188 Chapter 7
Root.
The root is the surface of the thread corresponding to the minor diameter of an external thread and the
major diameter of an internal thread. It represents the bottom or inner surface joining the sides of two
adjacent threads.
Depth.
The depth is the distance from the root of a thread to the crest, measured perpendicularly to the axis.
Pitch.
The pitch is the distance from any point on the screw thread to a corresponding point on the next thread
measured parallel to the axis. For example, nuts and bolts need to have the same pitch and diameter if
they are to be used together.
Lead.
The lead is the distance a screw thread advances in one complete turn, measured parallel to the axis.
On a single-thread screw the lead is equal to the pitch; on a double-thread screw the lead is twice the
pitch; on a triple-thread screw the lead is three times the pitch.
Gears.
Gears are enhanced assembly relationships and work by determining a relationship (or ratio) between
adjacent parts, such as gears and pulleys. Gears efficiently solve motion between related components
without the need to define physical contact relationships. They can be used to describe rotary to rotary
(gears), rotary to linear (rack and pinion), or linear to linear (telescopic or hydraulic cylinder) relation-
ships. When gears are drawn on machine drawings, the drafter usually draws only enough gear teeth to
identify the necessary dimensions. Figure 7.21 illustrates important gear nomenclature. The terms in the
figure are explained in the following list:
Pitch Diameter.
Pitch diameter (PD) is the diameter of the pitch circle (or line), which equals the number of teeth on the
gear divided by the diametral pitch. It represents the diameter of a thread at an imaginary point where
the width of the groove and the width of the thread are equal (i.e., the simple effective diameter of screw
thread). It is approximately halfway between the major and minor diameters. This is the critical dimen-
sion of threading, as it determines the fit of the thread (not used for metric threads).
Diametral Pitch.
Diametral pitch (DP) is the ratio of the number of teeth on a gear to the number of inches of pitch diam-
eter or the number of teeth to each inch of pitch diameter (i.e,. the number of teeth on the gear divided
by the pitch diameter). Diametral pitch is usually referred to as simply pitch.
Number of Teeth.
The number of teeth (N) is the diametral pitch multiplied by the diameter of the pitch circle (DP x PD).