heavier (larger) teeth permit an increased amount of
power to be transmitted while the size of the ring gear
and housing remain constant.
The tooth design is similar to the spiral bevel but
includes some of the characteristics of the worm gear.
This permits the reduced drive angle. The hypoid gear
teeth have a more pronounced curve and steeper angle,
resulting in larger tooth areas and more teeth to be in
contact at the same time. With more than one gear tooth
in contact, a hypoid design increases gear life and
reduces gear noise. The wiping action of the teeth
causes heavy tooth pressure that requires the use of
heavy grade lubricants.
Double-Reduction Final Drive
In the final drives shown in figure 5-13, there is a
single fixed gear reduction. This is the only gear
reduction in most automobiles and light- and some
medium-duty trucks between the drive shaft and the
Double-reduction final drives are used for heavy-
duty trucks. With this arrangement (fig. 5-14) it is not
necessary to have a large ring gear to get the necessary
gear reduction. The first gear reduction is obtained
through a pinion and ring gear as the single fixed gear
reduction final drive. Referring to figure 5-14, notice
that the secondary pinion is mounted on the primary
ring gear shaft. The second gear reduction is the result
of the secondary pinion which is rigidly attached to the
primary ring gear, driving a large helical gear which is
attached to the differential case. Double-reduction
final drives may be found on military design vehicles,
such as the 5-ton truck. Many commercially designed
vehicles of this size use a single- or double-reduction
Figure 5-14.Double-reduction final drive.