during processing is roughened or smoothed during operation. Small pits created by the circulation of very small foreign particles are also characteristic. From these traces conclusions can be drawn about the quality of the lubrication, the cleanliness of the lubricant and the direction and distribution of the load within the bearing. Normal track: If the lubricating oil film separates the rolling elements from the raceway sufficiently, the rolling elements rotating under load will leave a bright track on the raceway. However, the traces of each form are largely related to the brilliance of the surface, but almost all machining marks should be discernible, especially with a magnifying glass and microscope (compare the non-contact areas on the edge of the raceway!). Individual pits caused by small foreign particles are unavoidable. When lubrication is fairly good, those marks can only indicate the location of the load zone in the bearing, see Figure 23. The raceways or rolling elements usually discolor when the temperature is above about 80 C. It is caused by the chemical reaction of steel and lubricant or steel and additives in the lubricant, and usually has no adverse effect on the operating life of the bearing. On the contrary: these surface properties often show additive wear resistance. Usually these colors are brown or blue. However, these colors do not determine the operating temperature that causes discoloration. Despite similar operating conditions, very different colors are sometimes observed on bearing rolling elements. The discoloration caused by this oil should in no way be confused with the tempering color caused by excessive temperature, which is found in failed bearings, but rarely occurs, see section 3.3.5. Sometimes such traces can also be found in the equatorial region of the steel ball. Such trajectories are found when angular contact ball bearings always rotate about the same axis of rotation. Any such traces do not lead to a significant reduction in life, see Figure 24. 1923: Normal trajectory, surface structure still visible, only small dents caused by foreign particles 24: Steel ball with equatorial circumferential line 25: Towards Radial loads are carried by radial bearings, such as deep groove ball bearings. Under the action of a point load, in a bearing seat with sufficient stiffness, without radial preload, the trajectory on the stationary ring is shorter than half of the circumferential direction of the raceway. Under the action of circumferential load, the track is distributed in the circumferential direction of the raceway. a: point load on outer ring, circumferential load on inner ring b: point load on inner ring, circumferential load on outer ring The trajectory form is related to the direction of external load and rotation state (point load or circumferential load, axial load, combined load), see figure 25 to 27. Theoretical-practice comparisons can reveal important information about unexpected load states, such as disturbed floating bearing function. Under the action of pure radial load, the circumferential trajectory generated on the stationary ring is mainly related to the size of the load, the size of the bearing clearance, and the rigidity of the mating parts. The larger the load, the smaller the clearance, the smaller the bearing seat stiffness, and the larger the trajectory of the load zone. 26: Axial load of radial bearings, such as deep groove ball bearings. On the inner and outer rings, the eccentric distribution of the track in the circumferential direction of the entire raceway. 27: Deep groove ball bearings are subjected to combined axial and radial loads. On the inner ring (circumferential load), there is a continuous wider track in the circumferential direction of the entire raceway. On the outer ring (point load), the radial load zone trace is wider than the rest of the circumference.