Page 40 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 21

revolution, the mating gear (sometimes called the worm FIGURE 1-22 Bevel gears
gear) advances only one tooth. If the mating gear has
24 teeth, that gives the drive train a 24:1 mechanical
advantage. (This is technically only true for single-lead
worms; for a two-lead worm, two full revolutions are
needed to turn a mating gear one tooth.) Of course,
the mating gear will be moving very slowly, but a lot
of times, the trade-off is worth it.

Another great feature of worm gears is that the majority
of the time, they don’t back drive. This means that the
worm can turn the worm gear, but it won’t work the
other way around. The geometry and the friction just don’t allow it. So, a worm gear
drive train is desirable in positioning and lifting mechanisms where you don’t want to
worry about the mechanism slipping once a certain position is reached.

Planetary Gears Planetary, or epicyclic gears, are a combination of spur gears with
internal and external teeth. They are mostly used in places where a significant
mechanical advantage is needed but there isn’t much space, as in an electric
screwdriver or a drill. You can even layer planetary gear sets to increase the

FIGURE 1-23 Worm gears