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:
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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.
A wide range of lubricants are in play today to serve an eclectic mix of markets from food processing to renewable energy. Industry leaders like Paratherm, Q8 Oils and Fuchs design different products to suit specific applications from heat transfer oils to cleverly engineered cutting fluids. Two of the most well-known formats for lubricants are grease and oil.
The difference between grease and oil is directly related to viscosity. While oil is a liquid allowing it to flow through systems lubricating, grease is a semi-solid product with a higher viscosity allowing it to remain in place making it suitable for longer-term lubrication.
In the following sections, we’ll lift the lid on these two useful lubricant types to identify the advantages and disadvantages that make them suitable or unsuitable for an application.
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A thick, semi-solid solution, grease lubricates reduce friction and unwanted wear between moving components. It is composed of three main ingredients. A base oil, a thickening agent and additives. Base oils are typically a petroleum-based product or synthetic, while the thickener (often bentonite clay or lithium soap) holds the base oil in its place. Additives included tailor the grease to meet conditions helping them to perform at extreme pressure or resist higher temperatures.
Grease is employed in many applications, like forklifts and bearings as it can cope with high pressures and temperatures and sticks to surfaces. Under pressure, greases release oil that lubricates components then reabsorbs the fluid when the pressure lets up.
The consistency of grease lets it remain in place and effectively seal parts, protecting them from corrosive forces and heavy loads. Grease is a popular choice to defend metal surfaces from iron oxide (rust). A convenient option, grease can stay in bearings longer than oils can. Due to its ability to persists, grease requires less frequent reapplications than oils which reduces maintenance.
Grease is a powerful sealant and can protect components from a diverse selection of contaminants. For example, it can seal bearing from dirt, dust and other debris but is also resistant to water. As a result, it can stop leakage occurring in mechanical systems.
As it requires fewer reapplications, hard to access components are often lubricated with grease. It’s ability to stay in place also makes it’s a go-to solution for keeping equipment that does not see frequent use lubricated.
Grease does have some drawbacks, however. Unlike oils grease cannot flow through complex mechanical systems and its can’t be filtered. While oil can transfer heat away from parts of systems in need of cooling and warm other areas, grease is not an effective heart conductor. Grease can also cause problems of overheating in systems when it is applied to heavily.
While grease can seal parts against water, it has poor water separability unlike its less viscous counterpart, oil.
Lubricating oil is a fluid formulated with various viscosities to suit different applications. Like grease its key role is to reduces friction and wear between parts in motion. It is formulated with a base oils and additives, but unlike grease contains no thickening agent. It offers many benefits that have seen it become a vital component in an extensive array of mechanical operations.
Oil can transfer heat effectively which can stop areas where key processes are conducted from overheating, for example inside an internal combustion engine (ICE). Its fluid form allows it carry unwanted contaminants from the system cleaning parts and taking dirt and debris to a filter where it can be extracted.
The composition of oil allows it to be refined which means different levels of purity can be achieved and that oils can be adapted for a variety of applications from creating fuel to feedstocks for chemicals. Oil also supplies higher running speeds and excellent water separability helping the environment and companies to save money.
While oil has many useful attributes, there are some situations where grease is a better choice. Compared to grease which adheres to parts, oil may require continuous reapplication to lubricate parts effectively. As mentioned earlier, grease tends to see application when a mechanism is difficult to lubricate frequently because of the way it remains in place for longer. In a stop-start situation, grease stay on the component where it is required for instance, while oil will find its way back to the crankcase.
Before selecting an oil or grease for an application, always assess the lubricating needs of your equipment and operating conditions.
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