Machine Drawings, Continued
GEARS: Gears transmit power and rotating or reciprocating motion from
one machine part to another. When drawing gear teeth on machine drawings,
draw only enough gear teeth to identify necessary dimensions.
Relate gear nomenclature with the terms in the figure.
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 simply as 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 the addendum circle.
CIRCULAR PITCH (CP): The length of the arc of the pitch circle between
the centers or corresponding points of adjacent teeth.
ADDENDUM (A): The height of the tooth above the pitch circle or radial
distance between the pitch circle and the top of the tooth.
DEDENDUM (D): The length of the portion of the tooth from the pitch
circle to the base of the tooth.
CHORAL PITCH: The distance from center to center of teeth measured
along a straight line or chord of the pitch circle.
ROOT DIAMETER (RD): The diameter of the circle at the root of the teeth.
CLEARANCE (C): The distance between the bottom of the tooth and the top
of a mating tooth.
Continued on next page