brakes drum chapter_10

CHAPTER 10

Drum Brakes

Automotive Brake Systems, 5/eBy James D. Halderman

Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc.,Upper Saddle River, NJ 07458 • All rights reserved.2

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.

OBJECTIVES



KEY TERMS• Anchor eyes • Beehive holddown • Bonded linings • Brake fade • Brake shoe return springs • Brake shoe holddown • Brake shoes • Core charge • Double-trailing shoe • Dual-servo brake • Duo-Servo • Edge codes • Gas fade • Labyrinth seal • Leading shoe • Leading-trailing brakes • Ledges • Lining edge codes

• Lining fade • Lining table • Mechanical fade • Nibs • Non-servo brakes • Over-travel spring • Pawl • Piston stops • Platform • Primary shoe • Riveted linings • Secondary shoe • Self-energizing action • Shoe anchors • Shoe contact areas • Shoe rim • Shoe support pads • 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 DISADVANTAGESBRAKE 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.



DRUM BRAKE DISADVANTAGESBRAKE FADE

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



DRUM BRAKE DISADVANTAGES

• 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 PARTSBACKING 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 PARTSBACKING 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 PARTSSHOE 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 PARTSPISTON 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 PARTSSHOE 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 PARTSWHEEL 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 PARTSWHEEL 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 SHOESPRIMARY 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 SHOESPRIMARY AND SECONDARY BRAKE SHOES

FIGURE 10–13 Primary shoe lining may vary depending on the application.

FIGURE 10–12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing).



DRUM BRAKE SHOESLINING 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 SHOESLINING ASSEMBLY METHODS

FIGURE 10–15 Many brake linings are bonded.



DRUM BRAKE SHOESEDGE 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.




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 PARTSRETURN 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 PARTSRETURN SPRINGS

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



DRUM BRAKE PARTSBRAKE 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 PARTSBRAKE SHOE HOLDDOWNS

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



DRUM BRAKE PARTSPARKING 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 PARTSBRAKE 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 their outer edge to help dissipate heat.

FIGURE 10–21 An aluminium brake drum with a cast iron friction surface. The cooling fins around the outside help dissipate the heat from the friction surface to the outside air.



NON-SERVO BRAKE DESIGNPURPOSE 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.



NON-SERVO BRAKE DESIGNPARTS 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.



NON-SERVO BRAKE DESIGNPARTS AND OPERATION

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



NON-SERVO BRAKE DESIGNDOUBLE-TRAILING BRAKE

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

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



NON-SERVO BRAKE DESIGNLEADING-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 DESIGNPURPOSE 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 DESIGNDUAL-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 DESIGNDUAL-SERVO BRAKE CONSTRUCTION

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



DUAL-SERVO BRAKE DESIGNDUAL-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.



DUAL-SERVO BRAKE DESIGNSERVO 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.



DUAL-SERVO BRAKE DESIGNSERVO 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 ADJUSTERSSERVO 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.



AUTOMATIC BRAKE ADJUSTERSNON-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.



AUTOMATIC BRAKE ADJUSTERSNON-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.



AUTOMATIC BRAKE ADJUSTERSRATCHET-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?







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?

a. Technician A onlyb. Technician B onlyc. Both Technicians A and Bd. Neither Technician A nor B



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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