Figure 2-57.--Direction of gear rotation.
gear has the same number of teeth. With a 1 to 1 ratio,
revolutions) to turn the large gear one revolution (fig.
the driven gear rotates at the same speed as the driving
2-58). If the large gear drives the small gear, the
mechanical advantage is 1 to 2. The turning effort
required to rotate the gears is called torque. The torque
Two meshed spur gears are shown in figure 2-58,
ratio between gears varies with the mechanical
with the larger gear having twice the number of teeth as
advantage. Thus, if a small gear drives a larger gear, the
the smaller gear. This arrangement produces a speed
speed is decreased, but the torque is increased; if a
ratio of 2 to 1 with the small gear driving, since the
large gear drives a small gear, the speed is increased,
small gear rotates twice as fast as the larger one. Thus,
but the torque is decreased. This shows that through a
the gear ratio between two meshing gears is a
gear train, speed can be obtained by sacrificing torque,
comparison of the rpm of one gear to the rpm of the
or torque can be increased by sacrificing speed.
There are numerous types of gears used
Gears are not only used to produce speed ratios
throughout the vehicle power train. The most common
(same speed, increased speed, or reduced speed), they
gears found in automatic transmissions are the spur
are also used because the mechanical advantage of
and helical gears. The gears shown in figures 2-57 and
gears is directly related to the gear ratio of the driving
2-58 are SPUR gears. The HELICAL gear differs from
gear to the driven gear, as is the speed ratio. Therefore,
the spur gear in that its teeth are cut at an angle to the
if the small gear drives the gear twice its size, the
sides of the gear, while the spur gear teeth are cut
mechanical advantage is 2 to 1, since the small gear
straight and at right angles to the side of the gear.
m u s t exe r t i t s t o r q u e t w i c e t h e d i s t a n c e ( t wo
Figure 2-58.--Gear speed ratio.