Page 36 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 17

FIGURE 1-17 Screw jack used to lift a heavy load

one full revolution. In our example, let’s use 1/4 in. To use the screw jack, we need to
turn the 12-in handle through a full circle for the jack to raise up 1/4 in. The end of
the handle traces out a circle with a radius of 12 in., and the circumference equals
2π multiplied by the radius (C = 2 ×π× R). So, our mechanical advantage is the input
(2 ×π× 12 in) divided by the output (1/4 in), which is about 300!

Power screws like in our screw jack example can achieve very high mechanical
advantages in a compact space, so they are great for lifting jobs when rigging up a
pulley system wouldn’t be practical. A lot of this mechanical advantage is lost to
friction, and we’ll talk more about that in Chapter 4.

Another place you may have seen power screws at work is in turnbuckles. These are
used to tension ropes and cables that are already secured. As indicated in Figure 1-18,