Viscosity affects heat generation in bearings, cylinders and gear sets related to the internal friction of the oil. It controls the sealing effectiveness of the oil and the rate of oil consumption, and determines how easily the machine starts or operates under different temperature conditions, especially in cold climates.
Viscosity is a measure of an oil's resistance to flow. It decreases (thinner) as temperature increases and increases (or thickens) as temperature decreases. These conditions explain why oil flows more easily in the summer at temperatures of 25 degrees Celsius (minus 78 degrees Fahrenheit) than in the winter at temperatures of minus 25 degrees Celsius (minus 13 degrees Fahrenheit).
The viscosity of oil is most commonly measured by kinematic viscosity and is reported in units called centistokes (cSt). Kinematic viscosity is measured by the time it takes for a specific volume of oil to flow through a special device called a capillary tube.
Not all oils respond to a given temperature change in the same way. Many oils have the ability to resist changes in viscosity due to temperature changes. This property is called the viscosity index, or VI, of the oil. The higher the VI of an oil, the less its viscosity changes due to temperature changes.
The benefits of oils with higher VI are:
Viscosity generally increases at higher temperatures, resulting in lower fuel consumption and reduced wear.
Reduces viscosity at lower temperatures, which will improve starting and reduce fuel consumption.
Another factor in viscosity measurement is the oil's ability to resist shearing or "tearing one plane of lubricant away from another" during the hydrodynamic lubrication function.
However, under certain conditions, such as shock loading, continuous heavy loading, extremely high temperatures, and/or extremely low (thin) viscosity, the lubricant may not maintain its normal hydrodynamic film state.
One condition begins when there is intermittent contact between wear surfaces. This intermittent contact is called boundary lubrication, and damage begins to occur. If the above situation is not corrected immediately and boundary lubrication continues, failure due to lack of oil film may occur within hours.
Kinematic viscosity, viscosity index, and shear stress/shear rate are all factors that lubricant manufacturers should consider when blending lubricants, but what does all this mean to the end user? This means that the viscosity of the oil is the first and most important consideration when selecting an oil for a specific application.
Remember, for the most effective lubrication, the viscosity must match the speed, load and temperature conditions of the parts being lubricated.
