The Lubrication of Bearings.

Progress, Volume III, Issue 4, 1 February 1908, Page 122

The Lubrication of Bearings.

All machine users are interested in the subject of lubrication, since it so closely effects the efficiency of the machine. The subject is fully and suggestively treated b)Mr. F. H. Davies, in an article in the Eletfrical World, from which we reproduce the salient passages. The writer begins by explaining the function of the lubricant. In any bearing the function of the lubricant is to convert rubbing into rolling friction by the intervention of the minute globules of which the lubricant is composed ; consequently, in a well-lubricated bearing the metallic surfaces never actually touch one another, but are separated by a film of oil, more or less thick, according to its quality and the amount used. In the case of a bearing which is loaded in a downward direction, say to a pressure of 101b. per square inch of area, it is obvious that the oil between the lower surfaces has to transmit an equivalent force in order to keep them apart, and it

therefore follows that oil cannot be fed between these surfaces unless it is supplied at the above pressure. Now, the case in point is that of an ordinary bearing fed at the top by a drip lubricator having no mechanical arrangement to put pressure upon the oil, and the question is : How does the latter get between the lower surfaces where the oil is under a pressure of 101b. per square inch ? The result is brought about by the property of adhesion in conjunction with the velocity with which the oil is carried round by the shaft. In the above case the shaft naturally lies on the bottom surface of the bearing, leaving a small but important crescentshaped space at the top. The oil from the lubricator falls into this space, and spreads itself thickly over the upper surface of the shaft, adhering to it ; the film between the bottom surfaces is, of course, much thinner. As the shaft rotates, the freshly fed oil is carried into the gradually narrowing space, acquiring a high velocity, and by its adhesive properties still clinging to the journal. It is now, of course, considerably compressed, and the farther it is carried the greater its pressure becomes, until finally it acquires a pressure equal to that between the lower surfaces. The question of lubricating oils is a very important one, and by far too large a subject to be more than touched on here. The chief points to be considered in the selection of an oil aTe the pressure on the bearing or the size of the engine, and the temperature at which it is presumed the part to be lubricated will work. What is wanted is a fluid that possesses the lowest molecular friction, and from this point of view the thinner the oil the better. On the other hand, thin oil does not lend itself very well to what may be termed adhesive action of the bearing, and as the latter heats up it gets thinner, and consequently worse and worse in this respect. On the contrary, thick or heavy oil, owing to its greater adhesiveness, will force its way in even against the greatest pressure, but when there it is not so efficient as thin oil. It is obvious that in a case like this the idea is the happy medium, and it is at this result that most manufacturers of oils aim. In an engine or any machine it is customary to use the same oil to lubricate all the external parts. It should be remembered that while this is the only method possible in practice, it is theoretically wrong, as reciprocating bearings require a heavier oil than guides or journals, in order to reduce as much as possible the flow from top to bottom of the " brass " consequent upon the alternating impacts. It is conceivable that a greater proportion of a thin oil would be forced round to the side where there is no load than would be the case where a thick oil with its superior viscosity is used,