Load carrying ability, EP and AW additives

Bearing life is shortened if the lubricant film thickness is not sufficient to prevent metal-to-metal contact of the asperities on the contact surfaces. One option to overcome this is to use so-called EP (Extreme Pressure) additives. High temperatures induced by local asperity contact, activate these additives promoting mild wear at the points of contact. The result is a smoother surface, lower contact stresses and an increase in service life.

Many modern EP additives are of the sulphur/phosphorus type. Unfortunately these additives may have a negative effect on the strength of the bearing steel matrix. If such additives are used then the chemical activity may not be restricted to the asperity contacts. If the operating temperature and contact stresses are too high, the additives may become chemically reactive even without asperity contact. This can promote corrosion/ diffusion mechanisms in the contacts and may lead to accelerated bearing failure, usually initiated by micro pitting. Therefore, SKF recommends the use of less reactive EP additives for operating temperatures above 80 �C. Lubricants with EP additives should not be used for bearings operating at temperatures higher than 100 �C. For very low speeds, solid lubricant additives such as graphite and molybdenum disulphide (MoS2) are sometimes included in the additive package 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 fatigue life.

AW (Anti-Wear) additives have a function similar to that of EP additives, i.e. to prevent severe metal-to-metal contact. Therefore EP and AW additives are very often not differentiated between. However, the way they work is different. The main difference is that an AW additive builds a protective layer that adheres to the surface. The asperities are then sliding over each other without metallic contact. The roughness is therefore not reduced by mild wear as in the case of EP additives. Here too special care has to be taken; AW additives may contain elements that, in the same way as the EP additives, can migrate into the bearing steel and weaken the structure.

Certain thickeners (e.g. calcium sulphonate complex) also provide an EP/AW effect without chemical activity and the resulting effect on bearing fatigue life. Therefore, the operating temperature limits for EP additives do not apply for these greases. If the lubricant film thickness is sufficient, SKF does not generally recommend the use of EP and AW additives. However there are circumstances where EP/AW additives may be useful. If excessive sliding between the rollers and raceways is expected they may be beneficial. Contact the SKF application engineering service for further information.