Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/647

 DYNAMOMETERS gripped on the motor fly-wheel or pulley by a screw A, which, together with W, is adjusted to hold the brake steadily. Compensation is effected by the lever L inserted at B. This has a slotted end, engaged by a pin P fixed to the framing, and it will be seen that its action is to slacken the band if the load tend to rise and to tighten it in the contrary case. The external forces holding the brake from turning are W, distant R from the axis, and the reaction, Wj say, of the lever against the fixed pin P, distant Rx from the axis. The moment of Wj may be positive or negative. The torque T at any instant of steadyrunning is therefore {WRhWjRj}. Lord Kelvin patented a brake in 1858 (Fig. 2) consisting of a rope or cord wrapped round the circumference of a rotating wheel, to one end of which is applied a regulated force, the other end being fixed to a spring balance. The ropes are spaced laterally by the blocks B, B, B, B, which also serve to prevent them from slipping sideways. When the wheel is turning in the direction indicated, the forces holding the band still are W, and p, the observed pull on the spring balance. Both these forces usually act at the same radius R, the distance from the axis to the centre line of the rope, in which case the torque T is (W - i?)R, and consequently the brake horse-power is ■'hi'* (KK) ^7r^~‘ When /j. changes the -weight W rises or falls against the action of the spring balance until a stable condition W of running is obtained. The ratio — is given by where e = 2'7l8 : is the coefficient of friction and d the angle, measured in radians, subtended by the arc of contact between the rope and the wheel. In Fig. 2 d = 2ir. This ratio increases very rapidly as 6 is increased, and therefore by making 6 sufficiently large, p may conveniently be made a small —'kfraction of W, thereby rendering errors of observation of the spring balance negligible. Thus this kind of brake, though cheap to make, is, when 0 is large enough, an exceedingly accurate measuring instrument, readily applied and easily controlled. It has come into very general use in recent years, and has practically superseded the older forms of block brakes. Fig. 3. It is sometimes necessary to use water to keep the brake wheel cool. Engines specially designed for testing are usually provided with a brake wheel having a trough-shaped rim. Water trickles continuously into the trough, and the centrifugal action holds it as an inside lining against the rim, where it slowly evaporates. Fig. 3 shows a band - brake invented by Professor James Thomson, suitable for testing motors exerting a constant torque (see Engineering, 22nd October 1880). To maintain ev-Q constant,

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compensation for variation of //. is made by inversely varying 6. A and B are fast and loose pulleys, and the brake band is placed partly over the one and partly over the other. Weights W and w are adjusted to the torque. The band turns with the fast pulley if g. increase, thereby slightly turning the loose pulley, otherwise at rest, until 9 is adjusted to the new value of g. This form of brake was also invented independently

by Carpentier, and the principle has been used in the Raffard brake. A self-compensating brake of another kind, by Marcel Deprez, was described with Carpentier’s in 1880 {Bulletin de la Soci6t6 d'Encouragement, Paris). Ayrton and Perry have used a band or rope brake in which compensation is effected by the pulley drawing in or letting out a part of the band or rope which has been roughened or in which a knot has been tied. In an effective water-brake invented by W. Froude (see Eroc. Inst. M. E., 1877), two similar castings, A and B, each consisting of a boss and circumferential annular channel, are placed face to face on a shaft, to which B is keyed, A being free (Fig. 4). A ring tube of elliptical section is thus formed. Each channel is divided into a series of pockets by equally spaced vanes inclined at 45°. When A is held still and B rotated, centrifugal action -sets up vortex currents in the water in the pockets; thus a continuous circulation is caused between B and A, and the consequent changes of momentum give rise to oblique reactions. The moments of the components of these actions and reactions in a plane to which the axis of rotation is at right angles are the two aspects of the torque acting, and therefore the torque acting on B through the shaft is measured by the torque required to hold A still. Froude constructed a brake to take up 2000 H.P. at 90 revs, per min. by duplicating this apparatus. This replaced the propeller of the ship whose engines were to be tested, and the outer casing was held from turning by a suitable arrangement of levers carried to weighing apparatus conveniently disposed on the wharf. The torque corresponding to 2000 H.P. at 90 revs, per min. is 116,772 foot-pounds, and a brake 5 feet in diameter gave this resistance. Thin metal sluices were arranged to slide between the wheel and casing, and by their means the range of action could be varied from 300 H.P. at 120 revs, per min. to the maximum.