The ring and pinion gears are a matched set. They
are lapped (meshed and spun together with an abrasive
compound on the teeth) at the factory. Then one tooth
on each gear is marked to show the correct teeth
engagement. Lapping produces quieter operation and
assures longer gear life.
The spider gears are a set of small bevel gears that
include two axle gears (differential side gears) and hvo
pinion gears (differential idler gears). The spider gears
mount inside the differential case. A pinion shaft
passes through the two pinion gears and case. The two
side gears are splined to the inner ends of the axles.
A final drive is that part of a power transmission
system between the drive shaft and the differential. Its
function is to change the direction of the power
transmitted by the drive shaft through 90 degrees to the
driving axles. At the same time. it provides a fixed
reduction between the speed of the drive shaft and the
axle driving the wheels.
The reduction or gear ratio of the final drive is
determined by dividing the number of teeth on the ring
gear by the number of teeth on the pinion gear. In
passenger vehicles, this speed reduction varies from
about 3:1 to 5:1. In trucks it varies from about 5:1 to
11:1. To calculate rear axle ratio, count the number of
teeth on each gear. Then divide the number of pinion
teeth into the number of ring gear teeth. For example, if
the pinion gear has 10 teeth and the ring gear has 30 (30
divided by 10), the rear axle ratio would be 3:1.
Manufacturers install a rear axle ratio that provides a
compromise between performance and economy. The
average passenger car ratio is 3.50:1.
The higher axle ratio, 4.11:1 for instance, would
increase acceleration and pulling power but would
decrease fuel economy. The engine would have to run
at a higher rpm to maintain an equal cruising speed.
The lower axle ratio. 3:1, would reduce
acceleration and pulling power but would increase fuel
mileage. The engine would run at a lower rpm while
maintaining the same speed.
The major components of the final drive include
the pinion gear, connected to the drive shaft, and a
bevel gear or ring gear that is bolted or riveted to the
differential carrier. To maintain accurate and proper
alignment and tooth contact, the ring gear and
differential assembly are mounted in bearings. The
bevel drive pinion is supported by two tapered roller
bearings, mounted in the differential carrier. This
pinion shaft is straddle mounted. meaning that a
bearing is located on each side of the pinion shaft teeth.
Oil seals prevent the loss of lubricant from the housing
where the pinion shaft and axle shafts protrude. As a
mechanic, you will encounter the final drive gears in
the spiral bevel and hypoid design. as shown in figure
Spiral Bevel Gear
Spiral bevel gears have curved gear teeth with the
pinion and ring gear on the same center line. This type
of final drive is used extensively in truck and
occasionally in older automobiles. This design allows
for constant contact between the ring gear and pinion.
It also necessitates the use of heavy grade lubricants.
The hypoid gear final drive is an improvement or
variation of the spiral bevel design and is commonly
used in light and medium trucks and all domestic rear-
wheel drive automobiles. Hypoid gears have replaced
spiral bevel gears because they lower the hump in the
floor of the vehicle and improve gear-meshing action.
As you can see in figure 5-13, the pinion meshes
with the ring gear below the center line and is at a slight
angle (less than 90 degrees). This angle and the use of
Figure 5-13.Types of final drives.