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Brakes

Purpose of a Brake

When a vehicle is accelerated, energy supplied by theengine causes the vehicle speed to increase.

Some of this energy is instantly used up in overcomingfrictional and tractive resistances, but a large amountremains stored in the vehicle as kinetic energy (can be seenin neutral).

The vehicle does not immediately come to rest; instead, ittravels for a considerable distance before it becomesstationary.

In this case, the stored energy is slowly being convertedand used to drive the vehicle against the resistances thatoppose the vehicle motion, which is not feasible.

So an additional resistance, called a brake, is needed toconvert the energy at a faster rate. The purpose of a brakeis to convert kinetic energy to heat energy.

The speed of the energy conversion controls the rate ofretardation of a vehicle (i.e. its rate of deceleration).

Heat generation at the brake is obtained by rubbing a fixedpad or shoe against a rotating object driven by the motionof the vehicle.

Purpose of a Brake

Stopping Distance and Tyre Adhesion

The strength of the force pressing a shoe against a wheelgoverns the resistance to rotation of a road wheel.

During this operation the road surface has to drive thewheel around.

The limit of this driving force is reached when theresistance of the brake equals the maximum frictional forcethat is produced between the tyre and road.

The latter is called the adhesive force and can be calculatedfrom the expression:

Adhesive force = Load on wheel x Coefficient of friction

When the limit is reached, the wheel starts to skid.

So extra force on the brake shoe will not produce anyincrease in the rate of vehicle slows down, no matter howgood the braking system.

This situation is apparent when a vehicle is braked on aslippery surface: slight pressure on the brake soon locks upthe wheel and very poor braking results.

Road adhesion is affected by: type of road surface,condition of surface (wet, dry, icy, greasy), design of tyretread, composition of tread material and depth of tread.

Stopping Distance and Tyre Adhesion

With respect to method of braking contact Internal expanding brakes

Drum brakes

External contracting brakes Disc Brakes Band Brakes

With respect to mechanism Mechanical brakes Power brakes

With respect to application Foot Brake Hand Brake

Types of Brakes System

With respect to number of wheels Two wheel brake Four wheel brake

With respect to method of applying braking force Single acting Double acting

With respect to nature of power employed Vacuum brakes Pneumatic brakes Hydraulic brakes Electric brakes Magnetic brakes

Types of Brakes System

Drum Brake This internal expanding type of brake uses two shoes that

are attached to a back-plate, which is fixed to stub axle oraxle tube.

Each shoe has a T section and a friction lining is riveted orbonded to the outer face of the shoe.

At one end of the shoe is a device for expanding the shoewhen the brake pedal is depressed.

In a simple brake, a cam is used as a shoe expander, butmodern cars hydraulically operated pistons is used.

A shoe anchor is rigidly attached to the back-plate, takesthe form of a large pin that passes through the shoes (actas pivot).

Types - Method of Braking Contact(Drum Brakes)

Simple arrangements of springs are used to pull thebrake shoes return the shoes to the off position afterthe brake has been applied.

The inner cylindrical surface of the cast iron drum isground to give a smooth surface on to which the brakelinings can rub.

Drums should be exposed to good flow of air todissipate the heat.

Some form of adjuster is provided at each brake to takeup excessive clearance due to wear of the frictionfacing.


Types of shoe mountings Sliding and Pinned The end of each shoe locates at a fixed point on the brake

back plate can be mounted in two ways:

- Sliding - In the sliding (or floating) shoe type the end of theshoe has a radius, which can roll or slide on a flat surface. Itis held against the surface by the shoe return springs.

- Pinned - In the pinned shoe type the shoe pivots about apost mounted on the back plate.

- Sliding shoes give a degree of self-alignment so the liningscan be efficiently held against drum with minimum wear.


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Sliding Pinned


Leading and trailing shoes(Pinned)

Which shoe is ahead of itspivot point, is called leadingshoe.

Similarly the shoe trailsbehind its pivot point and iscalled a trailing shoe.

There is an importantdifference in the way leadingand trailing shoes act underbraking.


One key advantage of thedrum brake (pinned) systemover other systems is that itprovides a self-servo action.

When the force is applied onleading shoe. Notice that thefrictional drag force has amoment about the pivot point.

This increases the input loadand hence increases the drag.In other words, there is a self-servo action, which increasesthe braking effect.


But when force is applied ontrailing shoe. The moment ofthe frictional drag force aboutthe pivot point opposes theinput load, thereby reducingthe drag and the braking effect.

For many years, vehiclemanufacturers used drumbrakes as the main brakingsystem on both front and rearaxles because of their simplicityand low manufacturing costs.

To take more advantage thisproblem some modificationswere came into picture.


Types - Method of Braking ContactDrum Brakes - Duo-Servo Brake (Sliding)

This system is often called a self-energizing brake, which isvery powerful brake.

The operating principle is based on the use of drum energyto considerably boost the force applied by the driver.

When the leading shoe is pushed out into contact with theforward-moving drum, the frictional force causes it torotate partially with the drum.

The shoe movement produced by this self-wrapping actionis transmitted through a floating adjuster to the trailingshoe, which brings the shoe into contact with the drum.

So by this way now rotation of drum helps the brakeapplication for both shoes irrespective to leading andtrailing shoe arrangement.

To minimize the delaybefore the self-energizingassistance comes intoaction, the trailing shoe isheld on the anchor pin bya stronger return spring,by this means expanderonly moves the leadingshoe.

In this case only, theleading shoe is called theprimary shoe, becausethe shoe is made tocontact the drum beforethe secondary shoe.

Types - Method of Braking ContactDrum Brakes - Duo-Servo Brake (Sliding)

Before the universal adoption of the disc for front brakes,the 2LS was commonly used.

Each shoe had its own expander, which is positioned so thatboth shoes were subject to a self-servo action.

An interlinking pipe behind the back-plate provided anequal hydraulic pressure to each single-acting cylinder.

The cylinder housings acted as a shoe anchor for thefloating shoes, so the cylinders were rigidly attached to theback-plate.

Types - Method of Braking ContactDrum Brakes - Twin leading shoe brake (2LS)


Self Energizing action

Compared with a leading and trailing shoe brake, the 2LStype had the following advantages:

1. Equal self-servo there were two effective shoes so amore powerful, stable brake was obtained.

2. Even lining wear because both shoes did an equalamount of work, a longer life was achieved,

3. Greater resistance to fade less reliance was placed onone shoe to do the major share of the braking, so the self-servo action on this shoe could be reduced. This resultedin a more progressive brake and as a result, it was lesssensitive to heat.


One disadvantage of the 2LS type showed up duringreversing, because both shoes became trailing shoes whenthe car travelled backwards.


A shoe-type brake assembly uses two single-acting type wheel cylinders to expand the shoes in a twin leading shoe brake and a double-acting type for a leading and trailing shoe brake.

Wheel cylinders are fitted with a valve at the highest part to allow air to be bleed from the system.

Types - Method of Braking ContactDrum Brakes Wheel Cylinder

A major disadvantage of the drum brake operation is brake fade.

This condition occurs when the temperature between thetwo friction linings increases so much during braking that itcauses a reduction in the braking performance.

This condition normally arises when the vehicle is driven forlong periods and the brakes have been used regularly or ondownhill movement.

The driver then has to apply greater pressure to the brakepedal to try to obtain the same braking efficiency.

This is because in drum brakes the heated regions areshrouded by metal hence poor heat dissipation.


The disc brake was developed tominimize the fade problem seen indrum brakes.

They consist of an exposed disc, whichis attached to the hub flange and thetwo friction pads are pressed on to thisdisc to give a braking action.

The pads are moved by hydraulicpistons working in cylinders formed in acalliper that is secured to a fixed part ofthe axle.

Types - Method of Braking Contact(Disc Brakes)

When hydraulic pressure isapplied to the two cylindersheld in the calliper, the pistonsmove; this action forces thefriction pads into contact withthe rotating disc that is oftenmade from cast iron or othermaterials, such as carbon orceramic on high performancevehicles.

The sandwiching action of thepads on the disc gives aretarding action and heatgenerated from the energy ofmotion is conducted to thedisc.


As a large part of the disc is exposedto the air, heat is easily radiated, thisenable the brake to be usedcontinuously for long periodswithout serious fade occurs.

Hence this type of brake is not sosensitive to heat build-up.


Different types of callipers

- Two-piston calliper each shoe have individual piston.

- Four-piston calliper each shoe have

2 pistons

- Single piston calliper single piston used.


The limited road wheel-to-disc clearance on vehicleswith a steering geometry based on the negative offset(negative scrub radius) principle is often insufficient toaccommodate a calliper having two opposed pistons.

In these cases, a single-piston calliper is used.

The piston housing is keyed to the pad housing, whichis bolted to the wheel suspension member.

Hydraulic pressure moves the piston in one directionand the piston housing in the opposite direction.


Brake Disc

Brake discs are generally manufactured from grey cast iron and come in a number of different diameters.

The discs rotate at the same speed as the road wheels.

Brake discs have two frictional surfaces that the brake pads grip when brake pedal pressure is applied.

The radiated heat produced is dissipated throughout the disc frictional surface and housing area.


Types of Brake Discs

- Solid disc Used where the less braking force is there. Less manufacturing cost.


- Ventilated Disc

Like solid discs, ventilated discs alsohave two frictional surfaces, butthese are separated by an air space.

The frictional surfaces are joined byfins that are designed to draw coolair into the centre of the disc when itis rotating.

This enables the cooling of the discwhen high temperatures aregenerated during braking, thereforepreventing brake fade.

Ventilated discs are commonly usedon the front braking system


Cross drilled disc


Brake Pads

The pad is usually made by bonding africtional surface to a steel backing plate.

The brake pads are held in place by abrake calliper and are arranged in pairson each wheel.

The steel backing plates are mountedinto the calliper with the frictionalsurface facing the brake disc.

Earlier asbestos is used, but nowcellulose, mineral fibres, aramid,chopped glass, steel, copper fibres andceramics are in use.


1. The disc surface on which heat is generated is directlyexposed to the air, allowing easier dissipation of heat andgiving a greater resistance to fade.

2. Independence of self-servo effect. The non-assisted brakemay require more effort but its action is progressive (i.e.the brake gives a braking torque proportional to theapplied force).

3. The brake is not so sensitive to friction changes.4. Self-adjusting linings or pads are used, which are easily

replaced.5. Pedal travel does not increase as the disc heats up

heating a drum causes expansion that increases pedaltravel.

6. Weight of disc brake arrangements is generally lighterthan drum.

Types - Method of Braking ContactDisc Brakes vs Drum Brakes

This is simplest arrangement also called mechanicallyoperated braking system.

Four adjustable rods or cables link the brakeshoe operatinglevers to a transversely mounted cross-shaft.

The footbrake and handbrake controls are connected to thecross-shaft by links.

In this system, each brake will receive its share of the brakepedal force only when the mechanism is balanced (i.e. setup so that each shoe contacts the drum simultaneously).

If one brake has a much smaller shoe-drum clearance thanthe others, all of the drivers force will be directed to thatbrake; as a result, the unbalanced braking action will causethe vehicle to pull violently to that side.

Types With Respect to MechanismMechanical Brakes

Types With Respect to MechanismMechanical Brakes

Single-line hydraulic layout used to operate a drum and discbrake system.

When the driver applies brake pedal pressure to thesystem, the master cylinder feeds hydraulic fluid to each ofthe wheel cylinders.

Each cylinder then operates and moves the brake shoesoutwards towards the brake drum.

The master cylinder has an integral reservoir to store anadditional amount of fluid to support the systemsdemands.

Types With Respect to MechanismHydraulic Brakes

Regulations demand that a separate mechanical parkingbrake (handbrake) system must be provided on at least tworoad wheels.

This then enables the driver to stop the vehicle in the eventof the hydraulic system failing.


Brake manufacturers offer two main types of master cylinder

- Single-cylinder - for single-line layouts

- Dual-cylinder (tandem) - dual-line system layouts

Types With Respect to MechanismHydraulic Brakes Master Cylinder

Single Cylinder type Master Cylinder


When the brake pedal is depressed, the piston forces thefluid into the line until all shoe movement has been takenup.

Thrust on the pedal will then pressurize the system andforce each shoe against the drum.

If shoe movement is too great (i.e. if the shoes requireadjustment), the piston will reach the end of its travelbefore the brake applies.

To overcome this problem, the driver should pump thepedal: a quick return of the piston creates a depression inthe main chamber and causes fluid to flow through thefeed holes and over the main cup to recharge the cylinder.


On release of the brake, the shoe return springs pump thefluid back to the master cylinder.

Any excess fluid in the line, caused by pumping orexpansion due to heat, is returned to the reservoir via thebypass port.


Working Animation - https://www.youtube.com/watch?v=tS21Fhyt04U

Master Cylinder.mp4

Tandem (Dual) Master Cylinder

In order to avoid complete brake system failure, a tandem-type master cylinder is used.

In this type two independent brake circuits being controlled through the use of one brake master cylinder.

So hydraulic failure in one brake line will result in the loss of one brake circuit only.

While driver still able to apply brake through second brake circuit.


The tandem cylinder may be considered as two singlecylinders mounted end to end.

The cylinder contains two pistons, one directly connectedto the pedal and the other operated by fluid pressure.

At each of the two outlet points, a check valve is fitted.

One return spring is positioned between the pistons and astronger return spring, acting on the independent piston,ensures that the pistons are forced back to their stops.

Fluid is supplied through ports similar to the solo cylinderand the reservoir is divided into two parts to prevent a totalfluid loss when one line fails.


Under normal conditions, the movement of piston 1 causesan increase in the fluid pressure in the chamber controlledby piston 1.

Pressure from this chamber is transmitted to the frontbrake line and to piston 2, which, being free to move, willpressurize the rear brake line to the same extent as thefront line.

Assuming a failure occurs in the front line, the movementof piston 1 will discharge fluid at the fracture and will allowthe two pistons to contact.

Although this stage has taken up some of the pedal travelto operate the rear brakes.


Failure in the rear line causes the initial pressure to movepiston 2 to the limit of its travel.

Once this point is reached, the front brake can be operatedsuccessfully.

An additional rubber seal fitted to piston 2 prevents fluidfrom leaking from the serviceable section to the fracturedline.


To remove air from system through wheel cylinder valve iscalled bleeding.

Types With Respect to MechanismHydraulic Brakes Bleeding

Servo Assistance

In the past, the assistance given by brake drum rotation(self-servo) kept the pedal force low.

But when powerful disc brakes were introduced andvehicles became faster and more powerful, some otherform of assistance was needed.

Hence to lessen the effort, a boosting force is used toreinforce the drivers effort is called servo assistance.

Servo assistance may of several type depending upon theapplication:

- Vacuum assistance - medium cars

- Hydraulic assistance - heavy cars and vehicles fitted withABS

- Compressed air assistance - some light trucks andminibuses

Servo Assistance

Vacuum Assistance

Also called suspended vacuum assistance.

There are two main types of suspended-vacuum servo:

- Indirect-acting servo

- Direct-acting servo

Servo AssistanceVacuum Assistance

Indirect-acting Servo

This type is sometimes called a remote type because it ismounted remote from the pedal in the hydraulic linebetween the single master cylinder and the wheelcylinders.

Generally, this type is not used today in productionvehicles, although you may see it as an after-marketfitment to earlier vehicles.

This system consist of three major components:1. Vacuum cylinder containing a spring-loaded diaphragm2. Slave hydraulic cylinder3. Control valve actuated by hydraulic pressure

Servo AssistanceVacuum Assistance - Indirect-acting Servo

Under the brake off condition, the vacuum valve will be open and equal vacuum pressure will be there on both sides of the diaphragm.

Application of force on pedal produces a hydraulic pressure on the brakes and also raises the valve piston in the servo.

This movement closes the vacuum control valve and opens the air valve to allow a breakdown of the vacuum in the outer chamber of the vacuum cylinder.


Air pressure difference causes the booster diaphragm toapply a thrust on the slave-cylinder piston, which booststhe thrust given by the drivers foot on the brake pedal.

Release of the pedal drops the hydraulic pressure to allowthe valve piston to return and open the vacuum controlvalve.

Air is quickly evacuated from the outer chamber of thevacuum cylinder and a spring returns the diaphragm.


Direct-acting Servo

It can be used with either single or tandem mastercylinders.

It gets its name from the fact that the pedal linkage isdirectly connected to the servo.

When the brakes are off and the servo piston issuspended in vacuum.

Atmospheric pressure has been removed from both servocompartments and passed to the engine manifold throughthe pipe and non-return valve in the front of the vacuumchamber.

Servo AssistanceVacuum Assistance - Direct-acting Servo

Movement of the brake pedal initially closes the vacuumvalve and then opens the air valve.

This allows air to flow through the filter and valve to therear chamber.

Atmospheric pressure is now acting on the rear chamberand a vacuum acting on the front chamber.

The difference in air pressure between the two chambersproduces a force on the piston that boosts the effortapplied by the driver, as the atmospheric pressure isgreater than the vacuum or depression.


As the servo piston starts to move, it will exert a force onthe master cylinder push-rod through a rubber reactiondisc.

Pressure on this spongy disc will cause it to squeeze backand close the air valve.

In this position, the drivers efforts will be supplemented bya servo boost that is proportional to the effort applied: thisfeature enables the driver to feel the load applied to thebrake and also gives a progressive operation.

Further movement of the pedal will reopen the air valve iffurther assistance is required.


Release of the brake pedal causes the air valve to close andthe vacuum valve to open.

This will restore the vacuum suspension of the piston andallow the spring to return the piston to the off position.

In the event of vacuum failure, the brakes can still beapplied without servo assistance.


Vacuum assistance systems can becontrolled by vacuum in inductionmanifold.

Although in some of the dieselengine vacuum pump can be alsoused.

Servo AssistanceVacuum Assistance

x

Since the pressure difference of a vacuum arrangement islimited, a system is needed that provides a much greatersource of energy.

This can be achieved with a hydraulic servo because thistype of system operates on a pressure of 5582 bar

The hydraulic power is produced by the engine-drivenpump.

Two layouts are used in hydraulic system:

- Continuous flow system- Continuous flow system with accumulator

Servo AssistanceHydraulic Assistance

This system is mounted behind the master cylinder.

The servo valve is supplied with fluid from a pump.

During brakes off position, the fluid can easily passbetween the master-cylinder piston and servo valve to adrilling that leads it back to the reservoir.

Depression of the pedal initially closes the conical servovalve and causes the pump to build up a pressure in theregion A, which will tend to force the piston and valveapart.

Servo AssistanceHydraulic Assistance Continuous Flow type

The piston has a larger area than the valve, and so thethrust exerted on the piston will be greater than that actingon the valve and brake pedal.

As soon as a given pressure, which will depend on the forceapplied to the pedal, has built up, the servo valve willpartially open to maintain the pressure and give assistance.

If the pedal force is exceptionally high, a pressure reliefvalve will open and allow fluid to escape to the reservoir.

Release of the pedal returns and opens the servo valve,releases the brakes and restores uninterrupted flow of fluidfrom the pump to the reservoir.


The assistance given by the continuous-flow systemdepends on the pump speed, so a hard pedal is felt whenthe pump is stationary or rotating slowly.

To overcome this disadvantage, a hydraulic accumulator isnormally used.

The accumulator contains a spring-loaded piston, which isacted upon by the fluid: higher the fluid pressure, the morethe spring is compressed.

A cut-out valve maintains the accumulator pressure in therange 5582 bar.

A charging valve, activated by fluid pressure from theoutput or brake line side of the master cylinder, releasesfluid from the accumulator to act on the servo valve.

Servo AssistanceHydraulic Assistance Continuous Flow with Accumulator

Servo AssistanceHydraulic Assistance Continuous Flow with Accumulator

On light trucks and minibuses, an alternative to a hydraulicservo is used which is a compressed-air servo.

This is generally called an air/hydraulic (air over hydraulic)or Airpac system because compressed air is used to boostthe force applied by the driver to a hydraulically operatedbrake.

Air pressure, generated by an engine-driven compressor, isstored in a reservoir adjacent to the servo chamber.

This houses a piston that operates another piston thatcontrols the main hydraulic brake line.

When the brake is applied, a valve directs compressed airinto the servo cylinder to boost the pedal effort.

Servo AssistanceCompressed Air Assistance

Servo AssistanceCompressed Air Assistance

Antilock Braking System (ABS)

The ABS is designed to ensure that the driver is able tomaintain control of the vehicle during heavy or emergencybraking situations.

The ABS will provide shorter braking distances due to thelack of wheel lock, which allows the tyres to maintaingreater friction with the road surface.

Pressure on brake pedal should be as per road conditions.

Generally driver applies either too much or too littlepressure on the pedal. The effects of these actions:

1. Pedal pressure is too high one or more of the wheelsskid over the surface with the result that:

- a stopping distance is increased because the adhesionbetween a skidding wheel and the road is less than thatgiven by a wheel that is held due to locking

- directional control is lost. So in the case of a rear-wheelskid, the vehicle turns from front to rear.

2. Pedal pressure is too low stopping distance is increased,which can result in impact with an obstacle.


When the vehicle has to brake quickly, the road wheels willslow down quicker than the vehicle, at this point wheelslippage will occur.

This wheel slippage can be calculated using a simpleformula:

Slip Ratio = (Vehicle speed Wheel speed)/Vehicle Speed

Vehicle stability and control is maintained by limiting theamount of wheel slip while the vehicle is braking (slip ratioshould be low).

The ABS provides the driver with additional stability duringheavy braking by generally achieving a slip ratio of about1030 per cent.


Operation

When the pedal is pressed (1), the shuttle valve (2) ismoved by hydraulic pressure and opens line A.

As the shuttle valve moves, it also triggers valve switch (3),which informs the ECU that the pedal has been pressed.

So by these means pressure fed down to brakes.

During normal braking, the inlet valves (4) are in the openposition and the outlet valves (5) are in the closed position.

This allows the front and rear brakes to be applied in thenormal way.


If the ECU detects via a wheel sensor that a wheel is aboutto lock, the system will go into ABS mode.

The inlet valve of that wheel will be closed and the outletwill remain closed this is called pressure maintenance.

If, however, the wheel is still locking, then the outlet valvewill be opened and the pump (7) will be activated drawingfluid pressure away from the calliper or drum this is calledpressure decrease.

When the ABS ECU then recognizes that all wheels arerotating at the same speed, the process will start again withfull brake pressure being applied through the open inletvalves (4) to the brake callipers or wheel cylinders pressure increase.


This process can happen many times a second until thevehicle has come to a halt or the brakes are released andnormal driving is resumed.

Due to this operation driver feel pulses on brake pedal.

To recap on the operation, the three phases of ABSoperation are:

A. Pressure maintain phaseB. Pressure decrease phaseC. Pressure increase phase


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