Grease (lubricant)

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Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil.

Contents

A common feature of greases is that they possess high initial viscosities, which upon the application of shear, drop 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 be lubricated only infrequently and where a lubricating oil would not stay in position. They also act as sealants to prevent the ingress of water and incompressible materials. Grease-lubricated bearings have greater frictional characteristics because of their high viscosities.

Properties

A true grease consists of an oil or other fluid lubricant that is mixed with a thickener, typically a soap, to form a solid or semisolid. [1] Greases are usually shear-thinning or pseudo-plastic fluids, which means that the viscosity of the fluid is reduced under shear stress. After sufficient force to shear the grease has been applied, the viscosity drops and approaches that of the base lubricant, such as 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. A few greases are rheotropic, meaning they become more viscous when worked. [2] Grease is often applied using a grease gun, which applies the grease to the part being lubricated under pressure, forcing the solid grease into the spaces in the part.

Thickeners

An inverse micelle formed when a soap is dispersed in an oil. This structure is broken reversibly upon shearing the grease. Micelle scheme2-en.svg
An inverse micelle formed when a soap is dispersed in an oil. This structure is broken reversibly upon shearing the grease.

Soaps are the most common emulsifying agent used, and the selection of the type of soap is determined by the application. [3] Soaps include calcium stearate, sodium stearate, lithium stearate, as well as mixtures of these components. Fatty acids derivatives other than stearates are also used, especially lithium 12-hydroxystearate. The nature of the soaps influences the temperature resistance (relating to the viscosity), water resistance, and chemical stability of the resulting grease. Calcium sulphonates and polyureas are increasingly common grease thickeners not based on metallic soaps. [4] [5]

Powdered solids may also be used as thickeners, especially as clays. 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. Silicone greases are generally thickened with silica.

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. [6]

Additives

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 greases have increased resistance to film breakdown, form sacrificial coatings on the metal surface to protect if the film does break down, or include solid lubricants such as graphite or molybdenum disulfide to provide protection even without any grease remaining. [3]

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. [7] [8] Solid additives cannot be used in bearings because of tight tolerances. Solid additives will cause increased wear in bearings.[ citation needed ]

History

Grease from the early Egyptian or Roman eras is thought to have been prepared by combining lime with olive oil. The lime saponifies some of the triglyceride that comprises oil to give a calcium grease. In the middle of the 19th century, soaps were intentionally added as thickeners to oils. [9] Over the centuries, all manner of materials have been employed as greases. For example, black slugs Arion ater were used as axle-grease to lubricate wooden axle-trees or carts in Sweden. [10]

Classification and standards

Red wheel bearing grease for automotive applications. Wheel Bearing Grease.jpg
Red wheel bearing grease for automotive applications.

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”:

A given performance category may include greases of different consistencies. [11]

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. [12]

Other types

Silicone grease

Silicone grease is based on a silicone oil, usually thickened with amorphous fumed silica.

Fluoroether-based grease

Fluoropolymers containing C-O-C (ether) with fluorine (F) bonded to the carbon. They are more flexible and often used in demanding environments due to their inertness. Fomblin by Solvay Solexis and Krytox by duPont are prominent examples.

Laboratory grease

Grease is used to lubricate glass stopcocks and joints. Some laboratories fill them into syringes for easy application. Two typical examples: Left - Krytox, a fluoroether-based grease; Right - a silicone-based high vacuum grease by Dow Corning. Laboratory grease 1.jpg
Grease is used to lubricate glass stopcocks and joints. Some laboratories fill them into syringes for easy application. Two typical examples: Left - Krytox, a fluoroether-based grease; Right - a silicone-based high vacuum grease by Dow Corning.

Apiezon, silicone-based, and fluoroether-based greases are all used commonly in laboratories for lubricating stopcocks and ground glass joints. The grease helps to prevent joints from "freezing", as well as ensuring high vacuum systems are properly sealed. Apiezon or similar hydrocarbon based greases are the cheapest, and most suitable for high vacuum applications. However, they dissolve in many organic solvents. This quality makes clean-up with pentane or hexanes trivial, but also easily leads to contamination of reaction mixtures.

Silicone-based greases are cheaper than fluoroether-based greases. They are relatively inert and generally do not affect reactions, though reaction mixtures often get contaminated (detected through NMR near δ 0 [13] ). Silicone-based greases are not easily removed with solvent, but they are removed efficiently by soaking in a base bath.

Fluoroether-based greases are inert to many substances including solvents, acids, bases, and oxidizers. They are, however, expensive, and are not easily cleaned away.

Food-grade grease

Food-grade greases are those greases that may come in contact with food and as such are required to be safe to digest. Food-grade lubricant base oil are generally low sulfur petrochemical, less easily oxidized and emulsified. Another commonly used poly-α olefin base oil as well.[ clarification needed ] The United States Department of Agriculture (USDA) has three food-grade designations: H1, H2 and H3. H1 lubricants are food-grade lubricants used in food-processing environments where there is the possibility of incidental food contact. H2 lubricants are industrial lubricants used on equipment and machine parts in locations with no possibility of contact. H3 lubricants are food-grade lubricants, typically edible oils, used to prevent rust on hooks, trolleys and similar equipment.[ citation needed ]

Water-soluble grease analogs

In some cases, the lubrication and high viscosity of a grease are desired in situations where non-toxic, non-oil based materials are required. Carboxymethyl cellulose, or CMC, is one popular material used to create a water-based analog of greases. CMC serves to both thicken the solution and add a lubricating effect, and often silicone-based lubricants are added for additional lubrication. The most familiar example of this type of lubricant, used as a surgical and personal lubricant, is K-Y Jelly.

Cork grease

Cork grease is a lubricant used to lubricate cork, for example in musical wind instruments. It is usually applied using small lip-balm/lip-stick like applicators. [14]

See also

Related Research Articles

A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Guar gum</span> Vegetable gum from the guar bean, Cyamopsis tetragonoloba

Guar gum, also called guaran, is a galactomannan polysaccharide extracted from guar beans that has thickening and stabilizing properties useful in food, feed, and industrial applications. The guar seeds are mechanically dehusked, hydrated, milled and screened according to application. It is typically produced as a free-flowing, off-white powder.

<span class="mw-page-title-main">Motor oil</span> Lubricant used for lubrication of internal combustion engines

Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, detergents, dispersants, and, for multi-grade oils, viscosity index improvers. The main function of motor oil is to reduce friction and wear on moving parts and to clean the engine from sludge and varnish (detergents). It also neutralizes acids that originate from fuel and from oxidation of the lubricant (detergents), improves the sealing of piston rings, and cools the engine by carrying heat away from moving parts.

<span class="mw-page-title-main">Bearing (mechanical)</span> Mechanism to constrain relative movement to the desired motion and reduce friction

A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.

Saponification is a process of cleaving esters into carboxylate salts and alcohols by the action of aqueous alkali. Typically aqueous sodium hydroxide solutions are used. It is an important type of alkaline hydrolysis. When the carboxylate is long chain, its salt is called a soap. The saponification of ethyl acetate gives sodium acetate and ethanol:

<span class="mw-page-title-main">Stearic acid</span> Eighteen-carbon straight-chain fatty acid

Stearic acid is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a soft waxy solid with the formula CH3(CH2)16CO2H. The triglyceride derived from three molecules of stearic acid is called stearin. Stearic acid is a prevalent fatty-acid in nature, found in many animal and vegetable fats, but is usually higher in animal fat than vegetable fat. It has a melting point of 69.4 °C and a pKa of 4.50.

<span class="mw-page-title-main">Synthetic oil</span> Lubricant consisting of artificially made chemical compounds

Synthetic oil is a lubricant consisting of chemical compounds that are artificially modified or synthesised. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers.

<span class="mw-page-title-main">Plain bearing</span> Simplest type of bearing, comprising just a bearing surface and no rolling elements

A plain bearing, or more commonly sliding contact bearing and slide bearing, is the simplest type of bearing, comprising just a bearing surface and no rolling elements. Therefore, the journal slides over the bearing surface. The simplest example of a plain bearing is a shaft rotating in a hole. A simple linear bearing can be a pair of flat surfaces designed to allow motion; e.g., a drawer and the slides it rests on or the ways on the bed of a lathe.

The National Lubricating Grease Institute is an international trade association that serves the grease and gear lubricant industry. It exists to promote research and development of lubrication technology. It also publishes industry standards for greases.

The NLGI consistency number expresses a measure of the relative hardness of a grease used for lubrication, as specified by the standard classification of lubricating grease established by the National Lubricating Grease Institute (NLGI). Reproduced in standards ASTM D4950(“standard classification and specification of automotive service greases”) and SAE J310(“automotive lubricating greases”), NLGI's classification is widely used. The NLGI consistency number is also a component of the code specified in standard ISO 6743-9“lubricants, industrial oils and related products — classification — part 9: family X (greases)”.

Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this formulation, silicone grease is a translucent white viscous paste, with exact properties dependent on the type and proportion of the components. More specialized silicone greases are made from fluorinated silicones or, for low-temperature applications, PDMS containing some phenyl substituents in place of methyl groups. Other thickeners may be used, including stearates and powdered polytetrafluorethylene (PTFE). Greases formulated from silicone oils with silica thickener are sometimes referred to as silicone paste to distinguish them from silicone grease made with silicone oil and a soap thickener.

<span class="mw-page-title-main">Thickening agent</span> Increases the viscosity of a liquid without altering its other properties

A thickening agent or thickener is a substance which can increase the viscosity of a liquid without substantially changing its other properties. Edible thickeners are commonly used to thicken sauces, soups, and puddings without altering their taste; thickeners are also used in paints, inks, explosives, and cosmetics.

<span class="mw-page-title-main">Lithium soap</span>

Lithium soap is a soap consisting of a lithium salt of a fatty acid. Sodium-based and potassium-based soaps are used as cleaning agents in domestic and industrial applications, whereas lithium soaps are used as components of lithium grease.

Oil additives are chemical compounds that improve the lubricant performance of base oil. The manufacturer of many different oils can utilize the same base stock for each formulation and can choose different additives for each specific application. Additives comprise up to 5% by weight of some oils.

Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.

Crude oil is extracted from the bedrock before being processed in several stages, removing natural contaminants and undesirable hydrocarbons. This separation process produces mineral oil, which can in turn be denoted as paraffinic, naphthenic or aromatic. The differences between these different types of oils are not clear-cut, but mainly depend on the predominant hydrocarbon types in the oil. Paraffinic oil, for example, contains primarily higher alkanes, whereas naphthenic oils have a high share of cyclic alkanes in the mixture.

<span class="mw-page-title-main">Defoamer</span> Chemical additive that reduces and hinders the formation of foam in liquids

A defoamer or an anti-foaming agent is a chemical additive that reduces and hinders the formation of foam in industrial process liquids. The terms anti-foam agent and defoamer are often used interchangeably. Strictly speaking, defoamers eliminate existing foam and anti-foamers prevent the formation of further foam. Commonly used agents are insoluble oils, polydimethylsiloxanes and other silicones, certain alcohols, stearates and glycols. The additive is used to prevent formation of foam or is added to break a foam already formed.

Lithium 12-hydroxystearate (C18H35LiO3) is a chemical compound classified as a lithium soap. In chemistry, "soap" refers to salts of fatty acids. Lithium 12-hydroxystearate is a white solid. Lithium soaps are key component of many lubricating greases.

A metallic soap is a metallic salt of a fatty acid. Theoretically, soaps can be made of any metal, although not all enjoy practical uses. Varying the metal can strongly affect the properties of the compound, particularly its solubility.

References

  1. ↑ Dresel, Wilfried (2014). "Lubricating Greases". Encyclopedia of Lubricants and Lubrication. pp. 1076–1096. doi:10.1007/978-3-642-22647-2_16. ISBN   978-3-642-22646-5.
  2. ↑ Les Horve (12 June 1996). Shaft Seals for Dynamic Applications. CRC Press. pp. 449–. ISBN   978-1-4398-2255-5.
  3. 1 2 Richard L. Nailen, Engineering Editor (April 2002). "Grease: What it is; How it Works". Electrical Apparatus. Archived from the original on 2009-04-17. Retrieved 2008-10-23.{{cite journal}}: |author= has generic name (help)
  4. ↑ Arthur J. Caines; Roger F. Haycock; John E. Hillier (2004). Automotive Lubricants Reference Book. John Wiley & Sons. pp. 300–. ISBN   978-1-86058-471-8.
  5. ↑ Tan Jin (25 January 2013). Engineering Materials and Application. Trans Tech Publications Ltd. pp. 83–. ISBN   978-3-03813-994-2.
  6. ↑ Use of ozone depleting substances in laboratories. TemaNord 2003:516. "Archived copy" (PDF). Archived from the original (PDF) on 2008-02-27. Retrieved 2011-03-28.{{cite web}}: CS1 maint: archived copy as title (link)
  7. ↑ Ian Anderson (20 August 2017). How to be Handy [hairy bottom not required]: Build Money Saving DIY Skills, Create a Unique Home and Properly Look After Your Stuff. handycrowd media. pp. 204–. ISBN   978-82-93249-05-4.
  8. ↑ "Ceramic Grease". Powatec. Retrieved 2020-11-15.
  9. ↑ Thorsten Bartels et al. "Lubricants and Lubrication" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Weinheim. doi : 10.1002/14356007.a15_423
  10. ↑ Svanberg I (2006). "Blacks slugs (Arion ater) as grease: a case study of technical use of Gastropods in Pre-industrial Sweden". Journal of Ethnobiology . 26 (2): 299–309. doi: 10.2993/0278-0771(2006)26[299:BSAAAG]2.0.CO;2 . S2CID   62814828.
  11. ↑ Totten, George E.; Westbrook, Steven R.; Shah, Rajesh J., eds. (2003). Fuels and lubricants handbook: technology, properties, performance, and testing (volume 1). “ASTM manual” series, volume 37 (7th ed.). ASTM International. p. 560. ISBN   978-0-8031-2096-9.
  12. ↑ Rand, Salvatore J., ed. (2003). Significance of tests for petroleum products. “ASTM manual” series, volume 1 (7th ed.). ASTM International. p. 166. ISBN   978-0-8031-2097-6.
  13. ↑ Gottlieb, Hugo E.; Kotlyar, Vadim; Nudelman, Abraham (1997). "NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities". The Journal of Organic Chemistry. 62 (21): 7512–7515. doi:10.1021/jo971176v. PMID   11671879.
  14. ↑ "Cork Lubricants". Archived from the original on 2017-10-07. Retrieved 2017-03-25.

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