T-17
Table 1-5 (Cont.) Spur Gear Design Formulas
*All linear dimensions in millimeters
Symbols per Table 1-4
SECTION 2 INTRODUCTION TO GEAR TECHNOLOGY
This section presents a technical coverage of gear fundamentals. It is intended as a broad
coverage written in a manner that is easy to follow and to understand by anyone interested in
knowing how gear systems function. Since gearing involves specialty components, it is expected that
not all designers and engineers possess or have been exposed to every aspect of this subject.
However, for proper use of gear components and design of gear systems it is essential to have a
minimum understanding of gear basics and a reference source for details.
For those to whom this is their first encounter with gear components, it is suggested this
technical treatise be read in the order presented so as to obtain a logical development of the subject.
Subsequently, and for those already familiar with gears, this material can be used selectively in
random access as a design reference.
2.1 Basic Geometry Of Spur Gears
The fundamentals of gearing are illustrated through the spur gear tooth, both because it is the
simplest, and hence most comprehensible, and because it is the form most widely used, particularly
for instruments and control systems.
The basic geometry and nomenclature of a spur gear mesh is shown in Figure 2-1. The
essential features of a gear mesh are:
1. Center distance.
To Obtain
Dedendum
Outside Diameter
Root Diameter
Base Circle Diameter
Base Pitch
Tooth Thickness at
Standard Pitch Diameter
Center Distance
Contact Ratio
Backlash (linear)
Backlash (linear)
Backlash (linear) Along
Line-of-action
Backlash, Angular
Min. No. of Teeth
for No Undercutting
Use This Formula*
b = 1.25m
Do = D + 2m = m (N + 2)
DR = D – 2.5m
Db = D cosf
pb = m p cosf
p
Tstd = ––– m
2
m (N1 + N2)
C = ––––––––––––––––
2
––––––––– –––––––––
Ö1Ro – 1Rb + Ö2Ro – 2Rb – C sinf
mp = ––––––––––––––––––––––––––––––––––
m p cosf
B = 2(DC)tanf
B = DT
BLA = B cosf
B
aB = 6880––– (arc minutes)
D
2
Nc = ––––––
sin2f
From Known
Module
Module and Pitch Diameter or
Number of Teeth
Pitch Diameter and Module
Pitch Diameter and Pressure Angle
Module and Pressure Angle
Module
Module and Number of Teeth
Outside Radii, Base Circle Radii,
Center Distance, Pressure Angle
Change in Center Distance
Change in Tooth Thickness
Linear Backlash Along Pitch Circle
Linear Backlash
Pressure Angle
