WHOLE DEPTH (WD): The radial depth between
American Gear
Machinery
the circle that bounds the top of the gear teeth and the
Manufacturers
Repairman
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
DIAMETRAL PITCH SYSTEM
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
American Gear
Machinery
pitch circumference. In the diametral pitch system,
Manufacturers
Repairman
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