Greases are indispensable in certain applications where fluid lubricants cannot be contained effectively. Greases offer several advantages over oils in specific contexts.
Grease seals out contaminants is well-suited for incorporating insoluble solid additives like molybdenum disulfide and graphite and excels in stop-start performance because it doesn’t drain away like oil. Most dynamic machine operations rely on grease-lubricated bearings, making grease crucial for machinery operation.
Grease is essentially a semi-solid lubricant composed of base oil, additives, and a thickener. In simple terms, grease is motor oil enhanced with a thickening agent to achieve a semi-solid form. The thickener, often referred to as “the sponge that holds the lubricant,” is added to keep the lubricant in place where fluid lubricants would otherwise run off. The thickener constitutes 10 to 30 percent of the grease’s content.
Thickeners can be either simple or complex soaps. Simple soap thickeners, made of long fibers, have a smooth, buttery texture and include substances like lithium, polyurea, calcium, and silica. Complex soap thickeners, composed of both short and long fibers, have a more fibrous texture and include aluminum, sodium, and barium.
Lithium-based greases dominate the global market, accounting for over 75 percent of usage. These greases are versatile, suitable for both chassis and wheel bearing applications. Lithium-complex and calcium-sulfonate greases are top choices for high-performance multi-purpose greases due to their compatibility with widely used lithium greases.
Lithium-complex greases are known for their stability, high-temperature performance, and water resistance. Their performance in areas like extreme pressure, anti-wear, rust, and corrosion can be enhanced with additives. However, calcium-sulfonate greases often outperform lithium-complex greases both in lab tests and real-world applications. Unlike lithium-complex greases, calcium-sulfonate greases usually do not require additives to meet performance standards.
Calcium-sulfonate greases offer superior mechanical and shear stability, reducing leakage and run-out during operation. They can withstand higher temperatures, have inherent extreme-pressure and anti-wear properties, act as natural rust inhibitors, and provide excellent water resistance.
Greases, being non-fluid, are characterized by consistency rather than viscosity (though the viscosity of the base oil used in grease formulation is often reported). Consistency is measured using the Cone Penetration Test, which determines how far a cone of specified dimensions and mass penetrates into a standard amount of grease at 25°C within 5 seconds under gravity. This penetration is measured in tenths of a millimeter, providing a precise assessment of grease consistency.