EP and AW additives, and why lithium grease is losing share
Why gear oil says EP and hydraulic oil says AW, and why calcium sulfonate complex is replacing lithium in wet, hot and heavy duty.
AW is not the same as EP
Hydraulic oil usually says AW on the label (AW 32, AW 46) and gear oil says EP. That does not mean two separate systems for two separate products. Gear oil needs AW just as much as EP, it is just not what is printed on the label.
What AW additives do
Under normal conditions viscosity does the separating, and that is the base oil's job. Where the film thins enough that high points start touching (boundary lubrication), anti-wear additives, most commonly ZDDP, react at the contact points and form a thin, self-renewing film that viscosity alone cannot provide. Hydraulic pistons, vanes and spool valves run in near-pure sliding contact, so hydraulic fluids rely on AW as their main protection.
What EP additives do
At a gear's pitch line the contact is almost pure rolling, while towards the root and tip sliding takes over and contact stress climbs fast. Gears need both: AW backs up the rolling-dominant part whenever the film runs thin, and EP handles the split-second, high-sliding moments when the contact flashes to a very high temperature and reacts with the metal, forming a sacrificial coating that keeps the surfaces from welding.
EP is not just a backup
Some gearboxes run an aggressive mix as a matter of course, such as a crusher drive or a conveyor gearbox under constant shock loading. There, EP is the primary job from the first minute. That is why the equipment nameplate and the OEM spec matter more than habit: industrial gearbox specs call out the viscosity grade and whether EP is needed, usually referring to AGMA standards.
Why lithium grease is declining
Lithium and lithium-complex have long been the thickeners of choice. They offer good mechanical stability, good water resistance and a suitable temperature range, and they are easy to find. But the NLGI annual grease production survey shows a decline: a combined share of about 70 percent earlier in the decade, 62 percent in the 2022 report and about 57 percent in the 2024 report.
What is replacing it
- Calcium sulfonate complex is taking its place. Because the thickener supports load, it has inherent EP capability, so a lighter additive package can do the job.
- Water resistance is better. It shows strong performance in the ASTM D1264 water washout test.
- Dropping points are higher: roughly 355 to 390 degrees F (about 180 to 200 degrees C) for a standard lithium grease, against more than 500 degrees F (260 degrees C) for lithium-complex and calcium sulfonate complex greases.
- In wet, hot or heavy-load service this can make a big difference.
Choose by the application
Lithium grease is still excellent and right for many applications. Just make sure you chose it because the application calls for it and not because it is what everyone else uses.