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Brake Fundamentals

Brake Fundamentals

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Brake Fundamentals

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  1. Brake Fundamentals Chapter 57

  2. 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

  3. 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

  4. 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

  5. Drum and Disc Brakes • Drum brake systems • Metal brake drums bolted to wheels • Disc brake systems • Rotor and caliper, similar to bicycle

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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