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Summary: Greases*) are lubricating materials, used where the lubrication with oil is technically difficult. In this sense they are substitutes of the oil. The lubricant in the grease, however, is also an oil, which is contained in their composition. The greases are dispersions, a mixture of thickener (solid material) and liquid lubricant. The thickener (dispersed phase) forms a structure, which holds within it the liquid material (dispersion medium). The soap greases, as for example the lithium, have very special properties. They are non-Newtonian fluids. This means, that at high pressures the viscosity decreases and becomes similar to that of the oil. Usually they are regarded as an oil reservoir and as a temperature regulator of the lubrication. This means that at elevated temperatures, they bleed more oil, thereby improving lubrication. With the reduction of the temperature part of the oil comes back into the structure of the grease.
From a practical standpoint, the greases are pasty lubricants. From a scientific point of view, such a definition is not correct. The greases are thick liquids, dispersions with special properties. The exact definition currently is: "A solid or semisolid product, which constitutes a dispersion of a thickening agent in a liquid lubricant. May contain other ingredients, which give it special properties." An important property of the grease is, when there's no pressure on it, it behaves like a solid, i.e. it doesn't take the form of the container in which is placed. When it is under pressure, it behaves like a liquid and takes the form of the container. The appearance can be as smooth with homogeneous structure, also fibrous, with micelles. There are also greases with a slightly grainy texture.
Greases are a semi-solid mixtures, consisting of liquid lubricant, thickener and
additives. The liquid lubricant can be mineral, synthetic or vegetable oil. The thickener
gives the grease its characteristic texture and is sometimes regarded as "sponge",
which holds an oil. Thickeners are soaps, other organic or inorganic materials.
Most greases on the market are manufactured from mineral oils, thickened with soaps.
Additives extend the application and protect the lubricated surfaces. Greases are described
as temperature-regulated feeder: when the lubricating film between the friction surfaces
shrink, the increased temperature softened the grease in the volume, it expands and releases
oil, which restores the thickness of the film. The oil in the grease may be oxidized,
like any other oil. The main factor for the process flowing of the oxidation is the
elevated temperature. Such grease usually darkens and acidic substances are formed.
They can destroy the structure of the thickener, causing softening, oil leakage and
slipping the lubricant out. Since the grease is not a good heat conductor, oxidation
can start in the hottest point and spread in the volume. This leads to progressive
deterioration. If a soap grease /lithium, potassium or other/ is heated slowly,
its penetration increases /the grease softens/, until it melts. This temperature is
called "Dropping Point Temperature". When a grease is melted and then cooled, usually
it does not recover completely. Thereafter, its quality is changed considerably.
The working temperature should never elevate so much. The permissible operating
temperature of the greases is far below the dropping point temperature.
*) The greases are plastic lubricants, as far as the lubricants are gaseous, liquid, grease-like or solid. The term "grease" is the most widely used, as it means dispersions, having a wide range of properties. Another commonly used term, "consistent", is used incorrectly.
They are used in places, where oil lubrication is difficult.