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31 May 2012

What is viscosity

Viscosity is generally considered the most important property of a lubricant. What is it? And why should it matter to you?

Simply put, the viscosity of a lubricant (or any fluid) is a measurement of its resistance to flow. This example illustrates the concept: Suppose you have two glasses, one filled with water and the other with honey. If you were to tip the glasses over, the water would spill out of its glass faster than the honey. The honey has a higher resistance to flow, a higher viscosity, than water.

Simply put, the viscosity of a lubricant (or any fluid) is a measurement of its resistance to flow. This example illustrates the concept: Suppose you have two glasses, one filled with water and the other with honey. If you were to tip the glasses over, the water would spill out of its glass faster than the honey. The honey has a higher resistance to flow, a higher viscosity, than water.

Beware the highs and lows

Viscosity is important in lubrication because a lubricant’s viscosity must be:

  • low enough for the lubricant to flow to the parts that need it, providing ample protection in a timely manner, and
  • high enough for the lubricant to form a sufficient cushion between the surfaces it is supposed to prevent from contacting one another — a cushion substantial enough to withstand the loads and pressures of the application.

What happens if a lubricant’s viscosity is too low or too high?

  • If the viscosity is too low, the lubricant won’t provide a sufficient cushion between moving parts. This may lead to problems such as increased friction and wear, as well as increased heat and oxidation.
  • Too high a viscosity can cause problems as well. Inadequate flow of an overly viscous lubricant could lead to increased drag and friction, and in turn higher operating temperatures and energy consumption.

The bottom line: Improper lubricant viscosity — either too high or too low — may result in costly component damage and possibly equipment failure.

Consider the impact of temperature

A lubricant’s viscosity changes with temperature fluctuations. For this reason, lubricant viscosity readings, usually measured in centistokes (cSt), are generally accompanied by a temperature (most often 40°C and 100°C).

Because a lubricant may have to function across a broad range of temperatures, understanding the impact of temperature on viscosity is vital for ensuring consistently proper lubrication.

You need to know that:

  • The relationship between temperature and lubricant viscosity is inverse. Rising temperatures reduce viscosity, while falling temperatures cause viscosity to increase.
  • The sensitivity of viscosity to temperature fluctuations can vary widely from one lubricant to another. The viscosity of some lubricants will change significantly with modest temperature movements, while the viscosity of others will barely change even with wide swings in temperature.
  • A unitless measure known as the viscosity index (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).

Lubricant viscosity and refrigeration systems: a balancing act

Selecting a lubricant with the appropriate viscosity for a refrigeration application is particularly critical and challenging. Three key considerations:

  1. The lubricant must operate properly in both low-temperature and high-temperature conditions. It must providing balanced, consistent lubrication in all operating environments experienced by the system.
  2. The lubricant should be formulated so that its viscosity is not significantly diluted by the refrigerant. Depending on how a particular lubricant and refrigerant interact in the compressor, some refrigerant may solubilize in the lubricant and potentially reduce the lubricant’s viscosity. Too much viscosity dilution may decrease the protective, load-carrying capabilities of the lubricant.
  3. You cannot solve the dilution problem by choosing a fluid with a higher-than-necessary viscosity. The lubricant needs to have a low enough viscosity in cold temperatures so that as much lubricant as possible returns to the compressor. If the lubricant is too viscous, it may become trapped in the refrigeration system and cause a buildup of film in the evaporator. As the film accumulates, it can reduce the refrigeration system’s thermal efficiency, resulting in increased energy costs and potentially causing the system to shut down.

Selecting the right lubricant for a refrigeration system requires paying careful attention to viscosity as well as many other aspects that extend beyond the scope of this article. Seeking guidance from knowledgeable experts is recommended.

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