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Lubricant sampling and inspection of failed bearings

by:JNSN     2021-12-28
Lubricants can effectively reveal various failure causes of rolling bearings. But there must be suitable test samples (for bearings without seals), please refer to DIN 51750, ASTM standards D270-65 and 4057-81. Grease lubrication: Record the distribution and color of the grease in the bearing environment. Take samples from different positions inside and around the bearing and mark them accordingly. Take oil samples when the equipment is running or just after the equipment is stopped to obtain a typical distribution of impurities. Do not take oil samples from the bottom or just after filtering the lubricating oil (the particle concentration is incorrect). The objects should also be stored for testing (indicating the condition before failure). The general principle is: How often is the bearing relubricated or how often is the lubricant replaced? When was it last time? Check whether there are any components of the lubricating oil or grease that have fallen from the bearing or other parts. Use a clean container to hold the sample. They should be made of a suitable material (such as glass). There should be enough space in the container to stir the lubricant sample during the experiment. The sample can be analyzed in the customer's laboratory, an external lubricant laboratory, or in the company. The aspects that should be paid attention to are the degree of pollution and the type of pollutants (sand gravel, iron filings, soft particles, water, coolant) and the analysis of the lubricant itself (such as: aging, stability, color, coking, additive ratio). If possible, samples of fresh grease or lubricating oil should also be submitted for testing (when the lubricant and thermal effects are unknown). Check the bearing environment. Are there any parts that scratch the bearing parts? Has the component adjacent to the bearing been damaged (afterwards or pre-damage)? The cleanliness of the inside and outside of the seal (are there impurities in the bearing space?) Whether the bearing fasteners are loose (does the bearing deform under the action of force? The bolts are loose?). Evaluate whether the bearing in the installed state has broken or chipped areas? Is the seal broken? In particular, is there any deformation or hardening? Do you see the bearing deformed? Can abrasions from foreign substances be detected? In the installed state, can the bearing function normally? (According to the matching effect) 2.5 Disassembling the damaged bearing When disassembling the damaged bearing, care must be taken not to destroy the failure mode. If this is not possible, the bearing damage caused by disassembly shall be marked and recorded. If possible, follow the procedure below: Do not transmit the disassembly force through the rolling elements. A large disassembly force may indicate that the function of the floating bearing has been damaged. Seal, cage) Mark the bearing (installation position, installation direction) 2.6 Check the size of the shaft and the bearing seat on the mating surface (too tight fit, too loose fit) Shape and position tolerance of the mating surface (ovality) Roughness of the mating surface ( Excessive material loss) Fretting corrosion (the degree of corrosion indicates the degree of uneven support, the direction of the load) 2.7 Evaluation of the complete set of bearings The bearings to be evaluated should be uncleaned at the time of submission, that is, they still carry lubricant. The following points should be checked: the overall condition (cleanliness of the bearing and the condition of the mating surface, such as: installation marks, fretting corrosion, ring fracture, dimensional accuracy, scratches, discoloration) the condition of the sealing ring and the dust cover. Take pictures or describe the location and extent of any grease leaks. Condition of the cage Manual rotation test (indicating contamination, damage or preload) Measure the bearing clearance (the ring moves in the radial and axial directions) to ensure that the bearing is evenly loaded and rotated! 2.8 Return of failed bearings and evaluation of the cause of failure of a single part can basically be found by customers themselves or by field service engineers. The need for further special testing is related to each different failure characteristic. The following describes the process of inspecting each bearing component in detail. If it is obviously going to be tested, then the bearings to be shipped should be prepared as follows: Neither disassemble the bearing nor clean the bearing. Never use cold detergent or gasoline to clean (otherwise the clues in the lubricant will disappear and it will be corrosive). Avoid pollution after disassembly. If possible, use clean metal foil to individually wrap the bearings, because paper and cloth will absorb the oil in the grease. Use strong and thick packaging to avoid damage during transportation.
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