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In order to comprehend how temperatures and other operating conditions affect lubrication, we must first understand how lubricants behave in combating friction and wear.
During initial start-up or under heavily loaded conditions such as in gearsets, the metal surfaces in a lubricated system may actually come into contact with each other. To eliminate (or reduce) frictional wear during this lubrication condition, lubricants must have the correct viscosity as well as the appropriate anti-wear or extreme-pressure additive chemistry.
In boundary conditions, the chemical properties of the additive package will be just as important as the physical properties of the oil, particularly viscosity, but both will be affected by operating conditions such as temperature.
This lubrication condition occurs when a film of oil supports a rotating component, such as a shaft, and thus prevents the shaft from contacting the supporting bearing. This condition takes place after a machine has begun to rotate and the speeds and loads are such that a wedge of oil has actually moved between the shaft and bearing surfaces, lifting the shaft away from the bearing surface. Typical applications would be the crankshaft and journal bearing in an automobile engine or a turbine.
In order for hydrodynamic lubrication to be effective, the oil's viscosity must be such that the hydrodynamic condition will be maintained under every operating condition, such as high speed and high load, low speed and high load, low speed and low load, etc.
In contrast to boundary conditions, hydrodynamic lubrication requires that the physical properties of the oil, particularly its viscosity, be maintained under all conditions. If the operating conditions cause the viscosity to be reduced too much, metal-to-metal contact between the high spots or asperities could occur. If the oil's viscosity is too high (thick), the internal resistance to the shearing of the oil's molecules will reduce operating efficiency and temperatures will increase dramatically.
In equipment that has machine elements with concentrated areas of extremely high-pressure contacts, such as in roller and ball bearing applications, a lubrication condition called elastohydrodynamic lubrication (EHL) is very common. EHL conditions occur when a typical oil enters the contact zone between the moving elements. For example, as oil enters the contact zone between a ball and raceway, the oil's pressure rises sharply. This pressure in turn increases the oil's viscosity, which further increases pressure. The pressure becomes sufficient to cause the oil to become a pseudo-solid. This will slightly deform the contact zones of the elements, elastically affecting the rolling element's contact areas.
At the same time, assuming the oil's physical characteristics such as viscosity and viscosity index are correct for the application, the oil film thickness will prevent surface high points or asperities from breaking the oil film as the rolling elements rotate through the contact zone. Because an oil's viscosity is directly affected by temperature, it is obvious that incorrect or abnormal operating temperatures will interfere with the formation of the elastohydrodynamic lubricating film.
The formation of EHL films in bearings is always accompanied by an increase in temperature. If the temperature increase is sufficient to significantly reduce the oil's viscosity, severe damage can result.