chap98 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 98, the reader should
be able to:

•
•

•

Prepare for ASE Manual Drive Train and Axles
(A3) certification test content area “E (Rear

Axle Diagnosis and Repair).
Describe how the differential allows engine
torque to be applied to both drive wheels and
still allow a difference in the speed of the
drive wheels during cornering.
Determine the rear axle ratio.
Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 98, the reader should
be able to:

•
•
•
•

Perform a tooth contact pattern check and
determine corrective action.
Describe how to determine proper drive
pinion gear depth.
Explain how to achieve proper drive pinion
bearing preload.
Describe how to adjust the ring gear position
to provide the proper backlash.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
axle ratio
coast (or concave) side • companion flange • crown
diff • differential case • drive (or convex) side • drive
pinion gear
floating position
heel • hunting gear sets • hypoid gear set
lap
nonhunting gear sets
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
partially nonhunting gear sets • pinion gear • pinion shaft •
preloaded clutch
rear end • ring gear • ring gear runout • root
side gears • spider gears • straddle-mounted pinion
toe • turning torque • viscous coupling

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PURPOSE AND FUNCTION OF A DIFFERENTIAL

The differential allows engine torque to be applied to both drive 
axles, which rotate at varying speeds during cornering and while 
traveling over bumps and dips in the road. The differential also 
changes the direction of engine torque 90° from the rotation of the 
driveshaft lengthwise with the vehicle. 
These two purposes of a differential can be summarized as follows:
To change the direction of engine torque.
See Figure 98–1
To allow the drive wheels to rotate at different speeds.
See Figure 98–2

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Figure 98–1 The differential assembly
changes the direction of engine torque and
increases the torque to the drive wheels.

Figure 98–2 The difference between the travel
distance of the drive wheels is controlled by
the differential.

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A differential is a mechanical addition and subtraction assembly. 

By splitting the engine torque to the drive wheels when the 
vehicle is turning a corner, the torque forces cause the side gear 
and pinion mate gears to subtract torque from one side and add 
torque to the opposite side. 
Figure 98–3 When the vehicle turns a corner, the inner wheel slows and the outer wheel
increases in speed to compensate. This difference in rotational speed causes the pinion gears
to “walk” around the slower side gear.

Continued
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PARTS OF A DIFFERENTIAL
A differential is also called a rear end or abbreviated simply as a 

diff. Whenever any vehicle makes a turn, the outside wheel must 
travel a greater distance than the inside wheel. The driveshaft ­
applies torque to the drive pinion gear that meshes below the 
center line of a ring gear as shown here.

Figure 98–4 A hypoid gear set uses a drive
pinion that meshes with the ring gear below
the center line of the ring gear.

Continued
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This type of gear set is called a hypoid gear set and requires gear lubrication specifically designed 

for this type of service.
The ring gear is attached to a differential case that also contains small beveled spider gears or 
pinion gears.
A pinion shaft passes through the two pinion gears in the case.
In mesh with the pinion gears are two side gears that are splined
to the inner ends of the axles.
See Figure 98–5. 

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Figure 98–5 The differential case provides the support for the ring gear, side bearings, and side
gears. While a vehicle accelerates, the drive pinion contacts the ring teeth on its drive, or

convex side. While decelerating, the drive pinion contacts the ring teeth on its coast, or
concave side. See Figure 98–7. The intermediate position between drive and coast, when
neither the ring gear nor the pinion is driving each other, is called the “floating” position.

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The Pinion and Ring Gears During Operation  During operation, the 
position of the drive pinion gear on the ring gear changes.
The ring gear mounts onto the differential case.
Each slanted ring gear tooth has two ends. Its toe is closest to the ring gear 
center; its heel, closest to the outside circumference.
The tooth root is the depression lying between two teeth, and the crown is 

the very top of each tooth.
See Figure 98–6.

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Figure 98–6 The relationship among the ring gear and drive pinion as well as the side and
spider gears.

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Figure 98–7 The drive side is the convex side of the ring gear except for some front axles used
in four-wheel vehicles, and they often use the concave side on the drive side. they often use the
concave side on the drive side.

While a vehicle accelerates, the drive 
pinion contacts the ring teeth on its 
drive, or convex side. 
While decelerating, the drive pinion 
contacts the ring teeth on its coast,
or concave side.
The intermediate position between 
drive and coast, when neither the
ring gear nor the pinion is driving 
each other, is called the “floating” 

position.
Continued
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Change the Axle Ratio

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DIFFERENTIAL GEAR RATIOS
The final gear reduction in the drive train occurs in the differential 
assembly. The amount of torque a gear set delivers depends on the 
gear ratio between the drive pinion gear and the driven ring gear.
The ratio of the final drive (differential) is called the axle ratio. 

To determine the axle ratio, divide the number of teeth of the ring 
gear (driven gear) by the number of teeth of the drive pinion gear 
(driving gear):

The higher the axle ratio number, the faster the engine rotates for 
each rotation of the drive wheels. 
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Ring and Pinion Gear Set Types  The final drive gear ratio 
determines how many times a drive pinion tooth will make contact 
with a particular ring gear tooth during one revolution. This 
affects final drive gear set manufacture and service.
Final drive gear sets may be divided into three types, depending on 
the final drive gear ratio. 
1. Hunting gear sets are sets with final drive ratios expressible
in a fraction that cannot be reduced to any lower terms.
An example is one that has 41 teeth on the ring gear and 11 
teeth on the drive pinion. This combination creates a 3.73:1 
axle ratio. This type of gear set requires no timing marks or 
alignment during assembly. As the pinion gear drives the ring 
gear, each pinion tooth will hunt for, or seek, contact with 
every ring gear tooth.
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2. Nonhunting gear sets are gear sets with final drive ratios 
expressible as a whole number.
Nonhunting gear sets require ­timing marks. As the pinion gear 
drives the ring gear, each pinion tooth contacts only a few ring 
gear teeth during each revolution.
3. Partially nonhunting gear sets are sets with final drive ratios 
expressible as a reducible fraction not equaling a whole number.

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Partially nonhunting gear sets also require timing marks. During 
final drive operation, each pinion tooth contacts only some of the 
ring pinion teeth.
For the pinion teeth to make contact with the highest number of 
ring gear teeth, the pinion gear must drive the ring gear more 
than one revolution.
On nonhunting and partially nonhunting gears, manufacturers lap 
the contacting gear teeth to decrease wear. These gear sets are 
marked to ensure proper alignment during assembly procedures.
To preserve wear patterns, the gear sets should be reassembled 
using the same alignment. This prolongs the life of the gear set 
and decreases operational noise.
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DIFFERENTIALS
While a vehicle travels straight ahead, the speed of each driven wheel must be allowed to vary slightly 
as they go over bumps, potholes, railroad tracks, and other road surface irregularities.
While cornering, the wheels must be able to turn at much greater differences in speed. Without some 
mechanism to allow for a difference in speed between the wheels, the left wheel would skid through the 
turn.
Inside the differential gear housing four to six bevel gears help drive the axles. In most rear axles, two of 
these bevel gears are smaller pinions mounted on a shaft. They drive two side gears splined with each 
inner axle end.

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Torque Flow Through a Standard Open Differential
Open differentials deliver equal torque to both wheels at all times. As the case rotates (driven by the 
engine through the ring and pinion gears), the cross­shaft applies drive force to the spider gears.
The two side gears apply reaction forces that counter this drive force. Because the spider gear is 
free to rotate about the cross­shaft, the two reaction forces are equal. As the side gear applies a 
force to the spider gear, the spider gear applies an equal and opposite force to the side gear.
It is this force, on the side gear, that supplies the torque to the axle that drives the wheel. Because 
the force on each side gear is equal, the torque supplied to each wheel is also equal. This is true 
regardless of whether one wheel is rotating faster than the
other or at the same speed. See Figure 98–8

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Figure 98–8 A close-up view of the side gears and spider (pinion) gear. Note the ridges on the
gear teeth. These ridges are manufactured into the gear teeth to help retain lubricant so that no
metal-to-metal contact occurs.

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Traveling Straight­Ahead  Assuming that a vehicle has equal traction at both wheels, 
differential action does not occur. In other words, when traveling straight on smooth road surfaces 
the ring gear, carrier, and the drive axles are traveling at the same speed; they rotate as a unit.
Traveling Around Corners The ability of differential pinion gears to spin on their shafts allows 
each axle to rotate at a different speed.
Case speed is always equal to the average speed of the two side gears. Since the ring gear rotates 
with the case and each side gear rotates with its axle; when a vehicle corners, the outside wheel 
gains the same number of RPMs that the wheel on the inside loses, while ring gear RPM remains 
constant.

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Limited Slip Differentials When a vehicle equipped with a standard differential spins a tire, the 
opposite wheel does not receive enough torque to move the vehicle.
To solve this problem, most manufacturers use differentials that direct more power to the side 
gear attached to the spinning axle. Many differentials do this by forcing the side gear against the 
revolving case. This bypasses differential action, allowing the
case to drive the side gear directly.
A limited­slip differential distributes torque to both wheels equally or unequally, allowing the 
wheels to turn at the same or at different speeds.
See Figure 98–9.

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Figure 98–9 (a) A two-wheel-drive vehicle equipped with an open differential. (b) A two-wheel
drive vehicle equipped with a limited-slip differential.

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