Grease, Lubricating

a lubricant that exhibits properties of liquids or solids, depending on the stress. Under small stresses, lubricating greases preserve their structure, remain on vertical surfaces, and are retained on unsealed frictional surfaces. They consist of a liquid oil, a solid thickener (or gelling agent), and various additives. The particles of the gelling agent constitute the dispersed phase of a colloidal solution. They form a structural network of cells, which retain the dispersion medium (oil). Because of this structure, greases undergo deformation, like liquids of irregular viscosity, only under stresses in excess of their yield stress, which is usually 0.1–2 kilonewtons per m², or 1–20 grams-force per cm². The bonds of the structural network are restored immediately after the release of stress, and the lubricant once again assumes the properties of a solid.

The use of greases makes it possible to simplify the design and lower the weight of frictional surfaces, in addition to preventing pollution of the environment. Greases need to be replaced less frequently than ordinary lubricants; in fact, an initial application of grease often suffices for the entire service life of modern machines. In 1974, Soviet industry produced about 150 types of lubricating greases. World production totals about 1 million tons per year, which is 3.5 percent of the total production of lubricants.

Greases are produced by adding 5–30 (usually 10–20) percent of a solid thickener to petroleum oils or, less frequently, synthetic oils. The production process is of the batch type (that is, once a batch of the product has been obtained the starting materials are fed again and the production process is repeated). After being fused with the oil in a heated cauldron, the thickener crystallizes in the form of a network of fine fibers. Thickeners with melting points above 200°-300°C are dispersed in oil with the aid of homogenizers, such as colloid mills. Oxidation and rust inhibitors, antiwear agents, and other liquid additives are added to lubricating greases during manufacture, as are solid friction-reducing and sealant additives.

Greases are classified according to their gelling agents and areas of application. The most common greases are of the soap-base type, which are thickened with calcium, lithium, or sodium soaps of fatty acids of high molecular weight. Greases made with hydrated calcium can be used at temperatures of up to 60°-80°C, sodium greases can be used up to 110°C, and lithium and complex calcium greases can be used at temperatures as high as 120°-140°C. Hydrocarbon greases, thickened with paraffin or ceresin, constitute 10–15 percent of the total production of lubricating greases. They have low melting points (50°-65°C) and are mainly used for the preservation of metal articles.

Various greases are distinguished according to their uses. Antifriction greases, which lower sliding friction and reduce wear, are used in industrial machinery, instruments, and transport agricultural machinery on rolling or sliding bearings, in joints, and in gear drives and chain drives. Anticorrosion greases are used to protect metal articles. In contrast to such coatings as paint or chrome plating, grease is readily removed from frictional and other surfaces when the machine is put back into service. A third type, packing greases, includes sealants for the fittings on gate valves and stopcocks, greases that prevent threaded assemblies from seizing under heavy loads or high temperatures, and greases that are used to hermetically seal movable vacuum couplings.