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"Apart from oxidation, what can cause thickening of a gear oil?"
Gear oil viscosity typically trends downward as the oil ages. However, every so often a slight spike in viscosity may be seen in your oil analysis reports. A difference of a few centistokes could be due to inconsistencies in the lab or personnel running the sample. A greater increase in viscosity may indicate lubricant mixing. If there is a trend of rising viscosity over several samples, it may signify that other problems are occurring in your reservoir.
Mixing of lubricants is the most common cause of a viscosity increase. For example, topping off a gearbox that has a gear oil of 152 centistokes with an ISO 220 or 320 oil will cause the viscosity to rise. Results of a viscosity increase include high operating temperatures, power losses, decreased efficiency and improper oil flow.
Degradation is another possible reason why a gear oil’s viscosity can rise. Oxidation falls into this category but in different terms. When an oil is permitted to “run hot,” it will thermally break down, allowing it to oxidize. Applying the Arrhenius rate rule, which states that the chemical reaction rate doubles for every increase of 18 degrees F (10 degrees C), the oil life will be cut in half. Keep in mind that the more stress the oil is put under, the faster it will degrade.
Contamination also falls under degradation. Contaminants such as dirt and water can have a significant influence on the oil’s viscosity depending on their concentration. When a gear oil is contaminated with water and reaches the emulsified state, an increase in viscosity will be noticeable on the oil analysis report. This can be seen by looking at the water concentration and viscosity. While lab reports will indicate a rise in viscosity, due to the pressure viscosity coefficient of water vs. oil, the lubricated zone will see a decrease in film thickness.
A less common reason for rising viscosity is a poorly formulated gear oil suffering from evaporation loss. When higher temperatures are present, the lighter oil molecules are subject to evaporation, leaving the heavier molecules behind. The physical change will shift the molecules to the heavier end, resulting in an increase in viscosity.
While rising viscosity is usually associated with lubricant oxidation, there are other potential causes. Implementing a sound lubrication program and educating all parties involved should be the first steps toward combating irregular results in your oil analysis reports. Often the root cause of the problem can be eliminated before it becomes an issue.