Upload
molly-johnson
View
239
Download
4
Tags:
Embed Size (px)
Citation preview
© 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
© 2012 Delmar, Cengage Learning
Drum and Disc Brakes• Drum brake
systems– Metal brake drums
bolted to wheels
• Disc brake systems– Rotor and caliper,
similar to bicycle
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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