22-12-2012, 06:38 PM
Automotive Braking Systems
1Automotive Braking.ppt (Size: 1.42 MB / Downloads: 51)
Overview
Brakes convert kinetic energy into heat by creating friction
System must have very high reliability
Types of brakes:
- Disk
- Drum
System Basics
Hydraulic actuation allows multiplication of pedal force.
In this system, a 10lb force on the pedal produces 360 lbs of force at the friction surface.
Braking Dynamics
Dynamic weight transfer caused by braking shifts the weight balance towards the front of the car
Dynamic weight for the front and rear can be found using Newton’s Law
Because the maximum braking force is equal to the coefficient of friction times the normal force, the front wheels will have an increased capacity to provide braking force
Also, the system must be designed to distribute the application of force between front and rear
Drum Brakes
Expanding shoes create force on the inner surface of the drum
Used on the rear of some trucks and SUV’s
Self-energizing design requires less activation force
Require periodic adjustment
Master Cylinder
Provides a reservoir for brake fluid and contains the driving pistons in the hydraulic circuit
2 Types
Front - Rear split
-One piston for front brakes and one for rear
-If a leak occurs you could lose front brakes
Diagonally split
-One piston drives one front wheel and one rear wheel
-Diagonal layout allows you to maintain directional control if a leak occurs
Anti-Lock Brakes
A locked (sliding) wheel offers less braking force than a decelerating rolling wheel
The locked wheel also produces little lateral force, preventing steering control
Anti-Lock systems (ABS) monitor wheel lock-up and modulate brake pressure to provide controlled braking under most circumstances
System can have 2, 3 or 4 channels
Trucks typically use 3 channel with only one sensor for the rear axle
Most modern cars use 4 channel system
Wheel speed sensors monitor each wheel speed
ABS controller and high-pressure pump increase or reduce pressure to wheels in order to maintain consistent wheel speeds
Valves and Switches
Pressure Differential Switch
-Senses a pressure differential between the two master cylinder stages and warns the driver that the system has a leak
Metering Valve
-Prevents the front brakes from engaging until the rear drums have enough pressure to overcome the shoe return springs (Front disk-Rear drum systems only)
Proportioning Valve
-Reduces the pressure to the rear wheels to balance the effect of weight transfer to the front wheels.