T-81
SECTION 10 TOOTH THICKNESS
There are direct and indirect methods for measuring tooth thickness. In general,
there are three methods:
• Chordal Thickness Measurement
• Span Measurement
• Over Pin or Ball Measurement
10.1 Chordal Thickness Measurement
This method employs a tooth
caliper that is referenced from the
gear's outside diameter. Thickness
is measured at the pitch circle. See
Figure 10-1.
Fig. 10-1 Chordal Thickness Method
10.1.1 Spur Gears
Table 10-1 presents equations for each chordal thickness measurement.
Table 10-1 Equations for Spur Gear Chordal Thickness
10.1.2 Spur Racks and Helical Racks
The governing equations become simple since the rack tooth profile is trapezoid, as
shown in Table 10-2.
Table 10-2 Chordal Thickness of Racks
NOTE: These equations are also applicable to helical racks.
No.
Item
Example
1
Circular Tooth Thickness
2
Half of Tooth Angle at Pitch Circle
3
Chordal Thickness
4
Chordal Addendum
Symbol
Formula
s
q
sj
hj
p
(–– + 2x tan
a
)m
2
90 360x tana
–– + –––––––––
z pz
z m sinq
zm
––– (1– cosq ) + ha
2
m = 10
a = 20°
z = 12
x = +0.3
ha = 13.000
s = 17.8918
q = 8.54270°
sj = 17.8256
hj = 13.6657
No.
1
Chordal Thickness
2
Chordal Addendum
sj
hj
pm pmn
––– or ––––
2 2
ha
m = 3
a = 20°
sj = 4.7124
ha = 3.0000
Item
Example
Symbol
Formula
sj
hj
q
d
