Lubricant is vital to the operation of bearings; its quality directly affects the service life of the bearings, and precision bearings in particular have even higher requirements for lubricant. If the quality of the lubricant is too poor or it has deteriorated, this will directly affect the bearing’s performance and service life; therefore, it is essential to ensure that the lubricant does not deteriorate. The importance of lubricant to bearings goes without saying, so when using bearings, one must constantly check the lubricant to ensure that the grease has not deteriorated. However, some people do not know how to determine whether bearing lubricant has deteriorated. Today, I will explain several methods for identifying whether bearing lubricant has deteriorated.

1. Oil Drop Test

Take a clean sheet of white filter paper and place a few drops of oil on it. Once the oil has seeped through, if black powder is visible on the surface and the paper feels gritty to the touch, this indicates that the lubricant contains a significant amount of impurities. Good-quality lubricant will show no powder; it will feel dry and smooth to the touch, leaving a yellowish mark.

2. Oil Flow Observation Method

Take two measuring cups: fill one with the lubricant to be tested and place the other empty on the table. Raise the cup containing the lubricant 30–40 centimetres above the table and tilt it, allowing the oil to flow slowly into the empty cup. Observe the flow: high-quality lubricant should form a thin, even, continuous stream. if the flow fluctuates in speed or large clumps fall intermittently, the lubricating oil has deteriorated.

3. Rubbing Test

Rub the lubricant between your thumb and forefinger repeatedly. Good-quality lubricant will feel smooth, produce few particles, and cause no friction. If you feel a significant amount of friction, such as grit between your fingers, this indicates that the lubricant contains many impurities and should no longer be used; it must be replaced with new lubricant.

4. Light Inspection Method

On a sunny day, use a screwdriver to lift a drop of lubricating oil at a 45-degree angle to the horizontal. Hold it up to the sunlight and observe the droplet. If, under the light, the oil appears clear with no visible metal particles, it is in good condition and can continue to be used. If there are excessive metal particles, the lubricating oil should be replaced.

The above outlines methods for determining whether bearing lubricating oil has deteriorated. High-quality lubricating oil flows in a fine, even and continuous stream; if the flow rate fluctuates erratically, with large clumps occasionally falling, this indicates that the lubricating oil has deteriorated. Once bearing lubricating oil has deteriorated, it turns deep black, becomes foamy and shows signs of emulsification. When rubbed between the fingers, it lacks viscosity, feels rough or has an unpleasant odour. When dripped onto white paper, it appears deep brown, with no yellow wetting zone or numerous black spots. Failure to replace it promptly will accelerate component wear, shorten service life and may even lead to safety incidents.