Reasons for bearing blackening
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The bearings are SKF bearings, double-row cylindrical roller bearings. Before installation, they were heated to 120°C in oil; after being taken out of the oil, it was observed that certain areas of the outer ring had turned black. What caused this?
At that time, the half coupling was heated first; it was heated for over three hours until the oil temperature reached 180°, after which it dropped to around 20° degrees, and then the bearings were installed
It’s because the heating time is too long; when the temperature exceeds the specified level, the bearings turn black.
No, the bearing temperature did not exceed 120°C. The 180°C figure refers to the temperature of the heated half-coupling; after heating was complete, it was left there for 4 hours, after which the bearing was heated again
Are you still using the oil bath method? Nowadays, it’s possible to consider using electric heaters specifically for heating bearings. The heating temperature of bearings is influenced by several factors: the internal clearance of the bearing (bearing grade), the size of the bearing (shape, size, weight), and the material of the bearing cage. As for the oil temperature of 120°C mentioned by the original poster when heating the bearing, I think that’s probably a bit too high. For ordinary rolling bearings, I usually use a heating range of 90–110°C. If it’s indeed necessary to heat the bearing above 110°C, it’s important to refer to SKF’s technical specifications in order to determine the maximum allowable heating temperature for that bearing. In general, I believe that the blackening of bearings is caused by excessively high oil temperatures, which lead to changes in the crystal structure of the bearing surface; in other words, it’s as if the bearing has undergone another heat treatment process, which is extremely detrimental for its subsequent use
Sometimes, even when heated to 110 degrees, the assembly remains tight; it only becomes easier to assemble at around 130 degrees. There are no issues with usage either – it probably depends on the shaft diameter as well.