1902 Encyclopedia > Brake

Brake




BRAKE is an instrument by means of which mechanical energy may be expended in overcoming friction. It is used for several entirely different purposes, the principal of these being (a) to limit or decrease the velocity of, or in. some cases to bring completely to rest, the body or system

of bodies imparting the energy; (6) to measure directly the amount of frictional resistance between two bodies; or (c) to measure, indirectly, the amount of energy given out by the body or system of bodies producing it.
The first case is the familiar one occurring in the brakes of locomotives, railway carriages, and wheeled vehicles generally, and in those applied to such machines as cranes, winches, &c. Here some system of bodies,—or for simpli-city's sake we may say some body,—originally at rest has been set in motion, and has received acceleration up to a cer-tain velocity, the work which has been done in that accelera-tion being stored up as " actual energy" in the body itself. Before it can be brought to rest it must part with this energy, expending it in overcoming some external resistance. Very frequently the actual energy is very large in propor-tion to the usual resistance opposing the motion of the body, so that that motion would continue for a long time, or through a great distance, before the whole energy had been expended and the body brought to rest. For the sake of convenience, and in certain cases for the sake of safety, it is often necessary that this time or distance should be greatly shortened. This may be clone by artificially increasing the resistance for the time being, and the most convenient method of doing this is the use of a brake.
The construction of railway brakes falls to be treated in detail in the article RAILWAYS. In other vehicles the brake belongs generally to one of two classes—it is either a block which can be pressed against a wheel by a suitable arrangement of levers under the control of the driver, or a slipper or "skid" which can be placed under a wheel, and which is attached by a chain or otherwise to the body of the vehicle. The increased resistance is due in the one case to the friction between the block and the wheel, and in the other to that between the skid and the road.
In the case of hoisting-machines the brake is used very frequently as a means of controlling the velocity of the descent of the load. In the process of "lowering by a brake," its frictional resistance is alone opposed to the load, and suitable mechanical means are provided for varying that resistance so that the velocity of the descending weight may be kept within the desired limits. The brake used in these machines very frequently consists of a cylindrical pulley or narrow drum encircled by a flexible belt of iron or steel. One end of this belt is fastened to the framing, and it is so formed that in ordinary work the drum revolves in it without touching it. When necessary, however, the position of the movable end can be so altered as to bring a larger or smaller area into contact, the surfaces being held together with a pressure which can be varied to suit the requirements of each case. This is effected either by a simple lever (in small machines worked by the foot), or for heavier work by the aid of a screw and hand-wheel.
In what are known as " differential" brakes the brake-band is not fixed to the frame of the machine, but both its ends are attached to

points in a movable lever in such a way that motion of the lever affects them unequally, tightening one more than it loosens the other, or loosen-ing one more than it tightens the other. The principle of such an arrangement is shown in fig. 1. Here A. is the pul-ley, B the brakeband, and C the working lever; B being attached to the latteratpoints « and b unequally distant obvious that for any motion of C the angular motions of the arms Da and Db are equal, but the instantaneous linear motions of the points a and 5 in the directions of the band are unequal, varying directly as Da, D6, the ratio of the normals from D upon those directions. Thus any motion of C to the right tends to tighten the lower part of the belt and to slacken the upper part, but the slackening takes place through a larger distance than the tightening, and the belt is therefore released from the drum. By moving the lever to the left, on the other hand, the opposite action occurs, and the belt is correspondingly tightened.
Instead of using the friction between two solid bodies, in some special cases the frictional resistance of a fluid is employed, as in what Professor Rankine called fan brakes and pump brakes. In the one case the motion of revolving blades (commonly) is opposed by the resistance of the atmospheric or liquid medium surrounding them, and in the other the motion of a liquid is opposed by the resistance due to a narrow passage or orifice.
The measurement of the frictional resistance between two bodies of known material or form is often of great importance, and it is still more often of importance to measure, by means of the frictional resistance which it can balance, the amount of energy given out by some engine or machine. Both these measurements can be and are frequently made by means of brakes. For this purpose the apparatus must be so made that the actual resistance can be accurately measured,—that this resistance can be kept sensibly constant for any length of time, but can be altered at will,—and also that the brake can be kept continuously at work for any desired period. The brake used for this purpose commonly takes the form of a revolv-ing drum of iron, encircled by a ring of hard wood blocks connected together by thin iron bands. To this ring is attached a weight of known magnitude, at a known distance from the centre of the pulley. The wheel being set in motion the blocks can be gradually pressed upon it by a screw until the friction occurring is just sufficient to lift the weight and keep it off the ground or its support. So long as these conditions can be maintained the frictional resistance is exactly known, for its magnitude must be to that of the weight inversely as their distances from the centre of the wheel, and the energy expended in any given time will be equal to this resistance multiplied by the space passed through in the time by any point in the periphery of the drum.


Fia.l.—Differential Brake.
from the fulcrum D. It is


Figs. 2 and 3 show the construction of a brake of this kind, copied from drawings kindly furnished by Mr W. H. Maw, the designer. The

Fias. 2 and 3.—12 H.P. Friction Brake (designed by W. H. Maw).
drum A has a turned cylindrical surface 39 inches in diameter .and 34 inches wide ; it is fixed upon the shaft B with which it revolves. The brake-ring consists of fourteen wooden blocks CC, connected


„/ the straps DD of hoop-iron, and so arranged that they can he lightened up by the screw E. To the ring there is attached at G a pin with pointed ends, from the centre of which hangs the rod H carry-ing, by means of the plate at K, the weights. The blocks are made to exert such a pressure upon the wheel that the pin G, from which the weights are suspended, remains always in the position shown, its pointed ends coinciding with the top of gauges upon the fixed wooden beams beside the brake. This apparatus is made to a certain extent self-adjusting by means of two compensating levers. The upper end ot one of these, M, moves freely through an eye N some dis-tance below the centre of the shaft ; the hoop-iron rings are attached to its lower end at 0 and P, the latter further from N than the former. If N coincided with the centre of the shaft the lever would cause no impediment to the motion of the ring along with the drum ; as it is actually placed, however, if any motion occur the point 0 must move through a relatively smaller distance than in the other
. ON. , OB mi pl-
ease, because the ratio is less than p g- The consequence of this
is, that if through any cause the drum carry the ring round with it through any small distance, 0 tends to tighten the belt, and P to slacken it, but the relatively larger movement of the latter causes the final result to be a slackening, so that the weight drops back into'its proper place. If, on the other hand, the ring begins to slide back on the drum, a similar, but reversed, action at OP increases the pressure on the blocks, and the drum again gripping them, brings the weight back into its original position. The dash-pot L contains a piston very nearly fitting it, below which is water or oil, its object being to prevent too sudden a fall of the weight. It is really a little brake in itself, in which the energy given out by the descending weight is expended in overcoming the frictional resistance encountered by the water in passing upwards through the narrow space round the piston.
In order that a machine of this kind may be run continuously at a high velocity it requires careful and abundant lubrication. If the surface be too small, water will have to be used, but with a well-designed and not over-worked brake, tallow is the best lubricant.
AYith regard to the proper proportioning of surface, numerous experiments with brakes of different sizes run at different speeds seem to show that the surface required varies as the energy transmitted, and (approximately) inversely as the peripheral velocity of the drum. The conclusions drawn from them, put in the most general form, are (for a brake having a cast-iron drum and wooden blocks) as follows: —Let E be the energy (in foot lbs) to be absorbed per minute (that is, the work done per minute by the machine driving the brake), T the number of revolutions of the drum per minute, E its radius (in feet), and a the area (square inches), and b the breadth (inches) of the dram ; then in order that the latter may not heat, a should
E
not have a smaller value than "280 , while it is frequently and very properly made as much as '357 ~ . Expressing the same relation in
E E
other terms, we have b = from '0038 grj to "0048 g?p, or from
E E
o024 y- to '03 y-, V being the peripheral velocity of the drum in
feet per minute. If the work be expressed in horses' power (P) the
equation is very nearly equivalent to 6 = from ?M? to .
A brake may have automatic apparatus attached to it for showing
or registering its speed or performance. (A. B. W. K.)









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