Page 315 - Robot Builder's Bonanza

284 MOUNTING MOTORS AND WHEELS

Gear ratio
2 to 1

20 teeth
1200 RPM
25 teeth
60 teeth 960 RPM
400 RPM 30 teeth
24 teeth 12 teeth 600 RPM
Figure 24- 17 Two meshing gears,
showing a 2:1 gear ratio. The gear on the left
has twice the number of teeth as the gear on Figure 24- 18 The number of gear teeth directly relates to
the right. speed.

As with a lever, another important thing happens: decreasing the speed of the motor also
increases its torque. The power output is approximately twice the input. Some power is lost
in the reduction process due to the friction of the gears. If the drive and driven gears are the
same size, the rotation speed is neither increased nor decreased, and the torque is not affected
(apart from small frictional losses). You can use same- size gears in robotics design to transfer
motive power from one shaft to another.

ESTABLISHING GEAR REDUCTION
Gears are an old invention, going back to ancient Greece in about the third century BC.
Today’s gears are much refined, though they’re still based on the old designs in which teeth
from the two mating gears mesh with each other. Force is transferred from one gear to
another.
Gears with the same size teeth are usually characterized not just by their physical size but
also by the number of teeth around their circumference. In the example in Figure 24- 17, the
small gear contains 12 teeth, the large gear 24 teeth. And you can string together a number
of gears one after the other, all with varying numbers of teeth (see Figure 24- 18). Attach a
tachometer to the hub of each gear, and you can measure its speed. You’ll discover the follow-
ing two facts:

• The speed always decreases when going from a small to a large gear.
• The speed always increases when going from a large to a small gear.
There are plenty of times when you need to reduce the speed of a motor from 5000 RPM
to 50 RPM. That kind of speed reduction requires a reduction ratio of 100:1. To accomplish
that with just two gears you would need, as an example, a drive gear that has 10 teeth and a
driven gear that has 1000 teeth— quite impractical.
Instead, you reduce the speed of a motor using multiple gears, as in Figure 24- 19. Here,
the drive gear turns a larger “hub” gear, which in turn has a smaller gear permanently attached

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