Why is viscosity important?
Viscosity is extremely important when selecting gear oils as it significantly determines the formation of a lubricant film. By increasing the viscosity of the lubricating oil this creates a thicker lubricant film which in turn improves the anti-wear and dampening properties as well as scuffing load capacity.
As the temperature rises the viscosity reduces, as the load increases the viscosity also increases. Increased churning and squeezing losses can produce excessive heat, especially at high peripheral speeds. If the viscosity of the gear oil is too low then this can result in increased wear.
Temperature is a huge influencer of viscosity, when the temperature changes so does the viscosity. This is usually determined by means of the viscosity index (VI). The higher the viscosity index (VI) of a gear oil, the less the viscosity changes with temperature.
It is the base oil type that determines to what degree the viscosity changes. Different base oil types available are mineral oil, polyalphaolefin, ester and polglycol. The viscosity index (VI) improver additives contained in the lubricant can also affect how much the viscosity changes.
Viscosity indexes comparison
Mineral oil - Approx 85 - 100
Polyalphaolefin - Approx 130 - 160
Polyglycol - Approx 150 - 260