T-8
1.1 Comparison Of Metric Gears With American Inch Gears
1.1.1 Comparison of Basic Racks
In all modern gear systems, the rack is the basis for tooth design and manufacturing tooling.
Thus, the similarities and differences between the two systems can be put into proper perspective
with comparison of the metric and inch basic racks.
In both systems, the basic rack is normalized for a unit size. For the metric rack it is 1 module,
and for the inch rack it is 1 diametral pitch.
1.1.2 Metric ISO Basic Rack
The standard ISO metric rack is detailed in Figure 1-1. It is now the accepted standard for the
international community, it having eliminated a number of minor differences that existed between
the earlier versions of Japanese, German and Russian modules. For comparison, the standard
inch rack is detailed in Figure 1-2. Note that there are many similarities. The principal factors are
the same for both racks. Both are normalized for unity; that is, the metric rack is specified in terms
of 1 module, and the inch rack in terms of 1 diametral pitch.
Fig. 1-1 The Basic Metric Rack From ISO 53 Normalized For Module 1
Fig. 1-2 The Basic Inch Diametral Pitch Rack Normalized For 1 Diametral Pitch
p
p
––
2
p
––
2
0.02 max.
0.6 max.
rf = 0.38
Pitch Line
2.25
1.25
1
20°
a
h
Pitch Line
hw
hf
ha
c
rf
s
p
ha = Addendum
hf = Dedendum
c
= Clearance
hw = Working Depth
h
= Whole Depth
p
= Circular Pitch
rf
= Root Radius
s
= Circular Tooth Thickness
a = Pressure Angle
