Grease Applications, Properties and Types of Grease

Grease is a semi-liquid to solid mixture of liquid lubricant, thickener and additives. The liquid lubricant that performs the actual lubrication can be petroleum (mineral) oil, synthetic oil or vegetable oil. The thickener gives grease its characteristic consistency and is sometimes thought of as the "three-dimensional fibrous network" or "sponge" that holds the grease in place. Common thickeners are soaps and organic or inorganic non-soap thickeners. The majority of greases on the market consist of mineral oils mixed with soap thickener. Additives improve performance and protect grease and lubricated surfaces. Grease is defined as a temperature-regulated feeding device: When the lubricant film between wear surfaces is thinned, the resulting heat softens the adjacent grease, which expands and releases the oil to restore film thickness.

Function

The function of grease is to remain in contact with and lubricate moving surfaces without leaking under gravity or centrifugal action or compressing under pressure. Its main practical requirement is that it retains its properties under shear at all temperatures to which it is exposed during use. At the same time, grease must be able to flow into the bearing and from point to point in the lubricated machine via grease guns when necessary, but must not contribute significantly to the power required to start the machine, especially during start-up.

A. Applications suitable for grease

Grease and oil are not interchangeable. Grease is used when it is not practical or convenient to use oil. Lubricant selection for a particular application is determined by matching machine design and operating conditions with the desired lubricant properties. Grease is commonly used for:

• Machinery that operate intermittently or remain in storage for long periods of time. As the grease remains in place, a lubricant film can form instantly.
• Machinery that is not easily accessible due to frequent lubrication. High-quality greases can lubricate isolated or relatively inaccessible components for long periods of time without the need for frequent replenishing. These greases are also used in life-sealing applications such as some electric motors and gearboxes.
• Machinery operating under extreme conditions such as high temperature and pressure, shock loads or slow speed under heavy load. Under these conditions, grease provides the thicker film cushions required for protection and adequate lubrication, whereas oil films may become too thin and break.
• Worn components. Grease retains thicker films in gaps enlarged by wear and can extend the life of worn parts previously lubricated with oil. Thicker grease films also provide noise insulation.

B. Functional properties of grease

• Acts as a sealant to minimize leakage and keep contaminants out. Due to its consistency, grease acts as a sealant that prevents lubricant leakage and also prevents the entry of corrosive contaminants and foreign materials. It also acts to keep broken seals effective (whereas oil leaks easily).
• It is easier to preserve than oil. Oil lubrication may require expensive circulation equipment and a complex system of retention devices. By comparison, grease, due to its hardness, is easily confined to simplified, less costly retention devices.
• Holds solid lubricants in suspension. Finely ground solid lubricants such as molybdenum disulfide (moly) and graphite are mixed with grease in high temperature service (above 315 °C) or extremely high pressure applications. While grease holds the solids in suspension, the solids precipitate in the oil.
• It is not necessary to control and monitor the liquid level.

C. Notable disadvantages of grease

• Poor cooling. Because of its consistency, grease cannot dissipate heat through convection like a circulating oil.
• Resistance to movement. Grease has greater resistance to movement during start-up than oil, so it is not suitable for low-torque/high-speed operation.
• It is more difficult to dispense, drain and refill than using oil. Additionally, exact amounts of lubricant cannot be measured so easily.

Grease Characteristics

The features listed below are the features that should be taken into consideration in grease applications.

• Apparent viscosity
• Bleeding, migration, syneresis
• Consistency, penetration
• Contaminants
• Resistance to corrosion and rust
• Dropping point
• Evaporation
• Fretting wear and False Brinelling
• Oxidation stability
• Pumpability and feedability
• Shear stability
• Water resistance
• High temperature effects
• Low temperature effects
• Texture

Fluid Lubricants
Fluid lubricants used to formulate grease are normally petroleum or synthetic oils. In general, petroleum oils; Naphthenic oils tend to mix better chemically with soaps and additives and form stronger structures than paraffinic oils. Synthetic oils have a higher initial cost but are effective in high temperature and extremely low temperature conditions. With increasing environmental concerns, vegetable oils and some synthetic oils are also used in applications requiring non-toxic or biodegradable grease. The base oil chosen to formulate the grease must have the same properties as when lubricating the equipment with oil. For example, lower viscosity base oils are used for grease applications at lower temperatures or high speeds and light loads, while higher viscosity base oils are used for higher temperatures or low speed and heavy load applications.

Soap Thickeners

The soap thickener dispersed in the base liquid gives the grease its physical character. Soap thickeners not only provide the consistency of the grease, but also affect desirable properties such as water and heat resistance and pumpability. They can affect the amount of additives, such as rust inhibitors, required to achieve the desired quality. Soap affects how grease flows, changes shape, and wears when mechanically operated and in extreme temperatures. Each type of soap adds its own characteristic properties to the grease. The soap concentration can be varied to obtain different grease thicknesses. Additionally, the viscosity of the base oil also affects the thickness. Since the qualities of soap are also determined by the fatty acid from which the soap is prepared, not all greases made from soaps containing the same metals are the same. The name of the soap thickener refers to the metal (calcium, lithium, etc.) from which the soap is prepared.

Complex Soap

A. The high temperatures produced by modern equipment have required increasing the heat resistance of ordinary soap-thickened greases. As a result, “complex” soap greases were developed. The dropping point of a complex grease is at least 38 °C higher than its normal soap-thickened counterpart, and its maximum usable temperature is around 177 °C. Complex soap greases are limited to this temperature because mineral oil can flash, evaporate or burn above this temperature. In general, complex greases have good all-round properties and can be used in multi-purpose applications. Complex greases for extreme operating conditions are often produced with solid lubricants and more highly refined or synthetic oils are used.

B. A "complexing agent" made from a salt of said metal is an additional ingredient in forming a complex grease. A complex soap is formed when a fatty acid and alkali react to form a soap, and the alkali simultaneously reacts with a short-chain organic or inorganic acid to form a metallic salt (complexing agent). Basically, a complex grease is obtained when a complex soap is formed in the presence of a base oil. Common organic acids are acetic or lactic, while common inorganic acids are carbonates or chlorides.

Additives

There are applications where surface-protecting and performance-enhancing additives are used that can effectively increase the overall performance of a grease. Solid lubricants such as molybdenum disulfide and graphite are added to grease in certain applications at high temperatures (above 315 °C) and extremely high pressure applications. Adding solid additives requires frequent grease changes to prevent solids buildup (and the resultant wear) in components. Dyes are also available that improve the appearance of grease and are used for identification purposes.

Types of Grease

The most common greases are the following types:

A. Calcium grease

• Calcium (lime) grease

• Calcium complex grease

B. Sodium grease

• Sodium grease

• Sodium complex grease

C. Aluminum grease

• Aluminum grease

• Aluminum complex grease

D. Lithium grease

• Lithium grease

• Lithium complex grease

E. Other greases

• Polyurea grease

a) Polyurea grease

b) Polyurea complex grease

• Organo-clay grease

Compatibility

A. Greases are considered incompatible when the physical or performance properties of the blended grease fall below the original specifications. In general, greases with different chemical composition should not be mixed. Mixing greases with different thickeners can create a mixture that is too hard to provide adequate lubrication or, more commonly, a mixture that is too soft to stay in place.

B. Combining greases consisting of different base oils may produce a liquid component that does not provide a continuous lubricating film. When greases containing different additives are mixed, the additives may be diluted. Blended greases may become less resistant to heat or have lower shear stability. If a new brand of grease must be used, the component part should be disassembled and thoroughly cleaned to completely remove the old grease. If this is not practical, new grease should be injected until all traces of the previous product have been removed. Additionally, initial grease changes should be made more frequently than normally planned.