© 2012 delmar, cengage learning brake fundamentals chapter 57

© 2012 Delmar, Cengage Learning

Brake Fundamentals

Chapter 57

© 2012 Delmar, Cengage Learning

Objectives• Explain the basic principles of braking, including

friction, pressure, and heat dissipation• Describe hydraulic system operation, including

master cylinder, control valves, and safety switches

• Understand the operation of power brakes

© 2012 Delmar, Cengage Learning

Introduction• Kinetic energy: energy that wants to stay in

motion– Apply brakes to stop a car: dry friction changes

motion energy to heat energy• Temperature in brake linings can be 600°F

– Friction resists movement between surfaces

– Coefficient of friction varies• Temperature, rubbing speed, surface condition

– During a stop• Vehicle weight shifts to front brakes• Front breaks wear out faster

© 2012 Delmar, Cengage Learning

Brake Linings• Linings are bonded or riveted to disc backing

– Newer pads integrally molded

• Lining types– Asbestos linings: health hazard

– Semimetallic linings: sponge iron and steel fibers

– Metallic linings: used in heavy-duty and racing conditions

– Ceramic linings: use ceramic and copper fibers to control heat

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Drum and Disc Brakes• Drum brake

systems– Metal brake drums

bolted to wheels

• Disc brake systems– Rotor and caliper,

similar to bicycle

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Hydraulic Brake System Operation

• Brake pedal depression– Moves piston in master cylinder

• Fluid under pressure is pushed to slave cylinder• Slave cylinders are located at each wheel

• Pascal’s Law:– Pressure in an enclosed system is equal and

undiminished in all directions

– Force = Pressure x Area• Force applied to brake linings increases with

larger diameter wheel cylinder

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Hydraulic Brake Fluid• Glycol-based fluids are hygroscopic

– Absorb water

• Brake fluid – Higher boiling point than water

• DOT specifications – List both dry and wet boiling points

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Brake Hose and Tubing• Steel hydraulic brake tubing

– Runs the length of the vehicle

– Rubber hoses connect steel tubing to other components

• Flexibility needed to allow wheels to pivot

• Brake lines – Made of double-walled steel tubing coated with

rust-preventative material

– Replacing brake lines: copy originals as closely as possible

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Hydraulic System Operation• Driver depresses the brake pedal

– Linkage applies force to piston at rear of master cylinder

• Master cylinder operation– Supplies hydraulic pressure to wheel cylinders

• Primary cup compresses fluid when pedal is depressed

• Secondary cup keeps fluid from leaking out

• Seal lips are directional– Seal installed backwards will leak

© 2012 Delmar, Cengage Learning

Low Brake Pedal• Low pedal

– Brake pedal moves closer to floor before brakes applied

• Tandem master cylinder – Cylinder bore with two pistons and chambers

• Master cylinder reservoirs – Prevented from vacuum locking

• Rubber diaphragm in cover or plastic float

• Master cylinders – Mounted on bulkhead

© 2012 Delmar, Cengage Learning

Split Hydraulic System• Longitudinally split system

– Front and rear brakes: separate hydraulic systems

• Used on rear-wheel-drive vehicles

• Diagonally split system – Operates brakes on opposite corners of vehicle

• Used on front-wheel-drive vehicles

• Front suspension geometry – Negates brakes’ tendency to pull to one side

© 2012 Delmar, Cengage Learning

Quick Take-Up Master Cylinder• Some disc brake calipers are designed to have

less drag when brakes are not applied– More fluid needed to take up clearance

• Quick take-up master cylinder – Moves larger amount of fluid when pedal first

applied

– Rear of primary piston larger diameter than front

– Larger part of bore allows piston to move large volume of fluid more quickly

© 2012 Delmar, Cengage Learning

© 2012 Delmar, Cengage Learning

Drum Brakes• Found in some rear brake applications

– Good initial stopping • Inexpensive, mechanical parking brake

• Dual-servo drum brake– Self-energizing: during stopping, leading shoe digs

into brake drum

– Servo action: small force applied to make larger force

• Leading-trailing brake – Non-servo brake with anchor at bottom end of

each shoe

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Drum Brake Adjustment• Brakes wear: clearance increases between lining

and drum– Typical drum brake adjust has threaded shaft

attached to integral starwheel

– Dual-servo self-adjusters operate when brakes are applied during a stop when backing up

• Brake fade: results with excessive brake heat– Drum brakes do not dissipate heat as well as disc

brakes• Increased heat causes drum to expand• More effort required to stop the car

© 2012 Delmar, Cengage Learning

Disc Brakes• Disc brake system has rotor and caliper

– Caliper clamps friction pads against rotor

• Rotors are solid or ventilated – Lightweight solid used in lighter cars

– Ventilated have more surface area• Used in heavier vehicles

• Brake calipers– Fixed caliper: pistons on both sides

– Floating caliper: one to two pistons on one side

© 2012 Delmar, Cengage Learning

Disc Brakes (cont'd.)• Caliper pistons hollow and cup-shaped

– Installed with open side against friction pad back

• Rear disc brake systems – Have fixed or floating calipers

• Linings are fastened to metal back– May have tabs on pad back that need to be bent

during installation

– Some include wear sensor• Metal tab rubs against rotor when lining wears thin

© 2012 Delmar, Cengage Learning

Hydraulic System Valves and Switches

• Tandem systems have a hydraulic safety switch– Alerts drivers when half the system fails

• Some master cylinders have a fluid level switch– Several designs

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Hydraulic Control Valves• Metering valve

– Used on front disc brakes when car has rear drum brakes

– Prevents front brakes applying until rear shoes overcome spring pressure and contact drums

• Unnecessary with four-wheel disc brakes

• Proportioning valves – Prevent rear wheels from locking during hard stop

• Newer cars – Equipped with antilock brakes

© 2012 Delmar, Cengage Learning

Power Brakes• Brake booster

– Allows master cylinder to have larger bore• Brakes apply with less pedal travel

– Has check valve to provide reserve braking

• Vacuum-suspended power brake – Metal chamber divided by rubber diaphragm

• Air enters through filter behind pedal pushrod boot

• Other power brake types:– Hydraulic power assist, electric power assist, and

hydro-boost systems

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Parking Brake• Must operate independently of service brakes

– Cable connected hand brake or foot brake and to an equalizer

• Cable from each rear wheel is attached to both sides

• Pivots in center and applies each rear parking brake equally

• Warning light indicates when brake is applied– Helps prevent damage to braking system

© 2012 Delmar, Cengage Learning

Types of Parking Brakes• Drum brakes use integral-type parking brake

– Cable-actuated bar applies drum-type brake

• Drum-in-hat brake uses miniature drum and shoes housed in rotor center

• Parking brake may be integral to rear disc service brakes

• Independent-type emergency brake – May be internal-expanding type or external-

contracting type

© 2012 Delmar, Cengage Learning

Stoplight Switches and Antilock Brakes

• Stoplights are turned on by a stoplight switch– Pedal is depressed

• Contacts complete circuit

• Antilock brake systems (ABS) keep wheels from locking up– Sensors and computer monitor wheel speed

• Hybrid vehicle brake systems have same parts as conventional systems– Regenerative braking and computer controls

operate hydraulic brake