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Limitations of Extreme Pressure

Additives

Noria Corporation

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 sulfurphosphorus 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 oi l

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 extremepressure 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 oi ls 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.

itations of Extreme Pressure Additives http://www.machinerylubrication.com/Articles/Print/29031

1 5/9/2013 9:58 PM