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.
This viscosity behaviour is known as “Viscosity Index” or shortened as VI. It is a way to describe a lubricant’s viscosity variation, or sensitivity, to changes in temperature. As a lubricant is heated up, the viscosity will reduce, and as it cools down, viscosity will increase. 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. To ensure sufficient film thickness, each machine is designed to run with an optimum viscosity range. 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. However for higher temperatures or applications where the gaps between low and high temperatures are large, Oil A should be selected.
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.
Want to know if the VI of your oils are suitable for your operating conditions? Contact ILD technical service for assistance.
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 optimization, 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.