(5) Many troubles result from the use of unsuitable methods of oil application, and from failure to give the required attention in order to maintain a sufficient uniform supply of the oil. The effectiveness of hand oiling depends on the attendant. The oil is applied periodically in a quantity greater than is necessary for the momentary need, much of the oil escaping from the bearing without rendering service. Commonly the period between hand oilings is so great that lubrication becomes inadequate, and with neglect a condition of actual non-lubrication occurs. Drop feed oilers, although far superior to hand oiling, do not give a uniform feed, and may at any time become clogged and cease to provide lubrication. Drop feed oilers may be replaced by bottle oilers on small bearings and by wick-feed oilers on large bearings, with improved reliability and economy of lubrication. Whatever the method of oil application, neglect with regard to supply or cleanliness is certain to lead to trouble sooner or later.

Unsuitable Lubricants.

(6) The mechanical conditions, the operating conditions, and the lubricant used must conform one to the other in order that correct lubrication may be accomplished. A bearing once correctly lubricated may become incorrectly lubricated through a change in either the conditions or the lubricant.

The use of an oil, too light or too heavy in body for the existing mechanical and operating conditions, is one of the most common sources of bearing trouble that can be ascribed to the lubricant. For low speeds, heavy pressures, and poor film-forming conditions, the heavier-bodied oils are required; since a light-bodied oil would be squeezed out from the pressure area and fail to maintain a complete oil film. High speeds, light pressures, and good film-forming conditions call for oils of lighter body. The use of an oil too heavy in body results in excessive fluid friction and heating of the bearing.

Oil quality not suited to the service is also a frequent cause of trouble. Failure to use the special grades in a circulation or splash system will lead to breakdown of the oil film and the ultimate failure of lubrication. Where special conditions of high or low temperatures exist, the oil selected must have the specific qualities demanded for the service.

The use of a pure mineral oil for wet bearings may result in failure of the oil film, because the mineral oil will not adhere to a wet surface. Correctly compounded oils of suitable body should be used for this purpose. Heavy-bodied compounded oils are frequently advantageous in bearings subjected to excessive loads, on account of the strong adhesive properties resulting from compounding.

The use of grease on bearings subjected to high speeds and moderate loads, frequently leads to excessive film friction and high bearing temperatures. This effect is the same as that of using an oil with a body that is much too heavy.

Troubles caused by the use of an unsuitable lubricant are the result of failure to observe the principles set forth in earlier parts of this article. The application of these principles, through a correct analysis of the mechanical and operating conditions involved, will lead to the selection of the correct lubricants, and the avoidance of bearing troubles.

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Care of Hot Bearings.

Where it is essential that machinery be kept in continuous operation, it is of value to know how bearing troubles may be relieved temporarily while the real cause of the trouble is being removed. These emergency measures should not be relied upon for continuous service, as they do not overcome the cause of the trouble. A hot bearing is one whose temperature rises materially above its normal operating temperature from any of the causes previously described, leading to excessive friction and heat generation. Such a condition is often unstable, in that, unless the cause is corrected, friction and heat generation continue each to augment the other until a destructive temperature is reached.

When a small bearing heats up, it is not difficult to cool, as the total amount of heat present in the bearings is relatively small. Usually a liberal supply of oil is all that is required. When a large bearing becomes heated, a greater quantity of heat must be dissipated. The relatively small clearance in a large bearing tends to make it more sensitive to damage due to heating. The first thing to do when a large bearing heats is to increase the bearing brasses. If the bearing has not seized, but is extremely hot, it is usually sufficient to feed a liberal supply of steam cylinder oil (which possesses superior lubricating qualities at high temperatures) until the bearing cools, when normal oiling practice may be gradually resumed.

If a bearing has begun to seize, a little graphite or sulphur mixed with cylinder oil may be used to advantage. Castor oil and rapeseed oil are sometimes used for cooling bearings.

The use of cylinder oil or other emergency lubricants, in a circulation system, should be avoided. Where this system of lubrication is employed and a hot bearing is experienced, it is generally necessary to stop the machine and open up the bearing to find and remove the cause; this may be foreign matter in the bearing, or stoppage of the oil flow through obstructiions or bad joints.

In large circulation systems it is common practice to provide an oil cooler, by which the oil temperature is reduced before it returns to the bearings. In this case, a bearing that tends to heat may be cooled by increasing the flow of oil through it. The circulating oil is then both a lubricating and a cooling medium.

Bearings may often be cooled by water, which should be applied, if possible, to the shaft, near the bearing, and under no circumstance to the bearing; as, in the latter case, the shrinkage of the bearing around the heated and expanded shaft may result in seizure.

The Benefits of Correct Lubrication.

The correct lubrication of bearings, however, is not merely the avoidance of bearing troubles. Its effects are more far-reaching; reducing repair and replacement costs, cutting down power bills, and increasing the production of every machine in amounts that often constitute the difference between loss and profits.

If it were necessary for each operator to investigate in detail and learn the principles of correct lubrication from the ground up, the price would be prohibitive. This work, however, has been done. The facts are presented herein as fully as space will permit.

In its entirely, the subject of correct lubrication has been mastered completely by no individual. In so far as the problem has been solved, it has been the composite work of many, and the complete solution is only possible by the closest co-operation of operating and lubrication engineers who are making a special study of this phase of mechanical engineering.