WHOLE DEPTH (WD): The radial depth between

the circle that bounds the top of the gear teeth and the

circle that bounds the bottom of the gear teeth

Association

WORKING DEPTH (WKD): The whole depth

School

Spur Gear

Abbreviations

Abbreviations

Terms

minus the clearance, or the depth of engagement of two

mating gears; the sum of their addendums

Do

O u t s i d e diameter OD

CHORDAL THICKNESS (tc): The thickness of the

tc

tooth measured at the pitch circle or the section of the

Circular thickness CT

tooth that you measure to see if the gear is cut correctly

P

Circular pitch

CP

CHORDAL ADDENDUM (ac): The distance from

P

the top of a gear tooth to the chordal thickness line at

DP

Diametral

the pitch circle (used to set gear tooth vernier calipers

pitch

to measure tooth thickness)

N

NT

Number of teeth

DIAMETRAL PITCH (DP): The most important

DR

RD

calculation because it regulates the tooth size, or the

Root diameter

number of teeth on the gear divided by the number of

(none)

Chordal thickness tc

inches of pitch diameter

NUMBER OF TEETH (NT): The actual number of

(none)

ac

Chordal

teeth of the gear

addendum

BACKLASH (B): The difference between the tooth

thickness and the tooth space of engaged gear teeth at

the pitch circle

The diametral pitch system was devised to simplify

The symbols the American Gear Manufacturers

gear calculations and measurements. It is based on the

Association uses to describe gears and gear teeth are

diameter of the pitch circle rather than on the

different from those the Navy uses. The following list

circumference. Since the circumference of a circle is

will familiarize you with both sets of symbols:

3.1416 times its diameter, you always must consider this

constant when you calculate measurements based on the

pitch circumference. In the diametral pitch system,

however, the constant is in a sense "built into" the

system to simplify computation.

Association

School

Spur Gear

When you use this system, there is no need to

Abbreviations

Abbreviations

Terms

calculate circular pitch. Indexing devices based on the

diametral pitch system will accurately space the teeth,

PC

(none)

Pitch circle

and the formed cutter associated with the indexing

device will form the teeth within the necessary

PD

D

Pitch diameter

accuracy. This system simplifies all calculations such

C

as center distance between gears and working depth of

C-C

Center-to-center

teeth.

distance

Many formulas are used to calculate the dimensions

a

Addendum

ADD

of gears and gear teeth, but we will only use those

needed in this discussion. Appendix III of this manual

d

Dedendum

DED

contains a more complete list of such formulas.

Appendix IV contains explanations of how you

WKD

hk

Working depth

determine the formulas to calculate the dimensions of

CL

C

Clearance

Usually, you can get the outside diameter (OD) of

WD

ht

Whole depth

a gear and the number of teeth (NT) from a blueprint or

a sample gear. You may then use these two known

(none)

RC

Root circle

factors to calculate the necessary data.

14-3

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