31
mei
2012

Viscosity index

One tool to simplify the viscosity challenge

Viscosity, a fluid’s resistance to flow, is a critical factor to consider when selecting a lubricant. If the viscosity of a lubricant is too low or too high for the application, serious problems may result. Adding to the challenge, temperature fluctuations can affect the viscosity of a lubricant in use — sometimes substantially. Viscosity index (VI) offers a way to evaluate to what extent a lubricant’s viscosity is impacted by temperature changes.

Why viscosity matters

If a lubricant’s viscosity is too low, the lubricant will not be able to keep moving parts separated. This may lead to problems such as increased friction and wear, as well as increased heat and oxidation.

If viscosity is too high, the lubricant won’t flow adequately. This can lead to increased drag and friction, and in turn higher operating temperatures and energy consumption.

Either situation is harmful to equipment and could result in significant maintenance costs and downtime — and potentially serious damage and even system failure.

Temperature’s impact

Selecting a lubricant with an appropriate viscosity for an application is half the battle. You also need to consider the impact of temperature: Rising temperatures reduce viscosity, while falling temperatures cause viscosity to increase.

To complicate matters further, lubricant viscosities do not change to the same extent to a given shift in temperature. Some may move dramatically, while others move very little. Generally, the less sensitive a lubricant’s viscosity is to temperature changes, the better.

VI: a helpful evaluation tool

VI expresses the extent to which a lubricant’s viscosity changes in relation to temperature. The higher a lubricant’s VI, the more stable its viscosity is over a wide range of temperatures (i.e., the less its viscosity changes with temperature).

VI is very important in applications that experience wide temperature variations, such as refrigeration systems.

  • The lubricant in a refrigeration system must have a viscosity that is high enough to protect against friction and wear at high temperatures. It also needs to be high enough to overcome the potential for viscosity dilution (reduction of viscosity) that can occur as the lubricant and refrigerant interact.
  • At the same time, the lubricant’s viscosity must be low enough to ensure proper lubricant flow at cold temperatures so that the fluid is able to flow back to the compressor. Otherwise, if it becomes trapped in the refrigeration system, components may be damaged due to lack of sufficient lubrication. The system also could lose thermal efficiency, driving up energy costs.

A relatively high VI may indicate that a lubricant can withstand extreme temperature changes — high and low — and provide consistent protection to system components. Refrigeration lubricants with a relatively high VI also tend to resist viscosity dilution more effectively.

Of course, you should not rely solely upon VI to evaluate the quality or appropriateness of a lubricant. Bear in mind, for example, that many low-VI lubricants can be specifically formulated to offer protection comparable to their high-VI counterparts. Knowledgeable experts who understand the full scope of lubricant properties can help you select a suitable lubricant for your specific needs.

VI: How base fluids compare

Generally, the VI of a lubricant depends primarily on the base fluids used in the formulation. This chart illustrates the VI of common base fluids. As you can see, synthetic fluids do not always have a high VI. Alkyl benzene base fluids, which are typically used in R-22 refrigeration systems, are classified as synthetics and have a very low VI.

Some formulations contain a mixture of different base fluids, such as formulations blended with polyalphaolefin and alkyl benzene base fluids. These mixtures typically have a VI somewhere in between the VI of the component base fluids.

Why viscosity matters

If a lubricant’s viscosity is too low, the lubricant will not be able to keep moving parts separated. This may lead to problems such as increased friction and wear, as well as increased heat and oxidation.

If viscosity is too high, the lubricant won’t flow adequately. This can lead to increased drag and friction, and in turn higher operating temperatures and energy consumption.

Either situation is harmful to equipment and could result in significant maintenance costs and downtime — and potentially serious damage and even system failure.

Temperature’s impact

Selecting a lubricant with an appropriate viscosity for an application is half the battle. You also need to consider the impact of temperature: Rising temperatures reduce viscosity, while falling temperatures cause viscosity to increase.

To complicate matters further, lubricant viscosities do not change to the same extent to a given shift in temperature. Some may move dramatically, while others move very little. Generally, the less sensitive a lubricant’s viscosity is to temperature changes, the better.

VI: a helpful evaluation tool

VI expresses the extent to which a lubricant’s viscosity changes in relation to temperature. The higher a lubricant’s VI, the more stable its viscosity is over a wide range of temperatures (i.e., the less its viscosity changes with temperature).

VI is very important in applications that experience wide temperature variations, such as refrigeration systems.

  • The lubricant in a refrigeration system must have a viscosity that is high enough to protect against friction and wear at high temperatures. It also needs to be high enough to overcome the potential for viscosity dilution (reduction of viscosity) that can occur as the lubricant and refrigerant interact.
  • At the same time, the lubricant’s viscosity must be low enough to ensure proper lubricant flow at cold temperatures so that the fluid is able to flow back to the compressor. Otherwise, if it becomes trapped in the refrigeration system, components may be damaged due to lack of sufficient lubrication. The system also could lose thermal efficiency, driving up energy costs.
A relatively high VI may indicate that a lubricant can withstand extreme temperature changes — high and low — and provide consistent protection to system components. Refrigeration lubricants with a relatively high VI also tend to resist viscosity dilution more effectively.

Of course, you should not rely solely upon VI to evaluate the quality or appropriateness of a lubricant. Bear in mind, for example, that many low-VI lubricants can be specifically formulated to offer protection comparable to their high-VI counterparts. Knowledgeable experts who understand the full scope of lubricant properties can help you select a suitable lubricant for your specific needs.