General properties of a lubricating oil viscosity

· This indicates the resistance of a liquid to flow.

· There are several units for measuring viscosity. Formerly, the unit commonly used in America was Saybolt Universal Second (SSU), measured at 100°F or 210°F. In Europe, the former widely used unit was Redwood I second (RWI), measured at 100°F or 210°F. At present, most countries have switched over to the metric system that employs the unit Centistokes (cSt), measured at 40°C or 100°C.

· Oil with higher viscosity can stand greater pressure without being squeezed out of the lubricating surfaces. However, the high internal friction of the oil may offer greater resistance to the movement of the lubricating parts. An oil of lower viscosity offers less resistance to the moving parts but the oil can be easily squeezed out of the lubricating surfaces. It is therefore important to select a lubricating oil of appropriate viscosity to achieve optimum lubrication effect.

· Viscosity changes with temperature. Hence, the measuring temperature must be specified whenever the viscosity of a liquid is stated. When temperature rises, a liquid becomes less viscous. Similarly, a liquid becomes thicker when temperature drops.

· Viscosity Index (VI) is an indication of how the viscosity of a liquid varies with temperature. A high VI means the liquid does not thin out so much when temperature rises. VI improver additives that are usually high molecular weight polymers can increase the VI of lubricating oil.

· Increase in oil viscosity achieved by addition of polymers can be partially lost again through degradation of the polymer molecules by shear stress such as heavily loaded gears. Oil that can resist viscosity change due to shear are said to have high shear stability.