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.

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.

Compounded Oils.

Mineral oils are compounded with small proportions of animal and vegetable oils, for particular purposes. The vegetable and animal oils (commonly termed “fixed oils”) usually employed are refined rape oil, peanut oil, castor oil, neatsfoot oil, tal-low oil, lard oil, sperm oil and fish oils. These fixed oils vary in colour, viscosity and saponifying value. They also possess properties of great adhesion and “greasiness” or “oiliness.”

This property of greater adhesion is a particularly valuable qualification for lubrication; especially for large, slow moving journals, where the supply is furnished by means of drop or wick feed oiling. Under such conditions lubrication is attained through the formation of a complete oil film, resulting largely from the adhesive property of the fixed-oil content.

The fixed oils will emulsify readily with water, which qualification is also retained after the mixture or compounding of the fixed oil with the mineral oil. This property of a compounded oil is utilised to advantage where oil stains on fabrics, due to carelessness in handling the lubricant, must be removed by washing and scouring. Mineral-oil stains, if set by time or oxidised by sunlight, cannot be eliminated from fabrics by any scouring method.

This emulsifying property of compounded oils is also utilised where continuous operation is demanded and where adverse operating conditions, causing overheating of the bearing, are relieved through the application of water… Water which reaches the bearing surface, instead of washing away the oil film, produces a rich oily emulsion or lather of exceptional lubricating value.

The greater adhesiveness of a compounded oil is also of value under severe service conditions, where a heavy-bodied mineral oil is too thick to be distributed in a complete oil film, and where the use of a lighter-bodied compounded oil will assure the correct formation of the oil film and satisfy lubrication requirements.

Selection of Oils.

Bearings vary in size from small to very large; they run at speeds from slow to extremely high: they operate under temperatures from below zero F. (—18 deg. C) to 300 deg. F. (150 deg. C.) and over; and pressures range from light to abnormal. In every plant and on individual machines there are many bearings, on which the method of supply varies from hand oiling to circulation systems, in other words, from the crudest method to the most refined. In different industries the same machines are subjected to different service; some subject to water contamination, others subject to the presence of dirt and impurities.

Because of the many complexities and influencing factors, it is impossible to lay down simple rules, arbitrary specifications or other rule-of thumb methods, for the selection of oils. The correctness of an oil can be judged only by the actual service that it renders under the existing conditions of practice. For this reason, broad experience is essential in order that the correct selection of the most suitable lubricant can be made.

Bearing Troubles.

Correct lubrication may be regarded to-day as a science. Just as the chemist knows what results will follow the mixing of two substances under known conditions, so the fully-informed lubrication engineer can foretell what results will be the effect of applying a given lubricant to a bearing subjected to known mechanical and operating conditions.

Incorrect analysis of the factors influencing lubrication, failure to observe the true conditions, and lack of knowledge of the correct lubricants, necessarily lead to incorrect lubrication; and the penalties are troubles, wear and repairs, page 43 depreciation of equipment, and the expense of replacement. This negative aspect of lubrication calls for discussion, in order that incorrect conditions may be rectified.

Manifestations of Bearing Troubles.

Bearing troubles may make themselves known in a number of ways, each trouble resulting from one or more of several causes.

Hot bearings, eccentrics and guides, generally indicate excessive frictional losses. The temperature rise may be excessive, leading to further troubles, such as seizure of the bearing surfaces, the melting of the bearing metal and damage to the journal. A knock in the bearing may be the result of too great clearance, due to wear or improper adjustment or due to the need of a heavier oil than that in use. Rapid wear of bearings and journals may call for frequent repairs and renewals, or may cause the parts of the machine to run out of proper alignment. Unnecessary friction may be observed through an increase in the power that is demanded to operate the machinery.

Causes of Bearing Troubles.

A complete oil film is known to be the great essential of correct lubrication. Hence, incorrect lubrication which leads to troubles is due to conditions that interfere with the formation and maintenance of an effective oil film.

The causes of bearing troubles may be classified under the following six heads:—(1) Incorrect structure of the bearing, as influenced by materials, design, workmanship, adjustment and wear. (2) Excessive pressures, resulting from misalignment, heavy belt pull, or overload of the machine beyond its designed capacity. (3) Temperature extremes, resulting from surrounding conditions, hot or cold, or from frictional heat and deficient radiation. (4) Contamination of the lubricant before or during use in the bearing. (5) Incorrect methods and practices of lubrication. (6) Unsuitable lubricant for the conditions.

(To be continued.)