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Viscosity

Product News Thursday, August 27, 2009: Strub-SA cc

VISCOSITY Is the internal resistance to flow of a fluid substance. A fluid with a high viscosity has a higher resistance to flow than that of a low viscosity fluid. The viscosity of a fluid varies with temperature and pressure, the more pressure is applied the higher the viscosity will be, in terms of oil, this will increase the oils load carrying capability. Another factor that affects viscosity is operating speed, higher speed equals more pressure and therefore viscosity increases. Speed, and pressure raises temperature and with temperature increase an oil will get thinner, however a good oil will not thin out to the point where the fluid film on the moving parts is broken and wear takes place.

The Viscosity Index of an oil will give you an indication of how the oil behaves with changes in temperature. If the oil has a low VI (viscosity index) then that oil changes a lot with temperature, meaning it will be very thick at low temperatures and very thin at high temperatures. If the oil has a high VI then it is much more stable with changes in temperature, meaning that changes in temperature does not have a big effect on the oils viscosity. The Viscosity Index of an oil is a crucial factor to consider when selecting an oil.

There are numerous ways to express oil viscosity through SAE (Society of Automotive Engineers), SUS (Saybolt Universal Seconds), AGMA (American Gear Manufacturers Association), Centipose and Centistoke to mention a few. Further confusion is added because of temperature measurement in degrees Celsius and degrees Fahrenheit. Because there are so many ways to express oil viscosity by different Associations and Units an industry standard had to be found. The ISO (International Standards Organization) Viscosity Grade resolves much of the confusion and is generally accepted as the standard of viscosity grade measurement in the lubricating oil industry.