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"How important is the kinematic viscosity at 100 degrees C for a vacuum pump oil?"
Viscosity is the most important physical property of a lubricant. Whether you are looking at the viscosity at the standard 40 degrees C or at 100 degrees C, it is the viscosity at the machine’s operating temperature that is critical to understand. Most bearing temperatures are hotter than 40 degrees C (104 degrees F) at the core, which is where the temperature should be analyzed to match the correct viscosity for the application.
Vacuum pumps stress oil in a variety of different ways. Aside from the operating temperature, contamination is prevalent if these machines are not properly set up to exclude the ingress of contaminants. Also, the gas being processed has a huge impact not only on the viscosity but also on the life span of the oil in service. Process gases can reduce the oil’s viscosity, raise the acid number, increase the moisture level and lead to short lubricant life and low oxidative stability.
Most lubricant manufacturers will provide the viscosity for their products at 40 degrees C and 100 degrees C. The amount of change in the viscosity in relation to the temperature is known as the viscosity index of the lubricant. The viscosity index is most often reported in the technical data sheet of the candidate oil. The higher the viscosity index value, the less the viscosity changes with a change in temperature. This is a crucial factor to consider when looking to switch to a different type or brand of oil.
Kinematic viscosity is the measurement of a fluid’s resistance to flow due to the effects of gravity. It is the viscosity that most people are accustomed to in terms of how thick a fluid is. This will be one of the most noted changes as the oil is heated, the viscosity drops and the oil flows more readily. For all tribosystems, the oil’s ability to flow and support loads is what protects the component from surface degradation and ultimately the loss of usefulness and failure of the machine.
The viscosity at 100 degrees C is critical, not just for vacuum pumps but for all machines. It is important to understand the viscosity requirements of the bearings or gearsets at operating temperatures and ensure that they are being met by the lubricant in use. By verifying that the viscosity is appropriate, you will be generating less wear and prolonging the life of your machines.