FRICTION, in mechanics, the rubbing of the parts of engines and machines Against each other, by which means a great part of their effect is destroyed.
It is hardly possible to lay down gene ral rules concerning the quantity of fric tion, since it depends upon a multiplicity of circumstances, as the structure, firm ness, elasticity, &c. of the bodies rubbing against each other. Some authors make friction upon an horizontal plane, equal to one-third of the weight to be moved ; whilst others have found it to be consi derably less. Two objects must, how ever, be observed, viz. the loss of power which is occasioned by it, and the con trivances which have been made, and are in use, for the purpose of diminishing its effects. A body of a horizontal plane should be capable of being moved by the application of the least force ; but this is not the case, and the principal causes, which render a greater or less quantity of force necessary for it, are, _1, the rough ness of the contiguous surfaces ; 2, the irregularity of the figure, which arises either from the imperfect workmanship, or from the pressure of one body from the other ; 3, an adhesion, or attraction, which is more or less powerful, accord ing to the nature of the bodies in ques tion ; and 4, the interposition of ex traneous bodies, such as moisture, dust, &c.
Innumerable experiments have been made for the purpose of determining the quantity of obstruction, or of friction, which is produced in particular circum stances. 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 various concurring circumstances, produces a different result. Hence no certain and determinate rules can be laid down with respect to the subject of fric tion. Mr. Vince, who has done much on this subject, infers, 1, That friction is an uniformly retarding force in hard bodies, not subject to alteration by the velocity, except when the body is covered with cloth, woollen, &c. and in this case the friction increases a little with the velocity. 2, Friction increases in a less ratio than the quantity of matter or weight of the body. This increase, however, is differ ent for the different bodies, more or less, nor is it yet sufficiently known for any one body, what proportion the increase of friction bears to the increase of weight. 3, The smallest surface has the least friction, the weight being the same. But the ratio of the friction to the surface is not yet accurately known. Mr. Vince's experiments consisted in determining how far the sliding bodies wouldbe drawn in given times, by a weight hanging freely over a pulley. This method would both show him if the friction was a constant retarding force, and the other conclusions above stated. For as the spaces describ ed by any constant force, in given times, are as the squares of the times, and as the weight drawing the body is a con stant force, if the friction, which acts in opposition to the weight, should also be a constant force, then their difference, or the force by which the body is urged, will also be constant ; in which case the spaces described ought to be as the squares of the times, which happened ac cordingly in the experiments. The fric
tion, cxteris paribus, increases with the weight of the superincumbent body, and almost in the same proportion. The fric tion, or obstruction which arises from the bending of ropes about machines, is in fluenced by a variety of circumstances, such as their peculiar quality, the tempe rature of the atmosphere, and the diame ter, or curvature of the surface to which they are to be adapted. But when other circumstances remain the same, the diffi culty of bending a rope increases with the square of its diameter, as also with its tension ; and it decreases according as the radius of the curvature of the body to which it is adapted increases. Of the simple mechanical powers, the lever is the least subject to friction. In a wheel, the friction upon the axis is as the weight that lies upon it, as the diameter of the axis, and as the velocity of the motion. But upon the whole this sort of friction is not very great, provided the machine is well executed. In common pullies, es pecially those of a small size, the friction is very great. It increases in proportion as the diameter of the axis increases, as the velocity increases, and as the diame ter of the pully decreases. With a move able tackle or block of 5 pullies, a pow. er of 150 pounds will barely be able to draw up a weight of 500 pounds. The screw is subject to a great deal of fric tion ; so much so, that the power which must be applied to it, in order to produce a given effect, is at least double that which is given by the calculation, inde pendent of friction. But the degree of friction in the screw is influenced consi derably by the nature of the construction, for much of it is owing to the tightness of the screw, to the distance between its threads, and to the shape of the threads; the square threads producing, upon the whole, less friction than those which are sharp. The friction which attends the use of the wedge exceeds, in general, that of any other simple mechanical pow er. Its quantity depends so much upon the nature of the body upon which the wedge acts, besides other circumstances, that it is impossible to give even an approx imate estimate of it. The friction of me chanical engines not only diminishes the effect, or, which is the same thing, occa sions a loss of power, but is attended with the corrosion and wear of the prin cipal parts of the machine, besides pro ducing 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 of quite removing, the effects of friction.