Root Cause of Blackened Gear Box Lubricating Oil

The color change of gearbox lubricating oil is one of the important indicators of the health status of the equipment. Black lubricating oil often means that there may be abnormal conditions inside the equipment. Although many people think that lubricating oil blackening is mainly caused by metal wear particles, but in fact, oil discoloration may also be caused by lubricating oil oxidation, high temperature carbonization, pollutant mixing and other factors. This paper will systematically analyze the root causes of lubricating oil blackening, and discuss how to predict the failure through oil sample detection.

1. The main cause of black lubricating oil

(1) Effect of metal wear particles

During the gearbox runs, tiny metal particles will enter the lubricating oil due to the natural wear of the gears and bearings. These metal particles mainly come from:

Wear at the gear engagement site (normal wear or abnormal wear)

Wear of bearing roller or roller

The internal parts of the box are slightly worn

When the concentration of the metal particles is high, the color of the lubricating oil will gradually deepen, and eventually appear black.

(2) Oxidation and deterioration of lubricating oil

Lubricating oil in the high temperature, oxygen and water for a long time, will occur oxidation reaction, forming acid substances and mud, these oxidation substances will make the lubricating oil color darker.

Affected factors include:

The operating temperature is too high to accelerate the oil oxidation

The lubricating oil was not replaced for a long time, resulting in the depletion of antioxidants

Outside air or moisture enters the gearbox to facilitate the oxidation process

(3) Oil product degradation caused by high-temperature carbonization

Under high temperature conditions, the lubricating oil may carbonize to form deposits, making the lubricating oil color black.

The main reasons for carbonization:

Gearbox heat dissipation is poor, resulting in the high temperature state of lubricating oil for a long time

The lubricating oil is not replaced for a long time, and the heat resistance is decreased

The additives in the lubricating oil fail resulting cause sludge deposition

(4) the mixing of external pollutants

Dust, fuel oil, coolant or other external impurities mixed in the lubricating oil will accelerate the blackening of the oil product and reduce its lubrication performance.

Possible sources of contaminant:

Gearbox seal failure, causing dust or moisture to enter

Impurities enter the lubrication system during maintenance

Use of an unqualified or improperly stored lubricating Oil

2. How to predict the fault through the oil sample detection?

The detection of the lubricating oil is an effective means to predict the health status of the gearbox. By regularly analyzing the physical and chemical indexes of the lubricating oil, the potential problems can be found in advance.

(1) Common lubricating oil testing items

Viscosity test (Viscosity Test): test the lubricating oil for oxidation or contamination.

Moisture content test (Water Content Test): to identify whether the lubricating oil is damp.

Analysis of metal wear particles (Wear Metal Analysis): assess the wear of gear or bearings.

Total Acid Value (TAN) Test (Total Acid Number): Monitor the oxidation of lubricating oil.

Particle count (Particle Count Analysis): test the impurity content in the lubricating oil.

3. Effective measures to prevent the blackness of lubricating oil

To extend the service life of the gearbox lubricating oil and reduce the risk of equipment failure, the following measures can be taken:

✅ Replace lubricating oil regularly to avoid oxidation deterioration

✅ Use high-quality and standard lubricating oil

✅ optimize the heat dissipation design of gear box, reduce the operating temperature ✅ strengthen the gearbox seal, prevent external pollutants from entering

✅ Conduct lubricating oil detection regularly, and find abnormalities in time

Black lubricating oil is not only a normal use phenomenon, but also may indicate the potential of mechanical problems inside the gearbox. Through scientific oil product detection and regular maintenance, the service life of lubricating oil can be effectively extended, and the equipment failure rate can be reduced. In practical application, the running status of the gearbox should be comprehensively evaluated by combined with the oil analysis data to ensure the stability and reliability of the equipment.