The simple assumptions made in discussing fluid film lubrication are hardly ever valid in practice. A situation arises wherein there is intermittent contact between the surfaces, resulting in a significant rise in temperature and subsequent destruction of the contacting surfaces. Under these circumstances, the fluid film is no longer capable of adequately protecting the surfaces, and other approaches must be employed such as adding film-forming additives. Back to Top Lubricant Viscosity Viscosity is one of the most important properties of a lubricating oil. It is one factor responsible for the formation of lubricating films under both thick and thin film conditions.
Viscosity affects heat generation in bearings, cylinders and gears due to internal fluid friction.
It affects the sealing properties of oils and the rate of oil consumption. It determines the ease with which machines can be started at various temperatures, particularly cold temperatures. The satisfactory operation of any given piece of equipment depends on using an oil with the proper viscosity at the expected operating conditions. The basic concept of viscosity is shown in the figure, where a plate is being drawn at uniform speed over a film of oil. The oil adheres to both the moving and stationary surfaces.