gears have curved teeth. Spur gears are used very little
b. Worm (Fig. 29-9). The worm gear rear axle is
for this purpose because they are noisy. Spiral-bevel
gears are used most often. Hypoid gears are used in
used in some trucks because it allows a large speed
several passenger cars and in light trucks because they
reduction. The threads on the worm are similar to screw
permit the bevel drive pinion to be placed below the
threads and may be single, double, triple, or quadruple.
center of the bevel drive gear, thereby lowering the
The worm meshes with a worm gear having helical teeth
propeller shaft to give more body clearance. This gear
cut in its outside circumference. The worm may be
also operates more quietly. Worm gears are used
compared to a screw and the worm gear to a nut. As the
extensively in trucks because they allow a large speed
worm rotates, it pulls the worm gear around. The worm
reduction. These consist of helical worm, similar to
usually is made of steel and the worm gear of bronze.
screws, and meshing teeth gears. The worms have
The driving worm may be mounted at either the top or
single, double, triple, or quadruple threads. These
the bottom of the worm gear. But, usually it is necessary
type gears are shown in figure 29-8. Internal gear drives
to place the worm at the top in order to allow sufficient
were once popular and still are used in rare instances.
road clearance under the rear axle housing. The rear
They permit a large speed reduction like the double
worm bearing must be very strong and rugged because it
chain drive, which they resemble. A jackshaft is driven
takes the entire thrust reaction from driving the worm
by the propeller shaft through bevel gears and the
gear. If play develops in the worm because of wear, this
differential as it is in the double-chain drive, except that
bearing also must withstand repeated impact. When the
the jackshaft is mounted on the dead rear axle and
vehicle is operated in reverse, or when the road wheels
parallel to it. Spur pinions keyed on the ends of the
are driving the mechanism, the front bearing resists
jackshaft drive internal gears attached to the wheels.
these forces. Sometimes a worm of hourglass form is
The first gear reduction takes place in the bevel pinion
used in the worm gear as it provides more tooth bearing
and differential drive ring gear and the second in the
surface and, consequently, less stress in the teeth.
Figure 29-9. Worm Gear Drive.