"Lubricants " are substances employed to reduce friction. "Friction" may be described as the effect produced by two bodies sliding one. upon the other, which have upon their opposing surfaces minute asperities, that, interlock each other. The sliding movement, which forcibly removes these minute irregularities, creates what we call friction. Friction is reduced when these asperities are small, and lubrication is resorted to to prevent that loss of power caused by motion under these conditions. The chief lubricants used have a less coefficient of friction than the parts in contact. The term "co-efficient of friction" is an expression which indicates the proportion which resistance to sliding bears to the force which presses the surfaces together. There is little friction when this amounts to only 1/20; it is moderate when it is 1/10 and it is very high when it is 1/4, or 25 per cent., of the force which presses the surfaces together.
In a steam-engine, where many parts are moving, a large amount of friction is produced, which tends to stop those parts, and it would ultimately do it were they not continually re-supplied with fresh motion obtained by the burning of fuel. Hence it is apparent that the engine has not only to overcome the resistance of the work to be done, but also the resistance offered by its own parts. In other words, the amount of heat manifested in friction is the amount of extra heat that will have to be generated under the boiler, and the extra cost of working will be the cost of the fuel necessary to produce that heat. From the experiments of Morin, we find that the friction of a cast-iron shaft upon a dry bell-metal bearing amounts to 2 of the transmitted power, while with a wrought-iron shaft the friction is more" than '25; therefore, if such shafts were dry and unlubricated, 1/3 and 1/4 respectively of the total fuel cost would be wasted in overcoming friction. By careful lubrication of the same shafts, the loss may be reduced to •065 m the one case, and '089 in the other. Hence the importance of a good lubricant.
One of the next essential points is that it shall be properly distributed over the surface on which it is required, that just sufficient shall be used, and all waste avoided; otherwise, what would have to be spent in overcoming friction will have to be spent in buying oil.
For very heavy bearings, tallow and other solid lubricants are used, such as mixtures of sulphur and tallow, asbestos, soapstone with asbestos, graphite, caustic soda, beeswax, and other similar mixtures which find favour among locomotive engineers and those in charge of heavy machinery. The pressure that can be borne by a good lubricant for a useful length of time depends upon the nature of the bearings as well as upon the lubricant itself. The velocity of the rubbing action also must be taken into consideration. The maximum of pressure that solid lubricants will bear without destruction is unknown. For steel surfaces, lubricated with the best sperm-oil, moving slowly, 1200 lb. pressure per sq. in. of bearing surface has been found permissible. Under the pivots of swinging bridges, several thousand lb. per sq. in. have been found to work; for iron journals, 800 lb. per sq. in. should not be exceeded.
An efficient lubricant must exhibit the following characteristics:-(1) Sufficient " body " to keep the surfaces between which it is interposed from coming into contact; (2) the greatest fluidity consistent with (1); (3) a minimum co-efficient of friction; (4) a maximum capacity for receiving and distributing heat; (5) freedom from tendency to "gum or oxidize; (6) absence of acid and other properties injurious to the materials in contact with it; (7) high vaporization- and decomposition-temperatures, and low solidification-temperature; (8) special adaptation to the conditions of use; (9) freedom from all foreign matters. The modern methods of testing the lubricating qualities of oils are directed to a discovery of the following points:-(1) Their identification and adulteration; (2) density; (3) viscosity; (4) "gumming"; (5) decomposition- vaporization-, and ignition-temperatures; (6) acidity; (7) co-efficient of friction. The 1st and 2nd stages are described very fully in an original article by Dr. Muter in Spoils 'Encyclopaedia,. pp. 1462-1477. The viscosity and gumming tendency may be simultaneously detected by noting the time required by a drop to traverse a known distance on an inclined plane.
A 9 days' trial gave the following result:-Common sperm-oil, 5 ft. 3 in. on the 9th day; olive-oil, 1 ft. 9 1/2 in. on the 9th 'day; rape-oil, 1 ft. 7 3/4 in.on the 8th day; best sperm-oil, 4 ft. 6 1/3 in. on the 7th day; linseed-oil, 1 ft. 6 1/4 in. on the 7th day; lard-oil, 11 3/4 in. on the 5th day. The day given is in each case that on which the oil ceased to travel. There are several ways of applying the plane test. A very simple and general test of fluidity is. to dip blotting-paper in the oil, and hold it up to drain: symmetrical drops indicate good fluidity; a spreading tendency, viscosity. Retention of the oil on the paper for some hours at 200° F. (93 1/2o C), or for some days at ordinary temperatures, will show the rate of gumming. (Thurston.)
Putting aside the commoner characteristics of a good oil, such as the absence of acidity either natural or artificial, and the absence of gumminess, one of the most commonly believed ideas is, that an oil of high specific gravity is the best for lubricating purposes. Although this may be true in certain cases, yet from observations and experiments made oyer a long period it appears that they are not always the best, and that the point upon which we must rely is the viscosity. To test this, a French pourette graduated into 100 cc. is most useful. The pourette is fitted on a stand and filled with the oil to be tested; after allowing all buboles of the air to separate, it is permitted to run through, and the time it takes to do so is carefully noted. At the close of the experiments, it will be found that the viscosities are directly proportional to the time taken; thus, if a mineral oil takes 15 seconds, and rapeseed-oil 45 seconds, the viscosity of rapeseed-oil is 3 times that of the mineral. The temperature may be either 60° or 90° F. (15 1/2° or 32° C), but the latter is preferable, as the oil may be subjected to that temperature when in use.