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What does the term “viscosity index” mean for lubricants?

The viscosity of any form of lubricant will have a serious influence on not just the performance of your machinery, but its lifespan too. By choosing a lubricant with the right viscosity index (VI) for a specific application, companies can reduce maintenance costs and unwanted downtime.

The viscosity of any lubricant describes its resistance to shear and flow. It is impacted by several different factors, like emulsification, contaminants or other forms of lubricants, along with ageing.

The two most common kinds of viscosity are categorised as dynamic viscosity and kinematic viscosity. Dynamic viscosity, also known as absolute viscosity, presents data about the exact force required to make any lubricant flow. Kinematic viscosity, on the other hand, indicates how fast a lubricant will flow when force is then applied to it. Dynamic viscosity is measured in units called millipascal seconds (mPa·s), while the unit of measurement for kinematic viscosity is square millimetre per second (mm²/s).

What exactly is a lubricant’s viscosity index?

The viscosity of any lubricant is always affected by temperature. The higher the temperature is, the lower its viscosity will be. The quality and formulation of any lubricant is effectively determined by how much its viscosity decreases under increases of temperature.

The VI of the lubricant refers to rate of change in viscosity due to change in temperature. To identify whether or not a lubricant can meet an application’s requirements in line with the temperature range demanded by operation, you must be able to understand VI.

How is a lubricant’s viscosity index decided?

The viscosity index of any lubricant is decided by measuring its kinematic viscosity, which is conducted at a temperature of 40°C and 100°C. These two measurements will then be compared to results from two different reference oils.

A traditional-style mineral oil typically has a VI of somewhere between 95 and 100, while the VI of a mineral oil that has been highly refined can be anywhere up to 120. Base oils that are hydrocracked can have a VI even higher than 120, and synthetic oils formulated in labs can exceed this further.

Risks associated with low VI lubricants

While lubricants that have a low VI may possess the correct viscosity at a specific temperature, their viscosity can decrease rapidly in rising temperatures. This can result in an increase in friction, leading to unwanted wear when an oil’s protective film deteriorates.

Operating at lower temperatures, a low VI lubricant may have too high a viscosity, which can lead to several unwanted issues, like oil starvation, low oil flow and dry start-ups.

Always identify the VI of any lubricant you use

To enhance VI, lubrication manufacturers like Aeroshell, Q8 and Kluber, among others, include specially formulated additives in their oils and greases. These special formulations have been developed to answer the specifications of equipment manufacturers, offering superior levels of lubrication. It’s always wise to select the most effective lubricant with the VI you require for optimal levels of performance and equipment longevity.

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