350 APPLIED MECHANICS 
ance of friction, and so compensate for the increase in the coefficient of 
friction. A diminution in the coefficient of friction will produce the 
opposite action, the compensating lever will strike the upper stop K, and 
-the band will be tightened. 
When the compensating lever is floating between the stops, a load w, 
in addition to the weight of the lever ABC, may be required to balance 
the vertical components of the tensions in the band at A and B. The 
horizontal components of these tensions balance one another. 
If W is the load at D over and above that required to produce static 
balance when w is removed, and the whole brake is free to move about 
its axis, which is the axis of the wheel, then when the brake is in action, 
with the compensating lever floating and the loads W and w on, the 
driving torque is WR-—wr. If, however, the compensating lever is not 
floating, but rests against one of the stops, the driving torque is 
WR-wr+tp(r+a), where p is the reaction of the stop on the com- 
pensating lever. The + sign is to be taken when the lever is against 
the lower stop, and the — sign when the lever is against the upper stop. 
If N is the speed of the wheel in revolutions per minute, and if the 
forces are measured in lbs. and distances in feet, then the horse-power 
eb iah, 0s (r+ a)iN . If the brake is carefully adjusted 
the force p should be small, and may then be neglected. 
299. Pullen’s Friction Brake Dynamometer.— A form of dyna- 
absorbed is 
Ng aaa 
iy TT] 
Ss 
N 
SX ES 
K «CW (YW 
») 
Ss | 
eeeeeveee0d 
\\A \\aN 
KEE 
Section al XY. 
oe ee oe ee 
oa ee ee 
Fig. 541, a —T 
mometer suitable for testing small high-speed motors, such as petrol 
