copyright © 2007 by nelson, a division of thomson canada limited. 46-1chapter 46 brake systems 46

Copyright © 2007 by Nelson, a division of Thomson Canada Limited. 46-1Chapter 46

Brake Brake SystemsSystems

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ObjectivesObjectives

• Explain the basic principles of braking, including kinetic and static friction, friction materials, application pressure, and heat dissipation.

• Describe the components of a hydraulic brake system and their operation.

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• Perform both manual and pressure bleeding of the hydraulic system.

• Briefly describe the operation of drum and disc brakes.

• Inspect and service hydraulic system components.

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• Describe the operation and components of both vacuum-assist and hydraulic-assist braking units.

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FrictionFriction

• Kinetic friction– Kinetic or moving friction is created by

brake pads or shoes rubbing against rotating brake rotors or drums.

– Changes kinetic energy into thermal energy (heat).

• Static friction– Static or stationary friction holds the car in

place when stopped and should be present between the tire and road at all times.

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FrictionFriction

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Factors Governing BrakingFactors Governing Braking

• Pressure – The amount of friction generated between

moving surfaces contacting one another depends in part on the pressure exerted on the surfaces.

• Coefficient of friction – The amount of friction between 2 surfaces

(pads and rotors or shoes and drums) – Determined by dividing the force required to

pull an object across a surface by the weight of the object continued…


Factors Governing BrakingFactors Governing Braking

• Frictional Contact Surface – The amount of surface, or area, that is in

contact. – Simply put, bigger brakes stop a car more

quickly than smaller brakes used on the same car.

• Heat Dissipation – The tremendous heat created by the

rubbing brake surfaces must be conducted away from the pad and rotor (or shoe and drum) and be absorbed by the air.

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Friction Material ClassificationsFriction Material Classifications

• Fully metallic– Lining is made of sintered iron and has

been used for years in heavy-duty and racing applications because they have great fade resistance.

– They require very high pedal pressure and tend to quickly wear out drums and rotors.

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• Semi-metallic– Made of iron fibres moulded with an

adhesive matrix. – Offers excellent fade resistance with good

frictional characteristics so only a moderate amount of application pressure is needed.

– Semimetallic pads and shoes do not cause excessive wear on rotor or drum surfaces.

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• Organic (non-metallic)– For many decades, asbestos was the

standard brake lining material. – It offers good friction qualities, long wear,

and low noise. – New materials, such as composite/organic,

ceramics, and carbon fibres, are being used because of the health hazards of breathing asbestos dust.

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• Synthetic– Non-asbestos lining materials made of

synthetic substances are now available. – The major brake lining manufacturers are

constantly experimenting with new materials that meet all established criteria for long life, friction characteristics, drum and rotor wear, and heat dissipation.

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Brake System ComponentsBrake System Components

• Brake fluid

• Master cylinder

• Power booster

• Control valves

• Wheel brake assemblies

• Parking brake

• ABS system

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Hydraulic PrinciplesHydraulic Principles

• An example of how hydraulics can increase output force

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Dual Hydraulic Brake CircuitsDual Hydraulic Brake Circuits

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How the Master Cylinder WorksHow the Master Cylinder Works

• A pushrod connects the brake pedal to the master cylinder piston.

• When the pedal is pressed, the piston is pushed forward and the fluid in front of the piston is displaced into the brake system moving the pads and shoes into contact with the drums and rotors.

• As more force is placed on the brake pedal, the fluid transmits force throughout the system.

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Master Cylinder OperationMaster Cylinder Operation

• At rest– Fluid in the reservoir

can flow into the area ahead of the piston through the compensating port.

– This keeps the system full and prevents pressure build-up during temperature changes.

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• Applied– The piston is moved

forward blocking the compensating port forming a sealed pump chamber.

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• Brake release– When the pedal is

released the piston return springs force the piston back.

– The piston cup seal collapses and fluid flows past to allow speedy piston return and pressure decrease.

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Master Cylinder ComponentsMaster Cylinder Components

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Lines and HosesLines and Hoses

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Brake System ValvesBrake System Valves

• Metering valve– Delays application of front disc brakes until

pressure builds to rear drum brakes.

• Proportioning valve– Reduces the hydraulic pressure to the rear

drum brakes to balance braking.

• Pressure differential valve – Used to operate a warning light switch.

• Combination valve– Combines several functions in one valve.

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A Proportioning ValveA Proportioning Valve

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Pressure Differential ValvePressure Differential Valve

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Drum BrakesDrum Brakes

• A drum brake assembly consists of a cast-iron drum, which is bolted to and rotates with the vehicle’s wheel, and a fixed backing plate to which the shoes, wheel cylinders, automatic adjusters, and linkages are attached.– The shoes are surfaced with frictional

linings, which contact the inside of the drum when the brakes are applied.

– The shoes are forced outward by pistons located inside the wheel cylinder.

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Drum BrakesDrum Brakes

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Disc BrakesDisc Brakes

• Disc brakes use a cast iron rotor, inboard of the vehicle wheel.

• Both sides of the rotor are machined smooth for the brake pads to rub against.

• Usually the 2 surfaces are separated by a finned centre section for better cooling (ventilated rotors).

• The pads are attached to metal shoes, which are actuated by pistons housed in the brake caliper. continued…


Disc BrakesDisc Brakes

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Brake Fluid InspectionBrake Fluid Inspection

• Remove the cover and check the gasket.

• Check the fluid in the reservoir.– Do not overfill.

• Check the fluid for contamination.

• Check the master cylinder for signs of leaks.

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Vacuum Brake BoostersVacuum Brake Boosters

• All modern vacuum-assist units are vacuum suspended systems.

• The diaphragm inside the unit is balanced using engine vacuum until the brake pedal is depressed.

• Applying the brake allows atmospheric pressure to unbalance the diaphragm and allows it to move generating application pressure. continued…


Vacuum Brake BoostersVacuum Brake Boosters

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Vacuum Booster ChecksVacuum Booster Checks

• Pressure check

• Pedal travel

• Vacuum reading

• Release problems

• Hard pedal

• Grabbing brakes

• Internal bindingcontinued…


Hydraulic Brake BoostersHydraulic Brake Boosters

• Decreases in engine size, increased use of diesel engines, plus the continued use of engine vacuum to operate other engine systems, such as emission control devices, led to the development of hydraulic-assist power brakes.

• These systems use fluid pressure, not vacuum pressure, to help apply the brakes.

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Hydraulic Brake BoostersHydraulic Brake Boosters

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SummarySummary

• The four factors that determine a vehicle’s braking power are pressure, coefficient of friction, frictional contact surface, and heat dissipation.

• Three types of brake lining materials are used: metallic, semi-metallic, and non-asbestos substances.

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SummarySummary

• Two common brake system control valves are the metering valve and the proportioning valve.

• The brake lines transmit brake fluid pressure from the master cylinder to the wheel cylinders and calipers.

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SummarySummary

• Common brake problems include hard pedal, low pedal, leaks, grabbing, and longer than normal stopping distance.

• Brake system service includes checking the fluid, brake lines, and brake pedal movement.

copyright © 2007 by nelson, a division of thomson canada limited. 46-1chapter 46 brake systems 46

Documents

friction materials

stationary friction

coefficient of friction

static friction static

braking pressure

brake pads

hydraulic brake system

application pressure