Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/254

212 of bodies imparting tile etiergy ; (I) 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 tho 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 &quot; actual energy&quot; 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 done 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. 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 &quot; skid &quot; 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 &quot;lowering by a brake,&quot; 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 &quot;differential&quot; 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 latter at points a and 6 unequally distant from the fulcrum D. It is 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 6 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. FIG. 1. Differential Brake. 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 tho 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.

Fig. 2. FIGS. 2 and 3. 12 H.P. Friction Brake (designed by W. II. Maw).

