What Are the Properties of Lubricants? - Viscosity Index

Viscosity can vary enormously between different lubricants and temperature often plays a key role in changing internal resistance to flow. Temperature-triggered changes in the viscosity of a lubricant is known as Viscosity Index (VI). While VI is often an overlooked parameter when selecting lubricants, it can offer valuable insight into how the lubricant will behave when in use.

Viscosity Index vs ISO Viscosity Grade

Contrary to popular belief, Viscosity Index is not part of ISO Viscosity Grade, which calculates viscosity at a standard temperature of 40°C. VI is an independent parameter and is used to calculate how viscosity will change when exposed to temperatures higher or lower than 40°C.

The beginnings of Viscosity Index

The concept of Viscosity Index was first developed in the 1920s by a pair of American chemists. Using Pennsylvania crude as a benchmark for low viscosity and Texas Gulf crudes for high viscosity, the team developed a method for calculating changes in internal resistance to flow relative to temperature. Pennsylvania crude was given a VI of 100 while Texas Gulf crudes were given a VI of 0. A higher VI number indicated more stability when exposed to temperature changes.

Why Viscosity Index matters

As mentioned earlier, Viscosity Index is often disregarded during the lubricant selection process. This is usually a mistake as VI can have a big impact on how a lubricant performs when in circulation and exposed to temperatures other than the standard 40°C used to calculate viscosity.

VI allows operators to compare how different lubricants behave when exposed to temperatures outside normal operating conditions. Temperature changes can and do occur, triggered by anything from weather patterns to mechanical overheating. Viscosity Index helps operators make informed decisions to prepare for these changes and minimise the risk of unplanned downtime.

Calculating Viscosity Index

Today, ASTM D2270 Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100 °C is one of the most widely used methods to calculate VI. Most lubricants offer Vis ranging from between 90 to 160, though some can be as low as -60 or as high as 400.  

A holistic approach to lubricant analysis

Viscosity is just one of many properties used to analyse lubricants and assess suitability for different applications. Find out more about some of the other key properties of lubricants, including pour point, flash point and chemical stability in ‘What Are the Chemical & Physical Properties of Lubricants?'