Grease is discolored in the bearing, and it will change color even if it is not placed in the bearing.
This is because:
First, there is an oxidation problem with the grease. When the grease is placed (or operated) for a period of time, it will have a lot of contact with oxygen, and at the same time, the metal (the bearing itself) acts as a catalyst in this oxidation reaction. Therefore, when calculating the replenishment time interval of the grease in the bearing, the problem of oxidation should also be considered. Many greases now have antioxidant additives. This does not mean that it will not oxidize.
Second, the bearing itself, the temperature (the bearing itself, or the environment) will change, which causes the base oil in the grease to continuously enter and exit the thickener, and each time the base oil returns, it may not completely return. After a long time, the performance of the grease will change and the lubrication will not be satisfied, so re-lubricate. Grease with insufficient base oil may change color. Also, if the temperature is abnormal, the performance of the grease changes greatly, and thus, there is discoloration. If the temperature is high, it will carbonize. The color will become darker.
Third, during the operation of the bearing, impurities are mixed inside the grease. This kind of impurity may be caused by the bearing itself peeling off, or it may be entered by the outside world. It may be solid and may be liquid, which can affect the properties of the grease.