Grease

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Lubricating greases are semi-liquid lubricants that consist of a lubricating oil , a thickener and various additives or active substances (additives).

As a rule, lubricating greases consist of approx. 80% lubricating oil, approx. 5% to 10% thickener and approx. 10% to 15% additives. In the case of the most common fats, the thickener is a light or alkali metal soap, which forms a spongy structure that surrounds the oil droplets. The lubricating oil is released more or less quickly depending on the temperature, time and stress ( shear ). This process is also called "bleeding". A lubricating grease can also supply the friction point with oil at the edge of a tribological contact .

In addition to lubrication , lubricating greases should generally also offer protection against corrosion , which is usually achieved through additives . Dry lubricants are also added to protect against running dry at elevated temperatures.

By selecting the appropriate oils, thickeners and additives, the properties of the lubricating greases can be optimized for a wide variety of applications. There are greases for high or particularly low temperatures, for applications in a vacuum, particularly water-resistant and weather-proof, particularly pressure-resistant or creep-capable, food-safe or particularly adhesive greases.

Definitions

Lubricating greases are physical suspensions of lubricating oils, thickeners and various additives or active ingredients. According to DIN 51825, lubricating greases are consistent lubricants that consist of mineral oil and / or synthetic oil as well as a thickener. According to the ASTM , lubricating greases are solid to semi-liquid substances that result from the dispersion of a thickener in a liquid substance. Other additives that give special properties may be included.

Construction, structure and properties

As a rule, lubricating greases consist of 65–95% lubricating oil (base oil), 3–30% thickener and 0–10% additives. The suspension of base oil and thickener is also known as base fat.

Base oils

The viscosity and temperature dependency of the lubricating grease is determined by the base oil, to which the additives of the base oils also contribute. Thus cause VI Improver (additive of the base oil) such as to viscosity and the viscosity index at high temperatures developed.

A distinction is made between the following types of base oil: mineral oils and synthetic oils.

A rough classification of hydrocarbons in mineral oil-based lubricating oils is shown in the following table. A mineral oil is called paraffinic, naphthenic or aromatic, depending on which type of hydrocarbon dominates the overall properties.

Classification of mineral oils
designation Type of main component
Paraffins Chain-shaped, saturated
Olefins Chain-shaped, unsaturated
Naphthenes Annular, saturated
partly unsaturated naphthens Annular, unsaturated
Aromatics Ring-shaped, aromatic
Properties of paraffinic, naphthenic and aromatic mineral oils in simple comparison.
Oil type density Flash point Oxidation stability Wettability
Paraffinic mineral oil low high Well medium
Olefinic mineral oil medium medium Well Well
Aromatic mineral oil high low bad high

Typical base oils are:

Thickener

Thickeners, also known as thickeners, are the most essential component of lubricating greases. They are used to convey the state of aggregation typical of lubricating grease, i.e. the structure of the lubricating grease. The thickener forms the matrix in which the base oil is stored. The oil emerges from the matrix through milling and reaches the lubrication point. The amount of lubricating oil that emerges from the grease and acts at the lubrication point is controlled by the type of thickener. A matrix that is too rigid could cause insufficient lubrication, but if too much oil is released, the grease loses its lubricity too quickly because the matrix is ​​destroyed and the oil runs off. A suitably selected thickener can partially absorb the oil again during periods of rest.

Lubricating greases with different thickeners cannot always be mixed with one another, as the thickeners are not always compatible with one another and their properties influence each other when they come into contact (e.g. the dropping point can change because the soap structure breaks down).

Mixing different lubricants is one of the main causes of system problems. Miscibility (two substances are miscible if they completely dissolve in one another) of two lubricating greases does not necessarily indicate their compatibility (two lubricating greases are compatible if their properties do not influence each other when mixed).

Thickeners usually consist of alkali and alkaline earth salts of fatty acids (these lubricating greases are called metal soap greases) and / or other substances. Frequently used thickeners are:

Metal soap greases

Metal soap greases contain so-called metal soaps (salts of fatty acids with oxides or hydroxides of metals) as thickeners. Metal oxides / hydroxides used are those of the metals lithium, sodium, calcium, barium, aluminum, zinc and lead. The chemistry of metal soap formation offers a multitude of possible combinations when selecting bases and fatty acids:

  • Simple soap greases : Contain the so-called simple metal soaps as a thickener , they consist of one type of base and one type of fatty acid.
  • Complex soap fats: Contain the so-called metal complex soaps as a thickener , they consist of a base, a fatty acid and a typical non-fatty acid (e.g. acetic acid).
  • Mixed soap greases: Contain a mixture of different simple metal soaps as a thickener.

Additives

Additives are substances that give a product properties that it would not have or would only have insufficiently without these additives. Additives in lubricating greases must work well with the thickener system so that the grease does not soften or harden. Using suitable additives, for example, positive properties such as wear protection can be improved or undesirable properties such as the aging of the base oil can be reduced.

Additives can be divided into the following categories:

  • Antioxidants : Increasing the resistance to aging through unavoidable oxidative and / or thermal loads at the lubrication point. Oxidation can produce acids and oil-insoluble components that form impurities that can settle on the lubrication point. If the oxidation products are distributed in the grease, the viscosity often increases. Antioxidants used are phenolic or amine radical scavengers.
  • Metal deactivators : copper and its compounds are catalysts for the formation of peroxides and thus contribute to oxidation; Non-ferrous metal deactivators are z. B. heterocycles that passivate the copper surface .
  • Corrosion inhibitors : act in a similar way to non-ferrous metal deactivators, form a protective film on the metal surface that is impermeable to moisture and protects the surface from corrosion (so-called surface layer former). Corrosion inhibitors neutralize acidic reaction products from the breakdown of additives or the oxidation of the base oil. In this way a corrosive attack on the non-ferrous metal is weakened or prevented. If the effect specifically relates to ferrous metals or steel, one also speaks of rust inhibitors. Corrosion inhibitors are metal soaps of various acids, such as sulfonates , naphthenates , carboxylates , alkyl succinic acid derivatives , amine phosphates or partial polyol esters .
  • Extreme pressure additives: top layer formers, surface-active substances
  • Wear protection additives : wear reactions, such as B. fluid and mixed friction , are prevented by triboreactions.
  • Solid lubricants : Often substances with a layer structure (e.g. graphite or molybdenum disulphide ) have a positive effect on lubrication in areas of mixed friction, with small and oscillating movements and at very high or low ambient temperatures. Solid lubricants do not work with  hydrodynamic lubrication . Lubricants with more than 10% solid lubricant content are often categorized as paste.
  • Adhesive additives: long-chain polar polymers that give the grease increased adhesion to the surfaces to be lubricated.
  • Dyes

Parameters

Fat kneader or grease walker for fulling the grease, 3D model.

Important parameters and analysis methods include the following:

Grease-specific parameters

Cone penetration

The cone penetration of a lubricating grease is understood to be the penetration depth of a standard cone under defined conditions according to DIN ISO 2137. The measurement of the penetration depth of a cone allows assignment to a consistency class (in NLGI classes).

A distinction is made between resting and fulling penetration; after fulling the grease, the worked penetration is measured in a fat kneader or grease fuller.

Dropping point

The dropping point indicates the temperature at which a small amount of lubricating grease forms a long-drawn droplet under defined test conditions and drips out of the test device. The grease liquefies by dissolving the soap structure (ie the thickener). The transferability of the dropping point to practical properties is often difficult.

Base number

The Total Base Number (TBN,) often just the Base Number (BN) or in German for short base number, shows the ability of an engine oil to neutralize acidic combustion residues. Its unit of measurement is (mg KOH) / (g) and defines the amount of potassium hydroxide (KOH) in mg, which corresponds to the neutralization capacity of the alkaline active ingredients contained in one gram of lubricating grease. In the internal combustion engine it can, for. B. the combustion process leads to the formation of acidic gases that have to be neutralized if lubrication is to continue to be guaranteed. It follows from this that, among other things, the degree of the decrease in the base number during operation of an engine provides an indication of an oil change that is due.

Neutralization number

The Acid Number (AN), in English acid number , often also called neutralization number (NZ), indicates how many mg of potassium hydroxide (KOH) are necessary to neutralize the free acids contained in 1 g of oil. These can be contained as residues from refining. The NZ is measured, among other things, when the base number can no longer be determined or a measurement does not make sense.

Further characteristic values ​​and properties of lubricating greases and their determination can be found later in the article under "Analysis methods".

Shelf life

Lubricating greases are not subject to a shelf life, the central property for the duration of storage is therefore the shelf life or further usability. As a rule, manufacturers guarantee the specified data for periods of up to two years, and further usability is specified for up to 6 years. To ensure that a lubricating grease does not lose its lubricating properties during storage, it must be stored properly. The lubricants should be stored in the unopened original container, dry and protected from light and at moderate temperatures.

Further parameters / analysis methods

  • Flow pressure : The pressure required to force a strand of lubricating grease through a defined nozzle at a certain temperature.
  • Oil separation : procedure for determining the oil loss from lubricating greases.
  • SKF Emcor test : Testing of the corrosion -preventing properties of lubricating greases in the presence of water or salt solutions. The grease is in the SKF-Emcor method of adding water in aligning ball bearings tested (a lubricating grease should corrosion polluter, such. As water and oxygen away from the metal surface). The test is carried out in accordance with DIN 51802.
  • Lubricating grease service life (FAG-FE9): Determination of the lubricating grease service life at elevated temperatures. For testing purposes, the lubricating grease is filled in angular contact ball bearings and loaded at elevated temperatures until failure. The test is carried out in accordance with DIN 51821.
  • Copper corrosion : Determination of the corrosive properties of lubricating oils and greases on copper.
  • Viscosity : The rheological properties of a lubricating grease are determined by the shear viscosity and its stability after the shear load.
  • Water content: Too much water can cause corrosion, cavitation or oil oxidation, among other things, the water content is determined by Karl Fischer titration.
  • Color number : The change in the color of a lubricating grease compared to a standard can provide important information about aging (and the resulting oxidation) or about any contamination of an oil.
  • Analytical ferrography : With a used grease , iron particles indicate a wear process. By using ferromagnetism, it is possible to separate the (magnetizable) iron particles from the fat and sort them. Conclusions about the wear process can be drawn from the shape and size distribution of the iron particles.
  • Base oil extraction : Soxhlet extraction of the base oil from the thickener.

General methods of analysis

  • X-ray fluorescence analysis (XRF): element analysis with the possibility of a high sample throughput, recommended for fresh lubricating greases, malfunctions can occur with used greases.
  • Atomic emission spectrometry (AES): element analysis with the possibility of a high sample throughput, recommended for fresh lubricating greases and for used greases. AES can be carried out according to the RDE or the ICP method.
  • IR and Raman spectroscopy
  • NMR spectroscopy

Analysis methods for the raw materials used for the production of lubricating grease

  • Flash point : The flash point is the temperature at which so many easily flammable vapors develop in a crucible filled with the liquid to be tested that they can be briefly ignited by external ignition.
  • Color number: The change in the color of a base oil or additive (compared to a standard) can provide important information about possible impurities or indicate incorrect storage (aging processes).

Impurities

water

In addition to dust, water is a very common contaminant in lubricating greases. Due to its chemical and physical properties, water has massive direct and indirect effects on the lubricating grease.

The presence of water in the grease causes many problems, including:

  • The lubricity of water is significantly lower than that of greases, if there is water at the lubrication point, no stable lubricating film can form, there is abrasive material wear and possibly local welding.
  • Water promotes the oxidation of the metal and, as a result, the oil.
  • With combustion gases, water often forms acidic solutions that have to be neutralized by the base components of the grease.

Possible causes for a high water content include:

  • Leaky weld seams can allow water to penetrate the grease.
  • The stop-and-go operation of engines creates a constant change from cold to hot to cold, etc. When cooling down, condensation forms with water from the ambient air or from the combustion gases.

The water content of a grease or oil can e.g. B. can be determined by Karl Fischer titration . The quantitative use of IR spectroscopy is also possible if a standard lubricating grease is known (compare the intensity of the absorption bands).

Foreign particles

Foreign particles (including dust) can lead to premature wear at the lubrication point, and plastic components can be damaged in particular.

Production error

Soap

When using the soap, various production errors can occur:

  • If the proportion of soap in the grease is too low, oil may separate out during storage. Increased oil leakage can then also be observed at the lubrication point, which can lead to increased wear of the parts to be lubricated.
  • If the proportion of soap in the lubricating grease is too high, the soap particles can aggregate into larger particles and clog filling stations, lubrication points can run dry and when it is cold a higher flow pressure can be observed.
  • If the soap particles are too coarse, filling stations can clog and lubrication points can run dry.

Classification of lubricating greases

According to consistency class

This is indicated in the consistency index. The measurement is made with a penetrometer . The penetration depth of a cone allows it to be assigned to a consistency class. A distinction is made between consistencies from 000 (fluent) to 6 (hard). The consistency index is specified as a NLGI-class according to DIN 51818, and may be specified in Ruh- or worked penetration, wherein the fat prior to measurement according to the specifications of test instructions in the consistency measured according to DIN ISO 2137 drumming is the stress in a bearing mimic.

Classification of lubricating greases according to NLGI classes according to DIN 51818
NLGI Walk penetration consistency Applications
000 445 ... 475 fluently Gearbox, central lubrication
00 400 ... 430 weak flowing Gearbox, central lubrication
0 355 ... 385 semi-liquid Gearboxes, roller bearings, central lubrication
1 310… 340 very soft roller bearing
2 265 ... 295 soft Rolling bearings, plain bearings
3 220 ... 250 medium firm Rolling bearings, plain bearings, water pumps
4th 175… 205 firmly Roller bearings, water pumps
5 130 ... 160 very strong Water pumps, block grease
6th 85… 115 hard Block fat

According to the objects to be lubricated

Red bearing grease for automobiles.

According to scope

  • Normal fats
  • Multipurpose greases
  • EP greases
  • High temperature greases

Applications for lubricating greases

Stauffer rifles on the hoisting machine of the Hanover colliery

Like lubricating oils, lubricating greases are used to reduce mechanical friction and wear.

Lubricating greases work through a film that they build up between the lubricating surfaces. In this way, the grease prevents direct contact between the surfaces moving against one another. In practice, however, this is not enough to build up a stable lubricating film that completely separates the friction surfaces from one another.

Advantages of grease lubrication compared to oil lubrication:

  • The grease does not drip off the lubrication point
  • Suitable for seldom or slowly moving lubrication points
  • Sealing effect and protection of the lubrication point against direct ingress of dirt and water
  • Corrosion protection, provided that the grease has been appropriately added
  • The soap can also help separate the contact partners
  • With oil lubrication, the oil can completely separate the two friction partners, e.g. B. in a hydrodynamic plain bearing

Disadvantages of grease lubrication compared to oil lubrication:

  • With grease lubrication, the bearing friction remains in the range of mixed friction .
  • At higher speeds, e.g. B. in high-speed bearings, the fat heats up more because of its higher viscosity and reaches the critical temperature at which the base oil decomposes more quickly.
  • With small (oscillating) movements, lubricating greases often have problems supplying the lubricating point with base oil
  • No cooling of the lubrication point due to the lack of circulation
  • No cleaning effect at the lubrication point.
  • If the lubrication point is to be re-greased, it depends on the adhesive strength of the grease how easy the lubrication point can be cleaned.

If no permanent filling is planned and the lubrication point is open, grease nipples are used to be able to regularly bring fresh grease to the lubrication point with a grease gun as part of a maintenance or lubrication plan . Old grease and its impurities are pressed out of the lubrication point and can be removed if the collar made of old grease at the lubrication point is not desired as a seal against ingress of dust and dirt.

Comparison of some lubricating greases

Properties of various fats

Fat type Appearance Min. Temp. Max. Temp Cold behavior Water resistant Continuous walking Corrosion protection Dropping point price
Ca metal soap smooth, soft −35 ° C +80 ° C Well very good Well bad 80..100 ° C
Na metal soap fibrous, soft −30 ° C +120 ° C moderate inconsistent moderate Well 130..200 ° C
Li metal soap smooth, soft −40 ° C +140 ° C Well resistant very good very bad 170..220 ° C 1
Al metal soap smooth, clear −35 ° C +80 ° C Well swells up moderate very good ~ 120 ° C 3
Li / Pb grease 0 ° C +75 ° C bad resistant bad Well ~ 90 ° C 1.5
Ca / Pb fat 0 ° C +75 ° C bad resistant bad Well ~ 90 ° C 1.5
Ca complex smooth, soft +180 ° C moderate very good moderate Well > 240 ° C 0.9..1.2
Li complex smooth, soft + 180 ° C Well Well very good Well > 240 ° C
Al complex smooth, soft +180 ° C very good moderate Well > 200 ° C 3..4
Silicone grease smooth, soft +320 ° C Well resistant moderate - 30..50

The price column gives an approximate ratio that lithium metal soap assumes as 100%.

See also

  • DIN 51825 lubricating greases and solid lubricants
  • VDI guideline 2202 lubricants and lubrication devices for plain and roller bearings
  • Metal soaps are used to thicken the base oil in order to achieve the desired consistency of the fat
  • Despite its name, copper grease or paste is not actually a lubricating grease

Individual evidence

  1. ↑ a b c d Bartz, Wilfried J .: Lubricating greases: composition, properties, testing and application; with 33 tables and 126 references . In: Contact & Studies . tape 500 . Expert-Verlag, Renningen-Malmsheim 2000, ISBN 3-8169-1533-7 , p. 6 .
  2. ↑ DIN e. V .: DIN 51825 lubricants - lubricating greases K - classification and requirements. In: DIN e. V. DIN e. V., accessed April 5, 2019 .
  3. ↑ Information on lubricating greases. Retrieved April 5, 2019 .
  4. ↑ a b c d e f Bartz, Wilfried J .: Lubricating greases: composition, properties, testing and application; with 33 tables and 126 references . In: Contact & Studies . tape 500 . Expert-Verlag, Renningen-Malmsheim 2000, ISBN 3-8169-1533-7 , p. 33-50 .
  5. ↑ a b Lubricating Greases Basics. (PDF) Exxon Mobil Corporation, 2012, accessed April 14, 2019 .
  6. ^ Hans Beyer and Wolfgang Walter : Organische Chemie , S. Hirzel Verlag, Stuttgart, 22nd edition, 1991, p. 247, ISBN 3-7776-0485-2 .
  7. ↑ a b Bartz, Wilfried J .: Lubricating greases: composition, properties, testing and application; with 33 tables and 126 references . In: Contact & Studies . tape 500 . Expert-Verlag, Renningen-Malmsheim 2000, ISBN 3-8169-1533-7 , p. 111-130 .
  8. ↑ pastes | FUCHS LUBRITECH GMBH. Retrieved November 4, 2019 .
  9. ↑ a b c d e f g Totten, George E., Westbrook, Steven R., Shah, Rajesh J .: Fuels and Lubricants Handbook: Technology, Properties, Performance, and Testing. ASTM International, West Conshohocken, PA 2003, ISBN 978-0-8031-4551-1 .
  10. ↑ a b Lubricant ABC. Retrieved July 23, 2019 .
  11. ↑ Analytical ferrography - OELCHECK. Retrieved July 24, 2019 .
  12. ↑ Herbert Wittel; Dieter Jannasch; Joachim Vossiek; Christian Spura: Roloff / Matek machine elements . 23rd edition. Springer Fachmedien, Wiesbaden 2017, ISBN 978-3-658-17895-6 , table TB 4-9.
  13. ↑ Taisuke Maruyama, Tsuyoshi Saitoh, Atsushi Yokouchi: Differences in Mechanisms for Fretting Wear Reduction between Oil and Grease Lubrication . In: Tribology Transactions . tape 60 , no. 3 , May 4, 2017, ISSN  1040-2004 , p. 497–505 , doi : 10.1080 / 10402004.2016.1180469 (doi.org/10.1080/10402004.2016.1180469 [accessed June 2, 2020]).
  14. ↑ Werner Skolaut (Ed.): Mechanical engineering . Springer-Verlag, Berlin Heidelberg 2014, ISBN 978-3-8274-2553-9 , 27.2 stock.
  15. ↑ Fabian Schwack, Norbert Bader, Johan Leckner, Claire Demaille, Gerhard Poll: A study of grease lubricants under wind turbine pitch bearing conditions . In: Wear . tape 454-455 , ISSN  0043-1648 , p. 203335 , doi : 10.1016 / j.wear.2020.203335 ( sciencedirect.com [accessed June 2, 2020]).
  16. ^ Taken in parts from T. Braun: Fett-Schulung. (PDF) ZET-CHEMIE GmbH, December 2014, p. 26 , accessed on April 2, 2018 (German).