SUSIE’S LIST OF
ADVANCED GEAR TERMS
While you probably won’t need to worry about
many of these specifications when deciding what gear
to purchase for your bot, it is important to understand
the following terms in order to have a working
knowledge of how a gear works. Because gears are
circular, you should also be familiar with circle
terminology (diameter, chord, tangent, etc.).
• Addendum: The distance from the pitch circle to the
• Dedendum: The distance from the pitch circle to the
• Outer Diameter: The diameter of a gear measured
from the top of the teeth.
• Root Diameter: The diameter of a gear measured from
the base of the teeth.
• Pitch Diameter: The diameter of a gear measured
from the pitch circle.
• Addendum Circle: An imaginary circle drawn at the
top of the teeth.
• Root Circle: An imaginary circle drawn at the base of
• Pitch Circle: An imaginary circle drawn at the point of
contact when two gears mesh together.
• Circular Thickness: The thickness of a tooth measured
as an arc along the pitch circle.
• Chordal Thickness: The thickness of a tooth measured
between two chords that extend from the points
where the pitch circle passes through both ends of a
tooth. This measurement will be slightly less than the
circular thickness, as it is a measurement taken from a
straight line whereas circular thickness is the measure
of a curved line.
Cut out this sheet or download the file at the article link
and keep it handy as a quick reference for the next
time you sit down to plan a bot. These terms are
practical and will influence how well your gears work.
each other. The helical gear (Figure
5) has angled teeth to reduce the
impact between two teeth while
they rotate. Helical gears are used
when noise reduction is important
(less impact = less noise), but since
Susie doesn’t mind a loud bot, she
decides a spur gear will do the job.
Now that she has determined
the type of gear, Susie turns her
attention to the issue of how to
reduce her RPM. She discovers that
the key is the gear ratio. A gear ratio
is the relationship between the
number of teeth of two gears. Susie
can figure out the gear ratio she
needs by working backwards from
the existing RPM of the motor and
the desired RPM of her wheels.
Remember that she wants to reduce
her RPM from 1,000 to 100, so as a
ratio this would be 1,000:100 or 10:1.
wheel so that after 10 spins of smaller
gear, the larger gear will spin once,
thus slowing down the wheel’s RPM.
The gears Susie selected also
have options of “with” or “without”
hubs, face width, and material. Hubs
are the protruding part of a gear
that have a hole for attaching to a
motor or wheel using a setscrew.
This is a must for Susie’s bot. As for
the face width (the width of the
gear teeth), it is important to keep in
mind the purpose of the gear.
Newton’s third law says that for
every action, there is an equal and
opposite reaction. In the
world of combat robotics —
where the bot is being
designed for collisions — the
gear teeth will be exposed to
a lot of reactive force. If the
teeth are exposed to too
much force, they could
become stripped. So to
compensate for this, it is
generally better to use a gear
32 SERVO 09.2011