Fig.17. - A Fixed Pulley.
Figure 18 shows the arrangement of a single pulley block or shop tackle, consisting of one fixed pulley in the upper block and a movable one in the lower block. One end of the rope is fastened to the upper block. This arrangement 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 pulldown 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 lbs.; the worker has only to lift 50 lbs.; to raise the weight 1 ft. he must draw up 2 ft. of rope, that is, one on each side of the pulley. Without the pulley he would have 100 lbs. to raise 1 ft.
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 handle. To avoid this, and at the same time obtain the required mechanical advantage, it is common to have several pulleys, called sheaves, assembled in one block on the same pin. Sometimes three, four, or more sheaves are placed thus side by side, a strong pin serving as an axis. In this way a force can move two, three, or four times its own resistance. Thus in a three-sheaved movable block, 100 lbs. would balance 300 lbs. Since the entire movement of the pulley is made up of a series of stops and starts, the movable pulley acts during its motion on the principle of a lever of the second class. As a result, the force applied times the diameter of the pulley will always equal the weight lifted times the radius of the pulley. Figures 20, 21, and 22 show common forms of pulleys.
Fig.18. - Block and Tackle.
Fig. 19. - Series of Pulleys.