By Jim Fitch
Machinery Lubrication Magazine
Too often the viscosity index (VI) is disregarded as a lubricant selection parameter. One reason is simply because it is poorly understood. Some people think the viscosity index is incorporated in the ISO Viscosity Grade, but it is not. It stands alone as an independent lubricant performance differentiator.
We all know that viscosity is the single most important physical property of a lubricant. It is a crude measure of a lubricant’s molecular constitution from the standpoint of hydrocarbon chain size. Viscosity is determined by creating friction between the molecules from fluid movement. The higher the intermolecular friction (longer molecular chains), the higher the viscosity.
Viscosity determines film thickness and film strength in machines. It also influences other important factors such as those in the table below.