Automotive Greases: Types, Properties, and Applications
Automotive Greases: Applications
Greases are intimate mixtures of a soap solution in mineral oil, suitable for various types of machinery lubrication. The soap component holds the lubricant within the friction surfaces. Soap bases are obtained from fats through saponification with caustic soda, lime, alkali metals, and water.
Types of Greases
Greases are generally classified by the soap compound used in their manufacture, which influences their lubricating properties. Common grease types include:
- Fat-based calcium soap
- Fat-based sodium soap
- Fat-based bentonite
- Barium-based grease
- Lithium-based grease
- Silicone-based grease
- Lead-based grease
- Graphited grease
- Molybdenum disulfide grease
Fat-Based Calcium Soap Grease
Calcium soap base greases have a buttery soft appearance and are highly water-resistant, making them suitable for wet environments. However, they cannot withstand high temperatures (above 90°C) as they tend to disintegrate, and the oil separates from the soap base, potentially damaging moving parts.
Soap-Based Sodium Grease
Sodium soap greases can be used at high temperatures (100°C to 200°C) without oil separation. They are fibrous and can withstand heavy loads, making them ideal for mills, ball bearings, and roller bearings. However, they are water-soluble and unsuitable for moist environments.
Special Greases for High Temperatures
Special high-temperature greases are designed for use above 170°C. These include:
Fat Bentonite Grease
Bentonite greases, made from a special clay from Missouri (USA), have a high liquefaction point. They are suitable where sodium-based greases fail and are resistant to water and high temperatures. They are also ideal for difficult-to-access locations due to their long lubrication intervals.
Barium-Based and Lithium-Based Greases
Barium-based greases are water-resistant up to 200°C. Lithium-based greases perform well from -180°C to 190°C and are suitable for inaccessible lubrication points due to their long service life.
Fat Silicone Grease
Synthetic silicone greases offer long life, good stability, and high-temperature resistance but are expensive.
Extreme Pressure (EP) Greases
EP greases maintain a lubricant film under extreme conditions. They contain lead soap or other EP additives and are versatile due to advancements in industrial applications.
Manufacturers often combine soaps to create various greases for specific needs. Some examples include:
- Calcium greases: Low-speed bearings below 50°C.
- Calcium resin greases: For resin and chassis components.
- Sodium greases: For water contact at 100°C to 200°C.
- Calcium-sodium greases: For bearings.
- Silicone greases: High temperatures.
- Lithium greases: General use, air systems, 18°C to 120°C.
- Aluminum greases: Chains, gears, lifting equipment, some bearings.
- Barium greases: Water-resistant applications up to 200°C.
- Bentonite greases: Water-resistant, very high temperatures.
- Resin greases: As adhesives.
- Graphite greases: High temperatures, low-revolution applications.
- Molybdenum disulfide greases: High temperatures and pressures up to 450°C.
Physical Characteristics of Greases
Greases are selected based on their physical properties, similar to oils. Key characteristics include:
- Type of soap and its proportion
- Stability
- Consistency
- Reversibility
- Purity
- Dropping Point
- Adhesion
- Solubility in water
- Appearance
Soap Nature
Soaps play a crucial role in grease properties. The following table summarizes the behavior of individual soaps (table omitted as it was not provided in the original text).
Stability
Grease stability refers to its resistance to oil-soap separation under temperature, velocity, and pressure during operation or prolonged storage. Standards for stability have been established by experienced organizations. Instability is indicated by oil separation from the grease mass.
Consistency
Grease consistency depends on its composition, proportions, and manufacturing process. It must be appropriate for the operating temperature and application.
Reversibility
Reversibility is the ability of a grease to return to its original structure after oil-soap separation due to temperature elevation. This is crucial for bearing lubrication.
Purity
Greases should be free of ashes and acids.
Melting Point and Dropping Point
The dropping point is the temperature at which a grease ceases to be semi-solid and separates into oil and soap. It is measured by heating the grease in a tube until a drop forms. The grease’s maximum operating temperature should be below its dropping point.
Adhesion
Grease adhesion is primarily determined by the soap type. It is particularly important in high-revolution air systems, where it provides anti-centrifugal properties.
Solubility in Water
Water resistance is crucial for greases exposed to water contamination. The lubricant must be insoluble to prevent washout. While some soaps are soluble, others are highly insoluble. Other components also influence water resistance.