BRAKES AND DYNAMOMETERS 349 
Tn the brake shown in Fig. 538, the lubricating and cooling water enters 
at the top from the pipe P, and is distributed over the surface of the 
ey by grooves formed in the rubbing surfaces of the blocks, and 
leaves by the pipe Q at the bottom. Wooden shrouds L and M on the 
sides of the blocks prevent the water coming out at the sides. 
Oscillations of the brake may be damped by means of a dash-pot, 
such as that shown in Fig. 539. This dash-pot is a cylinder containing 
oil or water, and a piston, which is attached to 
a rod suspended from and jointed freely to the 
lever of the brake. The piston may be about 
1-16th inch smaller in diameter than the bore 
of the cylinder, and it should be thin at its 
edge. The oscillations of the lever are com- 
municated to the piston, but the motion of the 
piston is retarded by the liquid in the cylinder, 
a portion of which must move from one side 
of the piston to the other through the narrow Fic. 539. 
passage round the edge of the piston as the 
latter moves. In this way the amplitude of the oscillations of the brake 
is considerably reduced. 
The brake should be balanced, when unloaded, with the piston im- 
mersed in the liquid in the dash-pot. 
_ Instead of hanging the brake load on the end of the lever, the brake 
may be turned “‘end for end,” and the load end of the lever be made to 
rest on a pedestal placed on the platform of a weighing-machine. In 
many cases this is a very convenient arrangement. 
298. Use of Compensating Lever on Dynamometer.—When a band- 
block brake is used as a dynamometer, a compensating lever is often 
added. This lever provides a means of automatically adjusting the 
tension in the band to suit variations in the coefficient of friction 
between the brake blocks and the wheel. 
Referring to Fig. 540, ABC is the compensating lever jointed to the 
ends of the band at A and B, and D is the point of suspension of the brake 
load W. The band is 
tightened by the screw 
at E, so that when the 
brake is in action the 
compensating lever is 
horizontal and in line 
with D. 
The action of the 
compensating lever is as 
follows. Suppose that 
the coefficient of friction 
between the brake blocks Fia. 540. 
and the wheel should 
increase, this will cause the wheel to carry the brake and its levers round 
with it until the lever ABC strikes the stop H. The points A and B 
will continue to move round with the wheel, but A will move faster than 
B, because the outer end of the lever is resting on the stop H; the effect 
of this is obviously to slacken the brake strap and diminish the resist- 
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