Grease is a semisolid lubricant. It generally consists of a soap emulsified with mineral or vegetable oil. The characteristic feature of greases is that they possess a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. This change in viscosity is called shear thinning. Grease is sometimes used to describe lubricating materials that are simply soft solids or high viscosity liquids, but these materials do not exhibit the shear-thinning properties characteristic of the classical grease. For example, petroleum jellies such as Vaseline are not generally classified as greases.
Greases are applied to mechanisms that can only be lubricated infrequently and where a lubricating oil would not stay in position. They also act as sealants to prevent ingress of water and incompressible materials. Grease-lubricated bearings have greater frictional characteristics due to their high viscosity.
Powdered solid greases
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 greases 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.
Gear greases 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 General Materials
A true grease 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 grease
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.
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?”
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.
For more information on the NLGI, visit their website. To learn more about consistency numbers, see this Wikipedia page.
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.
1. Base Oil Viscosity
Base oil viscosity is one of the most important properties of any lubricant and is often overlooked and misunderstood when it comes to grease. The base oil viscosity for bearings can be calculated using a formula that takes the speed and dimensions of the bearing into consideration. It is critical to understand a grease’s base oil viscosity in order to choose the right grease for the application and conditions. Typically, a general-purpose grease will have a viscosity of ISO 460 or higher.
2. Base Oil Type
The base oil type is determined by the conditions in which the grease will perform, such as high and/or low temperatures, load and pressure. The load and pressure not only will influence the base oil type but also the additive package (extreme pressure, anti-wear, etc.). While synthetic oils will be better over a wider range of temperatures and conditions, mineral base oils can be used if you have a more constant running temperature.
3. Thickener Type
Most people believe the thickener is the lubricating part of the grease, but it actually is what holds the lubricant in place, like a sponge. A good thickener type will be compatible with the equipment manufacturer recommendations and will be able to withstand the conditions under which it must perform. There are many types of thickeners, but most are not compatible and should not be mixed. The most common thickeners are lithium, lithium complex, aluminum complex and polyurea.
4. Performance Properties
This is where the grease quality factors in. It’s performance properties include many of the same properties used for lubricating oils, as well as others exclusive to grease. Properties exclusive to grease include dropping point, mechanical stability, water washout, bleed characteristics and pumpability. The most important performance properties are determined by the application. If an application operates continuously at room temperature, properties like dropping and upper operating temperature limits are not as important. If an application operates under heavy loads at low speeds, load carrying tests such as four-ball EP or Timken OK load should be considered. It is important to remember that greases, like oils, have a careful balance of properties. A product may excel in one category and perform poorly in another. For this reason, it is important to weigh each property’s significance relative to the intended applications to select the best overall fit.
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.