Tài liệu học tập cơ bản khái quát về động cơ và các bộ phận trên ô tô phiên bản tiếng anh. Tài liệu này nhằm giúp chúng ta tiếp cận được cấu tạo chức năng của các bộ phận của xe. Với tài liệu này giúp các bạn sinh viên ngành cơ khí ô tô tiếp cận được những kiến thức cơ bản về ô tô và đồng thời nâng cao kiến thức tiếng anh. Bộ tài liệu này được soạn thảo bởi trường đào tạo và huấn luyện nghề ở Canada.
Chapter 48 Brake Systems Friction • Kinetic Friction – Is caused by rotating brake parts – Changes kinetic energy into thermal energy (heat) • Static Friction – Holds the car in place when stopped Factors Governing Braking • Four basic factors determine braking power • The first three govern the generation of friction: – Pressure – Coefficient of friction – Friction contact surface – Heat dissipation – Weight transfer also affects braking Pressure • The amount of friction generated depends on the pressure applied to the friction surfaces • Hydraulic systems provide the pressure Coefficient of Friction • The amount of friction generated between two surfaces • Most brake friction materials are between 0.25 and 0.55 Heat Dissipation • Heat must be conducted away from pads/rotor or shoes and drum • Inadequate heat dissipation can cause brake fade Brake Fade • Caused by excessive heat buildup • Outgassing from the pads can act as an air bearing Brake Lining Friction Materials • Non-asbestos organic (non-metallic) – Friction materials, graphite, nut shells, fillers • Fully metallic – Work poorly until fully warmed • Semi-metallic • Synthetic • Carbon-metallic/organic Principles of Hydraulic Brake Systems • Uses fluid to transfer pressure • Pressure applied to fluid in a closed system is consistent • Force can be increased by changing piston sizes • Output piston travel can be increased by using a smaller piston Stop Lamps • Stop lamps are part of rear tail lamp assemblies and the center high-mounted stop lamp (CHMSL) • Usually the stop lamp switch is mounted with the brake pedal • Brake lamp switch output often used for cruise control and TCC disengagement Brake Fluid Inspection • Remove the cover and check the gasket • Check the fluid in the reservoir – Do not overfill • • • • Check the fluid for contamination Test strips can check fluid condition Special testers can check boiling point Check the master cylinder for signs of leaks Master Cylinder Inspection • Check housing for cracks or damage • Leaks can be internal or external • Pump brake pedal and hold down, the pedal should not drop – If the pedal drops and there are no external leaks, suspect the fluid is bypassing the seals inside the master cylinder System Flushing • Many manufacturers specify periodic brake fluid flushing • Open each bleeder and pump fluid out until fresh new fluid appears • Flushing can remove sludge and moisture Brake Line Inspection • Check all tubing, hoses, and connections for leaks and damage • Check hoses for flexibility, bulges, and cracks Brake Pedal Inspection • Press and release the pedal several times • Check for friction and noise • Pedal reserve distance should be no lower than or inches above the floor • With the engine off, press the pedal several times to deplete vacuum – Hold the pedal and start the engine – The pedal should drop slightly Hydraulic System Bleeding • Any air in the system can be compressed • This decreases the amount of force that can be transmitted by the fluid • Bleeding removes the air trapped in the system • Two types of bleeding are common: – Manual bleeding – Pressure bleeding Manual Bleeding Pressure Bleeding Power Brakes • Standard brakes with a power assist unit located between the brake pedal and master cylinder • Two types of power assist used : – Vacuum assist – Hydraulic assist Vacuum Assist • Apply assist by opposing engine vacuum to atmospheric pressure • Modern systems are vacuum suspended • Boosters may be single or dual diaphragm • When the brake is applied atmospheric pressure is allowed to enter the rear chamber Hydraulic Brake Boosters • Fluid pressure supplied by the power steering system • Found mostly on diesel engines or engines with low vacuum Electric Parking Brakes • Replaces parking brake pedal or lever and cables • Some systems use electric motors mounted in the rear calipers • Other use an electric motor to pull the parking brake cables ... Electric Parking Brakes • Replaces parking brake pedal or lever and cables • Some systems use electric motors mounted in the rear calipers • Other use an electric motor to pull the parking brake cables... Valve – Delays application of front disc brakes until pressure builds to rear drum brakes • Proportioning Valve – Applies more hydraulic pressure to the disc brakes to balance braking • Combination... Usually the stop lamp switch is mounted with the brake pedal • Brake lamp switch output often used for cruise control and TCC disengagement Brake Fluid Inspection • Remove the cover and check