198 



STEAM ENGINES 



the pressure may be measured by the scales. The distance 

 between the center of the shaft to which the pulley is keyed 

 and the knife edge g should be measured carefully, as it repre- 

 sents the effective length of the brake arm. 



Rope Brake. A form of brake requiring less space than the 

 Prony brake is the rope brake, shown in Fig. 3. It is used in 

 determining the horsepower of engines of moderate size. The 

 pulley a on the engine shaft is encircled by a double rope b to 

 which are fixed blocks c that bear against the face of the 



pulley. One end of the 

 rope is attached to a 

 spring balance d sus- 

 pended from a beam e, 

 and the other end car- 

 ries a weight / that may 

 be varied. The friction 

 of the brake on the 

 pulley is increased by 

 adding to the weight /. 

 The direction of rota- 

 tion is indicated by the 

 arrow. The brake arm 

 in this type of brake is 

 the distance from the 

 center of the pulley to 

 the center of the rope, 

 and this distance should 

 be measured very care- 

 fully and quite accu- 



, 



FIG. 3 



rately, as it is required in calculating the horsepower. 



Cooling of Brake Pulleys. It is essential that the pulleys of 

 brakes should be well lubricated and for all except small powers, 

 means must be provided for conducting away the heat generated 

 by friction. If there are internal flanges on the brake wheel, 

 water can be run on the inside of the rim, the flanges serving 

 to retain the water at the sides and centrifugal force to keep it 

 in contact with the rim. A funnel-shaped scoop can be used 

 to remove the water. It should be attached to a pipe and 

 placed so as to scoop out the water, which should flow 



