T-72
SECTION 9 WORM MESH
The worm mesh is another gear type used for connecting skew shafts, usually 90°.
See Figure 9-1. Worm meshes are characterized by high velocity ratios. Also, they offer
the advantage of higher load capacity associated with their line contact in contrast to the
point contact of the crossed-helical mesh.
9.1 Worm Mesh Geometry
Although the worm tooth form
can be of a variety, the most popular
is equivalent to a V-type screw
thread, as in Figure 9-1. The mating
worm gear teeth have a helical lead.
(Note: The name “worm wheel” is
often used interchangeably with
“worm gear”.) A central section of
the mesh, taken through the worm's
axis and perpendicular to the worm
gear's axis, as shown in Figure 9-2,
reveals a rack-type tooth of the
worm, and a curved involute tooth
form for the worm gear. However,
the involute features are only true
for the central section. Sections on
either side of the worm axis reveal
nonsymmetric and noninvolute tooth
profiles. Thus, a worm gear mesh
is not a true involute mesh. Also,
for conjugate action, the center
distance of the mesh must be an
exact duplicate of that used in
generating the worm gear.
To increase the length-of-action,
the worm gear is made of a throated
shape to wrap around the worm.
9.1.1 Worm Tooth Proportions
Worm tooth dimensions, such as addendum, dedendum, pressure angle, etc., follow
the same standards as those for spur and helical gears. The standard values apply to the
central section of the mesh. See Figure 9-3a. A high pressure angle is favored and in
some applications values as high as 25° and 30° are used.
9.1.2 Number of Threads
The worm can be considered resembling a helical gear with a high helix angle. For
Fig. 9-1 Typical Worm Mesh
Fig. 9-2 Central Section of a Worm and
Worm Gear
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