Friction, in Mechanics, the rubbing of the parts of engines or machines against each other, by which means a great part of their effect is destroyed. A body upon a horizontal plane should be capable of being moved by the smallest application of force; but this is not the case, and the principal causes which render a greater or less application of force necessary are, first, the roughness of the contiguous surfaces; secondly, the irregularity of figure, which arises either from imperfect workmanship or the penetration of one body by another; thirdly, an adhesion or attraction, which is more or less powerful according to the nature of the bodies in question; and fourthly, the interposition of extraneous bodies, as dust, moisture, etc. Innumerable experiments have been made to determine the amount of friction or obstruction which is produced in particular circumstances; but the results of apparently similar experiments which have been made by different experimenters do not agree, nor is it likely they should, since the least difference of smoothness or polish, or of hardness, or, in short, of any of the concurring circumstances, produces a different result: hence no certain and determinate rules can be laid down on the subject of friction. Mr. Vince, who has done much on this subject, infers, first, that friction is a uniformly retarding force in hard bodies, not subject to alteration by the velocity except when the body is covered with cloth, woollen, etc, and in this case the friction increases slightly with the velocity; secondly, friction increases in a rather less ratio than the weight of bodies; the rate of increase, however, is various in different bodies, nor is it sufficiently determined for any one body what proportion the increase of friction bears to the increase of weight. The smallest surface, at least up to a certain point, has the least friction, the weight being the same; but the ratio of friction to the surface is not yet accurately known. The friction of mechanical engines not only diminishes the effect, or, which is the same thing, occasions a loss of power, but is attended with the corrosion and wear of the principal parts of the machine, besides producing a considerable degree of heat, and even actual fire; it is therefore of great importance in mechanics to contrive means capable of diminishing, if not quite removing, the effect of friction.
The methods of obtaining the important object of diminishing friction are of two sorts, viz. either by the interposition of unctuous or oily substances between the contiguous moving parts, or by particular mechanical arrangements. Olive oil is the best and perhaps the only substance that can be used in delicate work, as clocks and watches, when metal works against metal; but in large works the oil is liable to drain off unless some method be adopted to confine it. The best contrivance with which we are acquainted, for preventing the waste of oil, and for keeping gudgeons or axes properly supplied with it, is Barton's Patent Lubricator, a section of which is shown in the accompanying engraving, with the manner of applying it to the shafts of mill work, a shows a section of a metallic vessel filled with oil, and closed by a lid to prevent the admission of dust or other adventitious matter; b is a small tube rising to nearly the top of the vessel, and with the lower part extending an inch or two below it, and inserted into an aperture made through the plummer block, directly over the shaft c, shown also in section; through this tube a few threads of woollen yarn are drawn, which reach to the bottom of the vessel, and conduct the oil by capillary attraction, as a syphon, in minute but regular quantities to the shaft or gudgeon; the whole of the oil in the vessel is thus carried over, entirely free from dust or other impurities, and in the precise quantity required, which is easily regulated by the number of threads.
It must be obvious that the economy of this contrivance 5s very considerable; that machinery, where it is applied, win run with less friction, last longer, and require less power. Since the above cut was executed, Mr. Barton has greatly improved these lubricators, and materially extended their utility. From some experiments which have been made, it appears that when the strain is very great the solid unguents appear to be more effectual in diminishing friction than oils, and in this case tallow or swine's grease is generally employed. The celebrated "Anti-Attrition Composition" is simply a mixture of hog's lard and plumbago, in the proportion of four parts of the former to one of the latter. In launching ships the " ways" are smeared with soft soap. The mechanical contrivances for the diminution of friction consist either in avoiding the contact of such bodies as produce much friction, or by substituting a rolling for a sliding motion, as far as it may be practicable. As an instance of the first method we may notice that in mill work, the wooden axes of large wheels terminate in iron gudgeons, turning generally in brass bearings, which produces less friction than wood upon wood; and as the iron gudgeon can be made of smaller diameter than wooden ones of the same strength, the friction is also diminished from that cause in nearly the same ratio.
The conversion of a sliding motion into a rolling motion is effected by interposing cylindrical bodies between the moving parts of machines, which, according to their size and arrangement, are denomi-nated rollers, wheels, and (although improperly) friction rollers. In order to understand the nature of rollers, and the advantages attending their use, it must be considered that when one body is dragged over the surface of another, the inequalities of the surfaces of both bodies meet and oppose each other, which is the principal cause of friction or obstruction; but when one body, such as a cask, a cylinder, or a ball, is rolled upon another body, the surface of the roller does not rub upon the latter, but its parts are successively applied to or laid upon it, and are afterwards lifted up from it; therefore, in rolling, the principal cause of friction is avoided, and other advantages also obtained: thus, in mounting a carriage upon wheels, instead of placing it upon skids or a sledge, the only friction arising from the sliding of one part over another is that which takes place between the axle and the box in which it works.
The diminution of friction from this cause will be in the proportion of the diameter of the wheel to that of the axle, and it is further diminished by the friction being that of metal sliding upon metal, which offers much less resistance than the best made road could be brought to do, and also that the friction may be further reduced by means of lubricating substances. When the sliding motion in machinery is not in a rectilinear direction, but arises from the revolution of axes in their bearings, a great part of it may be converted into a rolling motion by supporting the axes upon the peripheries of four wheels instead of the usual fixed bearings; an instance of this is seen in the elegant machine invented by Mr. Atwood for illustrating the laws by which the descent of falling bodies is regulated. But although friction detracts from the effect of machines, and it is therefore generally an object to reduce it to the utmost, yet the action of some parts of machinery depends upon friction, as in the case of the brake of a crane or the drag of a coach.
For light machinery, also, wheels are sometimes made to turn by contact, by simply covering their periphery with buff leather, the resistance of the work not being sufficient to overcome the friction between the two surfaces.