CHAPTER XXI 
BRAKES AND DYNAMOMETERS 
291. Brakes.—A brake is an instrument for introducing an artificial 
resistance to the motion of a machine or moving body. The object of 
introducing the artificial resistance is either to stop the machine or retard 
it, or prevent its speed increasing. In acting, the brake converts work 
into heat by means of friction. The friction may be between solids, or 
between solids and a fluid, or it may be partly between solids and a 
fluid, and partly between the particles of the fluid. 
292. Band Brakes.—In a -band brake, a band, generally of metal, 
embraces a portion of 
the circumference of a 
wheel, as shown at (a), 
Fig. 534. One end 
of the band is jointed 
to one arm of a lever, 
and the other end is 
either jointed to an- 
other arm of the same 
lever, or it is jointed 
to a pin fixed on the 
frame of the machine. 
The required re- 
sistance is produced Fi. 534. 
by the friction between the band and the rim of the wheel, and when 
the brake is in action there are tensions T, and T, in the straight parts 
BE and CF of the band respectively, of which T, is the greater, and the 
turning action of these forces on the lever is balanced by a force P applied 
to the lever at D. 
By Art. 243, p. 277, a = on8, 
2 
The resisting torque due to the action of the band on the wheel is 
(T,—T,)R, where R is the effective radius of the wheel, that is, the 
radius measured to the middle of the thickness of the band. 
Referring to (a), Fig. 534, and assuming that the arms AB and AC 
of the lever are perpendicular to BE and CF respectively, then, taking 
moments about A, the fulcrum of the lever, Px AD=T, x AC—T, x AB. 
If ie is made equal tog! , then P=0, which means that once the brake 
2 : 
is in action it will remain in action without the application of any further 
ACelh 
effort on the lever. Again, if = is nearly equal ton only a small 
2 
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