High speed and extremely high temperature
High pressure and extreme load
When the moving or sliding surfaces are under very high pressure and speed, excessive frictional heat will be generated. The high local temperature thus produced at the surfaces, the lubricant fails to stick or get vaporized.
To meet these extra high-temperature high-temperature conditions, special additives called extreme pressure additive are added to mineral oils.
These additives form a more durable film on the metal surface which is capable of withstanding high temperature and having sufficient shearing strength to withstand the mechanical action of heavy loads.
The additives used in extreme pressure lubrication are of organic compounds containing certain active groups or radicals such as Cl (chlorinated esters), S (sulfur zed oils) or P form metallic chlorides, sulphides or phosphides, these surface metallic compounds possess low shearing strength and high melting point (E.g. Melting points of iron sulfide and iron chloride are 6500C and 11000C) hence they serve as good lubricants under high pressure and temperature conditions. This type of lubrication is called extreme pressure lubrication.
Applications: extreme pressure additives are used in lubricants in-
- Wire drawing (E.g. titanium can be drawn into wire only presence of a chlorine-containing additive which reacts with a stable oxide film of the metal surface )
- Cutting fluids (lubricant in machining of tough metals ). These lubricants consist of hydrocarbon oils and a small number of fatty acids and organic sulfide or chlorine additives.
- Hypoid gears, railway track joints and high-speed gears (as in cranes, aircraft, concrete mixture, tractor rollers etc).