Page 316 - Robot Builder's Bonanza

EVERYTHING YOU ALWAYS WANTED TO KNOW ABOUT GEARS 285

24-tooth
intermediate gear
(500 RPM)
12-tooth
driver
(1000 RPM)

Figure 24- 19 True speed
12-tooth 24-tooth reduction is accomplished by
pinion gear output gear ganging gears together. The idea is
fixed on (250 RPM)
intermediate gear to always go from fewer teeth to
more teeth; with each step, the speed
is reduced (and torque is increased).

to its shaft. The small hub gear turns the driven gear to produce the final output speed, in this
case 250 RPM. You can repeat this process over and over again until the output speed is but
a tiny fraction of the input speed.

G There are many other ways to achieve large- ratio gear reductions, and this is just one of them.
Other techniques include worm, planetary, and hypoid gears; you might find gearbox motors
that use these techniques, but they tend to be more expensive.

VARIATIONS IN GEAR TEETH
All of the examples shown so far have been of spur gears, which are the most common.
They’re used when the drive and driven shafts are parallel. Bevel gears have teeth on the
surface of the circle rather than the edge. They are used to transmit power to perpendicular
shafts. Miter gears serve a similar function but are designed so that no reduction takes place.
Spur, bevel, and miter gears are reversible— the gear train can be turned from either the
drive or the driven end. Conversely, worm and leadscrew gears transmit power perpendicu-
larly and are not usually reversible The lead screw resembles a threaded rod.
Rack gears are like spur gears unrolled into a flat rod. They are primarily intended to trans-
mit rotational motion to linear motion.

COMMON GEAR SPECIFICATIONS
Here are some common gear specifications to keep you warm at night.

Pitch: The size of gear teeth is expressed as pitch, which is roughly calculated by counting
the number of teeth on the gear and dividing it by the diameter of the gear. Common
pitches are 12 (large), 24, 32, 48, and 64. Odd- size pitches exist, of course, as do
metric sizes.
Pressure angle: The degree of slope of the face of each tooth is called the pressure angle.
A common pressure angle is 20°, although some gears, particularly high- quality worms
and racks, have a 14- 1/2° pressure angle.
Tooth geometry: The orientation of the teeth on the gear can differ. The teeth on most
spur gears is perpendicular to the edges of the gear. But the teeth can also be angled,
in which case it is called a helical gear. There are a number of other unusual tooth
geometries in use, including double- teeth and herringbone.

24-chapter-24.indd 285 4/21/11 11:51 AM