Friction match

“ But in a delicate machine like a watch, especially in the escapement, where the power is so light, and where the rubbing surfaces are so hard, smooth and regular, the solid friction is so minute as compared to the fluid friction, that the former is relatively very slight, as compared with the latter. ”

“ It is easy to see the reason why friction should diminish when the size of the pulley is increased. The friction acts at the circumference of the axle about which the wheel turns; it is there present as a force tending to retard motion. Now the larger the wheel the greater will be the distance from the axis at which the force acts which overcomes the friction, and therefore the less need be the magnitude of the force. ”

“ In the wheel and axle upon which we have been experimenting, we have found that about 30 per cent. of the power is consumed by friction. We shall be able to ascertain to what this loss is due, and then in some degree to remove its cause. From the experiments of Art. 165 [Pg 156] we learned that the friction of a small pulley was very much greater than that of a large pulley—in fact, the friction is inversely proportional to the diameter of the pulley. We infer from this that by winding the rope upon a barrel instead of upon the axle, the friction may be diminished. ”

“ If the magnitude of b exceeds a certain limit, then a is pulled along the table and b descends; but if b be smaller than this limit, both a and b remain at rest. When b is not heavy enough to produce motion it is supported by the tension of the cord, which is itself neutralized by the friction produced by a certain coherence between a and the table. Friction is by this experiment proved to be a force, because it prevents the motion of b. Indeed friction is generally manifested as a force by destroying motion, though sometimes indirectly producing it. ”

“ The latter illustrates what is called rolling friction, and in practical mechanics it is used constantly to decrease the loss—as, for example, in the wheels of wagons and cars. The use of lubricants to decrease friction is equally familiar. Without them, as everyone knows, it would be impossible to run any wheel continuously upon an axle at high speed for more than a very brief period, owing to the great heat developed through friction. Friction is indeed a perpetual antagonist of the mechanician, and we shall see endless illustrations of the methods he employs to minimize its influence. ”

“ Edward Rigg has this to say in regard to the apparatus of Jenkin and Ewing. "The friction, then, is true sliding friction without any rolling, and it will be evident that if the bearing were a circular hole just large enough to admit the pivot freely, the character of the friction would be in no way changed. In both a watch and clock the pivots are pressed against the sides of the pivot holes, either by the motive force or by gravity. There is no rolling round the pivot holes, so that the friction is all of the first kind. ”

“ Friction-Couplings.—Friction is useful as a means of communicating motion where sudden changes either of force or velocity take place, because, being limited in amount, it may be so adjusted as to limit the forces which strain the pieces of the mechanism within the bounds of safety. Amongst contrivances for effecting this object are friction-cones. A rotating shaft carries upon a cylindrical portion of its figure a wheel or pulley turning loosely on it, and consequently capable of remaining at rest when the shaft is in motion. ”

“ The upper limit should never be approached in machinery.
The coefficient of friction is that quantity which, being multiplied by the total pressure acting normally to the surfaces in contact, will give the measure of the maximum frictional resistance to motion.
34. Sliding Friction is Proportional to Pressure according to the third law quoted above. This is easily demonstrated by ascertaining what force is necessary to produce, or continue, motion in a body lying on a plane surface; double the weight of the body and the force required to produce, or continue, motion, will have to be doubled. ”

“ With heavy machinery, as the hardness and degree of polish of the surfaces cannot be increased in proportion to their weight, the solid friction is so great that while the interposition of a lubricant between the surfaces adds fluid friction, it also reduces the solid friction ”

“ The exceptions are, some increase when the rubbing surfaces under the same total pressure are very greatly extended, or when either surface is comparatively soft; and considerable lessening of friction when the bodies are very small, as in the runners of skates upon ice. For ordinary rates of motion, the total friction within a given space or distance is in like manner entirely independent of the velocity with which one surface is caused to move over the other ”

“ Hence, even with the assistance of the load, a force of 3·4 lbs. is necessary to overcome friction. This will enable us to determine the amount of friction in the same manner as we determined the friction in the pulley-block (Art. 207) . ”

“ The friction pulley is constructed with a friction ring upon its arms, the outer ring acting as a guard or protection, the friction ring being turned accurately to receive the wooden shoes attached to the driving arm; these wooden friction shoes are fitted to the outer surface of the ring as well as the inner, all being connected to one set of levers; so when force is applied to engage the friction to start the engine there is no tendency or danger of bursting the rim, as the pressure is equal on both sides of it. ”

“ For perfectly lubricated surfaces, were such attainable, assuming it practicable with complete separation of the surfaces, the laws of friction, according to Thurston, would become:
1. The coefficient is inversely as the intensity of the pressure, and the resistance is independent of the pressure.
2. The friction coefficient varies as the square of the speed.
3. The friction varies directly as the area of the journal bearing.
4. The friction varies as the temperature rises, and as the viscosity of the lubricant is thus decreased (80) . ”

“ In other cases the safety of the joint against displacement by sliding depends on its power of exerting friction, and that power depends on the law, known by experiment, that the friction between two surfaces bears a constant ratio, depending on the nature of the surfaces, to the force by which they are pressed together. In order that the surfaces which abut at the joint JK may be pressed together, the resistance required by the conditions of equilibrium CR, must be a thrust and not a pull ”

“ I need not
illustrate so well-known a principle by experiment. Solid
friction disappears when the liquid intervenes. In its place we
substitute the lesser difficulty of shearing one layer of the
liquid over the other; and if we keep up the supply of oil the
work required to do this is not very different, no matter how
great we make the pressure upon the bearings. Compared with the
resistance of solid friction, the resistance of fluid friction is
trifling. Here under the skate the lubrication is perhaps the
most perfect which it is possible to conceive. ”

“ The Laflin Friction Rowing Machine Do not use oil on friction cylinder. If its action is not perfectly smooth a little clear soap rubbed on its surface will properly correct its action. The means used to produce the resistance is a simple friction clutch, which takes instant hold at the commencement of the stroke and retains the pressure till its completion, when it instantly releases it precisely as in a boat. Quickly taken apart without loosening any bolts or screws. Each machine is adjustable to any amount of friction or resistance. ”

“ Friction is the resistance that must be overcome when one body moves over another. It is of two kinds, sliding and rolling. If one draws a block and then a car of equal weight along a board, the force employed in each case being measured [Pg 148] by a spring balance, a large difference in the force required will be noticed, showing how much less rolling friction is than sliding friction. ”

“ In either of these cases, the friction increases more rapidly than the ratio of normal pressure.
No surfaces can be made absolutely hard or smooth; when one surface is made to slide over another, the slight roughness of the one interlock with those of the other, so that the surfaces must be separated or the points abraded to allow of the motion; but if one surface roll upon another, the prominent points are successively raised, without the need of complete lifting of the body or wearing off those points. ”

“ This expectation is entirely borne out by experimental researches. Beauchamp Tower (“Report on Friction Experiments,” Proc. Inst. Mech. Eng., November 1883) found that when oil was supplied to a bearing by means of a pad the coefficient of friction was approximately constant with the value of 1⁄100, thus following the law of solid friction; but when the journal was lubricated by means of an oil bath the coefficient of friction varied nearly inversely as the load on the bearing, thus making μW = constant. ”

“ Those who have used friction rollers, know that it is a thing of great difficulty, to place their axes exactly parallel to that which they are intended to support: and even, if rightly placed at first, that a small degree of abrasion, greater on one pivot than another, will soon destroy that parallelism; and thus introduce a growing friction, capable, at length, of rendering the whole completely useless: for although the original friction is lessened by being transferred to a slower-moving axis, yet the latter still resists in some degree, say 1⁄4 of the whole ”

“ Referring to the friction as influenced by the nature of the metals in contact: the friction varies with the hardness of the metal; thus, with hard cast iron, there will, under equal conditions, be less friction than with soft cast iron. The friction is greater when the surfaces in contact are both of the same metal than when they are of different metals. ”

“ In transportation on land friction is a much more serious bar to motion than it is in marine or aerial transportation. There are two kinds of friction that we have to contend with; sliding friction when two contacting surfaces are dragged by each other; and rolling friction when one surface rolls upon another. It is impossible to obtain two surfaces that are absolutely smooth ”

“ It is hardly possible to lay down general rules for computing the quantity of friction, because it depends upon a multiplicity of circumstances, as the structure, firmness, elasticity, &c. of bodies rubbing against each other. Some authors make the friction upon a horizontal plane, equal to 1⁄3d of the weight to be moved; while others have found it to be considerably less. ”

“ With oil the friction appears to be a minimum when the pressure on the surface of a bearing is about 90 lbs. per square inch. The friction from too small a surface increases twice as rapidly as the friction from too large a surface, added to which, the bearing, when the surface is too small, wears rapidly away. ”

“ After several trials of this, however, it was clearly seen that the friction was too slight to be noted accurately by a gauge registering single pounds, that is, with a piston in contact with 6 in. of sand vertically, a friction of 25 lb. per sq. ft. would only require an increase of 1.8 lb. on the gauge. It is therefore assumed that the friction on so small a piston in sand need not be considered as a material factor in the experiments made. ”

“ Wherever two surfaces of metal must rub against each other, as in the case of the piston and the cylinder, there is bound to be a great amount of friction. This friction causes the parts to heat and in time it wears away the surfaces and destroys the efficiency of the engine. ”

“ In certain cases, where it is of great importance to remove the effects of friction, a contrivance called friction-wheels, or friction-rollers, is used. The axle of a friction-wheel, instead of revolving within a hollow cylinder, which is fixed, rests upon the edges of wheels which revolve with it; the species of motion thus becomes that in which the friction is of least amount. ”

“ Whenever one surface of matter is moved in contact with another surface there is a resistance called friction, [Pg 40] the moving body loses its rate of motion, and will presently be brought to rest unless energy be continuously supplied. This is true for masses of matter of all sizes and with all kinds of motion. Friction is the condition for the transformation of all kinds of mechanical motions into heat. The test of the amount of friction is the rate of loss of motion. A top will spin some time in the air because its point is small. ”

“ The angle made by the sides of the cones with the axis should not be less than the angle of repose. In the friction-clutch, a pulley loose on a shaft has a hoop or gland made to embrace it more or less tightly by means of a screw; this hoop has short projecting arms or ears. A fork or clutch rotates along with the shaft, and is capable of being moved longitudinally by a handle. When the clutch is moved towards the hoop, its arms catch those of the hoop, and cause the hoop to rotate and to communicate its rotation to the pulley by friction. ”

“ The coefficient of friction varies with the condition of lubrication of the surface of the pulley, which alters during the running and with every change in the velocity and temperature of the rubbing surfaces. ”