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40
3. Tooth Systems
1
A tooth system is a standard which specifies the relationships involving
addendum, dedendum, working depth, tooth thickness, and pressure angle.
The standards were originally planned to attain interchangeability of gears of
all tooth numbers, but of the same pressure angle and pitch.
Table III.1 contains the standards most used for spur gears. A 14.5º
pressure angle was once used for these but is now obsolete; the resulting
gears had to be comparatively larger to avoid interference problems.
Table III.2 is particularly useful in selecting the pitch or module of a
gear. Cutters are generally available for the sizes shown in this table.
Table III.3 lists the standard tooth proportions for straight bevel gears.
These sizes apply to the large end of the teeth. The nomenclature is defined
in Fig. III.2.
Standard tooth proportions for helical gears are listed in Table III.4.
Tooth proportions are based on the normal pressure angle; these angles are
standardized the same as for spur gears. Though there will be exceptions, the
face width of helical gears should be at least 2 times the axial pitch to obtain
good helical-gear action.
Tooth forms for worm gearing have not been highly standardized,
perhaps because there has been less need for it. The pressure angles used
depend upon the lead angles and must be large enough to avoid undercutting
of the worm-gear tooth on the side at which contact ends. A satisfactory tooth
depth, which remains in about the right proportion to the lead angle, may be
obtained by making the depth a proportion of the axial circular pitch. Table
III.5 summarizes what may be regarded as good practice for pressure angle
and tooth depth.
Table III.1
Standard and Commonly used
Tooth Systems for Spur Gears
1
Standardized by the American Gear Manufacturers Association (AGMA). Write
AGMA for a complete list of standards, because changes are made from time to
time. The address is: 1500 King Street, Suite 201, Alexandria, VA 22314.
Tooth
System
Pressure
Angle
F, deg
Addendum a Dedendum b
1.25/P
d
or 1.25m
20 1/P
d
or 1m
1.35/P
d
or 1.35m
1.25/P
d
or 1.25m
22.5 1/P
d
or 1m
1.35/P
d
or 1.35m
1.25/P
d
or 1.25m
Full
depth
25 1/P
d
or 1m
1.35/P
d
or 1.35m
Stub 20
0.8/P
d
or 0.8m 1/P
d
or 1m
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