Viscosity and Viscosity Index

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Industrial lubricants, with its proven function and importance to machineries, has various physical properties that directly influences its effectiveness. Here are some of the most common physical properties that most lubricants possess:


Viscosity is resistance to flow, and it is the physical property of lubricants that enables the separation of two sliding or rolling surfaces, thereby reducing friction.

It is measured in terms of centistokes (cSt) at 40oC (or mm2/sec), wherein a low number indicates a low viscosity (high flow) material, and a high number denoting a high viscosity (low flow) material.  As examples, water would have a viscosity of 1 cSt at 40oC, while honey registers 1,200 c

The International Standards Organization (ISO) created viscosity grades (VG’s) for the purposes of simplicity and practicality, and below are the standard ISO viscosity grades:

Viscosity Index (VI)

All lubricants experience a decrease in viscosity as application temperatures become higher.  Conversely, lubricants increase in viscosity upon exposure to lower temperatures.

It is therefore important to ensure, especially for high temperature applications, or for situations with wide temperature fluctuations, that the lubricant retains a sufficient level of oil film thickness despite elevated temperatures.   A lubricant’s failure to provide a sufficient oil film can cause a boundary lubrication condition wherein there are high friction and contact between surfaces, producing excessive wear.

In such situation, the selection of a lubricant with a ‘higher’ Viscosity Index may be appropriate.

Viscosity Index is a rating, or an empirical score of a lubricant’s rate of decrease in viscosity upon exposure to a higher temperature.   A lesser rate of decrease in viscosity is generally preferred, as the remaining lubricating film contributes better to friction reduction.  Such a lubricant merits a high viscosity index.

Viscosity Index or VI is derived by taking the viscosity of a lubricant at 40oC and at 100oC

Flash Point, Fire-Point, and Auto-ignition Temperature

The Flash Point of a lubricant is the lowest temperature at which vapors produced by the lubricant due to temperature will ignite, BUT with the presence of an ignition source, such as a spark. At flashpoint, measured in Celsius (oC), combustion is sustained only with the continued presence of the ignition source.

Fire Point, on the other hand, is the lowest temperature at which vapors shall continue to combust, even if the ignition source is removed.   Fire point is always higher than Flash Point.

This is not to be confused with Auto-Ignition temperature, which is the temperature at which the lubricant vapors ignite spontaneously WITHOUT any ignition source.

Flash Point is mainly considered in lubricant selection in consideration for safety – meaning select a lubricant with a Flash Point that is lower than application temperature.


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