As mentioned in the previous article, viscosity is a very important property of lubricant. However, knowing the right viscosity for the application is not enough. We need to know how the viscosity changes with temperature. This is especially true for machines that operate under wide temperature ranges.
Viscosity Index or VI is the viscosity behaviour in differing conditions. It is a way to describe a lubricant’s viscosity variation, or sensitivity, to changes in temperature. The viscosity will reduce as a lubricant heats and then increase when it cools down. The rate of which it increase and decrease is what VI is about.
Two lubricants may have the same viscosity at 40⁰C, but will not be at 100⁰C if their VI is different. Consider Figure 1 below: even though Oil A and Oil B have similar viscosity at 40⁰C, due to Oil A having a higher VI, its viscosity at 100⁰C decreased a lot lesser then that of Oil B. In other words, the higher the VI, the wider the operating temperature the oil can operate without losing its protection abilities.
The knowledge of VI helps in lubrication selection. Each machine is designed to run with an optimum viscosity range to ensure sufficient film thickness. However, changes in temperature may put the lubricant viscosity out of the range: either too thick or too thin, for the machine to operate reliably. Specifying the VI will ensure that variations in temperature will have no, or little impact, as the viscosity will still remain within optimum range.
To illustrate this, consider the same Figure 1 chart with the optimum viscosity range added (see Figure 2). Oil B meets the optimum viscosity range up until 60⁰C after which it will be too thin. Oil A has a wider temperature range as it will still be within the range until slightly beyond 100⁰C. For machines that operate under low to moderate temperature, Oil B may be suitable. Oil A should be selected for higher temperatures or applications where the gaps between low and high temperatures are large.
Lubricants with high VIs are typically synthetics or hydrocracked mineral base oils that sit on the higher side of API grouping such as Group III. VI also can be increased by adding a VI improver additive, which is basically a polymer that contracts or expands depending on temperatures.
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About the Author:
Aaron Said is ILD/Sinopec’s National Technical Manager. He is responsible for developing and delivering value-added lubricant technical services to client for reliability improvement, lubricant optimisation, and cost-savings. Among his professional credentials are Certified Lubricant Specialist (CLS) and Oil Monitoring Analyst I (OMA) from Society of Tribology & Lubrication Engineers (STLE) and the International Council of Machine Lubrication (ICML)’s Machine Lubricant Analysist I.