Tribology is the science of lubrication, and lubricants are produced for all purposes where friction needs to be overcome.
Due to the wide variety of circumstances in which friction is produced - at high or low temperatures and pressures, in the presence of water or corrosive acids, in a total vacuum or a radioactive environment - the oil manufacturer must be an expert in the science of lubricant formulation. All common lubricants are made up from one or more base oils, whether from crude oil or synthetic, plus a mixture of additives (chemicals), providing the specific qualities that the application requires.
Info Centre >> Fuel and Lube Oil Technical Manual >> 5. Lube Oil - Characteristics >> 5.1 Tribology
Lube Oil Viscosity
The most important parameter of the lubricant is the viscosity. Viscosity is a measure of its thickness or ability to flow.
The viscosity of an oil controls the thickness of the oil film under hydrodynamic lubrication conditions. Oils become thinner when heated and viscosity must always be related to temperature. This relationship between an oil viscosity and how it changes with temperature is called the Viscosity Index (VI). The higher the VI, the lower the change in viscosity with temperature. A typical VI range for straight mineral oils is 95-105. Sometimes the VI can improve by adding polymers to the oil: the most common application of these is multigrade oils used in automotive engine applications and hydraulic systems.
Surface Lubrication
To understand how the lubricant works, it is first necessary to consider the nature of the two surfaces to be lubricated.
Whenever a surface is machined there will be tiny irregularities in the surface. The nature of these irregularities will vary depending upon the machining process e.g. rolling, turning, grinding, milling or plateau honing, but the net effect is the same. Under microscopic examination, the surface is anything but smooth. When two such surfaces are forced to slide over each other, opposing high spots will contact, resisting any sliding action. The contact invariably alters the surface of the mating parts due to distortion, scuffing, microwelding and subsequent tearing. An engine or any machine operated under such conditions would not last long.
When a lubricant is spread copiously over the surfaces, it fills the depressions, resulting in a continuous film. Sliding is much easier as the lubricant assists the surfaces to move over each other. This condition is called boundary lubrication and is typical of the lubricant regime existing in slowly-moving bearings or grease lubricated slide ways.
If the surfaces move relatively quickly, and again sufficient oil is present, then the oil is dragged between the surfaces, filling the space between. One surface is forced to 'fly' clear of the other due to the upward lift generated by pressure within the oil film. If sufficient lift is generated then the surfaces are separated enough to ensure that no metallic contact occurs and the machine life becomes indefinite assuming the machine never stops. This condition is called hydrodynamic lubrication and is the type of lubrication that the engine designer seeks to ensure as it produces the least wear and friction losses.
If all relative motion between surfaces ceases, then the oil is slowly squeezed out from between the surfaces and boundary lubrication regimes are again established. Friction will increase as will wear. It is for this reason that the amount of wear occurring in bearings is far higher during an engine start-up before hydrodynamic lubrication conditions are properly established, than when the engine is operated at design speeds and correct oil pressures are established.