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Lubricant sampling and inspection of failed bearings

by:JNSN     2022-03-14
Lubricants can effectively reveal various failure causes of FAG rolling bearings. However, there must be a suitable test sample (for bearings without seals), please refer to DIN 51750, ASTM standards D270-65 and 4057-81. Grease Lubrication: Record the distribution and color of grease in the bearing environment Take samples from different positions inside and around the bearing and mark them accordingly Oil Lubrication: Take oil samples from the oil circuit near the bearing or from the middle of the oil supply Take oil samples while the equipment is running or just after the equipment has been shut down to obtain a typical distribution of impurities Do not take oil samples from the bottom or just after filtering the oil (incorrect particle concentration) In addition to the oil sample, residual oil in the filter The item is also preserved for testing (indicating the condition prior to failure). The general principle is: How often should the bearings be re-lubricated or how often should the oil be changed? When was the last time it was done? Check the oil or grease for any detachment from bearings or other components. Use a clean container for the sample. They should be made of a suitable material (eg glass) Sufficient space should be left in the container to agitate the lubricating oil sample during the experiment The sample can be analyzed in the customer's laboratory, in the external lubricants laboratory or at FAG. Aspects that should be concerned are the degree of contamination and the type of contamination (grit, iron filings, soft particles, water, coolant) and the analysis of the lubricant itself (eg ageing, stability, color, coking, additive ratios). If possible, a sample of fresh grease or oil is also submitted for testing (when lubricant and thermal effects are unknown) to check the bearing environment Are there any parts of the bearing parts that are scratched around? Have components adjacent to the bearing been damaged (after or before)? Cleanliness inside and outside the seal (are there any impurities in the bearing space?) Are the bearing fasteners loose (is the bearing deformed by force? Are the bolts loose?). Assess the mounted state of the bearing for any cracked or chipped areas? Is the seal broken? Is there deformation or hardening in particular? See if the bearing is deformed? Can scratches from foreign substances be detected? Can the bearing run normally in the installed state? (According to the matching effect) 2.5 Removing damaged bearings When removing damaged bearings, care must be taken not to destroy the failure mode. If this is not possible, the bearing damage due to disassembly should be marked and recorded. If possible, follow the procedure below: Do not transmit dismounting forces through the rolling elements High dismounting forces may indicate a functional impairment of the floating bearing Do not open sealed bearings Seal, cage) Mark the bearing (installation position, installation direction) 2.6 Check the shaft and bearing seat dimensions on the mating surface (too tight, too loose) Geometric tolerance of the mating surface (ovality) Roughness of the mating surface ( Excessive material missing) Fretting corrosion (the degree of corrosion indicates the degree of uneven support, direction of load) 2.7 Evaluation of the complete set of bearings Bearings to be evaluated shall be submitted uncleaned, ie still with lubricant. The following points are to be checked: General condition (cleanliness of the bearing and condition of mating surfaces, eg: mounting marks, fretting corrosion, broken rings, dimensional accuracy, scratches, discoloration) Condition of seals and dust caps. Take pictures or describe the location and extent of any grease leaks. Cage condition Manual rotation test (indicating contamination, damage or preload) Measure bearing play (rings moving radially and axially) to ensure that the bearing is loaded and rotated evenly! 2.8 Returning the failed bearing to FAG and evaluating the failure cause of a single part can often be discovered by the customer himself or by the FAG field service engineer. Whether further special testing is required is relevant for each different failure characteristic. The procedure for inspecting each bearing component is described in detail below. If it is obvious that it will be tested at FAG, the bearing should be shipped as follows: Do not disassemble nor clean the bearing. Never wash with cold cleaners or gasoline (otherwise the clues in the lubricant will disappear and it will be corrosive). Avoid contamination after disassembly. If possible, wrap the bearings individually in clean metal foil, as the paper and cloth will absorb the oil from the grease. Use strong and thick packaging to avoid damage in transit.
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