Page 252 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

230 Making Things Move

A ball screw has a semicircular groove that spirals up the screw and allows little steel
balls (housed in a ball nut) to ride up and down it. Ball screw and nut assemblies are
much more expensive than other types of power screws because of their efficiency.
Because the friction is so low, more of the input energy is transferred to useful work.

Regardless of the type of screw chosen, all power screws do one thing well: give
tremendous mechanical advantage. As you saw from the 600:1 ratio in the car jack
example in Chapter 1, this is pretty crucial in applications when you need to lift heavy
loads with a low input force. Power screws have been used in this capacity for many
years, and sometimes in reverse. The wooden ones in Figure 7-42 were actually

FIGURE 7-42 A diorama in a winery museum shows wooden
power screws that were used to press the grapes.