Page 31 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

12 Making Things Move

distance. As shown in Figure 1-12, an unfixed, movable pulley (also called a runner)
gives us a 2:1 mechanical advantage. Because each length of the rope carries half the
weight, the weight is twice as easy to pull up as it would be to lift the weight alone.
The trade-off is that you must pull the rope twice as far as the distance you want the
weight to move, since your effort is cut in half.
This last configuration is never very convenient. In order to be able to lift something
standing on the ground, most people would prefer to pull down instead of up. By
adding another pulley to the system, we maintain the 2:1 mechanical advantage but
change the pull force direction to be more convenient. The arrangement in Figure
1-13 is called a gun tackle and does exactly that. 1
The next logical step in this progression is to get a mechanical advantage of 3:1. There
are at least two ways to do this. One is called a luff tackle. This uses a compound
pulley (two independent pulleys in the same housing). Notice in the left image of
Figure 1-14 that the weight is suspended by three parts of rope that extend from the
movable single pulley at the bottom. Each part of the rope carries its share of the

FIGURE 1-13 A gun tackle
FIGURE 1-12 One unfixed arrangement gives a 2:1
pulley, or runner, gives a mechanical advantage, and
mechanical advantage. a convenient pull direction.