Engineering Braking Systems

Historical Development of Braking Systems

• Contracting Band Brakes

• Drum Brake

• Disc Brake

• Anti Lock Braking System ( ABS )

• Exhaust and Engine

• Regnerative Braking System

• Automotive Hand Break

1. Contracting Band Brake

• In 1895, cars used contracting band brakes as pneumatic rubber tyrers made “block brakes” less effective

• Had “Servo-assistance”

Servo Assistance- Brakes naturally tried to increase braking force

Advantage

• Less damage of road debris

Disadvantage

• Heat caused the drum to expand and drag againsnt the band

• Not enough force for faster cars

Drum Brake

• The shoes are lined with a friction material such as woven asbestos or a pressed asbestos composite

• The shoes are enclosed in the drum, so the friction lining is away from water and dirt

• Also has servo assistance

• Hydraulically operated pistons opened the shoes, as the hydraulic cylinder was connected by piping to a master cylinder

DISADVANTAGE

• Poor heat dissipation, difficulty to remove heat

Disc Brake

• Most common type of brakes used in modern cars

• Offers better heat dissipation other than drum brake

• Offer wet weather performance as water is thrown off the disc by centrifugal force

• Ventillated Discs to improve heat dissipation

Disadvantage

• No servo-assistance, so that means the force at the pedal is very large. To reduce the effort the pedal force is boosted, a Vacum Booster is used.

Anti Lock Braking System (ABS) - More in Depth as a short answer question

• It is a safety system that prevents the wheels from locking up during heavy braking.

Locking up- Wheels wheel stops rotating even though the car is still moving

• It can lead to a loss of control and consequently accidents

MAIN INFO

• In order for wheels to not lock up, it is achieved by “wheel sensors and computer control over the braking circuit”

1. The wheel begins to lock up, the wheel sensor will detect

2. Send a message to the computer

3. The computer will realease hydraulic pressure and allow the wheel to spin again.

It says ABS sensors are also used for:

Traction control → controls wheel spin during acceleration

Dynamic stability control → helps with handling and cornering

Exhaust and Engine Brakes

Engine Braking: Engine will tend to retard (slow down) the veichle

Exhaust Braking: Involves constricting the exhaust system

Regenerative Braking System ( Short Answer)

• More environmentally friendly approach

• In hybrid cars, the motor drives a generator that provides electricity to an electric motor. They also use batteries so in city areas the petrol or diesel motor does not need to run.

• The advantage of this means that it is also possible to use electricity generation to slow the car.

The car’s kinetic energy is converted into useful electrical energy instead of being given off as heat energy.

ADVANTAGE

So instead of only using normal friction brakes:

the car can also slow down through electricity generation

Generator- Mechanical to Electric

Motor- Electric to Mechanical

Automotive Hand Brake

• To hold a veichle in a parked position

Environmental Implications from the use of materials in Braking System

• When asbestos breaks into tiny fibres, people can breathe them in. These fibres can get stuck in the lungs and may eventually cause lung cancer.

ASBESTOS

Friction

• Mew= ff/rn

Friction from a ramp

• Use calculation method

• Break force into vertical and horizontal components

• Use fx=0 and fy=0

Friction from Ground

• Draw Diagram

• Flipping R in bewteen frictional force and RN

• Calculate angle

• Draw all measurements then sine rule

Friction for Disc Brakes

• 2 forces acting on disc brakes

• Refer to book

Stress

Stress= Force/ Area

MPA in mm2

Always convert KN to Newtons

Refer to book

Strain

• Strain= Extension or shortening/ Original Length

• As a decimal x100 to find percentage

Stress and Strain Diagram

Features of Stress and Strain Diagram

• Proportional Limit (Hookes Law)

- Straight line relationship between stress and strain

- E= STRESS/ STRAIN ( E = Young’s Modulus (Pa))

• Elastic Limit

- Beyond this point plastic deformation will occur

• Yield Points

- Increase in strain without an increase in stress

• Ultimate Tensile Strength

- Maximum stress a material can withstand

• Breaking Point

- The point the material will break or fracture

More features

• Necking: Undergo localised deformation

• Work Hardening: Increased strength and hardness with reduced ductility

• Toughness: The area under the whole curve, resistance to shock loading

• Resilience: The area under the curve ONLY in Elastic Deformation region

- It is also the amount of strain energy stored

• Stiffness: The Young modulus, slope of the straigt line

BOLTS AND NUTS

• Height of Nut= 0.8 D

• Height of Bolt= 0.7D

Washer= 2 x Diameter

Horizontal length Nut= 1.8 D

Vertical Length Nut= 1.6 D

MATERIALS for Braking System

1. Steel

Steel

• Binary alloy of iron and carbon

• No more than 2 percent carbon

• Steels are allotropic as is iron

Main structures in steel

• Austenite

• Ferrite

• Cementite

• Pearlite

• Diagram of 0.2, 0.83, 1.3 carbon steel percentages

Austentite

• Also called “gamma” iron

• Solid solution

• When a steel is heated to red hot, it becomes austentite in structure

Ferrite

• “Alpha” iron

• Soft and Ductile

• BCC structure present at room temp

Cementite

• Iron Carbide

• Increases Hardness at the expense of toughness and ductility, meaning more brittle easy to break

Pearlite

• This material is called a euctoid structure

• Forms when austentite cools to form two new solids

• FERRITE + CEMENTITE = PEARLITE

0.2 PERCENT CARBON STEEL

• More ferrite

• Less pearlite

• HYPO EUTECTOID

0.83 Percent Carbon Steel

• ALL PEARLITE

• EUTECTOID

1.2 PERCENT CARBON STEEL

• Pearlite and Cementite

• HYPER EUTECTOID

Martensite

• Martensite is when steel becomes heated until a Red Hot (FCC) stucture, then rapidly quenched to form a HCP Structure, resulting in the steel becoming very hard and brittle

Microstrutcure for Martensite

• Microstructures used for under the microscope

• Acicular Martensitte

• Needle shaped form

Tempered Martensite

• When martensite is reheated to make it less brittle, more tough, however retaining much of its hardness

• 200-600 degrees celcius

Low Alloy Steels

• Steels that have other elements added to improve hardness, toughnesss and strength

• Steels with less than 5 percent Alloying elements

• Replaced carbon steels for many applicatations

List of elements added to “Low Alloy Steels”

• Manganese

• Suplhur

• Nickel

• Chromium

• Molybdenum

• Vanadium

• Tungsten

• Copper

• Silicon

• Boron

• Aliminium

• High Strength Low Alloy Steel (HSLA)

1. Manganese

• Increase hardenability, making it more easy for it to be hardened

• NOT “HARDER”

Hardness = how resistant the steel is to scratching, indentation, or wear.

Hardenability = how easily the steel can be hardened by heat treatment (such as quenching), and how deeply the hardness penetrates into the steel.

2. Sulphur

• Combines with manganese, that produces good machining properties

3. Nickel

If added “2-5%”

- Improve toughness

- Impact resistance

If added “12-20%”

- Corrosion resistant steel is produced

4. Chromium

• Improve wear resistance

5. Molybdenum

• Improve hardnenability and increase strength properties at elevated temperatures

6. Vanadium

• Produces carbides that are stable at higher temperatures

• Improves elastic strength with little effect on ductility

7. Tungsten (tool)

• Added to tool steels to improve their stability

8. Copper

• Improve corrosion resistance

9. Silicon

• Same effect as Nickel

Used for:

• High strength structured steels

• Spring steels

• Electricical grade steels

10. Boron

• Has an immense impact on the hardenability of steels

• Added in tiny percentages (0.001-0.003%)

11. Alimininium

• Added to aid in nitriding a steel in amounts around 1 percent

12. High Strength Low Alloy Steels

• HSLA

• Improve yield strength and weldability

High Alloy Steels

1. Hadfield Steel

2. High speed steels

3. Stainless Steel

1. Hadfield steel

• Uses manganese in quantities of 12 percent with 1.2 percent carbon

• Results in a steel with an austentite structure at room temperature

• Outside is hard, inside soft

• Examples: Backhoe, excavator teetth, dredging equipment, rock breaking implements

2. High Speed Steels

18% Tungsten + 4% Chromium + 1% Vanadium + 0.75% Carbon

4. Stainless Steels

COVERED IN CHAPTER 4

Steels used in braking system

• Of carbon steels, 0.1 to 0.3% carbon is used for braking system

• Push rods, caliper pistons, brake pads, pad plates, shoes

Cast Iron

• By definition a steel cannot excess 2 percent carbon

IF IT DOES

• Cast iron are alloys that consist of iron with between 2.5 and 5 percent carbon, exceeding the 2 percent range

Why use cast iron?

• It is a cheap material

• Good compressive strength and rigidity

• Easily machined

• Great fluidity as it flows well in casting

Cast iron features “if mereley an alloy of iron and carbon”

• Very hard

• Very brittle

• Needed when high wear resistance was required

Types of Cast Iron

1. White cast iron

2. Grey cast iron ( Pearlitc or Ferritic)

3. High Duty cast iron

4. SG (spheroidal graphite) Cast Iron

5. CG (compacted graphite) Cast iron

1. White Cast Iron

• Formed due to fast cooling or low silicon

  

2. Grey Cast Iron

• Result of high silicon content, and graphite forming as flakes

Grey cast iron features

• Weak in tension

• Strong in compression

• Easily machined

• Excellent vibration dampening characteristics

1. Ferritic Grey Cast iron

• Wholely ferrite with Large graphite flakes

• Softer and weak

2. Pearlitic Grey Cast iron

• Good toughness and good compressive strength

• Used in Engine block, drum brakes, disc brakes

3. High Duty Cast Iron

• They act as stress concentrations at the tip of the flakes and under a tensile load where cracks can occur

4. SG Cast iron ( Nodular or ductile iron)

• AKA Nodular or ductile iron

• Achieved by adding small amounts of magnesium or cerium to the molten before casting

• Improved tensile strength

• Used in brake discs, drums

5. CG Cast iron

• Their vibration damping peformance is double that of SG cast iron

• Used in brake parts and pump housing

Definite Yield

• The graph shows a clear change from elastic behaviour to plastic behaviour

Indefinite Yield Graph

• There is no obvious point where it changes from elastic to plastic behaviour