STEAM ENGINES 



197 



nuts h and i are screwed up, the greater will be the pull exerted 

 on the spring balance. The arm e is extended and fits between 

 two stops I and n, so as to prevent excessive movement of the 

 brake. While a test is being made, the arm e should not 

 touch either stop. This can easily be attained by regulating 

 the tightness of the nuts h and i. The stops, however, should 

 never be dispensed with, as a sudden reversal of the engine or 

 an unexpected gripping of the pulley by the wooden blocks 

 might swing the brake arms around and injure the workmen. 

 The distance n from the center of the pulley c to a plumb- 

 line suspended from the center of the bolt o should be measured 

 very accurately, with the arm d in a level position. This 



FIG. 2 



distance n is used in determining the brake horsepower, as will 

 be explained later. 



Another form of friction brake is illustrated in Fig. 2. In 

 this type, there are a number of wooden blocks a fastened to an 

 iron band b and surrounding the pulley c. The friction 

 between the blocks and the pulley may be altered by loosening 

 or tightening the nut d on the bolt e that joins the ends of 

 the band b. To the bands are fastened the arms /, which are 

 bolted together at their outer ends and rest on a knife edge g 

 fastened to a block h resting on a platform scales *. When the 

 pulley is rotated in the direction indicated by the arrow, the 

 outer ends of the arms / press down on the block h and 



