ROOT—The surface of the thread corresponding to
identify the necessary dimensions. Figure 4-14 shows
the minor diameter of an external thread and the major
gear nomenclature, and the terms in the figure are
diameter of an internal thread
explained in the following list:
DEPTH—The distance from the root of a thread to
the crest, measured perpendicularly to the axis.
PITCH—The distance from a point on a screw
thread to a corresponding point on the next thread,
measured parallel to the axis.
LEAD—The distance a screw thread advances on
one turn, measured parallel to the axis. On a
single-thread screw the lead and the pitch are identical;
on a double-thread screw the lead is twice the pitch; on
a triple-thread screw the lead is three times the pitch
GEARS
When gears are drawn on machine drawings, the
PITCH DIAMETER (PD)—The diameter of the
pitch circle (or line), which equals the number of teeth
on the gear divided by the diametral pitch
DIAMETRAL PITCH (DP)—The number of teeth
to each inch of the pitch diameter or the number of teeth
on the gear divided by the pitch diameter. Diametral
pitch is usually referred to as simply PITCH.
NUMBER OF TEETH (N)—The diametral pitch
multiplied by the diameter of the pitch circle (DP x PD).
ADDENDUM CIRCLE (AC)—The circle over the
tops of the teeth.
OUTSIDE DIAMETER (OD)—The diameter of
draftsman usually draws only enough gear teeth to
the addendum circle.
Figure 4-14.—Gear nomenclature.
4-5
