TOOTH THICKNESS0 pages

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.
10.1.1 Spur Gears
Table 10-1 presents equations for each chordal thickness
measurement.
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.
10.1.3 Helical Gears
The chordal thickness of helical gears should be
measured on the normal surface basis as shown in Table
10-3. Table 10-4 presents the equations for chordal
thickness of helical gears in the radial system.
10.1.4 Bevel Gears
Table 10-5 shows the equations of chordal thickness
for a Gleason straight bevel gear.
Table 10-6 presents equations for chordal thickness of
a standard straight bevel gear.
If a standard straight bevel gear is cut by a Gleason
straight bevel cutter, the tooth angle should be adjusted
according to:
tooth angle (º) = 180º ( s + h1tana) (10-1)
pRe 2
This angle is used as a reference in determining the
circular tooth thickness, s, in setting up the gear cutting
machine.
Table 10-7 presents equations for chordal thickness of
a Gleason spiral bevel gear.
The calculations of circular thickness of a Gleason spiral
bevel gear are so complicated that we do not intend to go
further in this presentation.
10.1.5 Worms and Worm Gears
Table 10-8 presents equations for chordal thickness of
axial module worms and worm gears.
Table 10-9 contains the equations for chordal
thickness of normal module worms and worm gears.
10.2 Span Measurement Of Teeth
Span measurement of teeth, Sm, is a measure over a
number of teeth, Zm, made by means of a special tooth
thickness micrometer. The value measured is the sum of
normal circular tooth thickness on the base circle, Sbn
and normal pitch, Pen (Zm - 1).
10.2.1 Spur and Internal Gears
The applicable equations are presented in Table
10-10.
Figure 10-4 shows the span measurement of a spur
gear. This measurement is on the outside of the teeth.
For internal gears the tooth profile is opposite to that of
the external spur gear. Therefore, the measurement is
between the inside of the tooth profiles.
10.2.2 Helical Gears
Tables 10-11 and 10-12 present equations for span
measurement of the normal and the radial systems,
respectively, of helical gears.
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