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Technically, the viscosity of oil is a measure of the oil’s resistance to shear, but is more commonly known as resistance to flow. A high viscosity implies a high resistance to flow while a low viscosity indicates a low resistance to flow. Viscosity varies inversely with temperature. Viscosity is also affected by pressure; higher pressure causes the viscosity to increase, and subsequently the load-carrying capacity of the oil also increases. This property enables use of thin oils to lubricate heavy machinery. The load carrying capacity also increases as operating speed of the lubricated machinery is increased.
Viscosity may be considered the most important physical property of a lubricant because it determines the lubricant load carrying ability as well as its ability to flow through a lubricant system.
Viscosity as determined with the Spectro-Visc is one of the most important analytical tests in a condition monitoring program because it provides early warning of lubricant degradation or contamination. A viscosity increase could be due to solid contamination, improper top-off oil grade, coolant/water, or oxidation. On the other hand, a viscosity decrease could be due to fuel contamination, improper top-off oil grade, or additive degradation (shear).
Viscosity Index - The viscosity index, commonly designated VI, is an arbitrary numbering scale that indicates the changes in oil viscosity with changes in temperature. Viscosity index can be classified as follows:
- low VI - below 35
- medium VI - 35 to 80
- high VI - 80 to 110
- very high VI - above 110.
A high viscosity index indicates small oil viscosity changes with temperature. A low viscosity index indicates high viscosity changes with temperature. Therefore, a fluid that has a high viscosity index can be expected to undergo very little change in viscosity with temperature extremes and is considered to have a stable viscosity. A fluid with a low viscosity index can be expected to undergo a significant change in viscosity as the temperature fluctuates.
Applicable Spectro Products: