Brakesdrumchapter10 111221140048-phpapp01 (1)

OBJECTIVES
After studying Chapter 10, the reader will be able to:
1. Prepare for the Brakes (A5) ASE certification test
content area “B” (Drum Brake Diagnosis and
Repair).
2. Identify drum brake component parts.
3. Describe the operation of non-servo brakes.
4. Explain the operation of dual-servo brakes.
5. Discuss drum brake adjusters.

Automotive Brake Systems, 5/e Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc.,
By James D. Halderman 2 Upper Saddle River, NJ 07458 • All rights reserved.

KEY TERMS
• Anchor eyes • Lining fade
• Beehive holddown • Lining table
• Bonded linings • Mechanical fade
• Brake fade • Nibs
• Brake shoe return springs • Non-servo brakes
• Brake shoe holddown • Over-travel spring
• Brake shoes • Pawl
• Core charge • Piston stops
• Double-trailing shoe • Platform
• Dual-servo brake • Primary shoe
• Duo-Servo • Riveted linings
• Edge codes • Secondary shoe
• Gas fade • Self-energizing action
• Labyrinth seal • Shoe anchors
• Leading shoe • Shoe contact areas
• Leading-trailing brakes • Shoe rim
• Ledges • Shoe support pads
• Lining edge codes • Shoe web
• Trailing shoe
• Water fade


DRUM BRAKE ADVANTAGES
• Drum brakes were the first type of brakes used on
motor vehicles.
• Even today, over 100 years after the first
“horseless carriages,” drum brakes are still used
on the rear of most vehicles.



FIGURE 10–1 Typical brake system components showing disc
brakes on the front and drum brakes on the rear.



FIGURE 10–2 An exploded view of a typical drum brake assembly.


• SELF-ENERGIZING AND SERVO ACTION
• PARKING BRAKE SERVICE


DRUM BRAKE DISADVANTAGES
BRAKE FADE

• Drum brakes are not very efficient at dissipating
heat.
• The brake drum covers the linings, and most of the
heat produced during braking must pass through
the drum, from the inside out into the air.
• The greatest drawback of drum brakes is that they
are susceptible to fade.
• Brake fade is the loss of stopping power that
occurs when excessive heat reduces the friction
between the brake shoe linings and the drum.


BRAKE FADE

• The four types of brake fade include:
• 1. Mechanical Fade
• 2. Lining Fade
• 3. Gas Fade
• 4. Water Fade


• BRAKE ADJUSTMENT
• BRAKE PULL


Quick-and-Easy Drum Brake Adjustment
Check

• Tap the brake drum lightly with a hammer or
wrench. If the brake shoes are not contacting
the drum, the drum will ring like a bell. If the
shoes are contacting the drum, the sound will
be muffled.


DRUM BRAKE PARTS
BACKING PLATE
• The foundation of every
drum brake is the backing
plate that mounts to the
steering knuckle on the
front brakes, or to the
suspension or axle housing
on the rear brakes.

FIGURE 10–3 The backing plate is the
foundation of every drum brake.


DRUM BRAKE PARTS
BACKING PLATE

FIGURE 10–4 A labyrinth seal is created between the lip of
the backing plate and the groove in the brake drum.


DRUM BRAKE PARTS
SHOE ANCHORS
• Shoe anchors prevent the
brake shoes from rotating
with the drum when the
brakes are applied.
• The majority of drum
brakes have a single
anchor, but some drum
brake designs use two or
more.

FIGURE 10–5 A keystone anchor allows the
brake shoes to self-center in the drum.


DRUM BRAKE PARTS
PISTON STOPS
• Some backing plates
incorporate piston stops
that prevent the wheel
cylinder pistons from
coming out of their bores
when the friction assembly
is disassembled for
servicing.

FIGURE 10–6 Piston stops prevent the wheel
cylinder from coming apart.


DRUM BRAKE PARTS
SHOE SUPPORT PADS

• The shoe support pads are stamped into the
backing plate and contact the edges of the brake
shoes to keep the linings properly aligned with the
center of the friction surface inside the brake drum.
• These pads are also called ledges or shoe
contact areas.
• The support pads are slightly coated with special
high-temperature silicone brake grease to
minimize wear, prevent rust, and eliminate
squeaking that can occur when the shoes move
slightly on the pads during a stop.


DRUM BRAKE PARTS
WHEEL CYLINDERS
• Hydraulic pressure is
transferred from the master
cylinder to each wheel
cylinder through brake
fluid.
• The force exerted on the
brake fluid by the driver
forces the piston inside the
wheel cylinder to move
outward.
FIGURE 10–7 Cross-section of a wheel cylinder
that shows all of its internal parts. The brake line
attaches to the fluid inlet. The cup extender
prevents the cup seal lip from collapsing when the
brakes are released.


DRUM BRAKE PARTS
WHEEL CYLINDERS

FIGURE 10–8 The pushrods are held in place by the rubber dust boots. As
the wheel cylinder pistons move outward, the pushrods transfer the movement
to the brake shoes.


DRUM BRAKE SHOES
• The linings of drum brakes
are attached to curved
metal assemblies called
brake shoes.
• Most shoes are made of
two pieces of sheet steel
welded together in a T-
shaped cross section.

FIGURE 10–9 Steelbrake shoes are made
from two stampings welded together—the web
and the lining table.


DRUM BRAKE SHOES

FIGURE 10–11 Typical drum brake shoe and
the names of the parts. (Courtesy of Allied Signal
Automotive Aftermarket)
FIGURE 10–10 Tapered ends on the linings
help to reduce brake noise.


DRUM BRAKE SHOES
PRIMARY AND SECONDARY BRAKE SHOES

• In a dual-servo drum brake system, the shoes in a
dual-servo brake perform different jobs.
• The primary shoe (forward facing shoe) is self-
energized by drum rotation to create a servo action
that forces the secondary shoe more firmly against
the drum.
• Because of this, the two shoes have physical
differences and cannot be interchanged.


DRUM BRAKE SHOES
PRIMARY AND SECONDARY BRAKE SHOES

FIGURE 10–12 The primary (forward facing) FIGURE 10–13 Primary shoe lining may
brake shoe often has a shorter lining than the vary depending on the application.
secondary shoe (rearward facing).


DRUM BRAKE SHOES
LINING ASSEMBLY METHODS
• There are two main
methods used to mount
brake linings to the brake
shoe.
• RIVETING
• BONDING

FIGURE 10–14 Riveted brake linings are
quiet and reliable at high temperatures.


DRUM BRAKE SHOES
LINING ASSEMBLY METHODS

FIGURE 10–15 Many brake linings are bonded.


DRUM BRAKE SHOES
EDGE CODES
• Starting in 1964, brake linings have been using a
standardized way to identify the brake lining
materials.
• The edge codes follow the Society of Automotive
Engineers (SAE) Standard J866a.
• The edge codes contain three groups of letters
and numbers:
• The first group is a series of letters that identify the
manufacturer of the lining.
• The second group is a series of numbers, letters, or
both that identify the lining compound or formula.
• The third group is two letters that identify the
coefficient of friction.

DRUM BRAKE SHOES
EDGE CODES


DRUM BRAKE SHOES
EDGE CODES

FIGURE 10–16 Typical drum brake lining edge codes.


Purchase Quality Brake Linings for Best
Performance

• While many brands of replacement brake
lining provide acceptable stopping power and
long life, purchasing factory brake lining from
a dealer is usually the best opportunity to get
lining material that meets all vehicle
requirements. Aftermarket linings are not
required by federal law to meet performance
or wear standards that are required of original
factory brake linings.


DRUM BRAKE PARTS
RETURN SPRINGS
• The brake shoe return
springs retract the shoes
to their unapplied positions
when the brake pedal is
released.
• This helps prevent brake
drag, and aids the return of
brake fluid to the master
cylinder reservoir.
• Most brakes use closed-
coil return springs to retract
the brake shoes.
FIGURE 10–17 A typical drum brake
assembly showing the support plate
(backing plate), anchor pin, and springs.


DRUM BRAKE PARTS
RETURN SPRINGS

FIGURE 10–18 A single spring-steel spring is used on some drum brakes.


DRUM BRAKE PARTS
BRAKE SHOE HOLDDOWNS

• While the return springs retract the brake shoes to
their unapplied positions, the brake shoe
holddowns keep the shoes securely against the
support pads on the backing plate.
• The holddowns prevent noise, vibration, and wear,
but still allow the shoes to move out and back as
the brakes are applied and released.
• The holddowns also provide enough freedom of
movement to allow adjustments of the shoes
outward as the linings wear.


DRUM BRAKE PARTS
BRAKE SHOE HOLDDOWNS

FIGURE 10–19 Various types and styles of hold-down springs.


DRUM BRAKE PARTS
PARKING BRAKE LINKAGE
• Most rear drum brake
friction assemblies include
a parking brake linkage.
• The linkage commonly
consists of a cable, lever,
and strut system that
spread the brake shoes
apart to apply the brake
mechanically.
• The parking brake strut
plays a large part in many
of the automatic brake
adjusters.
FIGURE 10–20 A mechanical brake
linkage is part of most drum brake
assemblies.

DRUM BRAKE PARTS
BRAKE DRUMS
• The brake drum is not
connected to the backing
plate, but turns with the
wheel.
• The drum mounts on the
hub or axle, and covers the
rest of the brake assembly.
• Brake drums are made of
cast iron or cast aluminum
with a castiron liner.
• Many of these drum types
may have ribs or fins on FIGURE 10–21 An aluminium brake drum with
their outer edge to help a cast iron friction surface. The cooling fins
around the outside help dissipate the heat from
dissipate heat. the friction surface to the outside air.


NON-SERVO BRAKE DESIGN
PURPOSE AND FUNCTION

• A non-servo brake feature is that each brake
shoe is applied individually.
• The action of one shoe has no effect on the action
of the other.
• Many non-servo drum brakes use self-energizing
action to improve their braking performance.


PARTS AND OPERATION

• Self-energizing action occurs when the forward or
leading shoe contacts the drum and the drum
attempts to rotate the shoe along with it.
• However, the shoe cannot rotate because its far
end (relative to drum rotation) is fixed in place by
an anchor.
• As a result, drum rotation energizes the shoe by
forcing it outward and wedging it tightly against the
brake drum.


PARTS AND OPERATION

FIGURE 10–22 Self-
energizing action can increase
or decrease the stopping
power of a brake shoe.


DOUBLE-TRAILING BRAKE
• The least powerful non-
servo drum brake is the
double-trailing brake.

FIGURE 10–23 A double-trailing
non-servo drum brake.


LEADING-TRAILING BRAKE DESIGN
• The non-servo leading-
trailing brake has one
leading shoe and one
trailing shoe.

FIGURE 10–24 A leading-trailing
non-servo brake.


DUAL-SERVO BRAKE DESIGN
PURPOSE AND FUNCTION
• The dual-servo brake is the most common drum brake
design.
• The name “servo” comes from the fact that one shoe
“serves” the other to increase application force.
• All servo brakes used on automobiles are of the dual-servo
design that works with equal force in both directions.
• The primary advantage of the dual-servo brake is that it is
more powerful than any of the non-servo designs.
• Another advantage of the dual-servo brake is that it makes
a good parking brake.
• Dual-servo action not only makes the brake very powerful,
it allows the brake to hold equally well in both directions.


DUAL-SERVO BRAKE CONSTRUCTION
• The basic dual-servo brake
uses one anchor and a
single two-piston wheel
cylinder.
• ADJUSTING LINK
• PRIMARY AND
SECONDARY BRAKE
SHOES

FIGURE 10–25 A typical dual-servo drum brake.


DUAL-SERVO BRAKE CONSTRUCTION

FIGURE 10–26 A typical dual-servo brake adjusting link assembly.


DUAL-SERVO BRAKE OPERATION
• When a dual-servo brake is
applied, the wheel cylinder
attempts to force the tops
of both brake shoes
outward against the drum.

FIGURE 10–27 Dual-servo brake
operation. The primary shoe on the left
exerts a force on the secondary shoe on
the right.


SERVO ACTION
• Once all clearance is taken up between the brake
shoes, adjusting link, and anchor, both brake
shoes become self-energized like the leading
shoes in a non-servo brake.
• The anchor pin prevents the secondary shoe from
rotating, and the adjusting link (held in position by
the secondary shoe) serves as the anchor for the
primary shoe.
• Servo action then occurs as a part of the braking
force generated by the primary shoe is transferred
through the adjusting link to help apply the
secondary shoe.


SERVO ACTION

FIGURE 10–28 Dual servo action greatly
increases the application force on the
secondary shoe.


Rear Wheel Lockup? Check the
Adjustment

• Servo action enables a drum brake to provide
increased stopping power, but it can also cause the
brakes to grab and lock if they get too far out of
adjustment. As clearance between the shoes and
drum increases, the primary brake shoe is allowed a
greater range of movement. The farther the shoe
moves, the more speed it picks up from the rotating
brake drum. At the moment the slack is taken up
between the brake shoes, adjusting link, and anchor,
the speed of the primary shoe is converted into
application force by servo action. If the primary shoe
is moving too quickly, it will apply the secondary shoe
very hard and fast, causing the brakes to grab and
possibly lock the wheels.


AUTOMATIC BRAKE ADJUSTERS
SERVO BRAKE STARWHEEL AUTOMATIC ADJUSTERS

• Servo brakes use three styles of starwheel
adjusters:
• Cable
• Lever
• Link



FIGURE 10–29 A cable-actuated starwheel adjuster. This type of
adjuster makes the adjustment as the brakes are released.



FIGURE 10–30 A lever-actuated starwheel
automatic adjuster. This type of adjuster makes the
adjustment when the brakes are applied.



FIGURE 10–31 A link-actuated starwheel adjuster. This type of
adjuster makes the adjustment when the brakes are released.



FIGURE 10–32 The operation of a
typical self-adjuster. Notice that the
adjuster actually moves the starwheel.



FIGURE 10–33 A cable-actuated
starwheel adjuster with an over-travel
spring.


NON-SERVO STARWHEEL AUTOMATIC ADJUSTERS

• OPERATION.
• The starwheel automatic
adjusters used on
nonservo brakes may be
mounted on either the
leading or trailing shoe.
• These types of adjusters
work whenever the brakes
are applied—in either the
forward or reverse
direction.

FIGURE 10–34 A non-servo brake with a
lever-actuated starwheel automatic adjuster on a
leading shoe. This type of adjuster makes an
adjustment as the brakes are applied.


NON-SERVO STARWHEEL AUTOMATIC ADJUSTERS

FIGURE 10–35 A non-servo brake
with a lever-actuated starwheel
automatic adjuster on the trailing shoe.
This type of adjuster makes the
adjustment as the brakes are released.


RATCHET-TYPE AUTOMATIC ADJUSTERS

• Most ratchet automatic adjusters use movement of
the brake shoes to adjust the lining-to-drum
clearance.
• The adjustment of a ratchet adjuster is carried out
by two parts that have small interlocking teeth.
• As the adjustment is made, the two toothed
elements ratchet across one another.
• Once adjustment is complete, the teeth lock
together to hold the brake shoes in their new
positions.



• LEVER-LATCH RATCHET AUTOMATIC
ADJUSTER
• STRUT-QUADRANT RATCHET AUTOMATIC
ADJUSTER



FIGURE 10–36 A lever-latch ratchet automatic adjuster.



FIGURE 10–37 A strut-quadrant
ratchet automatic adjuster.


Cool the Brakes before Backing

• Self-adjusters can over-adjust the rear drum brakes if
the brake drums are hot and have increased in
diameter due to the heat. For example, if a pickup
truck towing a boat had to brake while backing down a
long, steep grade to the boat ramp, the rear brake
drums could become larger in diameter due to the
heat created during braking. The brakes could
overadjust if the driver repeatedly depresses and
releases the brake pedal while backing the trailer
down the boat ramp. Then, after the boat has been
removed from the trailer and the rear brakes have
cooled, the drums will shrink and keep the rear brakes
from releasing, preventing the truck from moving up
the ramp.


SUMMARY
1. Drum brake shoes include the lining table and shoe web
plus holes for the springs to attach and semicircular
anchor eyes.
2. Brake linings can be attached using rivets or bonding.
3. Lining edge codes identify the manufacturer and include
two letters at the end, which identify the coefficient of
friction of the material. The first letter indicates the
coefficient when the lining is cold and the second indicates
the coefficient when the lining is hot.
4. Brake shoes are forced outward against a brake drum by
hydraulic action working on the brake shoes by the piston
of a wheel cylinder.
5. The curved arch of the brake shoe causes a wedging
action between the brake shoe and the rotating drum. This
wedging action increases the amount of force applied to
the drum.


SUMMARY
6. Dual-servo brakes use primary and secondary brake
shoes that are connected at one end. The wedge action
on the front (primary) shoe forces the secondary shoe into
the drum with even greater force. This action is called
servo self-energizing.
7. Leading-trailing brakes use two brake shoes that are not
connected. Leading-trailing brakes operate on a more
linear basis and are therefore more suited than dual-servo
for ABS.
8. Most self-adjusting mechanisms usually operate from the
secondary or rearward facing brake shoe and adjust the
brakes as the brakes are released.
9. Some self-adjusters operate on the primary shoe and
adjust the brakes as they are being applied.


REVIEW QUESTIONS

1. Describe the difference between a dual-servo and
a leading–trailing drum brake system.
2. List all the parts of a typical drum brake.
3. Explain how a self-adjusting brake mechanism
works.
4. What do the last two letters of the lining or pad
edge code identify?


CHAPTER QUIZ

1. Two technicians are discussing drum brake
shoes. Technician A says that forward and
rearward facing shoes are the same and can be
installed in either position on any drum brake
system. Technician B says that the darker color
lining should always be placed toward the front of
the vehicle. Which technician is correct?
a. Technician A only
b. Technician B only
c. Both Technicians A and B
d. Neither Technician A nor B


CHAPTER QUIZ

2. Two technicians are discussing brake lining edge
codes. Technician A says that the code can
identify the manufacturer. Technician B says that
all friction material from the same manufacturer
will have the same edge codes. Which technician
is correct?


CHAPTER QUIZ

3. Technician A says that starwheel adjusters use
different threads (left- and right-handed) for the
left and right sides of the vehicle. Technician B
says that a pawl controls the teeth of the
starwheel adjuster. Which technician is correct?


CHAPTER QUIZ

4. Technician A says that drum brakes can fail to
slow the vehicle if driven through deep water.
Technician B says that when drum brakes get
hot, the brake pedal will drop because the drum
expands away from the shoes. Which technician
is correct?


CHAPTER QUIZ

5. Technician A says
that self-adjusters
used on most drum
brakes work when the
brakes are applied,
then release while
traveling in reverse.
Technician B says
that some adjusters
can overadjust if the
brake drums are hot.
Which technician is
correct?
Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc.,
Automotive Brake Systems, 5/e

CHAPTER QUIZ

6. Drum brake shoes ________.
a. Can have riveted linings
b. Can have bonded linings
c. Can have one shoe that is longer than the other
on the same wheel brake
d. All of the above are possible


CHAPTER QUIZ

7. Which lining does most of the braking on a dual-
servo brake?
a. Front (forward facing)
b. Rear (rearward facing)
c. Both contribute equally
d. Depends on the speed of the vehicle


CHAPTER QUIZ

8. Which lining does most of the braking on a
leading-trailing brake?
a. Leading (forward facing)
b. Trailing (rearward facing)
c. Both contribute equally
d. Depends on the speed of the vehicle


CHAPTER QUIZ
9. Two technicians are discussing drum brake self-
adjusters. Technician A says that a frozen
starwheel adjuster can cause the brakes to lock
up due to the adjusting lever being unable to
move the adjuster causing the linkage to bind.
Technician B says that some brakes self-adjust
when the brakes are applied rather than when
released. Which technician is correct?


CHAPTER QUIZ

10.A typical drum brake backing plate has how many
shoe support pads (also called ledges)?
a. 3
b. 4
c. 6
d. 8


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