High-temperature Grease for Bearings: The modern machinery must work seamlessly under the specific operating conditions. During operation, friction is caused, which can increase corrosion and wear and reduce the lifetime of machines. Therefore, greases are used to lubricate the machines and reduce friction, corrosion and wear.
In the industry, there is a need for high-temperature-resistant lubricants, and for this, high-temperature grease is required. This grease is designed to provide effective lubrication at elevated temperatures and does not break down like other conventional greases. It has properties like excellent mechanical stability, thermal stability, high load-bearing capacity and corrosion resistance.
Constituents of grease
The main constituents of grease are base oil (mineral or synthetic), thickener and additives.
Base oil
The percentage of base oil is 85% – 90% of the total weight of grease. The base oil acts as the main lubricant and determines the performance in terms of temperature, speed and load-carrying capacity. Usually, mineral oil, synthetic oil, or a combination of both is used as a base oil. Mineral oils are preferred as they are affordable and suitable for a specific temperature range. However, they cannot be used for high-temperature applications.
Synthetic oils are preferred as they show enhanced stability at high temperatures, lower volatility, improved resistance to oxidation, and superior viscosity. They can fulfil the demands of high-temperature applications. Synthetic hydrocarbons, ester oil, fluorine oil and silicone oil outperform mineral oils in high-temperature applications.
The base oil used in high-temperature grease should have a high decomposition temperature, high boiling point, chemical stability and oxidation resistance.
Thickener
The percentage of thickener used is 10% – 15%, and it determines the stability, viscosity and longevity of the grease. A specific amount of thickener is added to get the required consistency of grease. It prevents the grease from leaking out of the bearings.
The thickener thickens the base oil and transforms it into a semi-solid state. The thickener forms a framework in the base oil, and then the lubricant is absorbed in the framework, which creates a paste-like grease. Lubrication depends on the absorbed base oil.
Thickeners can impact the properties of grease, like thermal stability, dropping point, rheology and mechanical shear stability. Two-types of thickeners are used: Soap-based and Non-soap-based. Soap thickeners are made of metal salts of high-grade fatty acids, which are also called metal soaps. However, they have a limited temperature tolerance.
Non-soap thickeners are divided into two categories: Organic thickeners and Inorganic thickeners
Organic thickeners refer to substances without solid hydrocarbons or metal soaps, which have thickening effects like PTFE, urea and phthalocyanine. They have outstanding high-temperature lubricating properties, and urea-based thickeners are used in ⅓ of the organic thickeners worldwide.
Inorganic thickeners and inorganic substances, which exhibit excellent high-temperature oxidation stability and high-temperature resistance. They are widely used in the manufacture of high-temperature greases. Some of the examples of inorganic thickeners are silica gel, bentonite, and boron nitride.
The thickeners used in high-temperature grease should enhance its friction-reducing properties and anti-wear capability.
Additives
The percentage of additives in greases is 2% – 5%, and they are used to improve the performance of the greases in conditions like extreme pressure, elevated temperatures, and heavy loads. Some of the additives are corrosion inhibitors, extreme pressure additives, anti-wear additives, oxidation inhibitors, and tacky additives.
The additives used in high-temperature greases should be compatible with the thickeners to avoid damaging the grease.
How should grease be used in bearings?
Grease is mainly used with 80% -90% of rolling bearings. For bearings operating at elevated temperature high-temperature grease for bearings has to be used. Grease helps to reduce friction and wear by preventing direct contact between the metallic parts. Grease forms a thin lubricating film between the two moving metallic parts. It helps to maintain the temperature by dissipating the heat of the bearing.
If the bearing is packed completely with grease, then after a few minutes of operating the bearings, the temperature inside the bearing will start to rise uncontrollably. The bearing will continue to operate until the lubricating property is lost, and the bearing will fail prematurely.
When the bearing is packed fully, the speed of the individual rolling elements will be 4 – 5 times higher than the shaft speed, which leads to grease churning. In this, the grease has to push through the excess grease, which results in overheating and degradation of the grease.
Always cover 30% – 40% of the grease volume, as it is sufficient to lubricate the bearing. It will work effectively until the re-lubrication interval starts. Bearings need optimised lubricant, and they should never be under- or over-lubricated.