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212 Making Things Move

FIGURE 7-26 Shaft collars (credit: McMaster-Carr)

Gears

Gears are easy to use if you know the vocabulary (introduced in Chapter 1) and can
space them apart at the correct distance. One nice thing about gears is that if you
know any two things about them, such as outer diameter and number of teeth, you
can use some simple equations to find everything else you need to know, including
the correct center distance between them.

Before we talk about the types of gears, let’s review the anatomy of a spur gear drive
train in Figure 7-27 and the related vocabulary.

• Number of teeth (N) The total number of teeth around the outside of the
gear.

• Pitch diameter (D) The circle on which two gears effectively mesh, about
halfway through the tooth. The pitch diameters of two gears will be tangent
when the centers are spaced correctly.

• Diametral pitch (P) The number of teeth per inch of the circumference of
the pitch diameter. Think of it as the density of teeth—the higher the number,
the smaller and more closely spaced the teeth. Common diametral pitches for
hobby size projects are 24, 32, and 48.