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Trajectory Analysis of Failed Bearings

by:JNSN     2022-03-14
Trajectory phenomena with radial over-preloading of failed rolling bearings: Figure 33 shows the circumferential trajectories on both rings with radial over-preloading, which can cause thermal damage under extreme conditions, see chapter 3.3.5. Reasons: Shaft/bearing seat interference is too large Temperature difference between inner and outer rings is too large Bearing clearance is too small There is an elliptical deformation of the trajectory phenomenon: There are several separate trajectory areas on the circumference of the stationary ring raceway, see Figure 34. Reason: The bearing seat or shaft is oval, for example, due to circumferential deformation caused by machining or tapped holes near the bearing mating surface. The bearing seat has different stiffness in the circumferential direction, and has a large interference with the outer ring. Measure the thin-walled bearings and store 23 pairs of the removed bearings vertically to evaluate running characteristics and damage. Rolling contact mode 33: Radial over-preloaded deep groove ball bearings. There are tracks along the entire circumference, even ferrules that carry point loads. 34: Elliptical deformation of deep groove ball bearings. Two opposing radial load zones are formed on the elliptically deformed outer ring (point load) track. Abnormal trajectory Judging whether the trajectory is normal or not depends largely on the bearing application. For example, when the bearing is mainly subjected to radial loads, there is a normal trajectory, but when the same trajectory occurs in the bearing condition with axial preload, it is abnormal, which indicates incorrect bearing installation. Therefore, it is necessary to know the bearing conditions in order to correctly evaluate the trajectory. Of course, some basic failure phenomena can usually be assessed by trajectory. Trajectory phenomenon when lubrication is insufficient: the visible trajectory and the surface topography observed through the microscope may be related to the quality of lubrication. Under moderate loads and without a fully separated lubricating film, dark, rough tracks can be formed. The thinner the lubricant film, the greater the impact on the surface. For this, reference can be made to an example where the surfaces are not completely separated, see Figure 28. When the load in the contact zone is high, the track is bright and pressure-polished, in stark contrast to the non-load bearing zone of the track, see Figure 29. Cause: Insufficient amount of lubricant in the bearing Insufficient lubricant viscosity at this operating temperature and speed (see 'FAG Rolling Bearings' brochure, Corrected rating life calculation) Remedy: Improve the lubricant supply so that the viscosity of the lubricant matches the operating conditions Accommodating the use of lubricants with effective additives Using surface-coated bearing parts When there is contamination in the bearing or in the lubricant, we must first distinguish the difference between solid and liquid contamination. Phenomenon caused by solid contaminants: pits caused by external particles circulating on the raceway. When observing the trajectory under the microscope, soft particles, hardened steel particles and hard gravel particles can be distinguished by the form of pits, see Figures 30, 31, and 32. Especially large and hard foreign particles have a great influence on bearing life. More specific information on this can be found in the description of fatigue failure, see chapter Fatigue due to cyclic motion of foreign particles. A large number of small and hard foreign particles can cause roughening of the contact surface, as shown in Figure 28, and accelerate abrasive wear. Evaluation of running characteristics and damage on dismantled bearings Rolling contact mode 30: Indentation caused by soft particles 31: Indentation formed by hardened steel particles 32: Indentation formed by hard grit Phenomenon caused by liquid contamination : Water is one of the major liquid pollutants. A small amount of water is drawn into the lubricant. It reduces lubrication and leaves marks as shown in Figure 29. There is a dull track when there is a lot of moisture in the bearing. The pressure polishing trajectory of fatigue damage is also caused by corrosion or overload, please refer to chapter Fatigue caused by poor lubrication: Unsuitable sealing installation environment is not clean Residues during processing, such as foundry sand particles with large temperature changes ( Condensed water) Remedial measures for lubricating oil not clean: Improve the sealing installation environment and clean the fittings. If necessary, before coating work (before the first rotation of the bearing), clean the entire oil circuit system. Trajectory Phenomenon: Only fixed-end bearings in a fixed-floating bearing arrangement have such a trajectory, as shown in Figure 35b, which is produced by axial forces (Figure 26). In most cases, small (preferably none) axial loads will also be found in floating bearings. Causes: Impaired floating function of the floating end bearing (wrong fit, radial thermal expansion, overturning, fretting corrosion) Unexpected large axial thermal expansion Remedy: Check fit and dimensional tolerances of mating parts Change mounting and Operating conditions Using bearings with internal axial displacement: Cylindrical roller bearings N, NU, NJ24 Evaluate the running characteristics and damage of the removed bearings. Rolling Contact Mode 35: Fixed-floating arrangement of two deep groove ball bearings. a: The deep groove ball bearing at the working end is used as a locating bearing, and the deep groove ball bearing at the driving end is used as a floating bearing. b: Bearing trajectory under normal operation. The trajectory of the locating bearing shows the characteristics of the bearing under combined load, and the trajectory of the non-locating bearing shows the pure radial load or the main radial load. c: Bearing trajectory under detrimental axial preload (non-moving outer ring of floating bearing). The trajectory of each bearing shows the characteristics of the combined load. The symmetrical trajectories of the two bearings indicate detrimental axial preload.
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