Viscosity Index: What It Is and Why It’s Important

A lubricant’s viscosity is its most important property. Viscosity can be defined as an oil’s resistance to flow and shear. It is influenced by several factors such as contamination with water, particles or other lubricants. Taking a proactive approach to monitoring your lubricant’s viscosity can make a big difference in the health and life of your machinery.

Viscosity is also affected by temperature. The lubricant’s formulation and quality will impact how much its viscosity will drop with an increase in temperature. The change in an oil’s viscosity due to a temperature change is called the viscosity index (VI). Understanding VI is important to discern whether the lubricant in question meets the asset’s requirements based on the operating temperature range.

To determine an oil’s VI, the viscosity is measured at two temperatures: 40 degrees C and 100 degrees C. This is then compared to a scale based on two reference oils. While the VI has no units of measure, it is well known that the viscosity index of a conventional mineral oil is between 95 to 100. Highly refined mineral oils have a VI of approximately 120. Synthetics may have a VI of nearly 250. A higher number means the lubricant changes viscosity at a lower rate based on the temperature.

If drawn on a chart with viscosity on a vertical axis and the temperature on a horizontal axis, the slope would be more horizontal with higher viscosity indexes. A higher VI is more desirable because it enables the lubricant to provide a more stable lubricating film over a wider temperature range.

Some oils may have the right viscosity at certain temperatures but do not quite meet the needs of both ends of the temperature range. Keep in mind that a slight difference in temperature can equate to a huge viscosity change that could be detrimental to the asset.

For example, if the viscosity is too high, it could result in low oil flow, causing oil starvation and dry startups. When the viscosity is too low, greater mechanical friction may occur, which can lead to heavier loads. You also would see more wear due to film loss.

Additives can be used to improve the viscosity index. This allows a lubricant to be formulated to meet the original equipment manufacturer’s specifications. It is always a good idea to check the equipment manufacturer’s specifications to determine the proper lubricant and to consider the climate and operating conditions when making your selection. With proper installation and an accurate viscosity index, you can be sure to achieve maximum life expectancy and reliable operation of your machinery.

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