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The New Zealand Railways Magazine, Volume 4, Issue 10 (February 1, 1930)

Petroleum (mineral) Oils

Petroleum (mineral) Oils.

Oils used for bearing lubrication are principally derived from petroleum. Crude petroleum is a more or less mobile liquid, obtained from different strata below the earth's surface. Some crudes are very liquid and flow freely, while others are thick and viscous. In density, petroleum crude is somewhat lighter than water. Its colour varies from pale to amber to black, depending on its composition. Each crude has a characteristic odour, affected by the impurities it contains.

A Famous Mountain Railway. Three engine train ascending the Rimutaka Incline, North Island. (Grade 1 in 15.)

A Famous Mountain Railway.
Three engine train ascending the Rimutaka Incline, North Island. (Grade 1 in 15.)

All petroleum crudes consist of chemical combinations of carbon and hydrogen in varying proportions, sometimes chemically combined with impurities and usually contaminated with water, salts and earthy matter. They are exceedingly complex mixtures of many components or groups of substances, such as naphtha (gasoline), kerosene, paraffin, wax, lubricating oils and heavier substances of varying nature. These components appear in widety varying proportions in the various crudes.

The processes employed in oil refineries are for the purpose of separating the crude into such components as will find a market readily. Where the process is designed to produce a maximum quantity of gasoline and kerosene, their quantity is increased by employing a cracking process, which breaks down some of the heavier fractions. This is spoken of as destructive distillation; due to its effect upon heavier components of the crude, including the lubricating oils. Lubricants resulting from this process are therefore generally of inferior quality.

In the manufacture of high-quality lubricants the crude is selected on the basis of the lubricating oil components contained and the freedom of the crude from objectionable and injurious impurities. The fractional method of distillation is employed, carefully avoiding all excessive temperatures that might injure or break down the lubricating-oil fractions or residues.

Durability, resulting from extreme purity and chemical stability, is a special qualification demanded of oils subjected to long continued service in the automatic oiling systems of modern machines.

In the production of such oils, a crude petroleum is selected with a view to the presence of those hydrocarbons which possess the highest lubricating qualities; and to the absence of oxidised and unstable hydrocarbons, sulphur compounds, and other chemical impurities. These crudes are distilled under most favourable moderate temperatures; they are frequently produced by distillation in a vacuum, and are refined by chemical and absorptive methods to standardise purity, stability, viscosity and colour.

Where the method of lubrication requires that page 42 the oil serve its purpose but once in the oil film, the special high quality which establishes extreme durability is not essential. In bearings so lubricated, however, it is seldom that the mechanical conditions are ideal for the formation of an effective oil wedge, with the result that high lubricating value and adhesiveness of the oil are of great importance.

While an oil adapted to this purpose may not require all of the refining process employed in the manufacture of oil for continued service, a crude must be selected which contains the required elements necessary to high lubricating value and adhesiveness, and the process of manufacture must preserve these properties. The further elimination of all impurities, which would in any way impair the value of the oil, produces an oil of high quality and standardised viscosity.

A ring oiled bearing as constructed for a large electric generator.

A ring oiled bearing as constructed for a large electric generator.