Halderman ch099 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

DRUM BRAKES

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ObjectivesObjectives• 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.

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DRUM BRAKE DRUM BRAKE ADVANTAGESADVANTAGES

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Drum Brake AdvantagesDrum Brake Advantages• Drum brakes were first type brake used

on motor vehicles• Still used on the rear of many vehicles

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Figure 99-1 Typical brake system components showing disc brakes on the front and drum brakes on the rear.

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Figure 99-2 An exploded view of a typical drum brake assembly.

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Drum Brake AdvantagesDrum 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

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Drum Brake AdvantagesDrum Brake Advantages• Self-Energizing and Servo Action

– Brake drums use servo action• One brake shoe helps the other, providing

for increased stopping power

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Drum Brake AdvantagesDrum 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

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DRUM BRAKEDRUM BRAKEDISADVANTAGESDISADVANTAGES

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Drum Brake DisadvantagesDrum Brake Disadvantages• Brake Fade

– Four types of brake fade1. Mechanical Fade

– Occurs when drum gets so hot if expands away from brake linings

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– Brake shoes move outward to reach drum

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– Brake pedal drops toward floor

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– Four types of brake fade2. Lining Fade

– Occurs when friction coefficient drops due to intense heat

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– Four types of brake fade2. Lining Fade

– Brake lining becomes slippery

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– Four types of brake fade3. Gas Fade

– Occurs under extended hard braking from high speeds

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– Thin layer of gases build up between linings and drum

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– Gas acts as lubricant and reduces friction

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– Four types of brake fade4. Water Fade

– Occurs when moisture is trapped between shoes and drum

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– Moisture acts as lubricant

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– Friction eventually creates enough heat to evaporate water

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Drum Brake DisadvantagesDrum Brake Disadvantages• Brake Adjustment

– Drum brake design requires adjusting– Brake shoe lining wears– Clearance between lining and drum

increases

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Drum Brake DisadvantagesDrum Brake Disadvantages• Brake Adjustment

– Longer brake pedal travel results– Most vehicles have automatic adjustments

to maintain proper drum to lining clearance

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Drum Brake DisadvantagesDrum Brake Disadvantages• Brake Pull

– Brake pull occurs when friction assemblies on opposite sides of vehicle have different amounts of stopping power

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DRUM BRAKE PARTSDRUM BRAKE PARTS

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Drum Brake PartsDrum Brake Parts• Backing Plate

– Foundation of drum brake is backing plate– Mounts to suspension or axle housing

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– Shoe support pads are stamped into backing plate

– Support pads contact edges of brake shoes to keep linings aligned

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– Pads are also called ledges or shoe contact areas

– Pad lightly coated with silicone brake grease to minimize wear

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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.

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– Backing plate serves as mounting surface for friction assembly parts

– Plate protects brake assembly from contaminants

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– 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

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Figure 99-4 A labyrinth seal is created between the lip of the backing plate and the groove in the brake drum.

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– Plate has openings to permit inspection and adjustment

– Openings sealed with metal plugs

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– Metal plugs must be punched out to allow inspection or adjustment

– Rubber replacement seals available

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Drum Brake PartsDrum 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

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Drum Brake PartsDrum 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

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Figure 99-5 A keystone anchor allows the brake shoes to self-center in the drum.

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Drum Brake PartsDrum Brake Parts• Piston Stops

– Piston stops prevent wheel cylinder pistons from coming out of their bores

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Figure 99-6 Piston stops prevent the wheel cylinder from coming apart.

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Drum Brake PartsDrum 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

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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.

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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.

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DRUM BRAKE SHOESDRUM BRAKE SHOES

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Drum Brake ShoesDrum 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

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Figure 99-9 Steelbrake shoes are made from two stampings welded together—the web and the lining table.

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Figure 99-10 Tapered ends on the linings help to reduce brake noise.

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Drum Brake ShoesDrum 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

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Drum Brake ShoesDrum 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

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Figure 99-11 Typical drum brake shoe and the names of the parts.

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Drum Brake ShoesDrum 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

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– Because of different forces involved, primary and secondary shoes are different

– Secondary shoe lining extends nearly the full length of lining table

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– Secondary shoe lining material has higher coefficient of friction

– Primary shoe undergoes far less stress

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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.

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– Primary shoe may run half the length of lining table

– Primary shoe lining has lower friction coefficient

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Figure 99-13 Primary shoe lining may vary depending on the application.

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Drum Brake ShoesDrum Brake Shoes• Lining Assembly Methods

– Two ways to mount linings to brake shoe– Riveting

• Lining attached to lining table with copper or aluminum rivets

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Figure 99-14 Riveted brake linings are quiet and reliable at high temperatures.

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– Riveting• Advantages of riveting

– Riveted brakes operate more quietly than bonded brakes

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– Riveting• Advantages of riveting

– Rivets will not loosen at high temperatures

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– Riveting• Disadvantages of riveting

– Rivet holes create stress points where cracks may occur

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– Reduced service life because linings must be replaced before rivets contact drum

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– Rivet can cause deep groves in drum, often requiring drum replacement

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– Bonding• Linings bonded to shoe with high-

temperature adhesive

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Figure 99-15 Many brake linings are bonded.

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– Bonding• Advantages of bonding

– Without rivets, bonded linings have longer service life

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– If worn too long, will do less damage to drum

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– Fewer problems with cracking

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– Bonding• Disadvantages of bonding

– If bonded lining gets too hot, bonding may fail and lining can separate from shoe

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– Bonding• Disadvantages of bonding

– Bonded linings can be noisier

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Drum Brake ShoesDrum Brake Shoes• Edge Codes

– Edge codes identify brake lining materials• First group of letters identify manufacturer

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– Edge codes identify brake lining materials• Second group is numbers and/or letters

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– Edge codes identify brake lining materials• Identifies lining compound or formula

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– Edge codes identify brake lining materials• Third group is two letters that identify

coefficient of friction

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– Coefficient of friction indicates amount of friction between two surfaces

• Coefficient is always less than 1

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• The higher the coefficient, the more friction

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• Coefficient codes include letters representing range of friction

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CHART 99–1 Edge code letters represent a range of coefficient of friction of the linings.

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Figure 99-16 Typical drum brake lining edge codes, showing the coefficient of friction codes for cold and hot circled.

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DRUM BRAKE PARTSDRUM BRAKE PARTS

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Drum Brake PartsDrum 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

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Figure 99-17 A typical drum brake assembly showing the support plate (backing plate), brake shoes, and springs.

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Figure 99-18 A single spring-steel spring is used on some drum brakes.

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– Type, location, and number of springs varies

– All springs attached in one of two ways• Connect directly from shoe to shoe

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– All springs attached in one of two ways• Connect from one shoe to the anchor post

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Drum Brake PartsDrum Brake Parts• Brake Shoe Holddowns

– Keep shoes securely against support pads on braking plate

– Help prevent noise, vibration, and wear

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– Allow free movements to allow adjustments of shoes

– Many forms of holddowns

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Figure 99-19 Various types and styles of hold-down springs. The hold down pins are commonly called nails.

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– Most common holddown design is steel pin installed through hole in backing plate

• Corresponding hole in shoe web

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• Spring fits over end of pin against shoe web

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• Washer compresses spring and locks onto flattened end of pin

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– Another holddown is taper-wound coil spring with hook on its end

• Sometimes called beehive holddown

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• Hook installed through hole in shoe web

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• Hook attaches to retaining clip in backing plate

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Drum Brake PartsDrum Brake Parts• Parking Brake Linkage

– Consists of cable, lever, and strut system that spreads brake shoes apart

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Figure 99-20 A mechanical brake linkage is part of most drum brake assemblies.

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Drum Brake PartsDrum Brake Parts• Brake Drums

– Turns with wheel– Mounts on the hub or axle

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Drum Brake PartsDrum 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

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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.

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NON-SERVO BRAKE NON-SERVO BRAKE DESIGNDESIGN

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Non-Servo Brake DesignNon-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

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Non-Servo Brake DesignNon-Servo Brake Design• Parts and Operation

– Self-energizing action occurs when leading shoe contacts drum

– Drum attempts to rotate shoe with it

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– Shoe is fixed in place by anchor– Drum rotation energizes shoe by wedging it

tightly against brake drum

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– 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

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– Leading shoe becomes trailing shoe when vehicle is in reverse

– Trailing shoe becomes leading shoe when vehicle is in reverse

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Figure 99-22 Self-energizing action can increase or decrease the stopping power of a brake shoe.

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– 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

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Non-Servo Brake DesignNon-Servo Brake Design• Leading-Trailing Brake Design

– Non-servo leading-trailing brake has one leading shoe and one trailing shoe

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Figure 99-23 A leading-trailing non-servo brake.

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– 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

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DUAL-SERVO DUAL-SERVO BRAKE DESIGNBRAKE DESIGN

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Dual-Servo Brake DesignDual-Servo Brake Design• Purpose and Function

– Most common brake design– Servo implies that one shoe serves the

other to increase application force

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– 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

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– NOTE: Dual-servo brakes are also called Duo-Servo, which is a brand name of the Bendix Corporation.

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Dual-Servo Brake DesignDual-Servo Brake Design• Dual-Servo Brake Construction

– Uses one anchor and one two-piston wheel cylinder

– Anchor usually mounted at top of backing plate

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Figure 99-24 A typical dual-servo drum brake.

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– Tops of brake shoes held against anchor by individual return springs

– Bottoms of shoes spaced apart by adjusting link and return spring

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– Adjusting Link (Starwheel Adjuster)• Adjusting link consists of starwheel that is

part of adjusting screw

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– Adjusting Link (Starwheel Adjuster)• Starwheel and adjusting screw helps reduce

brake squeal

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Figure 99-25 A typical dual-servo brake adjusting link assembly commonly called a starwheel adjuster.

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– NOTE: Adjusting links generally have specific left- or right-hand threads, and must be installed on the correct side of the vehicle.

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– Primary and Secondary Brake Shoes• Two brake shoes are not called leading and

trailing parts

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– Primary and Secondary Brake Shoes• Identified as primary and secondary shoes

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– Primary and Secondary Brake Shoes• Primary shoe always faces front of vehicle

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– Primary and Secondary Brake Shoes• Secondary shoe provides about 70% of total

braking power in dual-servo brake

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Dual-Servo Brake DesignDual-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

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Dual-Servo Brake DesignDual-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

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Figure 99-26 Dual-servo brake operation. The primary shoe on the left exerts a force on the secondary shoe on the right.

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Dual-Servo Brake DesignDual-Servo Brake Design• Servo Action

– Once all clearance is taken up, both brake shoes become self-energized

– Anchor pin prevents secondary shoe from rotating

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Dual-Servo Brake DesignDual-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

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Figure 99-27 Dual servo action greatly increases the application force on the secondary shoe.

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AUTOMATIC BRAKEAUTOMATIC BRAKEADJUSTERSADJUSTERS

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Automatic Brake AdjustersAutomatic Brake Adjusters• Servo Brake Starwheel Automatic

Adjusters– Three styles of starwheel adjusters

• Cable• Lever• Link

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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.

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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.

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Figure 99-30 A link-actuated starwheel adjuster. This type of adjuster makes the adjustment when the brakes are released.

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Adjusters– All three adjusters mount on secondary

brake shoe– Adjust when brakes are applied while

vehicle moves in reverse

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Adjusters– As brakes are applied, secondary shoe

moves away from anchor– Movement causes cable or linkage to pull

up on adjuster pawl

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Figure 99-31 The operation of a typical self-adjuster. Notice that the adjuster actually moves the starwheel.

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Adjusters– If brakes are worn, pawl engages next

tooth of starwheel– When brakes released, pawl return spring

pulls the pawl down

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Adjusters– Action rotates starwheel, moving brake

shoes apart – Movement reduces lining-to-drum

clearance– Some servo brakes have over-travel spring

assembly

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Figure 99-32 A cable-actuated starwheel adjuster with an overtravel spring.

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Adjusters– Adjuster is mounted under starwheel– Adjustment is made as brakes are applied

rather than when released

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Automatic Brake AdjustersAutomatic Brake Adjusters• Non-Servo Starwheel Automatic

Adjusters– Can be mounted on either leading or

trailing shoe– Work whenever brakes are applied

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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.

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Adjusters– When not in use, adjuster pawl is held in

place by parking brake strut– When brakes applied, primary shoe moves

toward brake drum

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Adjusters– Pawl spring pivots the pawl downward– Pawl mounts on brake shoe and rotates

starwheel to adjust brake

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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

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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.

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Automatic Brake AdjustersAutomatic 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

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– During adjustment, toothed parts ratchet across one another

– When adjustment is complete, the teeth lock and hold brake shoes in new position

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– Lever-latch ratchet automatic adjuster installs on leading shoe of non-servo brake

• Operates whenever brakes are applied

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Figure 99-35 A lever-latch ratchet automatic adjuster.

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• Design consists of large lever and smaller latch with interlocking teeth

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• As brakes applied, shoes move outward toward drum

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• Parking brake strut pulls on adjuster lever

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• Lever forced to pivot inward where it attaches to top of leading shoe

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• If brakes are worn, lever ratchets one or more teeth on latch

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• When brakes released, parking brake strut holds shoes farther apart to reduce lining-to-drum clearance

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– Strut-quadrant ratchet automatic adjuster used on some non-servo brakes

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Figure 99-36 A strut-quadrant ratchet automatic adjuster.

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• Strut-quadrant adjuster has three basic parts

– Parking brake strut

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– Adjusting quadrant

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– Quadrant spring

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• When brakes applied, leading shoe moves out toward brake drum

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• If there is sufficient wear, the edge of the slot in shoe web contacts inner side of adjusting quadrant arm and pulls it outward

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• Toothed section of quadrant is lifted away from post

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• Quadrant spring rotates quadrant until pivot pin is bottomed in slot

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• When brakes released, quadrant returns inward with leading shoe

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• Toothed section of quadrant engage teeth on strut post

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• Quadrant remains in its new extended position, reducing the lining-to-drum clearance

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Halderman ch099 lecture