‘ BRAKES AND DYNAMOMETERS 361 
of slight circumferential adjustment. The end of the eye-piece next the 
dise B is masked, except for a slot similar and opposite to the slot in 
the dise. When the four slots are set in line, as shown at (b), Fig. 549, 
a flash of light is seen at the eye-piece every revolution, and if the shaft 
revolves quickly enough the light will appear to be continuous. This 
effect is apparent at any speed over 100 revolutions per minute. At 
lower speeds the flash is seen to be intermittent, but this in nowise 
affects the accuracy and reliability of the result. 
Suppose now that the shaft is transmitting power. One disc lags 
behind the other, and although the slots in C, A, and D are still in line, 
the light is cut off by the displacement of the slot in B, due to the lag 
just mentioned. This cutting off of the light is clearly shown at (c). 
Now if the torque finder D be moved round by an amount equal to the ° 
lag of the disc B the slot in D will then be opposite to the slot in B 
when the slot in A is opposite to the slot in C, and the flash will now be 
received by D, as shown at (d). The torque finder is moved by operating 
a micrometer spindle, and by means of a scale and vernier the angular 
movement can be measured to the zt, of a degree. 
The Bevis-Gibson torsion meter as just described will evidently give 
the twist of the shaft at one definite point of each revolution, and in the 
ease of turbine shafts, where the torque is practically uniform, this is 
all that is required. For reciprocating engines, 
where the torque varies considerably during each 
revolution, a simple modification enables the 
operator to take several readings, usually twelve, 
at definite points of a revolution. The discs are 
perforated with slots arranged in the form of a 
spiral, as shown in Fig. 550. The lamp and 
torque finder must be moved radially so as to bring 
them into line with each corresponding pair of 
slots in the dises. Plotting the readings on squared Fia. 550. 
paper, the actual twisting moment diagram can be drawn, and from this 
the mean torque is readily found. 
Exercises XXI. 
1. The drum of a band brake is 18 inches in diameter. The band is ,‘, inch 
thick, and it embraces three-quarters of the circumference of the drum. The 
hand lever is arranged as shown at (c), Fig. 534, p. 344. AC=3 inches, and 
AD=18 inches. If the force P at the end of the lever is 40 lbs., and the co- 
efficient of friction between the band and the drum is 0-2, what is the resisting 
torque, in ft.-lbs., exerted on the brake drum ? 
2. In a band and block brake (Fig. 535, p. 345) the wheel is 24 inches in 
diameter, and the band is ys inch thick. There are twelve wood blocks, each 
2 inches thick, and each subtending an angle of 18 degrees at the centre of the 
wheel. The coefficient of friction between the blocks and the wheel is 0°35, 
The brake is operated by means of a lever, arranged as shown at (d), Fig. 534, 
p- 344, AB=4 inches, and AD=24 inches. What force must be applied to the 
end D of the lever when a weight of 300 lbs. is being lowered at a uniform 
velocity, the weight being hung by a rope which is coiled round a barrel on the 
axle of the brake wheel, the effective diameter of the barrel being 20 inches? 
3. A wheel 12 feet in diameter, rotating at the rate of one revolution in 
2 seconds, is acted on by a brake which applies normal pressures of 1 cwt. each 
at opposite ends of a diameter. If the coefficient of friction be 0°6, find (in 
horse-power) the rate at which work is being absorbed ? [Inst.C.E.] 
