Chevrolet Light Duty Truck 1973 Service Manual page 244

4 -2 0 REAR S U S PEN SIO N A N D DRIVELINE
THEORY OF OPERATION-
STANDARD DIFFERENTIAL
The differential has two basic functions. First, it
transmits torque from the propeller shaft, through a 90°
turn, to the rear wheels. Second, it takes the torque
provided by the propeller shaft and divides this torque
as necessary to allow each wheel enough torque to rotate
as conditions require.
Gears
The rear axle transmits power through a 90° turn. To do
this, bevel gears as shown in figure 38 were previously
used. Straight bevel and spiral bevel gears were used for
the ring and pinion gears; but the drive and driven gear
center line must intersect, or meet, each other. This is
satisfactory for differential side gears, but, the desire to
lower the driveshaft brought about another variation of
the bevel gear—the hypoid gear, shown in figure 39.
Meshing hypoid gears do not require a meeting of their
center lines. The drive pinion gear may then be placed
below the centerline of the ring gear, thereby lowering
the drive shaft.
Gear Ratio
The drive axle of a vehicle is said to have a certain axle
ratio. Most people are aware of this term, and are
familiar with the ratios most frequently used on specific
vehicles.
At this time, let's look into the what, how, and why of
axle ratios. W hat is a ratio? How is it achieved? Why is
it needed?
The term axle ratio is, to be more exact, the ring gear
and pinion ratio. This means it is a comparison of the
number of teeth of the driving and driven gears. It was
found that the relative sizes of the driving and driven
SPIRAL BEVEL GEARS
Fig. 38—Typical Bevel Gears
gears affect the output speed and torque. As a simple
example, a ring gear with twice as many teeth as the
drive pinion, would turn at one half of pinion speed.
This also means that since the speed is cut in half, the
output torque is doubled. By applying basic laws of
gearing, each axle can be designed with the most
desirable axle ratio for specific applications.
The ratio numbers of an axle express the simplest
comparison of the number of ring gear teeth to each
drive pinion tooth. For example, the 4.11 to 1 ratio
means that for each pinion tooth there are 4.11 ring gear
teeth or, by the same token, the pinion turns 4.11
revolutions to each revolution of the ring gear.
When the actual component parts are seen, it is obvious
that there are more than 4.11 teeth on the ring gear and
1 tooth on the pinion. The 3.11:1 gear set has 37 teeth on
the ring gear and 9 on the drive pinion, or 37 -s- 9 =
4.11. These same divisions show that a gearset with 43
14 = 3.07, 41 - 11 = 3.73, and 39
h
Therefore, the axle ratio not only gives the gear speed
reduction and torque multiplication, but also states the
simple ratio of the number of ring gear teeth to each
drive pinion tooth.
The question of why this choice of ratios are available
has several answers. First, each engine has a definite
speed range in which it operates most efficiently, and
supplies maximum usable torque. To take advantage of
this built-in feature, a multiple speed transmission is
employed. This permits the operator to maintain proper
engine speed regardless of road speed. But, to make the
transmission as simple and practical as possible, high
gear is normally direct drive. This means that the engine
and drive shaft turn at the same relatively high rate of
speed with no torque multiplication. Since each vehicle's
job application, engine, wheel size, and terrain condition
requires a specific speed and torque rate at the axle
shaft, the gear ration of the axle allows the tailoring of
the power train to the job.
10 = 3.90.
LIGHT DUTY TRUCK SERVICE MANUAL