The Importance of Viscosity and the Viscosity Index for Lubricant Selection

The Importance of Viscosity and the Viscosity Index for Lubricant Selection

You need to understand how lubricants function and why each of their qualities is significant if you want your engine oil to survive and function well. Viscosity, which is likely the most significant physical quality of any lubricant, whether it be oil or grease, will be the focal point of these qualities.

Viscosity Is The Fundamental Component Of Lubrication

It is also usually referred to as the thickness or thinness of the oil. Viscosity, in its most basic form, is the resistance to the lubricant flow that it presents. The hydrodynamic wedge, often called film thickness, is essential for the machines. This wedge is responsible for splitting the machines’ surfaces smoothly. This layer prevents direct contact with moving metal surfaces, which is why it is an important component.

If it were not present, the machines would age far more quickly. When it is presented in that manner, it proves its importance. So, that’s a good thing. 

The lubricant’s viscosity is a significant factor in determining the film’s effectiveness. Viscosity, on the other hand, is not a constant feature but rather a characteristic that shifts over time and is influenced by various factors. Viscosity is also determined by temperature, which is another well-known feature.

When the temperature starts to drop, it is not uncommon for the viscosity of an oil to increase, also known as thickening. As the temperature increases, the viscosity decreases, making the material thinner. Because viscosity shifts in response to variations in temperature, the viscosity index is an essential part of lubricants that consider this phenomenon.

An Explanation of the Viscosity Index

One way to characterize the relationship between temperature and oil viscosity is with the viscosity index improver (VI). If you want to know if a lubricant can keep machinery running smoothly in different temperatures, you must know VI. This is more than just a technical specification; it is a map for choosing the best lubricant polymer for efficient machinery maintenance.

The viscosity of an oil is measured at two standard temperatures—40°C and 100°C—to calculate its VI. Next, a scale based on two reference oils is used to compare these values. A VI of 95 to 100 is common for conventional mineral oils and approximately 120 for highly refined mineral oils. 

A VI of 250 or more is not uncommon for synthetic oils, which are renowned for their exceptional properties. This is a unitless quantity, but to put it simply, a lower viscosity change rate with temperature fluctuations means a higher VI, which is better.

A useful way to visualize the relationship between temperatures and oil viscosity is through a graph with temperature on the X-axis and viscosity on the Y-axis. 

Oils with a greater viscosity index (VI) have a flatter graph, which means they can keep a consistent lubricating coating across a larger temperature spectrum.

How the Viscosity Index Affects Various Pieces of Equipment

How the Viscosity Index Affects Various Pieces of Equipment

There is a specific set of lubrication requirements for various machinery, including gearboxes, compressors, crankcase engines, and hydraulics. The machinery’s design, operational loads, and speed significantly impact these requirements, which in turn determine the necessary lubricant viscosity and, consequently, the Viscosity Index.

Choosing the correct VI is required in gearboxes and compressors to keep the lubricant’s film thickness sufficient under different operating temperatures and loads. Reduced operating temperature, as occurs during cold weather or downtime, causes the viscosity to become excessively thick, which in turn causes several problems, such as increased energy consumption due to higher fluid friction or limited oil circulation.

Mechanical wear could be accelerated if working temperatures become too high, as a drop in viscosity would occur. The end outcome is the same in both cases: failure. A higher VI, if chosen correctly, can lessen the impact of either situation.

It’s obvious that equipment subjected to large temperature swings needs much more VIs. As an example, the lubricant in an outdoor compressor on a chilly morning can encounter very different circumstances by midday due to the temperature change. With minimal viscosity variation, a lubricant with a high VI will offer more consistent protection across these temperature variations.

However, controlling temperature-induced viscosity fluctuations is only one of the many functions of the VI. It’s important to check that the lubricant can handle the unique design and operating requirements of the machinery. Lubricant perfection is the ability to generate a layer that is both thick enough to keep metals from touching and thin enough to provide smooth motion.

Enhanced Viscosity Index Agents

Enhanced Viscosity Index Agents

Lubrication research has progressed over the years to accommodate the varied and rigorous requirements of contemporary machinery, which has resulted in the creation of Viscosity Index Improvers (VII). These chemicals may be added to lubricants to make them work better at a wider range of temperatures by increasing their VI.

VII are usually polymers that are added to oil to slow the rate at which viscosity changes with temperature, increasing the VI. Their main function is to combat the oil’s natural propensity to thin down at higher temperatures by expanding as they heat up. Using VIIs requires some finesse; on the one hand, they increase the oil’s useful operating temperature range, but on the other, they might add complications.

In high-stress areas like gearboxes or engines, VI improvers might shear down during service, which is a problem. The shearing process might cause the lubricant’s viscosity to diminish, which in turn reduces its efficacy permanently.

One important consideration when choosing a lubricant containing viscosity improver additive, particularly for high-shear applications, is the phenomenon known as Temporary Viscosity Loss (TVL). In these cases, synthetic oils with a naturally higher VI are preferable to mineral oils, which often have a lower VI.

Regardless of these warnings, VII’s advantages, especially for temperature-sensitive equipment, are certain. The trick is to find a lubricant that satisfies the machinery’s demands while avoiding shear-induced viscosity loss by selecting the correct balance of VI improvers.

How to Determine the Viscosity Index

There is no learning curve for using the VI calculator. To find the oil’s VI, enter its viscosity at 40°C and 100°C into the calculator. You may use this tool to compare different lubricants and discover the VI of the one you’re currently using. By entering a known viscosity and VI at one of the temperature points, the viscosity at the other temperature point may be calculated.

Calculation Example: A hypothetical oil with 100 cSt at 40°C and 15 cSt at 100°C is considered. A VI or 157 will result when you input these numbers into the calculator. This is probably synthetic and has a higher viscosity than mineral oil. When deciding if this oil is right for your machinery and the conditions in which it operates, this calculation should shed light on how it reacts to variations in temperature.