The oil’s chemistry also plays a significant role in determining its air content. All finished lubricants consist of base oil and additives. The characteristics of your base oil can determine important factors such as your viscosity, interfacial behavior, density, and gas solubility. The surface tension of your oil can also be affected by the size of the bubbles and how long they stay in that formation of the bubble.
Depending on the oil application, the appropriate ratios and types of additives vary. As such, there may be more emphasis on certain characteristics such as oxidation stability, viscosity behavior, or foam control. These additives all affect how long air can remain in the oil and the oil’s state, which can affect our machinery.
As shown in the video below, we can compare the air content percentage of an oil at varying temperatures and observe significant differences.
As shown in the first video, for a wind turbine gearbox using Optigear Syn CT320, the oil contains less air as the temperature increases, decreasing from 2% at 80°C to 0.7% at 110°C. At 110°C, we see a further decrease in air content to 0.65%. As the temperature starts decreasing again toward 80°C, we observe a volume with 2.2% air content in the oil. This is simply due to a temperature change in the oil, not to any additional air ingress.
As such, for the Optigear Syn CT320 oil in this wind turbine gearbox application, we can conclude that if the oil operates at temperatures around 80°C, we can expect up to 2% air volume in the oil. We observe that for lower temperatures, the air content may increase due to the impact of viscosity on air-release capability.
But if the temperatures increase (to a temperature that is tolerated within the system), then the volume of air will decrease, which is a good thing. However, as temperature increases, your chances of thermal and non-thermal oxidation also increase.
In the video above, we see a completely different behavior with the Fuchs Titan EG ATF D VI oil in an automotive gearbox, which starts off at 45 °C. There is a low air volume in the oil at 0.1%. However, when the temperature reaches 73°C, the volume of air increases by 0.7%.
The air bubbles are much larger, increasing the contact surface and their count within the oil. As the temperature decreases to 49°C, the volume decreases by 0.4%, and the number of bubbles decreases. With the continued drop in temperature to 44°C, the air volume decreases to 0.2% and then tapers off to 0.1%, with smaller, fewer bubbles.
In a wind turbine gearbox application using industrial gearbox oil, we observe that the air content decreases as temperature increases. Conversely, in an automotive gearbox using transmission gear oil, the air content increases with rising temperatures. This is very specific to the oils tested in these examples, as different oils will have varying ratios and types of additives and base oils, which can be affected in diverse ways.
The chemistry of the oil is, therefore, another critical part of understanding the air in your oil. If this is properly understood and measured, it can be very useful for monitoring your oil’s health in the field.
Find out more in the full article, "Air in Oil: The Reliability Variable Many Programs still treat too late" featured in Reliable by Sanya Mathura, CEO & Founder of Strategic Reliability Solutions Ltd, David Placzek, Dr. Lukas Hafner
