36 



APPLIED SCIENCE 



Figure 18 shows the arrangement of a single pulley block or shop 

 tackle, consisting of one fixed pulley in the upper block and a mov- 

 able one in the lower block. One end of the rope 

 is fastened to the upper block. This arrange- 

 ment is merely a single movable pulley with its 

 rope extended up and around another pulley, thus 

 enabling the operator to pull down when raising 

 the weight. The upper pulley therefore does not 

 affect the amount of the force, but merely 

 changes its direction from a pull-up to a pull- 

 down on the rope. The advantage of this type 

 of block and tackle is that the force is decreased 

 one-half, while the space the worker pulls through 

 is twice that of the movement of the weight. 

 W is 100 Ibs.; the worker has only to lift 50 Ibs.; 

 to raise the weight 1 ft. he must draw up 2 ft. of 

 rope, that is, one on each side of the pulley. With- 

 out the pulley he would have 100 Ibs. to raise 1 ft. 



FIG. 18. Block 

 and Tackle. 



Increasing the number of pulleys decreases the weight per 

 strand, and allows a 

 smaller force to overcome 

 a larger at the expense of 

 space and loss of time. 

 (Fig. 19.) The pulley 

 ropes used are called 

 tackle, and the pulley, 

 a block. A number of 

 pulleys placed together 

 occupy much space and 

 are inconvenient to han- 

 dle. To avoid this, and 

 at the same time obtain 

 the required mechanical 

 advantage, it is common to have several pulleys, called 



a a 



KIG. 19. Series of Pulleys. 



