This section is from the book "A Practical Treatise On Friction, Lubrication, Fats And Oils", by Emil F. Dieterichs. Also available from Amazon: A practical treatise on friction, lubrication, fats and oils.
To overcome friction and put its resisting power to as low a point as possible, we use lubricants that can absorb the frictional heat and, becoming vaporized by it, will carry the heat into space.
Lubricating is a necessity, and is the most important factor in the mechanical world. Without lubrication all the power we can obtain from Steam, Electricity, Gas, Water, Air and Horse Power, Spring and Wind Power could be of no use to us, and travel on railroads and steamboats, the running of factories, the riding in automobiles, in carriages and wagons, the use of sewing machines, the riding on bicycles, the keeping record of time by our watches ; in short, the using of anything that is dependent on mechanical motion would be utterly impossible.
We have then to investigate, "What is Lubricating?"
Nature teaches us the laws of lubrication by its wonderful workings in the human body and in the bodies of all animals. All the joints of the bones in the body would be useless and stiff were it not for their being constantly lubricated by the so-called "Joint Water," an unctuous fluid which surrounds all the joints of the skeleton part of the body. This " Joint Water" is constantly produced and supplied by nature, and is constantly consumed by the frictional heat created by our exertions and movements, and is likewise constantly disposed of and frees the body from the otherwise accumulating frictional heat by transferring it, with the perspiration and exhalations from the body, into space. When the recuperating powers of the body fail to operate properly, from one cause or another, the inflammatory condition of the joints gives evidence of the absence of proper lubrication, and the final failing and drying-up of this lubricating " Joint Water " under diminished generating power in advanced age cause the joints to move with difficulty and pain, and in the end make them lose their usefulness altogether.
So it is with machinery.
Whenever the surface of one part of machinery in motion is bearing on the surface of another, friction is created and friction creates heat. This heat is involved and increases with the velocity and continued motion, and if not taken up and carried away by lubrication, will finally increase and accumulate to such an extent that the machinery will have to be stopped to allow the metal to cool off.
Metal is able to absorb a large amount of fric-tional heat, but it is unable to free itself of it as fast as it is generated by continued and rapid motion, and the heat finally accumulates to such an extent as to overheat the bearings, and if further continued will so increase and expand the metal as to cause the parts to weld themselves tightly together. This has frequently been the case with the old style flour-mill spindles, which, after becoming overheated, were found to be so tightly welded in their steps, that they had to be chiseled out. To avoid this we have to keep the revolving parts well lubricated in their bearings, and we have to continue doing this as long as the machinery is kept in motion, and in exact proportion to the frictional heat evolved and the amount of work we expect to have done.
That frictional heat is created by motion and that it vaporizes the lubricant, is shown by the following convincing experiments made many years ago by the celebrated scientist, Count Rumford:
He had a metal vessel constructed, with hollow bottom, had a perpendicular shaft fitted to it, and had it rapidly moved by mechanical power.
He then filled the vessel with water, and in the course of four hours ascertained that the water, by the absorption of the frictional heat generated, had attained a temperature of 140° F., and in eight hours had reached the boiling-point. After this he found the water to evaporate and gradually diminish in bulk as long as he kept the shaft in rapid motion.
If we now substitute oil for the water, we will find the oil also gradually attain the temperature of its evaporating point, commonly known as "flash or fire test," and will find it also to vaporize and gradually reduce its bulk until the shaft be stopped moving.
This shows conclusively that frictional heat accumulates, and that it is disposed of by evaporating into space; this evaporation, though continuous, is invisible. Lubricating, therefore, cannot be simply an interposition of some substance, as a sort of cushion, between the metallic surfaces of machinery in motion. Soft metallic compounds, such as plumbago and some of the finer grades and inert matter, asbestos, mica, sulphur, lime and soapstone, have been recommended and have been tried for that purpose; but it has been found that while all such substances serve well, in small quantities, to fill the interstices which exist in all metallic surfaces of bearings however highty polished, and thereby presenting a smoother bedding for the revolving shaft, they can only absorb so small a portion of the heat created by the friction as the metal itself of which the machinery is constructed.
The capacity of plumbago and other inert matter for absorbing and carrying away the fric-tional heat is very low, as they cannot vaporize, while the capacity of oil and fatty matter is very great, and we are, therefore, compelled to use oil and fatty matter for lubricating.
If lubricating would be simply a mechanical action, and if a cushion of soft metal or other inert matter, or of oil or fat, would be sufficient to prevent the gradual increase and accumulation of frictional heat, then a very limited amount of oil, fat or other inert matter, once applied, should be sufficient. We find, however, that we are obliged to renew the lubricant with regularity and in exact proportion to the frictional heat created by the motion, and in exact proportion to the amount of work we expect to do with the machinery and we have to do this as long as the latter is kept in motion.
We have then to ask : What has become of the large quantities of oil which we were compelled to constantly apply to the bearings of the machinery ?
As coal and water are constantly consumed to keep the supply of steam up to move the machinery; so is oil constantly consumed to draw the frictional heat away from the bearings. We can see how the coal and water are consumed, but we are unable to see how the oil is consumed. We can, however, find silent proof that it is so, and that lubricating is a strictly chemical process and not a mechanical one.
We know that metal cannot absorb oil, and if we allow most liberally for all possible wasting and for transformation of much of it into gummy accumulations around the bearings and in cylinders, we must admit that a very limited number of gallons from every barrel of oil used could thus be accounted for, and it remains to be seen what has become of the balance.
We well know at what temperature water is evaporated and converted into steam, and after serving its purpose to move the machinery is lost into space. Precisely the same chemical process, the transformation from a fluid into a gaseous state, takes place when oil is used for lubricating.
When the oil becomes heated by the frictional heat until its evaporating temperature is reached, it becomes, like steam, a gas, and is lost into space with the frictional heat it has absorbed in exactly the same manner as oil when distilled from a still is transformed from its fluid state into a gaseous one to be rendered to a liquid state by passing through a condenser.
This transformation takes place on every bearing, although on so small a scale as to be almost entirely imperceptible to our senses. Where the revolving shaft rests heaviest in the hollow of the bearing, there is the line to be drawn where this invisible transformation of the oil from the liquid into the gaseous state takes place. This line is exceedingly small - perhaps no more than the thickness of the finest sheet of paper - but on this small line the frictional heat starts to be generated, and being taken up by a few atoms of the oil at a time, is carried with their vapors into space. When from neglect or insufficient lubrication bearings become overheated, and under the rapidly increasing temperature the few particles of oil vaporize too fast and become decomposed under the increased heat, the arising vapors, with a penetrating burning smell, prove to us the slow and mysterious process by which the oil disappears.
Lubricating is, therefore, a chemical process, and requires the interposing of such substances between the moving parts of machinery as are capable to absorb the frictional heat, and vaporized by it, carry it into space. Such qualifications are best possessed by oil and fatty matter, and we, therefore, use them for lubricating our machinery.
A continuous stream of water or the application of ice will likewise absorb and vaporize with the frictional heat, but not possessing sufficient adhesive body, cannot prevent gradual abrasion of the metal.
It has lately been claimed that molasses could be used, like oil, to lubricate machinery, but while molasses possesses adhesive quality (viscosity) its lubricating power consists in the water contained in its body which is not efficient enough to prevent abrasion of the metal.
For over forty years I have contended in my writings and lectures that lubrication of machinery is not a mechanical process, a mere interposition of some substance as a cushion between the surfaces, but is a strictly chemical process, the transformation of a substance by heat from a liquid into gaseous state. I have theretofore elaborately explained my observations on the subject of the theory of lubrication and have asked for refutation or endorsement of my contention by scientists and those acquainted with the oil industry, but I have met in publications and printed compilations only with endless chat-terings about viscosity in connection with the old cushion theories, but not a word about of " what is lubrication," while it is so distinctly explained by the immense amounts of fats and oils constantly consumed and lost in the process and the constant necessity for replacement with new supplies.