Extreme pressure: a class of additives, mostly sulfur-phosphorus compounds plus some borate and chlorinated types, that react with hot asperity tips to form a sacrificial iron sulfide or phosphate layer that shears instead of welding. They switch on at the high flash temperatures of severe sliding contacts such as hypoid and heavily loaded spur gears. EP gear oils carry the familiar sulfur smell, and their reactivity can attack bronze and other yellow metals when hot.
That's an EP gear oil, keep it out of the bronze worm drive or you'll eat the wheel.
Check the gearbox builder's list before putting a sulfur-phosphorus EP oil into anything with bronze worm wheels or internal backstops.
Also heard as
- extreme pressure
- EP additive
Do not mix it up with
- AW: Antiwear additives protect under moderate boundary contact; EP additives react only at the higher temperatures of severe, near-welding contact.
Related terms
AW
Antiwear: additives or oils that build protective films to cut wear under moderate boundary contact.
Four-ball EP test
Four-ball test that raises load in ten-second steps until the balls weld, rating extreme pressure capacity.
Scuffing
Sudden, severe adhesive damage where the oil film fails and sliding surfaces locally weld and tear.
Boundary lubrication
Regime where surfaces touch through the oil and protection comes from chemical surface films, not fluid pressure.
Additive package
Pre-blended mix of performance chemicals that a blender adds to base oil to make a finished lubricant.