Load carrying ability
If the thickness of the hydrodynamic lubricant film is not sufficient to prevent metal-to-metal contact, bearing service life can be extended significantly by using a lubricant containing extreme pressure (EP) or anti-wear (AW) additives.
In applications where κ > 1 (→ Viscosity ratio κ), SKF does not generally recommend using EP or AW additives. However, there are circumstances where EP/AW additives may be beneficial such as if excessive sliding between the rolling elements and raceways is expected.
Extreme pressure additives
EP (extreme pressure) additives are an option to overcome metal-to-metal contact of the asperities on the contact surfaces. Elevated temperatures, induced by local asperity contact, activate these additives, which promote mild chemical wear at the points of contact. The result is a smoother surface, lower contact stresses and extended bearing service life.
Some modern EP additives contain sulphur-phosphorus, which may become chemically active even without asperity contact. In these cases, high operating temperatures and/or contact stresses become the catalyst. The resulting chemical activity may not be restricted to the asperity contacts and can have a detrimental effect on the strength of the bearing steel matrix. This can promote corrosion/diffusion mechanisms in the contacts and may lead to accelerated bearing damage, usually initiated by micro spalls.
Therefore, SKF recommends using EP additives that are less reactive in applications with operating temperatures > 80 °C (175 °F) and < 100 °C (210 °F). For very low speeds, solid lubricant additives such as graphite and molybdenum disulfide (MoS2) can be used to enhance the EP effect. These additives should have a high purity level and a very small particle size. Otherwise, dents due to overrolling of the particles might reduce bearing service life.
For additional information about EP additives, contact the SKF application engineering service.
AW (anti-wear) additives, like EP additives, prevent direct metal-to-metal contact. However, the way they work is very different. The main difference is that AW additives build a protective layer that adheres to the surface of the metal. The asperities can then slide over each other, avoiding direct contact. The surface finish is therefore not affected by mild wear as is the case with EP additives. AW additives, like EP additives, may contain elements that can weaken the steel structure close to the surface.
Certain thickeners (e.g. calcium sulphonate complex) also provide an EP/AW effect without chemical activity and the resulting effect on bearing fatigue life. The temperature limits for these thickeners are well within the temperature limits of most greases and oils.
For additional information about AW additives, contact the SKF application engineering service.