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Halderman ch099 lecture

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The document discusses drum brakes, including their components, operation, advantages, and disadvantages. It describes drum brake parts such as the backing plate, shoe anchors, wheel cylinders, brake shoes, and linings. It explains the differences between primary and secondary brake shoes and how drum brakes use self-energizing and servo action to provide stopping power. The document also discusses drum brake fade issues and methods for adjusting drum brakes.

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DRUM BRAKES 99
Objectives The student should be able to: Prepare for the Brakes (A5) ASE certification test content area “B” (Drum Brake Diagnosis and Repair). Identify drum brake component parts. Describe the operation of non-servo brakes. Explain the operation of dual-servo brakes. Discuss drum brake adjusters.
DRUM BRAKE ADVANTAGES
Drum Brake Advantages Drum brakes were first type brake used on motor vehicles Still used on the rear of many vehicles
Figure 99-1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.
Figure 99-2 An exploded view of a typical drum brake assembly.
Drum Brake Advantages Self-Energizing and Servo Action Primary advantage Drum brakes apply more stopping power for given amount of force applied to brake pedal than do disc brakes
Drum Brake Advantages Self-Energizing and Servo Action Brake drums use servo action One brake shoe helps the other, providing for increased stopping power
Drum Brake Advantages Parking Brake Drum brakes make excellent parking brakes Low effort from driver holds heavy vehicle in place Disc brakes require complex set of extra parts to serve as parking brakes
DRUM BRAKE DISADVANTAGES
Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Occurs when drum gets so hot if expands away from brake linings
Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Brake shoes move outward to reach drum
Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Brake pedal drops toward floor
Drum Brake Disadvantages Brake Fade Four types of brake fade Lining Fade Occurs when friction coefficient drops due to intense heat
Drum Brake Disadvantages Brake Fade Four types of brake fade Lining Fade Brake lining becomes slippery
Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Occurs under extended hard braking from high speeds
Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Thin layer of gases build up between linings and drum
Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Gas acts as lubricant and reduces friction
Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Occurs when moisture is trapped between shoes and drum
Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Moisture acts as lubricant
Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Friction eventually creates enough heat to evaporate water
Drum Brake Disadvantages Brake Adjustment Drum brake design requires adjusting Brake shoe lining wears Clearance between lining and drum increases
Drum Brake Disadvantages Brake Adjustment Longer brake pedal travel results Most vehicles have automatic adjustments to maintain proper drum to lining clearance
Drum Brake Disadvantages Brake Pull Brake pull occurs when friction assemblies on opposite sides of vehicle have different amounts of stopping power
DRUM BRAKE PARTS
Drum Brake Parts Backing Plate Foundation of drum brake is backing plate Mounts to suspension or axle housing
Drum Brake Parts Backing Plate Shoe support pads are stamped into backing plate Support pads contact edges of brake shoes to keep linings aligned
Drum Brake Parts Backing Plate Pads are also called ledges or shoe contact areas Pad lightly coated with silicone brake grease to minimize wear
Figure 99-3 The backing plate is the foundation of every drum brake. There are normally six pads where the brake shoes contact the backing plate.
Drum Brake Parts Backing Plate Backing plate serves as mounting surface for friction assembly parts Plate protects brake assembly from contaminants
Drum Brake Parts Backing Plate Edge of backing plate curves outward to form lip Lip fits into groove in edge of brake drum to form water barrier Seal is called labyrinth seal
Figure 99-4 A labyrinth seal is created between the lip of the backing plate and the groove in the brake drum.
Drum Brake Parts Backing Plate Plate has openings to permit inspection and adjustment Openings sealed with metal plugs
Drum Brake Parts Backing Plate Metal plugs must be punched out to allow inspection or adjustment Rubber replacement seals available
Drum Brake Parts Shoe Anchors Prevent brake shoes from rotating with drum when brakes applied Most drum brakes have one anchor; some have two or more
Drum Brake Parts Shoe Anchors Many anchors are round post mounted on backing plate Brake shoes have cutouts that contact anchor Another anchor is keystone anchor
Figure 99-5 A keystone anchor allows the brake shoes to self-center in the drum.
Drum Brake Parts Piston Stops Piston stops prevent wheel cylinder pistons from coming out of their bores
Figure 99-6 Piston stops prevent the wheel cylinder from coming apart.
Drum Brake Parts Wheel Cylinders Hydraulic pressure transferred from master cylinder to each wheel cylinder through brake fluid Pressure forces piston inside wheel cylinder to move outward Pushrods or links force brake shoes outward against brake drum
Figure 99-7 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.
Figure 99-8 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
Drum Brake Shoes Linings of drum brakes attached to curved metal assemblies called brake shoes Outer edge of lining is lined with friction material Friction material contacts drum brake to generate stopping power
Figure 99-9 Steelbrake shoes are made from two stampings welded together—the web and the lining table.
Figure 99-10 Tapered ends on the linings help to reduce brake noise.
Drum Brake Shoes Curved outer portion of the shoe is lining table, or shoe rim or platform Lining table supports the block of friction material Metal piece of shoe under the lining table is shoe web
Drum Brake Shoes All of force actuating shoe is transferred through web to the lining table Web usually has number of holes and notches where hardware attaches Brake shoes can be relined and reused if web and lining table are not damaged
Figure 99-11 Typical drum brake shoe and the names of the parts.
Drum Brake Shoes Primary and Secondary Brake Shoes Primary shoe (front facing shoe) is self-energized by drum rotation to create servo action Servo action forces secondary shoe more firmly against drum
Drum Brake Shoes Primary and Secondary Brake Shoes Because of different forces involved, primary and secondary shoes are different Secondary shoe lining extends nearly the full length of lining table
Drum Brake Shoes Primary and Secondary Brake Shoes Secondary shoe lining material has higher coefficient of friction Primary shoe undergoes far less stress
Figure 99-12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing). The color of the primary and secondary lining can also be different due to differences in friction and wear requirements.
Drum Brake Shoes Primary and Secondary Brake Shoes Primary shoe may run half the length of lining table Primary shoe lining has lower friction coefficient
Figure 99-13 Primary shoe lining may vary depending on the application.
Drum Brake Shoes Lining Assembly Methods Two ways to mount linings to brake shoe Riveting Lining attached to lining table with copper or aluminum rivets
Figure 99-14 Riveted brake linings are quiet and reliable at high temperatures.
Drum Brake Shoes Lining Assembly Methods Riveting Advantages of riveting Riveted brakes operate more quietly than bonded brakes
Drum Brake Shoes Lining Assembly Methods Riveting Advantages of riveting Rivets will not loosen at high temperatures
Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Rivet holes create stress points where cracks may occur
Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Reduced service life because linings must be replaced before rivets contact drum
Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Rivet can cause deep groves in drum, often requiring drum replacement
Drum Brake Shoes Lining Assembly Methods Bonding Linings bonded to shoe with high-temperature adhesive
Figure 99-15 Many brake linings are bonded.
Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding Without rivets, bonded linings have longer service life
Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding If worn too long, will do less damage to drum
Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding Fewer problems with cracking
Drum Brake Shoes Lining Assembly Methods Bonding Disadvantages of bonding If bonded lining gets too hot, bonding may fail and lining can separate from shoe
Drum Brake Shoes Lining Assembly Methods Bonding Disadvantages of bonding Bonded linings can be noisier
Drum Brake Shoes Edge Codes Edge codes identify brake lining materials First group of letters identify manufacturer
Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Second group is numbers and/or letters
Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Identifies lining compound or formula
Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Third group is two letters that identify coefficient of friction
Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces Coefficient is always less than 1
Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces The higher the coefficient, the more friction
Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces Coefficient codes include letters representing range of friction
CHART 99–1 Edge code letters represent a range of coefficient of friction of the linings.
Figure 99-16 Typical drum brake lining edge codes, showing the coefficient of friction codes for cold and hot circled.
DRUM BRAKE PARTS
Drum Brake Parts Return Springs Brake shoe return springs retract shoes when brake pedal is released Springs prevent brake drag Most brakes use closed coil springs
Figure 99-17 A typical drum brake assembly showing the support plate (backing plate), brake shoes, and springs.
Figure 99-18 A single spring-steel spring is used on some drum brakes.
Drum Brake Parts Return Springs Type, location, and number of springs varies All springs attached in one of two ways Connect directly from shoe to shoe
Drum Brake Parts Return Springs All springs attached in one of two ways Connect from one shoe to the anchor post
Drum Brake Parts Brake Shoe Holddowns Keep shoes securely against support pads on braking plate Help prevent noise, vibration, and wear
Drum Brake Parts Brake Shoe Holddowns Allow free movements to allow adjustments of shoes Many forms of holddowns
Figure 99-19 Various types and styles of hold-down springs. The hold down pins are commonly called nails.
Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Corresponding hole in shoe web
Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Spring fits over end of pin against shoe web
Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Washer compresses spring and locks onto flattened end of pin
Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Sometimes called beehive holddown
Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Hook installed through hole in shoe web
Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Hook attaches to retaining clip in backing plate
Drum Brake Parts Parking Brake Linkage Consists of cable, lever, and strut system that spreads brake shoes apart
Figure 99-20 A mechanical brake linkage is part of most drum brake assemblies.
Drum Brake Parts Brake Drums Turns with wheel Mounts on the hub or axle
Drum Brake Parts Brake Drums Covers the rest of the brake assembly Made of cast iron or cast aluminum with cast-iron liner May have ribs or fins to dissipate heat
Figure 99-21 An aluminum 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 DESIGN
Non-Servo Brake Design Purpose and Function Non-servo brake allows each brake to work separately Action of one shoe has no effect on action of the other Many non-servo brakes have self-energizing action
Non-Servo Brake Design Parts and Operation Self-energizing action occurs when leading shoe contacts drum Drum attempts to rotate shoe with it
Non-Servo Brake Design Parts and Operation Shoe is fixed in place by anchor Drum rotation energizes shoe by wedging it tightly against brake drum
Non-Servo Brake Design Parts and Operation When trailing shoe contacts drum, rotation de-energizes shoe by forcing it away from brake drum Trailing brake shoe is always de-energized by drum rotation
Non-Servo Brake Design Parts and Operation Leading shoe becomes trailing shoe when vehicle is in reverse Trailing shoe becomes leading shoe when vehicle is in reverse
Figure 99-22 Self-energizing action can increase or decrease the stopping power of a brake shoe.
Non-Servo Brake Design Parts and Operation Leading shoes wear faster than trailing shoes because they are applied with greater force Leading shoe will often have thicker lining or larger surface area
Non-Servo Brake Design Leading-Trailing Brake Design Non-servo leading-trailing brake has one leading shoe and one trailing shoe
Figure 99-23 A leading-trailing non-servo brake.
Non-Servo Brake Design Parts and Operation Design has one energized and one de-energized shoe Non-servo brake designs not as powerful as servo brake Less prone to lockup than servo brake
DUAL-SERVO BRAKE DESIGN
Dual-Servo Brake Design Purpose and Function Most common brake design Servo implies that one shoe serves the other to increase application force
Dual-Servo Brake Design Purpose and Function Primary advantage: more powerful than any non-servo design Makes good parking brake—holds equally well in either direction More susceptible to pull than other brake designs
Dual-Servo Brake Design Purpose and Function NOTE: Dual-servo brakes are also called Duo-Servo, which is a brand name of the Bendix Corporation.
Dual-Servo Brake Design Dual-Servo Brake Construction Uses one anchor and one two-piston wheel cylinder Anchor usually mounted at top of backing plate
Figure 99-24 A typical dual-servo drum brake.
Dual-Servo Brake Design Dual-Servo Brake Construction Tops of brake shoes held against anchor by individual return springs Bottoms of shoes spaced apart by adjusting link and return spring
Dual-Servo Brake Design Dual-Servo Brake Construction Adjusting Link (Starwheel Adjuster) Adjusting link consists of starwheel that is part of adjusting screw
Dual-Servo Brake Design Dual-Servo Brake Construction Adjusting Link (Starwheel Adjuster) Starwheel and adjusting screw helps reduce brake squeal
Figure 99-25 A typical dual-servo brake adjusting link assembly commonly called a starwheel adjuster.
Dual-Servo Brake Design Dual-Servo Brake Construction NOTE: Adjusting links generally have specific left- or right-hand threads, and must be installed on the correct side of the vehicle.
Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Two brake shoes are not called leading and trailing parts
Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Identified as primary and secondary shoes
Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Primary shoe always faces front of vehicle
Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Secondary shoe provides about 70% of total braking power in dual-servo brake
Dual-Servo Brake Design Dual-Servo Brake Operation When dual-servo brake is applied, wheel cylinder attempts to force tops of both brake shoes outward against drum Primary shoe not directly anchored to backing plate so it rotates
Dual-Servo Brake Design Dual-Servo Brake Operation As primary shoe rotates, it forces secondary shoe to rotate Rotation seats secondary shoe firmly against anchor, increasing braking force
Figure 99-26 Dual-servo brake operation. The primary shoe on the left exerts a force on the secondary shoe on the right.
Dual-Servo Brake Design Servo Action Once all clearance is taken up, both brake shoes become self-energized Anchor pin prevents secondary shoe from rotating
Dual-Servo Brake Design Servo Action Adjusting link serves as anchor for primary shoe Servo action occurs as part of the braking force generated by primary shoe is transferred to help apply secondary shoe
Figure 99-27 Dual servo action greatly increases the application force on the secondary shoe.
AUTOMATIC BRAKE ADJUSTERS
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Three styles of starwheel adjusters Cable Lever Link
Figure 99-28 A cable-actuated starwheel adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are released.
Figure 99-29 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are applied.
Figure 99-30 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters All three adjusters mount on secondary brake shoe Adjust when brakes are applied while vehicle moves in reverse
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters As brakes are applied, secondary shoe moves away from anchor Movement causes cable or linkage to pull up on adjuster pawl
Figure 99-31 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters If brakes are worn, pawl engages next tooth of starwheel When brakes released, pawl return spring pulls the pawl down
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Action rotates starwheel, moving brake shoes apart Movement reduces lining-to-drum clearance Some servo brakes have over-travel spring assembly
Figure 99-32 A cable-actuated starwheel adjuster with an overtravel spring.
Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Adjuster is mounted under starwheel Adjustment is made as brakes are applied rather than when released
Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters Can be mounted on either leading or trailing shoe Work whenever brakes are applied
Figure 99-33 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 Adjusters Non-Servo Starwheel Automatic Adjusters When not in use, adjuster pawl is held in place by parking brake strut When brakes applied, primary shoe moves toward brake drum
Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters Pawl spring pivots the pawl downward Pawl mounts on brake shoe and rotates starwheel to adjust brake
Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters When brake released, return springs retract shoes Pawl levered back into resting position Trailing-shoe non-servo starwheel adjuster works like leading-shoe design
Figure 99-34 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 Adjusters Ratchet-Type Automatic Adjusters Use movement of brake shoes to adjust lining-to-drum clearance Adjustment of ratchet adjuster performed by two parts with interlocking teeth
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters During adjustment, toothed parts ratchet across one another When adjustment is complete, the teeth lock and hold brake shoes in new position
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Operates whenever brakes are applied
Figure 99-35 A lever-latch ratchet automatic adjuster.
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Design consists of large lever and smaller latch with interlocking teeth
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake As brakes applied, shoes move outward toward drum
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Parking brake strut pulls on adjuster lever
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Lever forced to pivot inward where it attaches to top of leading shoe
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake If brakes are worn, lever ratchets one or more teeth on latch
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake When brakes released, parking brake strut holds shoes farther apart to reduce lining-to-drum clearance
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes
Figure 99-36 A strut-quadrant ratchet automatic adjuster.
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Parking brake strut
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Adjusting quadrant
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Quadrant spring
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes When brakes applied, leading shoe moves out toward brake drum
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes If there is sufficient wear, the edge of the slot in shoe web contacts inner side of adjusting quadrant arm and pulls it outward
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Toothed section of quadrant is lifted away from post
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Quadrant spring rotates quadrant until pivot pin is bottomed in slot
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes When brakes released, quadrant returns inward with leading shoe
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Toothed section of quadrant engage teeth on strut post
Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Quadrant remains in its new extended position, reducing the lining-to-drum clearance
TECH TIP 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. BACK TO PRESENTATION
TECH TIP 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. BACK TO PRESENTATION Aftermarket linings are not required by federal law to meet performance or wear standards that are required of original factory brake linings.
TECH TIP 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. BACK TO PRESENTATION 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.
TECH TIP Cool the Brakes before Backing Self-adjusters can overadjust 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. BACK TO PRESENTATION 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. NOTE: Some drum brakes are equipped with a bimetallic heat sensor that prevents the self-adjusters from working if the brakes are hot.

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Halderman ch099 lecture

  • 2. Objectives The student should be able to: Prepare for the Brakes (A5) ASE certification test content area “B” (Drum Brake Diagnosis and Repair). Identify drum brake component parts. Describe the operation of non-servo brakes. Explain the operation of dual-servo brakes. Discuss drum brake adjusters.
  • 4. Drum Brake Advantages Drum brakes were first type brake used on motor vehicles Still used on the rear of many vehicles
  • 5. Figure 99-1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.
  • 6. Figure 99-2 An exploded view of a typical drum brake assembly.
  • 7. Drum Brake Advantages Self-Energizing and Servo Action Primary advantage Drum brakes apply more stopping power for given amount of force applied to brake pedal than do disc brakes
  • 8. Drum Brake Advantages Self-Energizing and Servo Action Brake drums use servo action One brake shoe helps the other, providing for increased stopping power
  • 9. Drum Brake Advantages Parking Brake Drum brakes make excellent parking brakes Low effort from driver holds heavy vehicle in place Disc brakes require complex set of extra parts to serve as parking brakes
  • 11. Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Occurs when drum gets so hot if expands away from brake linings
  • 12. Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Brake shoes move outward to reach drum
  • 13. Drum Brake Disadvantages Brake Fade Four types of brake fade Mechanical Fade Brake pedal drops toward floor
  • 14. Drum Brake Disadvantages Brake Fade Four types of brake fade Lining Fade Occurs when friction coefficient drops due to intense heat
  • 15. Drum Brake Disadvantages Brake Fade Four types of brake fade Lining Fade Brake lining becomes slippery
  • 16. Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Occurs under extended hard braking from high speeds
  • 17. Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Thin layer of gases build up between linings and drum
  • 18. Drum Brake Disadvantages Brake Fade Four types of brake fade Gas Fade Gas acts as lubricant and reduces friction
  • 19. Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Occurs when moisture is trapped between shoes and drum
  • 20. Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Moisture acts as lubricant
  • 21. Drum Brake Disadvantages Brake Fade Four types of brake fade Water Fade Friction eventually creates enough heat to evaporate water
  • 22. Drum Brake Disadvantages Brake Adjustment Drum brake design requires adjusting Brake shoe lining wears Clearance between lining and drum increases
  • 23. Drum Brake Disadvantages Brake Adjustment Longer brake pedal travel results Most vehicles have automatic adjustments to maintain proper drum to lining clearance
  • 24. Drum Brake Disadvantages Brake Pull Brake pull occurs when friction assemblies on opposite sides of vehicle have different amounts of stopping power
  • 26. Drum Brake Parts Backing Plate Foundation of drum brake is backing plate Mounts to suspension or axle housing
  • 27. Drum Brake Parts Backing Plate Shoe support pads are stamped into backing plate Support pads contact edges of brake shoes to keep linings aligned
  • 28. Drum Brake Parts Backing Plate Pads are also called ledges or shoe contact areas Pad lightly coated with silicone brake grease to minimize wear
  • 29. Figure 99-3 The backing plate is the foundation of every drum brake. There are normally six pads where the brake shoes contact the backing plate.
  • 30. Drum Brake Parts Backing Plate Backing plate serves as mounting surface for friction assembly parts Plate protects brake assembly from contaminants
  • 31. Drum Brake Parts Backing Plate Edge of backing plate curves outward to form lip Lip fits into groove in edge of brake drum to form water barrier Seal is called labyrinth seal
  • 32. Figure 99-4 A labyrinth seal is created between the lip of the backing plate and the groove in the brake drum.
  • 33. Drum Brake Parts Backing Plate Plate has openings to permit inspection and adjustment Openings sealed with metal plugs
  • 34. Drum Brake Parts Backing Plate Metal plugs must be punched out to allow inspection or adjustment Rubber replacement seals available
  • 35. Drum Brake Parts Shoe Anchors Prevent brake shoes from rotating with drum when brakes applied Most drum brakes have one anchor; some have two or more
  • 36. Drum Brake Parts Shoe Anchors Many anchors are round post mounted on backing plate Brake shoes have cutouts that contact anchor Another anchor is keystone anchor
  • 37. Figure 99-5 A keystone anchor allows the brake shoes to self-center in the drum.
  • 38. Drum Brake Parts Piston Stops Piston stops prevent wheel cylinder pistons from coming out of their bores
  • 39. Figure 99-6 Piston stops prevent the wheel cylinder from coming apart.
  • 40. Drum Brake Parts Wheel Cylinders Hydraulic pressure transferred from master cylinder to each wheel cylinder through brake fluid Pressure forces piston inside wheel cylinder to move outward Pushrods or links force brake shoes outward against brake drum
  • 41. Figure 99-7 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.
  • 42. Figure 99-8 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.
  • 44. Drum Brake Shoes Linings of drum brakes attached to curved metal assemblies called brake shoes Outer edge of lining is lined with friction material Friction material contacts drum brake to generate stopping power
  • 45. Figure 99-9 Steelbrake shoes are made from two stampings welded together—the web and the lining table.
  • 46. Figure 99-10 Tapered ends on the linings help to reduce brake noise.
  • 47. Drum Brake Shoes Curved outer portion of the shoe is lining table, or shoe rim or platform Lining table supports the block of friction material Metal piece of shoe under the lining table is shoe web
  • 48. Drum Brake Shoes All of force actuating shoe is transferred through web to the lining table Web usually has number of holes and notches where hardware attaches Brake shoes can be relined and reused if web and lining table are not damaged
  • 49. Figure 99-11 Typical drum brake shoe and the names of the parts.
  • 50. Drum Brake Shoes Primary and Secondary Brake Shoes Primary shoe (front facing shoe) is self-energized by drum rotation to create servo action Servo action forces secondary shoe more firmly against drum
  • 51. Drum Brake Shoes Primary and Secondary Brake Shoes Because of different forces involved, primary and secondary shoes are different Secondary shoe lining extends nearly the full length of lining table
  • 52. Drum Brake Shoes Primary and Secondary Brake Shoes Secondary shoe lining material has higher coefficient of friction Primary shoe undergoes far less stress
  • 53. Figure 99-12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing). The color of the primary and secondary lining can also be different due to differences in friction and wear requirements.
  • 54. Drum Brake Shoes Primary and Secondary Brake Shoes Primary shoe may run half the length of lining table Primary shoe lining has lower friction coefficient
  • 55. Figure 99-13 Primary shoe lining may vary depending on the application.
  • 56. Drum Brake Shoes Lining Assembly Methods Two ways to mount linings to brake shoe Riveting Lining attached to lining table with copper or aluminum rivets
  • 57. Figure 99-14 Riveted brake linings are quiet and reliable at high temperatures.
  • 58. Drum Brake Shoes Lining Assembly Methods Riveting Advantages of riveting Riveted brakes operate more quietly than bonded brakes
  • 59. Drum Brake Shoes Lining Assembly Methods Riveting Advantages of riveting Rivets will not loosen at high temperatures
  • 60. Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Rivet holes create stress points where cracks may occur
  • 61. Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Reduced service life because linings must be replaced before rivets contact drum
  • 62. Drum Brake Shoes Lining Assembly Methods Riveting Disadvantages of riveting Rivet can cause deep groves in drum, often requiring drum replacement
  • 63. Drum Brake Shoes Lining Assembly Methods Bonding Linings bonded to shoe with high-temperature adhesive
  • 64. Figure 99-15 Many brake linings are bonded.
  • 65. Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding Without rivets, bonded linings have longer service life
  • 66. Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding If worn too long, will do less damage to drum
  • 67. Drum Brake Shoes Lining Assembly Methods Bonding Advantages of bonding Fewer problems with cracking
  • 68. Drum Brake Shoes Lining Assembly Methods Bonding Disadvantages of bonding If bonded lining gets too hot, bonding may fail and lining can separate from shoe
  • 69. Drum Brake Shoes Lining Assembly Methods Bonding Disadvantages of bonding Bonded linings can be noisier
  • 70. Drum Brake Shoes Edge Codes Edge codes identify brake lining materials First group of letters identify manufacturer
  • 71. Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Second group is numbers and/or letters
  • 72. Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Identifies lining compound or formula
  • 73. Drum Brake Shoes Edge Codes Edge codes identify brake lining materials Third group is two letters that identify coefficient of friction
  • 74. Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces Coefficient is always less than 1
  • 75. Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces The higher the coefficient, the more friction
  • 76. Drum Brake Shoes Edge Codes Coefficient of friction indicates amount of friction between two surfaces Coefficient codes include letters representing range of friction
  • 77. CHART 99–1 Edge code letters represent a range of coefficient of friction of the linings.
  • 78. Figure 99-16 Typical drum brake lining edge codes, showing the coefficient of friction codes for cold and hot circled.
  • 80. Drum Brake Parts Return Springs Brake shoe return springs retract shoes when brake pedal is released Springs prevent brake drag Most brakes use closed coil springs
  • 81. Figure 99-17 A typical drum brake assembly showing the support plate (backing plate), brake shoes, and springs.
  • 82. Figure 99-18 A single spring-steel spring is used on some drum brakes.
  • 83. Drum Brake Parts Return Springs Type, location, and number of springs varies All springs attached in one of two ways Connect directly from shoe to shoe
  • 84. Drum Brake Parts Return Springs All springs attached in one of two ways Connect from one shoe to the anchor post
  • 85. Drum Brake Parts Brake Shoe Holddowns Keep shoes securely against support pads on braking plate Help prevent noise, vibration, and wear
  • 86. Drum Brake Parts Brake Shoe Holddowns Allow free movements to allow adjustments of shoes Many forms of holddowns
  • 87. Figure 99-19 Various types and styles of hold-down springs. The hold down pins are commonly called nails.
  • 88. Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Corresponding hole in shoe web
  • 89. Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Spring fits over end of pin against shoe web
  • 90. Drum Brake Parts Brake Shoe Holddowns Most common holddown design is steel pin installed through hole in backing plate Washer compresses spring and locks onto flattened end of pin
  • 91. Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Sometimes called beehive holddown
  • 92. Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Hook installed through hole in shoe web
  • 93. Drum Brake Parts Brake Shoe Holddowns Another holddown is taper-wound coil spring with hook on its end Hook attaches to retaining clip in backing plate
  • 94. Drum Brake Parts Parking Brake Linkage Consists of cable, lever, and strut system that spreads brake shoes apart
  • 95. Figure 99-20 A mechanical brake linkage is part of most drum brake assemblies.
  • 96. Drum Brake Parts Brake Drums Turns with wheel Mounts on the hub or axle
  • 97. Drum Brake Parts Brake Drums Covers the rest of the brake assembly Made of cast iron or cast aluminum with cast-iron liner May have ribs or fins to dissipate heat
  • 98. Figure 99-21 An aluminum 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.
  • 100. Non-Servo Brake Design Purpose and Function Non-servo brake allows each brake to work separately Action of one shoe has no effect on action of the other Many non-servo brakes have self-energizing action
  • 101. Non-Servo Brake Design Parts and Operation Self-energizing action occurs when leading shoe contacts drum Drum attempts to rotate shoe with it
  • 102. Non-Servo Brake Design Parts and Operation Shoe is fixed in place by anchor Drum rotation energizes shoe by wedging it tightly against brake drum
  • 103. Non-Servo Brake Design Parts and Operation When trailing shoe contacts drum, rotation de-energizes shoe by forcing it away from brake drum Trailing brake shoe is always de-energized by drum rotation
  • 104. Non-Servo Brake Design Parts and Operation Leading shoe becomes trailing shoe when vehicle is in reverse Trailing shoe becomes leading shoe when vehicle is in reverse
  • 105. Figure 99-22 Self-energizing action can increase or decrease the stopping power of a brake shoe.
  • 106. Non-Servo Brake Design Parts and Operation Leading shoes wear faster than trailing shoes because they are applied with greater force Leading shoe will often have thicker lining or larger surface area
  • 107. Non-Servo Brake Design Leading-Trailing Brake Design Non-servo leading-trailing brake has one leading shoe and one trailing shoe
  • 108. Figure 99-23 A leading-trailing non-servo brake.
  • 109. Non-Servo Brake Design Parts and Operation Design has one energized and one de-energized shoe Non-servo brake designs not as powerful as servo brake Less prone to lockup than servo brake
  • 110. DUAL-SERVO BRAKE DESIGN
  • 111. Dual-Servo Brake Design Purpose and Function Most common brake design Servo implies that one shoe serves the other to increase application force
  • 112. Dual-Servo Brake Design Purpose and Function Primary advantage: more powerful than any non-servo design Makes good parking brake—holds equally well in either direction More susceptible to pull than other brake designs
  • 113. Dual-Servo Brake Design Purpose and Function NOTE: Dual-servo brakes are also called Duo-Servo, which is a brand name of the Bendix Corporation.
  • 114. Dual-Servo Brake Design Dual-Servo Brake Construction Uses one anchor and one two-piston wheel cylinder Anchor usually mounted at top of backing plate
  • 115. Figure 99-24 A typical dual-servo drum brake.
  • 116. Dual-Servo Brake Design Dual-Servo Brake Construction Tops of brake shoes held against anchor by individual return springs Bottoms of shoes spaced apart by adjusting link and return spring
  • 117. Dual-Servo Brake Design Dual-Servo Brake Construction Adjusting Link (Starwheel Adjuster) Adjusting link consists of starwheel that is part of adjusting screw
  • 118. Dual-Servo Brake Design Dual-Servo Brake Construction Adjusting Link (Starwheel Adjuster) Starwheel and adjusting screw helps reduce brake squeal
  • 119. Figure 99-25 A typical dual-servo brake adjusting link assembly commonly called a starwheel adjuster.
  • 120. Dual-Servo Brake Design Dual-Servo Brake Construction NOTE: Adjusting links generally have specific left- or right-hand threads, and must be installed on the correct side of the vehicle.
  • 121. Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Two brake shoes are not called leading and trailing parts
  • 122. Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Identified as primary and secondary shoes
  • 123. Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Primary shoe always faces front of vehicle
  • 124. Dual-Servo Brake Design Dual-Servo Brake Construction Primary and Secondary Brake Shoes Secondary shoe provides about 70% of total braking power in dual-servo brake
  • 125. Dual-Servo Brake Design Dual-Servo Brake Operation When dual-servo brake is applied, wheel cylinder attempts to force tops of both brake shoes outward against drum Primary shoe not directly anchored to backing plate so it rotates
  • 126. Dual-Servo Brake Design Dual-Servo Brake Operation As primary shoe rotates, it forces secondary shoe to rotate Rotation seats secondary shoe firmly against anchor, increasing braking force
  • 127. Figure 99-26 Dual-servo brake operation. The primary shoe on the left exerts a force on the secondary shoe on the right.
  • 128. Dual-Servo Brake Design Servo Action Once all clearance is taken up, both brake shoes become self-energized Anchor pin prevents secondary shoe from rotating
  • 129. Dual-Servo Brake Design Servo Action Adjusting link serves as anchor for primary shoe Servo action occurs as part of the braking force generated by primary shoe is transferred to help apply secondary shoe
  • 130. Figure 99-27 Dual servo action greatly increases the application force on the secondary shoe.
  • 132. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Three styles of starwheel adjusters Cable Lever Link
  • 133. Figure 99-28 A cable-actuated starwheel adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are released.
  • 134. Figure 99-29 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are applied.
  • 135. Figure 99-30 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.
  • 136. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters All three adjusters mount on secondary brake shoe Adjust when brakes are applied while vehicle moves in reverse
  • 137. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters As brakes are applied, secondary shoe moves away from anchor Movement causes cable or linkage to pull up on adjuster pawl
  • 138. Figure 99-31 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.
  • 139. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters If brakes are worn, pawl engages next tooth of starwheel When brakes released, pawl return spring pulls the pawl down
  • 140. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Action rotates starwheel, moving brake shoes apart Movement reduces lining-to-drum clearance Some servo brakes have over-travel spring assembly
  • 141. Figure 99-32 A cable-actuated starwheel adjuster with an overtravel spring.
  • 142. Automatic Brake Adjusters Servo Brake Starwheel Automatic Adjusters Adjuster is mounted under starwheel Adjustment is made as brakes are applied rather than when released
  • 143. Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters Can be mounted on either leading or trailing shoe Work whenever brakes are applied
  • 144. Figure 99-33 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.
  • 145. Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters When not in use, adjuster pawl is held in place by parking brake strut When brakes applied, primary shoe moves toward brake drum
  • 146. Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters Pawl spring pivots the pawl downward Pawl mounts on brake shoe and rotates starwheel to adjust brake
  • 147. Automatic Brake Adjusters Non-Servo Starwheel Automatic Adjusters When brake released, return springs retract shoes Pawl levered back into resting position Trailing-shoe non-servo starwheel adjuster works like leading-shoe design
  • 148. Figure 99-34 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.
  • 149. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Use movement of brake shoes to adjust lining-to-drum clearance Adjustment of ratchet adjuster performed by two parts with interlocking teeth
  • 150. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters During adjustment, toothed parts ratchet across one another When adjustment is complete, the teeth lock and hold brake shoes in new position
  • 151. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Operates whenever brakes are applied
  • 152. Figure 99-35 A lever-latch ratchet automatic adjuster.
  • 153. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Design consists of large lever and smaller latch with interlocking teeth
  • 154. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake As brakes applied, shoes move outward toward drum
  • 155. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Parking brake strut pulls on adjuster lever
  • 156. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake Lever forced to pivot inward where it attaches to top of leading shoe
  • 157. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake If brakes are worn, lever ratchets one or more teeth on latch
  • 158. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake When brakes released, parking brake strut holds shoes farther apart to reduce lining-to-drum clearance
  • 159. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes
  • 160. Figure 99-36 A strut-quadrant ratchet automatic adjuster.
  • 161. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Parking brake strut
  • 162. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Adjusting quadrant
  • 163. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Strut-quadrant adjuster has three basic parts Quadrant spring
  • 164. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes When brakes applied, leading shoe moves out toward brake drum
  • 165. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes If there is sufficient wear, the edge of the slot in shoe web contacts inner side of adjusting quadrant arm and pulls it outward
  • 166. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Toothed section of quadrant is lifted away from post
  • 167. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Quadrant spring rotates quadrant until pivot pin is bottomed in slot
  • 168. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes When brakes released, quadrant returns inward with leading shoe
  • 169. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Toothed section of quadrant engage teeth on strut post
  • 170. Automatic Brake Adjusters Ratchet-Type Automatic Adjusters Strut-quadrant ratchet automatic adjuster used on some non-servo brakes Quadrant remains in its new extended position, reducing the lining-to-drum clearance
  • 171. TECH TIP 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. BACK TO PRESENTATION
  • 172. TECH TIP 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. BACK TO PRESENTATION Aftermarket linings are not required by federal law to meet performance or wear standards that are required of original factory brake linings.
  • 173. TECH TIP 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. BACK TO PRESENTATION 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.
  • 174. TECH TIP Cool the Brakes before Backing Self-adjusters can overadjust 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. BACK TO PRESENTATION 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. NOTE: Some drum brakes are equipped with a bimetallic heat sensor that prevents the self-adjusters from working if the brakes are hot.

Editor's Notes

  • #6: Figure 99-1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.
  • #7: Figure 99-2 An exploded view of a typical drum brake assembly.
  • #30: Figure 99-3 The backing plate is the foundation of every drum brake. There are normally six pads where the brake shoes contact the backing plate.
  • #33: Figure 99-4 A labyrinth seal is created between the lip of the backing plate and the groove in the brake drum.
  • #38: Figure 99-5 A keystone anchor allows the brake shoes to self-center in the drum.
  • #40: Figure 99-6 Piston stops prevent the wheel cylinder from coming apart.
  • #42: Figure 99-7 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.
  • #43: Figure 99-8 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.
  • #46: Figure 99-9 Steelbrake shoes are made from two stampings welded together—the web and the lining table.
  • #47: Figure 99-10 Tapered ends on the linings help to reduce brake noise.
  • #50: Figure 99-11 Typical drum brake shoe and the names of the parts.
  • #54: Figure 99-12 The primary (forward facing) brake shoe often has a shorter lining than the secondary shoe (rearward facing). The color of the primary and secondary lining can also be different due to differences in friction and wear requirements.
  • #56: Figure 99-13 Primary shoe lining may vary depending on the application.
  • #58: Figure 99-14 Riveted brake linings are quiet and reliable at high temperatures.
  • #65: Figure 99-15 Many brake linings are bonded.
  • #78: CHART 99–1 Edge code letters represent a range of coefficient of friction of the linings.
  • #79: Figure 99-16 Typical drum brake lining edge codes, showing the coefficient of friction codes for cold and hot circled.
  • #82: Figure 99-17 A typical drum brake assembly showing the support plate (backing plate), brake shoes, and springs.
  • #83: Figure 99-18 A single spring-steel spring is used on some drum brakes.
  • #88: Figure 99-19 Various types and styles of hold-down springs. The hold down pins are commonly called nails.
  • #96: Figure 99-20 A mechanical brake linkage is part of most drum brake assemblies.
  • #99: Figure 99-21 An aluminum 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.
  • #106: Figure 99-22 Self-energizing action can increase or decrease the stopping power of a brake shoe.
  • #109: Figure 99-23 A leading-trailing non-servo brake.
  • #116: Figure 99-24 A typical dual-servo drum brake.
  • #120: Figure 99-25 A typical dual-servo brake adjusting link assembly commonly called a starwheel adjuster.
  • #128: Figure 99-26 Dual-servo brake operation. The primary shoe on the left exerts a force on the secondary shoe on the right.
  • #131: Figure 99-27 Dual servo action greatly increases the application force on the secondary shoe.
  • #134: Figure 99-28 A cable-actuated starwheel adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are released.
  • #135: Figure 99-29 A lever-actuated starwheel automatic adjuster. This type of adjuster makes the adjustment when the vehicle is being driven in reverse and the brakes are applied.
  • #136: Figure 99-30 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.
  • #139: Figure 99-31 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.
  • #142: Figure 99-32 A cable-actuated starwheel adjuster with an overtravel spring.
  • #145: Figure 99-33 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.
  • #149: Figure 99-34 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.
  • #153: Figure 99-35 A lever-latch ratchet automatic adjuster.
  • #161: Figure 99-36 A strut-quadrant ratchet automatic adjuster.