What causes the discoloration of the grease in the bearing?

Grease is usually used in friction parts of machinery to lubricate and seal. It is also used on metal surfaces to fill voids and prevent rust. It is mainly formulated with mineral oil (or synthetic lubricant) and thickener. There are three reasons for the discoloration of motor bearing grease:

1.Grease has an oxidation problem. After a period of storage (or operation), the grease will have a lot of contact with oxygen. On the other hand, the metal (the bearing itself) acts as a catalyst in this oxidation reaction. Therefore, when calculating the replenishment interval of grease in the bearing, the problem of oxidation should also be considered. Many greases now have antioxidant additives, but this does not mean that they will not oxidize.

2. The temperature of the bearing itself (the bearing itself, or the environment) will change, which causes the base oil in the grease to continuously enter and exit the thickener. At the same time, when the base oil goes back, it may not go back completely. Over time, the performance of the grease will change and become unsatisfactory for lubrication, so re-lubrication is required. The color of grease with insufficient base oil may change。also, if the temperature is abnormal, the performance of the grease will change greatly, which will also change color. If the temperature is high, it will carbonize and the color will become darker.

3. During the operation of the bearing, impurities are mixed inside the grease. This kind of impurity may be peeled off by the bearing itself, or it may be entered by the outside world. It may be solid, it may be liquid, these will affect the properties of the grease.

Is it feasible to judge whether the situation is normal based on the color of the grease?

It is very difficult to judge based on this point alone. Different greases have different internal additives, different operating conditions, and different temperature ranges, so it is impossible to have a unified standard. Even if it is the same grease, the formula may be slightly different due to different batches of products. So please do not use this color change as a basis, it is unreliable.