chap96 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 96, the reader should
be able to:

•
•
•

Prepare for ASE Suspension and Steering
(A4) certification test content area “C”
(Related Suspension and Steering Service).

Name driveshaft and U-joint parts, and
describe their function and operation.
Describe how CV joints work.

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OBJECTIVES:
After studying Chapter 96, the reader should
be able to:

•
•

Explain how the working angles of the Ujoints are determined.
List the various types of CV joints and their
applications.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
cardan joints • center support bearing • CV joint boot • CV
joints
double-cardan joints • drive axle shaft • driveshaft
fixed joint
half shaft

plunge joint • propeller shaft
Rzeppa joint • spider
trunnions • universal joints
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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A drive axle shaft transmits engine torque from the
transmission or transaxle (if front wheel drive) to the
rear axle assembly or drive wheels.
Driveshaft is the term used by the Society of
Automotive Engineers (SAE) to describe the shaft
between the transmission and the rear axle assembly
on a rear-wheel-drive vehicle.
The SAE term will be used throughout this textbook.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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A drive axle shaft transmits engine torque from the transmission or 
transaxle (front wheel drive) to the rear axle assembly or
drive wheels. 

Figure 96–1 Typical rear-wheel-drive power train
arrangement. The engine is mounted longitudinal
(lengthwise).
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Figure 96–2 Typical front-wheel-drive power train arrangement. The engine is usually mounted
transversely (sideways).

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Figure 96–3 Typical driveshaft (also called a
propeller shaft). The drivershaft transfers
engine power from the transmission to the
differential.

A typical driveshaft is a hollow steel tube. 
A splined end yoke is welded onto one end 
that slips over the splines of the output shaft 
of the transmission. An end yoke is welded 
onto the other end of the driveshaft. Some 
driveshafts use a center support bearing.
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DRIVESHAFT DESIGN
Most driveshafts are constructed of hollow steel tubing. The forces 
are transmitted through the surface of the driveshaft tubing. The 
surface is in tension, and cracks can develop on the outside surface 
of the driveshaft due to metal fatigue. Driveshaft tubing can bend 
and, if dented, can collapse. 

Figure 96–4 This driveshaft failed
because it had a slight dent
caused by a rock. When engine
torque was applied, the
driveshaft collapsed, twisted, and
then broke.

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Most rear­wheel­drive cars and light trucks use a one­ or two­piece driveshaft. A steel tube driveshaft has a 
maximum length
of about 65 in. (165 cm). Beyond this length, a center support bearing, called a steady bearing or hanger 
bearing, must be used.
Some vehicle manufacturers use aluminum driveshafts; these can be as long as 90 in. (230 cm) with no 
problem. Many extended­cab pickup trucks and certain vans use aluminum driveshafts to eliminate the need 
(and expense) of a center support bearing.
Composite­material driveshafts are also used in some vehicles. These carbon­fiber­plastic driveshafts are very 
strong yet lightweight, and canbe made in extended lengths without the need for a center support bearing.
See Figure 96–5.

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Figure 96–5 A center support bearing is used on many vehicles with long driveshafts.

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To dampen driveshaft noise, it is common to line the inside of the 
hollow driveshaft with cardboard. This helps eliminate the tinny 
sound whenever shifting between drive and reverse in a vehicle 
equipped with an automatic transmission. 
Figure 96–6 Some driveshafts use rubber between an inner and outer housing to absorb
vibrations and shocks to the driveline.

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DRIVESHAFT BALANCE
All driveshafts are balanced. Generally, any driveshaft whose 
rotational speed is greater than 1000 RPM must be balanced.
Driveshaft balance should be within 0.5% of the driveshaft 
weight. (This is one reason why aluminum or composite 
driveshafts can be longer because of their light weight.)
Driveshafts are often not available by make, model, and year of 
the vehicle. There are too many variations at the factory, such as 
transmission type, differential, or U­joint type.
To get a replacement driveshaft, it is usually necessary to know 
the series of U­joints (type or style of U­joint) and the center­to­
center distance between the U­joints.
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U-JOINT DESIGN AND OPERATION
Universal joints (U­joints) are used at both ends of a driveshaft. 
U­joints allow the wheels and the rear axle to move up and down, 
remain flexible, and still transfer torque to the drive wheels.
A simple universal joint can be 
made from two Y­shaped yokes 
connected by a crossmember 
called a cross or spider.
Figure 96–7 A simple universal joint (U-joint).

The four arms of the cross are 
called trunnions. A similar 
design is the common U­joint 
used with a socket wrench set.
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Most U­joints are called cross­yoke joints or Cardan joints, named for a sixteenth­
century Italian mathematician who worked with objects that moved freely in any 
direction.
Torque from the engine is transferred through the U­joint. The engine drives the U­joint at 
a constant speed, but the output speed of the U­joint changes because of the angle of the 
joint. The speed changes twice per revolution.

See Figure 96–8.

The greater the angle, the greater the change in speed (velocity).

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Figure 96–8 How the speed difference on the output of a typical U-joint varies with the speed
and the angle of the U-joint. At the bottom of the chart, the input speed is a constant 1000 RPM,
While the output speed varies from 900 RPM to 1100 RPM when the angle difference in the joint
is only 10°. At the top part of the chart, the input speed is a constant 1000 RPM, yet the output
speed varies from 700 to 1200 RPM when the angle difference in the joint is changed to 30°.

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If one U­joint were used in a driveline, change in speed of the 
driven side (output end) would generate vibrations in the driveline.
To help reduce vibration, another U­joint is used at the other end of 
the driveshaft. If the angles of both joints are nearly equal, the 
acceleration and deceleration of one joint is offset by the alternate 
deceleration and acceleration of the second joint. 
It is very important that both U­joints operate at about
the same angle to prevent excessive driveline vibration. 
Figure 96–9 The joint angle
is the difference between
the angles of the joint.
(Courtesy of Dana
Corporation)

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Acceptable Working Angles  Universal joints used in a typical driveshaft should have a working 
angle of 1/2 to 3 degrees. The working angle is the angle between the driving and driven end
of the joint.
If the driveshaft is perfectly straight (0­degree working angle), then the needle bearings inside the 
bearing cap are not revolving because there is no force (no difference in angles) to cause the rotation 
of the needle bearings.
If the needle bearings do not rotate, they can exert a constant pressure in one place and damage the 
bearing journal. 
See Figure 96–10. 

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If a two­piece driveshaft is used, one U­joint (usually the front) 
runs at a small working angle of about 1/2 degree, just enough to 
keep the needle bearings rotating. The other two U­joints (from the 
center support bearing and rear U­joint at the differential) operate 
at typical working angles of a single­piece driveshaft.

Figure 96–10 The angle of
this rear U-joint is noticeable.

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If the U­joint working angles differ by more than 1/2 degree, a vibration is usually 
produced that is torque sensitive. As the vehicle is first accelerated from a stop, engine 
torque can create unequal driveshaft angles by causing the differential to rotate on its 
suspension support arms.
This vibration is most noticeable when the vehicle is heavily loaded and being accelerated 
at lower speeds. The vibration usually diminishes at higher speeds due to decrease in the
torque being transmitted.
If the driveshaft angles are excessive (over 3 degrees), a vibration is usually produced that 
increases as the speed of the vehicle (and driveshaft) increases.

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CONSTANT VELOCITY JOINTS
Constant velocity joints, commonly called CV joints, are designed 
to rotate without changing speed. Regular U­joints are usually 
designed to work up to 12 degrees of angularity. 

If two Cardan­style U­
joints are joined together, 
the angle at which this 
double­Cardan joint
can function is about
18 to 20 degrees.
Figure 96–11 A double-Cardan U-joint.

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What Is a 1350­Series U­Joint?
Most universal joints are available in sizes to best match the torque that
they transmit. The larger the U-joint, the higher the amount of torque.
Most U-joints are sized and rated by series numbers.

See the chart on Page 1242 of your textbook. 
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Double­Cardan U­joints were first used on large rear­wheel­drive vehicles to help reduce drive­
line­induced vibrations, especially when the rear of the vehicle was fully loaded and driveshaft 
angles were at their greatest.
As long as a U­joint (either single or double Cardan) operates in a straight line, the driven shaft 
will rotate at the same constant speed (velocity) as the driving shaft. As the angle increases, the 
driven shaft speed or velocity varies during each revolution.
This produces pulsations and a noticeable vibration or surge.
The higher the shaft speed and the greater the angle of the joint,
the greater the pulsations.

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NOTE: Many four­wheel­drive light trucks use standard Cardan­style  U­
joints in the front drive axles. If the front wheels are turned sharply   and 
then accelerated, the entire truck often shakes due to the pulsations created 
by the speed variations through the U­joints. This vibration is normal and 
cannot be corrected. It is characteristic of this type of
design and is usually not noticeable in normal driving.

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The first constant velocity joint was designed by AlfredH. Rzeppa (pronounced shep’pa) in the mid­1920s. 
The Rzeppa joint transfers torque through six round balls that are held in position midway between the two 
shafts. This design causes the angle between the shafts to be equally split regardless of the angle. 
Because the angle is always split equally, torque is transferred equally without the change in speed (velocity) 
that occurs in Cardan­style U­joints.
This style of joint results in a constant velocity between driving and driven shafts. It can also function at 
angles greater than
simple U­joints can, up to 40 degrees.
See Figure 96–12. 

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