THEORY AND GENERAL DISCUSSION:
Viscosity is one of the most important properties of a lubricating oil. Viscosity is a measure of the internal resistance to the motion of a fluid and is mainly due to the forces of cohesion between the fluid molecules. The formation of a fluid film of a lubricant between the moving friction surface and the generation of the frictional heat under the particular condition of load, bearing speed and lubricant supply is mainly depends upon the viscosity of the lubricant and to some extent on its oiliness. If the viscosity of the oil is too low, the fluid lubricant film cannot be maintained between the moving surfaces as a result of which excessive wear may take place. On the other hand, if the viscosity of the oil is too high, excessive friction due to the shearing of oil itself would result. Hence in hydrodynamic lubrication, the
lubricant should posses the proper viscosity. So, it is of vital importance to have knowledge of the viscosity of the lubricating oil. Viscosity of the fluid may be measured in several ways, one of which is determining time required for a definite amount of the liquid to flow through a capillary. Such method includes the use of Saybolt, Engler and Redwood Viscometers.
Viscosity may be expressed as dynamic (or absolute) viscosity, kinematic viscosity or the viscosity called after the name of the apparatus used for its determination.