Viscosity Index (VI) is the measure of a fluid’s ability to resist changes in viscosity due to temperature fluctuations. In other words, it measures how much the lubricant will thin out or thicken up when subjected to varying temperatures. A high VI rating means that the lubricant maintains its viscosity better over a wider range of temperatures and is less likely to experience significant changes in thickness. This quality is important for industrial lubricants, as it helps ensure that the lubricant continues performing effectively even as the equipment heats up and cools down.
Viscosity Index vs. Viscosity
Despite their similarities, viscosity index and viscosity are not the same. However, you may hear people use the terms interchangeably, which makes it even more unclear which rating you should consider when choosing a lubricant. Since lubricant viscosity is its most important attribute, understanding the difference between these two viscosity terms is crucial.
The viscosity of your lubricant measures its resistance to flow. A simple example of high viscosity is honey, which does not flow easily. Water, on the other hand, has a low viscosity. Lubricants have viscosity ratings known as an ISO Viscosity Grade (VG). Lubricant manufacturers measure VG at a temperature of 40°C to provide a consistent rating system.
A simple example to illustrate viscosity index is to think about the oil you use for cooking. Cooking oil has a higher viscosity at room temperature, meaning that it resists flow. However, the viscosity of that oil changes when you heat it in a pan. Its viscosity decreases as it is more fluid at higher temperatures. This is basically the same for the lubricant in your equipment! During operation and when your equipment warms up, lubricant viscosity changes. This means that viscosity index measures the change in viscosity of your lubricant as temperature rises or falls. In lubricants, both formula and quality of ingredients can affect VI.
The Importance of Viscosity Index
Your equipment will function better if you choose a lubricant with the right viscosity. It is even better to check the lubricant’s VI for the operating temperature range of your equipment. Your lubrication management will be even more effective if you make sure the viscosity of the lubricant is right for all operating temperatures of your equipment. The following are some of the problems caused by viscosity changes:
- High viscosity leads to low oil flow resulting in a dry startup or oil starvation.
- Low viscosity leads to the loss of oil film strength resulting in heavier loads, more mechanical wear, and increased friction.
Viscosity errors in either direction can lead to equipment failure and an increase in wear and tear.
How Lubricants with the Same Viscosity and Different Viscosity Index Perform
Here’s an example of how two lubricants with the same viscosity grade can behave differently at various temperatures. The VG of oil #1 is 150 and the VI is 95, while the VG and VI of oil #2 are both 150. Oil #2 has a higher viscosity index than oil #1, so its viscosity fluctuation is less extreme. In terms of viscosity grade, these lubricants are similar. It’s clear, however, that their viscosity index makes a big difference in how they perform.
| Temperature | -20°C / -4°F | 20°C / 68°F | 40°C / 104°F | 60°C / 140°F | 80°C / 176°F | 100°C / 212°F |
| Oil #1 (VG = 150 / VI = 95)Viscosity in cSt | 37,000 | 550 | 150 | 56 | 26 | 15 |
| Oil #2 (VG = 150 / VI = 150)Viscosity in cSt | 11,000 | 435 | 150 | 65 | 34 | 20 |
| Viscosity Difference in % | +236% | +26% | 0% | -14% | -23% | -25% |
What is a Normal VI for Lubricant?
Lubricants can have VIs ranging from -60 to over 400. Most lubricants, however, are in the 90 to 160 range. The VI of traditional mineral oil is typically 95 to 100. Refined mineral oil tends to have a higher VI though, sitting around 120. Synthetic oils will have a VI of up to 250. When a lubricant’s VI number increases, its viscosity is less affected by temperature variations. A wider temperature range provides greater stability and reliability for your equipment.
Many lubricants have a VI-improver additive. This additive helps to stabilize viscosity changes. However, it is important to note that additives deplete over time. As an additive is used up, a lubricant that relies on an additive to maintain its viscosity eventually loses its quality.
What VI Does Your Equipment Need?
Consider the operating conditions of your equipment when determining your lubricant’s VI.
High VI lubricants are best for equipment with:
- A fluctuating load
- A variable speed
- Temperature shifts
- Changing environmental variables
- Uncertain viscosity (if you are uncertain about the right viscosity, go with a higher VI lubricant).
Low VI lubricants are suitable for equipment with:
- A consistent speed
- A constant load
- Consistent temperature
- Determined viscosity needs
Viscosity and VI are Essential to Lubrication Management
Each asset in your facility will have a different environment and lubrication needs. Trial and error isn’t a bad thing, as it’s a natural part of continuous improvement. It’s possible that the first lubricant you pick doesn’t do the best job. Next time, you might want to consider using a lubricant with a higher VI. The secret is to keep an eye on the health and dependability of your asset following a modification. You can determine from the statistics whether changing to a higher VI was a wise decision or whether there is another factor at play.
