Chlorine, potassium-borate and sulfur-phosphorus extreme pressure (EP) additives are primarily used for industrial gear lubricants. These additives are temperature-activated and react with metal asperities to form a sacrificial film. Unfortunately, these types of EP additives have some limitations, such as:
1. They can be detrimental to slow-speed gear applications (less than 10 feet per minute), causing high rates of wear known as “polishing.”
2. Extreme pressure additives like sulfur‑phosphorus types can be “too chemically reactive,” resulting in polishing wear. This type of wear is undesirable because it reduces gear accuracy by wearing away the tooth profiles. In these cases, potassium-borate additives can be used to deposit EP films without a chemical reaction with the metal.
3. The rate of reaction of EP additives is greatest where the gear tooth contact temperatures are highest; therefore, some difficulties are experienced in low-temperature applications when operating temperatures do not become high enough to fully activate the reactive EP agents. The viscosity of the base oil is of extreme importance. Anything that reduces the bulk oil temperature or the flash temperature will reduce the total contact temperature and increase the risk of wear. If the total contact temperature is not at the necessary level, the extreme pressure additive may not react correctly or at the right rate.
4. Solid lubricants such as molybdenum disulfide, graphite or tungsten disulfide are sometimes used when the operating temperatures are too high or low for an oil in which the reaction rate may not be sufficient; however, these solid films have limited wear lives and may not carry the loads necessary for long gear and bearing life.
5. Sulfur-phosphorus EP additives have a high-temperature limit of approximately 95 degrees C. This restricts the temperature range in which these oils can be used.
6. Sulfur-phosphorus EP additives are somewhat corrosive to yellow metals, particularly at temperatures higher than 60 degrees C. Worm gearsets frequently contain phosphor-bronze materials, and it is for this reason that gear oils using sulfur-phosphorus EP additives may not provide satisfactory service in worm gear drives.
7. Depending upon the amount used, sulfur-phosphorus EP additives may not be compatible with oils containing zinc anti-wear (AW) additives. This is why it is not recommended to mix AW gear oils with EP gear oils.
8. Chlorine and borate EP additives may not be fully effective or may cause corrosive conditions where water is present.