Grease (lubricant) Definition
Grease (lubricant), as a lubricant, is a semi-solid substance composed of a base oil, a thickening agent, and various additives. Here is a more detailed definition:
Grease (lubricant) is a semi-solid or solid lubricant consisting of a base oil thickened to a semi-solid consistency by the addition of a thickening agent and enhanced with various additives to improve performance and protect against wear, corrosion, oxidation, and extreme pressure.
Components of Grease (lubricant)
- Base Oil :
– Typically mineral oil, synthetic oil, or a blend of both.
– The base oil provides the primary lubricating properties. - Thickening Agent :
– Common thickeners include metal soaps (such as lithium, calcium, sodium), complex soaps, and non-soap thickeners (such as polyurea, clay).
– The thickener gives grease its characteristic consistency and helps it stay in place during use. - Additives :
– Additives are included to enhance performance, such as antioxidants, anti-wear agents, corrosion inhibitors, extreme pressure additives, and tackiness agents.
Functions and Applications
- Lubrication :
– Grease (lubricant)reduces friction and wear between moving parts, preventing metal-to-metal contact. - Protection :
– Provides a protective barrier against contaminants such as dust, dirt, and water.
– Helps prevent corrosion and rust on metal surfaces. - Sealant :
– Grease (lubricant) can act as a sealant to prevent the ingress of contaminants into machinery. - Consistency :
– Stays in place, even under gravity, centrifugal forces, or when subjected to pressure and temperature variations.
Common Applications
- Automotive :
– Used in wheel bearings, chassis components, and universal joints. - Industrial Machinery :
– Applied to bearings, gears, and other moving parts in various types of machinery. - Household Items :
– Found in appliances, tools, and other equipment requiring lubrication. - Construction and Heavy Equipment :
– Used in the lubrication of heavy machinery and equipment operating under harsh conditions.
Advantages of Grease
- Long-Lasting :
– Remains effective for extended periods, reducing the need for frequent reapplication. - Water Resistance :
– Effective in environments where water contamination is a concern. - Versatility :
– Suitable for a wide range of applications and operating conditions.
Powdered solid Grease (lubricant)
Powdered solids may also be used as thickeners, especially as clays, which are used in some inexpensive, low performance greases. Fatty oil-based greases have also been prepared with other thickeners, such as tar, graphite, or mica, which also increase the durability of the grease.
Engineering assessment and analysis
Lithium-based greases are the most commonly used; sodium and lithium-based greases have higher melting point (dropping point) than calcium-based Grease oil (lubricant) but are not resistant to the action of water. Lithium-based grease has a dropping point at 190 to 220 °C (350 to 400 °F). However the maximum usable temperature for lithium-based grease is 120 °C. The amount of grease in a sample can be determined in a laboratory by extraction with a solvent followed by e.g. gravimetric determination.
Additives
Gear Grease oil consist of rosin oil, condensed with lime and stirred with mineral oil, with some percentage of water. Special-purpose greases contain glycerol and sorbitan esters. They are used, for example, in low-temperature conditions. Some greases are labeled “EP”, which indicates “extreme pressure”. Under high pressure or shock loading, normal grease can be compressed to the extent that the greased parts come into physical contact, causing friction and wear.
EP grease contains solid lubricants, usually graphite and/or molybdenum disulfide, to provide protection under heavy loadings. The solid lubricants bond to the surface of the metal, and prevent metal-to-metal contact and the resulting friction and wear when the lubricant film gets too thin.
Solid additives such as copper or ceramic powder are added to some greases for static high pressure and/or high temperature applications, or where corrosion could prevent dis-assembly of components later in their service life. These compounds are working as a release agent. Solid additives cannot be used in bearings because of tight tolerances. Solid additives will cause increased wear in bearings.
Grease (lubricant) General Materials
A true Lubricator consists of an oil and/or other fluid lubricant that is mixed with a thickener, typically a soap, to form a solid or semisolid. Greases are a type of shear-thinning or pseudo-plastic fluid, which means that the viscosity of the fluid is reduced under shear. After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base lubricant, such as the mineral oil. This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic. It is often applied using a grease gun, which applies it to the part being lubricated under pressure, forcing the solid grease into the spaces in the part.
Classification and standards
Jointly developed by ASTM International, the National Lubricating Grease Institute (NLGI) and SAE International, standard ASTM D4950“standard classification and specification for automotive service greases” was first published in 1989 by ASTM International. It categorizes greases suitable for the lubrication of chassis components and wheel bearings of vehicles, based on performance requirements, using codes adopted from the NLGI’s “chassis and wheel bearing service classification system”:
- LA and LB: chassis lubricants (suitability up to mild and severe duty respectively)
- GA, GB and GC: wheel-bearings (suitability up to mild, moderate and severe duty respectively)
A given performance category may include greases of different consistencies.The measure of the consistency of grease is commonly expressed by its NLGI consistency number.
The main elements of standard ATSM D4950 and NLGI’s consistency classification are reproduced and described in standard SAE J310“automotive lubricating greases” published by SAE International.
Standard ISO 6743-9 “lubricants, industrial oils and related products (class L) — classification — part 9: family X (greases)”, first released in 1987 by the International Organization for Standardization, establishes a detailed classification of greases used for the lubrication of equipment, components of machines, vehicles, etc. It assigns a single multi-part code to each grease based on its operational properties (including temperature range, effects of water, load, etc.) and its NLGI consistency number.\
The difference between Grease & oil
Grease and oil are both common lubricants used in bearings. The main difference between Lubricator
and oil is that grease consists of an oil and a thickener. The thickener acts like a sponge that retains the oil.
Grease can also contain various additives such as rust inhibitors, EP (extreme pressure) additives, oxidation
preventatives, etc. Typically greases will have a semi-solid to solid consistency. This consistency allows
grease to stay in place longer than oil. For many applications, the ability to stay in place and slowly release
oil gives grease an advantage over oil alone.
Oil and grease are two very familiar terms, but most people aren’t clear on exactly what the difference is… unless they happen to work in lubrication, that is! There are some situations where an oil is the better choice, and some when a grease works better. So, what is the difference between an oil and a grease? read below
- Greases are actually oils with thickener added.
- At room temperature, greases are usually solid, while oils are usually liquid.
- All oils can be turned into greases, but not all greases come from oils.
- Greases are typically only used on machinery, tools, or equipment, while oils have a multitude of other, non-industrial uses.
Food-grade Lubricator
Some greases need to be food safe, or food-grade (the terms mean the same thing). This means that if they accidentally end up in an items that is meant to be consumed by people or animals, they will not cause any harm, as long as they are found below a specific concentration. (Note: just because a grease is labeled ‘Food Safe’ doesn’t mean you can cook with it. You can’t!) Interflon makes several food-safe lubricants. Read more in this post called “What is a food-grade lubricant?”
The NLGI
Grease is an extremely important part of our modern, industrialized world… so much so that it even has its own institute, where people do nothing but think about grease all day long! It’s called the National Lubricating Grease Institute (NLGI).
The NLGI offers a scale for consistency. This rates greases based on their relative firmness, from 000 (fluid, like cooking oil) to 6 (very hard, like cheddar cheese). Again, your choice of grease is going to be determined by the type of application you’re using it for. Very fluid greases can be used in low-speed applications where there is no danger of leakage. Harder greases can be used in high-speed applications.
Choosing Right Grease Tips
When selecting a grease, you must consider the application and operating conditions in which the lubricant will function. For a better understanding of what goes into a good grease, concentrate on its components, including the base oil type, thickener type and base oil viscosity.
What are Grease Types?
Grease is classified into two main soap and non-soap groups, based on the type of thickener:
Soap based grease
This grease is divided to three groups as below:
Simple soap grease
This grease is made of base oil and a metal soap which is a product of a fatty acid and a metal hydroxide such as aluminum, sodium, calcium, lithium or barium reaction and neutralization.
Mixed soap grease
This grease is made of base oil and a mixture of two different metal soaps such as lithium and calcium soaps.
Complex soap grease
This grease was developed to improve the heat resistance of soap based greases. It is similar to regular soap grease except that its thickener is consisted of two dissimilar acids one fatty and one organic. This grease has higher dropping point and better resistance under high loads, temperatures and speeds. The most popular complex soap greases are aluminum, lithium, calcium, and barium complex greases.
Non-Soap based grease
This grease is divided to two groups as below:
Mineral based grease
This grease is made of base oil and a mineral thickener such as mineral clay like bentonite, modified mineral silicate or graphite.
Polymer based grease
Examples of this grease are poly urea grease and thermoplastic powders grease.
Types of Thickeners
The thickener defines the type of grease. There are three or four different types of materials that go into thickeners. The focus in this article is on organic thickeners such as lithium stearate, sodium dodecylsulfate and diurea. There are simple greases and complex greases, depending on the types of fatty acids used.
Lithium: Because lithium soaps are very efficient thickeners, lithium 12-hydroxystearate greases are the most prevalent. Lithium greases provide good lubricity and have great shear stability, thermal resistance and relatively low oil separation. Antioxidants are added to improve oxidative resistance (see Figure 2).
Calcium: These greases have better water resistance than lithium greases. They also have good shear stability. However, they have low-dropping points, do not have good operating temperature range and can only be used in operating conditions up to 110 C (230 F).
Sodium: These greases offer high-operating temperature, up to 175 C (347 F) but are confined to operating conditions no higher than 120 C (248 F) because of poor oxidative stability and high oil bleed. They also are not very water resistant. However, they do provide good lubricity and shear stability.
Aluminum: These have excellent oxidative resistance and good water resistance. But they have a low-dropping point of only 110-115 C (230-239 F). Their usage is generally limited to operating conditions less than 80 C (176 F). When these greases overheat in bearings, they cause sharp torque increases.
