An Introduction To Brake Types , Selection And Application

An Introduction To Brake Types , Selection And Application



Introduction
A COMMON MISCONCEPTION ABOUT BRAKES IS THAT BRAKES SQUEEZE AGAINST A DRUM OR DISC, AND THE PRESSURE OF THE SQUEEZING ACTION SLOWS THE VEHICLE DOWN. THIS IS IN FACT A PART OF THE REASON FOR SLOWING DOWN A VEHICLE.
•ACTUALLY BRAKES USE FRICTION OF BRAKE SHOES AND DRUMS TO CONVERT KINETIC ENERGY DEVELOPED BY THE VEHICLE INTO HEAT ENERGY.
•WHEN WE APPLY BRAKES, THE PADS OR SHOES THAT PRESS AGAINST THE BRAKE DRUMS OR ROTOR CONVERT KINETIC ENERGY INTO THERMAL ENERGY VIA FRICTION.

Brake Classification 
There are numerous brake types as shown in Figure 1

FIG 1

Brakes are used to decelerate a rotating component or system of components by absorbing power from it. Most types use simple sliding friction. Figure 2 shows some basic models.
• The simple band brake comprises a flexible band bearing on the circumference of a drum; these are used on simple winches.
• The external shoe brake has external shoes with friction linings, rigidly connected to pivoted posts. The brake is operated by a linkage, which provides an actuation force, pulling the brake shoes into contact with the drum.
• Internal drum brakes, used on older designs of motor vehicle, operate by the friction-lined brake shoes being pushed into contact with the internal surface of a brake drum by a single cam (leading/trailing leading shoe type) or twin hydraulic cylinders (twin leading shoe type).
• Hydraulic disc brakes, as found on most road vehicles, aircraft, etc. and many industrial applications comprise twin opposing hydraulic pistons faced with pads of friction material. The pads are forced into contact with the disc by hydraulic pressure, exerting forces normal to the disc which
transfer into tangential friction forces, thereby applying a deceleration force to the disc.

FIG 2

Drum Brakes
 The drum brake has a metal brake drum that encloses the brake assembly at each wheel.Two curved brake shoes expand outward to slow or stop the drum which rotates with the wheel.

FIG 3

A further classification can be made in terms of the length of the brake shoe: short, long or complete band.
1-Short shoe external drum brakes

FIG 4

2-Long shoe external drum brakes

FIG 5

3-Double long shoe external drum brake.

FIG 6

4-Long shoe internal drum brakes

FIG 7

5- Band brakes

FIG 8

Advantages 
Drum brakes are simple in design and operation. They have been used in automotive braking systems for decades.Drum brakes have a servo action. This action means that wheel rotation helps the brakes to apply, therefore requiring less pedal force to stop the vehicle.The emergency brake mechanism can easily be incorporated to the brake system.
Disadvantages  
The drum brake assembly, although well suited for wheeled vehicles, has some disadvantages. One problem that occurs during heavy braking is brake fade. During panic stops or repeated harsh stops, the brake linings and drum develop large amounts of heat that reduce the amount of friction between the brake shoe and drum. This reduction in friction greatly decreases the stopping ability of the vehicle, and in most cases additional pressure directed on the brake pedal would not increase the stopping performance of the vehicle.
Disc Brakes
In a disc brake, the fluid from the master cylinder is forced into a caliper where it presses against a piston.
The piston in turn squeezes two brake pads against the disc (rotor), which is attached to wheel, forcing it to slow down or stop.

FIG 9

ADVANTAGES OF DISC BRAKES
The auto industry changed from drum brakes to  disc brakes for a number of reasons: :
   - More resistant to brake fade.
   - Better cooling.
   - Water and dirt resistant.
   - Less maintenance.

Magnetic Brakes

Magnetic braking works because of induced currents and Lenz’s law. If you attach a metal plate to the end of a pendulum and let it swing, its speed will greatly decrease when it passes between the poles of a magnet. When the plate enters the magnetic field, an electric field is induced in metal and circulating eddy currents are generated.These currents act to oppose the change in flux through the plate, in accordance with Lenz’s Law.The currents in turn heat the plate, thereby reducing its kinetic energy. The practical uses for magnetic braking are numerous and commonly found in industry today. This phenomenon can be used to damp unwanted mutations in satellites, to eliminate vibrations in spacecrafts, and to separate nonmagnetic metals from solid waste

FIG 10

Electric brakes
When the electrical current is fed into the system by the controller, it flows through the electromagnets in the brakes. The high capacity electromagnets are energized and are attracted to the rotating armature surface of the drums which moves the actuating levers in the direction that the drums are turning. The resulting force causes the actuating cam block at the shoe end of the lever to push the primary shoe out against the inside surface of the brake drum. The force generated by the primary shoe acting through the adjuster moves the secondary shoe out into contact with the brake drum.Increasing the current flow to the electromagnet causes the magnet to grip the armature surface of the brake drum more firmly. This results in increasing the pressure against the shoes and brake drums until the desired stop is accomplished.

FIG 11

 Types Of Brake Systems
The power source which carries the pedal force applied by the driver on brake pedal to the final brake drum or brake disc in order to de accelerate or stop the vehicle the braking systems are of 6 types-
1- Mechanical braking system
2- Hydraulic braking system
3- Air or pneumatic braking system
4- Vacuum braking system
5- Magnetic braking system
6- Electric braking system
1- Mechanical Brakes System
It is the type of braking system in which the brake force applied by the driver on the brake pedal is transferred to the final brake drum or disc rotor through the various mechanical linkages like cylindrical rods, fulcrums, springs etc. In order to de accelerate or stop the vehicle.

   - Mechanical brakes were used in various old automobile vehicles but they are obsolete  now  days due to their less effectiveness.

FIG 12

Mechanical brake system :
1: pressure in the cylinder;
2: piston movement;
3, 4: forces of the brake blocks (measured in couples for all the wheels of one bogie).


2- Hydraulic braking System
It is the type of braking system in which the brake force applied by the driver on brake pedal is first converted into hydraulic pressure by master cylinder (for reference read article on master cylinder) than this hydraulic pressure from master cylinder is transferred to the final brake drum or disc rotor through brake lines.
    - Instead of mechanical linkages, brake fluid is used in hydraulic brakes for the transmission of              brake pedal force in order to stop or de accelerates the vehicle.
     - Almost all the bikes and cars on the road today are equipped with the hydraulic braking system          due to it high effectiveness and high brake force generating capability

FIG 13

In the 1600s, French scientist Blaise Pascal determined that pressure exerted on a confined liquid caused an increase in pressure at all points. This meant that in a closed container or system, any pressure against the liquid would be transmitted without any pressure loss through the system. This led to what is called Pascal’s Law.
Pascal’s Law states that pressure = force/area or

 P = F/A

This is the basis for hydraulics.
Where:
F : Pushing force Kg (lb)
A: Piston area cm²( inch²)
p : Fluid pressure Kg/cm² (psi)

FIG 14

We can write a simple formula for the relationship between the forces on the two pistons:

FB = (FA×AB)/AA

Hydraulic system and Components :
The hydraulic system is comprised of all of the brake system components that operate using brake fluid to transfer force. This generally consists of the master cylinder, hydraulic valves, lines and hoses, calipers, and wheel cylinders.

1- Master Cylinder
The master cylinder is the primary unit in the brake system that converts the force of the operator's foot into fluid pressure to operate the wheel cylinders.

FIG 15

2- HYDRAULIC VALVES, LINES, AND HOSES To get the fluid from the master cylinder to thewheel brakes, a network of steel lines and reinforced rubber hoses is used. In addition, many systems use valves to control fluid pressure to either the front or rear brakes.
For many years, different types of brake system valves were used to control brake application, mostly to prevent excessive application pres¬sure and wheel lockup. On many vehicles these valves have been eliminated by the ABS system, but some remain in use. On vehicles with combination brake systems, meaning front disc brakes and rear drum brakes, two hydraulic valves are commonly used to help control brake application, shown in fig 16

FIG 16

Metering Valve 
The metering valve is designed to equalize braking action at each wheel during light brake applications. A metering valve is used on vehicles with front disc brakes and rear drum brakes, and is located in the line to the disc brakes. The metering valve functions by preventing the disc brakes from applying until a set pressure has built up in the system; then the metering valve opens and applies full pressure to the disk brakes. When the brakes are released, fluid will bypass the main metering valve and return to the master cylinder.

FIG 17

Proportioning Valve 
The proportioning valve also equalizes braking action with front disc brakes and rear drum brakes. It is located in the brake line to the rear brakes. The function of the proportioning valve is to limit pressure to the rear brakes when high pressure is required to apply the front disc. This prevents rear wheel lockup and skidding during heavy brake applications.

FIG 18

4- Calipers And wheel cylinders
Calipers and wheel cylinders are the outputs of the hydraulic system, using the pressure of the hydraulic system to apply the pads and shoes against the discs and drums to slow the wheel.

FIG 19

ADVANTAGES OF HYDRAULIC BRAKES
- Equal braking effort to all the four wheels
- Less rate of wear (due to absence of joints compared to mechanical brakes)
- Force multiplication (or divisions) very easily just by changing the size of one piston and cylinder relative to other.
DISADVANTAGES OF HYDRAULIC BRAKES
- Even slight leakage of air into the breaking system makes it useless.
- The brake shoes are liable to get ruined if the brake fluid leaks out.

3- Air or Pneumatic Brakes System
It is the types of braking system in which atmospheric air through compressors and valves is used to transmit brake pedal force from brake pedal to the final drum or disc rotor.
 - Air brakes are mainly used in heavy vehicles like busses and trucks because hydraulic brakes fails to transmit high brake force through greater distance and also pneumatic brakes generates higher brake force than hydraulic brake which is the need of the heavy vehicle.
 - The chances of brake failure is less in case of pneumatic brakes as they are usually equipped with a reserve air tank which comes in action when there is a brake failure due to leakage in brake lines.
 - High end cars these days are using air brakes system due to its effectiveness and fail proof ability.

FIG 20

4- Vacuum Brakes System
It is the conventional type of braking system in which vacuum inside the brake lines causes brake pads to move which in turn finally stops or de accelerate the vehicle.
 - Exhauster , main cylinder , brake lines , valves along with disc rotor or drum are the main components that combines together to make a vacuum braking system
 - Vacuum brakes were used in old or conventional trains and are replaced with air brakes now days because of its less effectiveness and slow braking.
 - Vacuum brakes are cheaper than air brakes but are less safe than air brakes.

FIG 21

5- Magnetic Brakes System
In this types of braking system, the magnetic field generated by permanent magnets is used to cause the braking of the vehicle.
 - It works on the principle that when we pass a magnet through a cooper tube, eddy current is              generated and the magnetic field generated by this eddy current provide magnetic braking.
 - This is the friction less braking system thus there is less or no wear and tear.
 - This is the advanced technology in which no pressure is needed to cause braking.
 - The response to the braking in this is quite quick as compared to other braking systems.

FIG 22

6- Electrical Brakes System
It is type of braking used in electric vehicle in which braking is produced using the electrical motors which is the main source of power in electric vehicles, it is further divided into 3 types-
(i) Plugging Brakes-When the brake pedal is pressed in the electric vehicle equipped with plugging braking, the polarity of the motors changes which in turn reverses the direction of the motor and causes the braking.
(ii) Regenerative Braking- It is the type of electrical braking in which at the time of braking the motor which is the main power source of the vehicle becomes the generator i.e. when brakes are applied, the power supply to the motor cuts off due to which the mechanical energy from the wheels becomes the rotating force for the motor which in turn converts this mechanical energy into the electric energy which is further stored in the battery.

        - Regenerative braking saves the energy and are widely used in today’s electric vehicles.
        - Tesla Model-S provides the most effective regenerative braking.

FIG 23

(iii) Dynamic or Rheostat Braking-
It is the type of electrical braking in which resistance provided by the rheostat causes the actual braking, in this type a rheostat is attached to the circuit that provides the resistance to the motor which is responsible for de acceleration or stopping of the vehicle.

Parking Brake
When a driver applies the parking brake on a vehicle equipped with rear drum brakes, it pulls cables that are attached to actuator levers and struts inside the brake drum. These actuator levers and struts mechanically apply the brakes by pushing both brake shoes outward into the drum (fig.24 ). 

FIG 24

The Basic Principles
The brake system converts the kinetic energy of vehicle motion into heat.The brake system converts the kinetic energy of the moving vehicle into heat

Newton is always right 

F=ma

kinetic energy of car :
                                        KE = (m×V²)/2G
where:
KE  : kinetic energy Kw
m    : Vehicle weight  kg
v     : Vehicle speed m/s

kinetic energy of crane load :

   KE= J × ωm² / 2 (Joules) 

where
J    : Total inertia referred to braked shaft [kgm²]
ωm :Maximum disc speed [rad/sec]

Braking Force (Fb) is the tangential friction force acting between the brake pads and disc

Fb = 2 . μ . Fn

Where:
 μ  : is the coefficient of friction between the pad and the disc
 Fn : Clamping Force  is the force pressing each brake pad against the disc. N



Braking Torque (Tb) is the moment of braking force about the center of rotation.

Tb = Fb . r

Where
r  : is the effective disc radius.

FIG 25

 Total breaking force 

f : is the force applied by the driver’s foot
Rp : is the pedal lever ratio
Fb : is the booster assist force
Am : is the area of the master cylinder
Aw : is the area of the front caliper piston
μ    :is the coefficient of friction of the lining
 r    :is the effective radius of the caliper
R :   is the loaded radius of the tire.
Stopping distance = reaction distance + braking distance

FIG 26

Deceleration

       Υ= V² / 2 D

Υ: deceleration, expressed inm.s-²
V: initial speed, expressed in m.s-¹
D: breaking distance, expressed in m


Braking force
                                              F = M ×Υ
F: braking force, expressed in N
M: mass, expressed in kg
Υ: deceleration, expresseden m.s⁻¹
Reaction distance
The reaction distance is the distance you travel from the point of detecting a hazard until you begin braking or swerving.

The reaction distance is affected by
The car’s speed (proportional increase):
         2 x higher speed = 2 x longer reaction distance.
         5 x higher speed = 5 x longer reaction distance.
Your reaction time.
       - Normally 0.5–2 seconds.
       - 45–54 year-olds have the best reaction time in traffic.
       - 18–24 year-olds and those over 60 have the same reaction time in traffic. Young people have
         sharper senses but older people have more experience

                                   d = (V × t) / 3.6
d = reaction distance in metres (to be calculated).
v = speed in km/h.
t = reaction time in seconds.
3.6 = fixed figure for converting km/h to m/s.


Braking distance
The braking distance is the distance the car travels from the point when you start braking until the car stands still.
The braking distance is affected by  -The vehicle’s speed (quadratic increase; “raised to the power of 2”):
               2 x higher speed = 4 x longer braking distance.
               3 x higher speed = 9 x longer braking distance.
       - The road (gradient and conditions).
       - The load.
       - The brakes (condition, braking technology and how many wheels are braking).

D = V² / (250 * μ )

D= braking distance in metres (to be calculated).
V = speed in km/h.
250 = fixed figure which is always used.
μ  = coefficient of friction, approx. 0.8 on dry asphalt and 0.1 on ice.
The coefficient of friction(CoF) is a number that expresses the ratio of force required to move an object divided by the mass of the object.
The Society of Automotive Engineers (SAE) developed a Friction Identification System forBrake Linings and Brake Blocks (SAE Recommended Practice SAE J866a)

Stopping distance

Stopping distance = reaction distance + braking distance

 Brake parts 

FIG 27

Drum brake: mechanism that slows and stops a car by fiction, by pression brake shoes against a drum.
Drum: cylindrical part attached to the wheel, against which the brake shoes are pressed to stop the car.
Brake lining: frictional part on the outside edges of the brake shoes.
Return spring: part of the brake mechanism that returns the brake shoes to their initial position.
Piston: cylindrical part that transmits the pressure to and receives pressure from the brake shoes.
Wheel cylinder: type of roller that applies a uniform pressure to the wheel then the brake is activated.
Brake shoe: part on which the brake lining is mounted.
Brake pads: part activated by the piston.
Wheel hub: central part crossed by the axel.
Stud: metal pin.
Disk: round, flat, piece of metal, pressed against the wheel to slow or stop the car.
Brake line: system liquid-transporting tubes.
Splash shield: protector that prevents dirt from fouling the braking system.
Disk brake: mechanism that slows and stops a car by friction, by pressing a disk against the wheel axel.
Brake Fluid
The specifications for all automotive brake fluids are defined by the Federal Motor Vehicle Safety Standards and are assigned Department of Transportation (DOT) numbers.
Qualities that brake fluid must have:
• Free flowing at low and high temperatures
• A boiling point over 400 degrees F. (204 degrees C.)
• Low freezing point • Non-corrosive to metal or rubber brake parts
• Ability to lubricate metal and rubber parts
• Hygroscopic - Ability to absorb moisture that enters the hydraulic system

The three (3) brake fluids currently assigned DOT numbers are DOT 3, DOT 4 and DOT 5. DOT 3 and DOT 4 are polyalkylene-glyco-ether mixtures while DOT 5 is silicone based. All domestic and most import car manufacturers specify and require DOT 3 brake fluid (some imports require DOT 4 as it has a higher boiling point). DOT 5 brake fluid is not currently used in any domestic or import vehicles .

Precautions must always be observed when working with brake fluids:
• Brake fluid is toxic to the human body.
• Brake fluid can damage painted surfaces.
• Brake fluid contaminated with moisture, dirt, petroleum or other foreign material will damage the        hydraulic system internally.
• Only denatured alcohol or other approved cleaners should be used when cleaning brake hydraulic        parts.
• Use only fresh, clean brake fluid (never reuse old brake fluid).
• Never mix brake fluids with any other fluids, including other types of brake fluid (e.g. DOT 3 and    DOT 4).