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.
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.
