7.4 Features of Internal Gears
General advantages:
1. Lend to compact design since the center
distance is less than for external gears.
2. A high contact ratio is possible.
3. Good surface endurance due to a convex profile surface
working against a concave surface.
General disadvantages:
1. Housing and bearing supports are more complicated,
because the external gear nests
within the internal
gear.
2. Low velocity ratios are unsuitable and in many cases
impossible because of interferences.
3. Fabrication is limited to the shaper generating process,
and usually special tooling is required.
8.0 WORM MESH
The worm mesh is another gear type used for
connecting skew shafts, usually 90º, see Figure 1.38. Worm meshes are characterized by
high velocity ratios. Also, they offer the advantage of the higher load
capacity associated with their line contact in contrast to the point contact of the
crossed-helical mesh
8.1 Worm Mesh Geometry
The worm is equivalent to a V-type screw thread, as evident from Figure 1.39. The mating worm-gear teeth have a helical lead. A central section of the mesh, taken through the worm’s axis and perpendicular to the wormgear’s axis, as shown in Figure 1.39, reveals a rack-type tooth for the worm, and a curved involute tooth form for the wormgear. However, the involute features are only true for the central section. Sections on either side of the worm axis reveal non-symmetric and non-involute 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 wormgear. To increase the length of action the wormgear is made of a throated shape to wrap around the Worm.
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