Page 38 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 19

Spur Gears The most commonly FIGURE 1-19 Spur gears in a drive train
used gear is called a spur gear. Spur
gears transmit motion between parallel
shafts, as shown in Figure 1-19.
Individual spur gears are primarily
described by three variables:

1. Number of teeth (N)

2. Pitch diameter (D)

3. Diametral pitch (P)

The last two variables sound alike,
which can be confusing, because they
represent very different things. The
pitch diameter of a spur gear is 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. This means that half the
pitch diameter of the first gear plus half the pitch diameter of the second gear will
equal the correct center distance. This spacing is critical for creating smooth running
gears.

The diametral pitch of a gear refers to the number of teeth per inch of the
circumference of the pitch diameter. Think of it as tooth density—the higher the
number, the more teeth per inch along the edge of the gear. Common diametral
pitches for hobby-size projects are 24, 32, and 48.

NOTE The mating gears can have different pitch diameters and number of
teeth, but the number of teeth per inch, or diametral pitch (P), must be the
same for the gears to mesh correctly.

Rack-and-Pinion Gears A pinion is just another name for spur gear, and a rack is a
linear gear. A rack is basically a spur gear unwrapped so that the teeth lay flat, as
shown in Figure 1-20. The combination is used in many steering systems, and it is