Definition
Extreme Pressure (EP) additives protect metal surfaces under severe contact conditions — high loads and sliding speeds that generate temperatures exceeding 200°C at the contact point, conditions typical of hypoid gears and worm gears.
EP additives function through tribochemical reactions: at elevated surface temperatures, they react with the metal to form inorganic salt layers (iron sulfide, iron chloride, iron phosphide) that shear more easily than the base metal, sacrificially preventing metal-to-metal welding and scoring.
Common EP additive chemistries:
- Sulfur-phosphorus compounds — most prevalent; used in API GL-4 and GL-5 gear oils
- Chlorinated paraffins — historically common in metalworking fluids; being phased out due to environmental concerns
- Borate esters — ashless, used in some specialized gear oils