The basic properties required of lubricating oil is, that oil protects the
sliding surfaces of machine components against wear.
Conditions in between sliding surfaces can be classified according to the degree of
interaction between the surfaces. It is usually desirable to separate surfaces completely
by a film of lubricant since wear is eliminated and the friction comes solely from
shearing the fluid.
Some oils can be deficient in this respect. The ability of a
lubricant to achieve full separation depends on its viscosity, the greater the viscosity
the greater the separation effect.
Complete fluid film separation of surfaces also depends on the loads and speeds involved
and on the type of component. Because of the clinging effect of APL, efficient upper
cylinder lubrication can be achieved. Added to this benefit, the reduction in " blow
by", and a decrease in oil contamination in the sump by combustion chamber gases.
At relatively high loads between surfaces complete separation cannot be achieved and
there is some degree of contact. All machined surfaces are to some extent rough such that
"hills" and "valleys" occur on a microscopic scale. When contact
occurs, then the surfaces interact, causing friction, heat, cold welding, and destruction
of bearing surfaces.
The function of the lubricant now changes to protecting the surfaces against seizure and
tearing which would occur with dry metals by a form of 'cold welding'. This process of
protection depends on active molecules in the lubricant, which attach themselves to the
surfaces, known as boundary lubrication.
As the load and speed are increased then eventually the boundary lubricating film breaks
down and intensive metallic contact occurs.
EP (extreme pressure) Lubricants, (gear oil, bearing grease), are used as lubricants for
severe duties, such as hypoid gears, to give a good degree of protection up to very high
Adding APL enhances
the efficiency of EP Lubricants. Indeed, a temperature reduction is effected
which is of added benefit since the task of the EP Lubricants is consequently reduced with