chapter10 brakes

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Brakes

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CHAPTER 10

BRAKES

10.1 PRINCIPLE

It goes without saying that brakes are one of the most important control

components of vehicle. They are required to stop the vehicle within the smallest

possible distance and this is done by converting the kinetic energy of the vehicle

into the heat energy which is dissipated into the atmosphere.

10.1 Functions of a braking system:

The function of the brakes is to develop suitable retarding force to slow, stop, or

hold and convert the kinetic energy of the vehicle into heat and dissipate thisheat.

10.2 Requirements of a good braking system:

a.) The brakes should develop maximum possible retarding force and

deceleration under all conditions of operation. This must happen

irrespective of the road being flat or uneven; dry or wet; up or down hill;

straight or winding; the vehicle being heavily loaded or unloaded; and the

speed being low or high.

b.) The deceleration as produced should be preferably be uniform throughout

its application. The pedal effort required should not vary with road

condition or load, i.e. the pedal effort should always be same for the given

deceleration.

c.) The pedal effort required to perform an emergency stop on a fully laden

vehicle must be within the comfortable capabilities of the driverbut at the

same time the pedal effort should increase regularly with the deceleration

required and the pedal control should not be over-sensitive for lightly

laden vehicle.

d.) Theresponse time of the braking system should be as short as possible.


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e.) The braking system should be very reliable to promote highest degree of

safety on the road. It should not be affected by water, heat, road grit or

dust, etc.

f.) Brake performance should not be affected by the wear of this components

g.) The braking system should require very little maintenance and

adjustments. It should have a long and economical life.

h.) The effect of braking operation on other vehicle systems should be

minimum. It should not interfere with the operation of suspension and

steering systems.

i.) Braking system assembly should be as light as possible. This will

minimize unsprung weight.

j.) Due to braking, the vehicle should not drift to right or left.

k.) The noise and vibrations generated by the braking system should be

minimum.

l.) There should be a provision for an independent secondary braking

system which can be used if the main system fails.

m.)A parking brake must also be provided. This can be combined with a

secondary brake.


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10.3 Operation of drum brakes: A drum brake consists of a cast-iron drum

containing a pair of semi-circular brake shoes. The drum is attached to a wheel

and revolves with it, so that the drum is slowed and halted, the wheel slows and

stops too. Friction to slow the drum is applied from inside by the shoes, which do

not rotate but are mounted on a stationary metal back-plate. Each shoe consists

of a curved length of steel or light alloy cast curving faced with a hard-wearing

lining.

In most drum brakes, the shoes are forced against the rotating drum by a

pivoting arrangement; one end of each shoe hinges on a pivot point and the

other end can be moved by a cam, or by hydraulic fluid forced into the wheel

cylinder.

In one hydraulic system, the wheel cylinder is rigidly fixed to the back-plate and

contains two pistons which operate the shoes individually. An alternative plan is

to use a single piston in a cylinder which can side on the back-plate. When the

brakes are applied, fluid pressure acts equally on the piston and on the closed

end of the cylinder, pushing them apart, so that the linings rub against the drum.

Return springs, stretched between the shoes, ensure that when the brake pedal

is released the shoes are retracted until the linings are clear of the drum.

When two shoes are hinged on the same pivot point, the braking system has one

leading shoe and one trailing shoe. An alternative arrangement is to hinge the

shoes separately, at opposite points on the back-plate. Both then act as leading

shoes when the car is running forwards.

A leading shoe tends to be forced into closer contact by the frictional drag of the

rotating drum a self-applying action which increases the braking force on the

wheel. A trailing shoe tends to be pushed away by the drum, and so does

considerably less work than a leading shoe.


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10.4 Two leading shoe type brakes:

In this type the ends of the two brake shoes are hinged separately, at opposite

points of the back plate. Two wheel cylinders are there at the other ends of the

brake shoes. Each shoe is operated by the piston in a wheel cylinder, which is

located at the end opposite the hinge point of that shoe. This arrangement gives

rise to both shoes acting as leading shoes. As such during brake operation, both

shoes exert equal force on the wheel. If the rotation of the drum is reversed, the

brake becomes the two trailing brake shoe. A two leading arrangement gives an


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augmented response to pedal effort. This is because of the self applying action of

the leading shoes. This braking system is usually used on the front wheels of a

vehicle where extra weight is transferred during braking.


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10.5 One leading and one trailing shoe type brakes:

A simple two-shoe brake shown in figure 2 consists of two shoes of which one is

termed as leading and the other as trailing shoe. The upper shoe whose tip is

dragged along with the drum with its direction of rotation is called leading shoe.


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During braking, the leading shoe offers more braking torque than the trailing

shoe; therefore its efficiency is also higher. The lower side shoe is also dragging

or trailing shoe offers lesser braking torque and so is less efficient. Such drum

brakes have been used on the rear wheels of the Mitsubishi lancer car, Tata LPT

1612 truck, etc.

The two leading shoe system is not suitable for the rear wheels. This is because,

when the vehicle runs backward, the leading shoes become trailing shoes and

the braking effort on the wheels gets reduced. With this reduced braking effort, it

may not be possible to hold the vehicle against running backwards on a slope, by

applying the rear brakes. A leading trailing shoe brake is a cheaper and better

alternative for the rear wheels since it is equally effective whether the vehicle is

going forward or backward.


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Comparison of drum brakes

Description

Two leading

shoe (2 LS)brake

Two trailing

shoe (2 TS)brake

One leading

and one trailing

shoe (1LS +

1TS) brake

Braking force on wheels Highest Lowest Medium

Rate of braking liming wear Maximum Minimum Medium

Nature of lining wear Almost uniform More uniform Non-uniform

Behavior in reverse motion Changed, since

it behaves as

2TS

Changed, since

it behaves as

2LS

Unaffected

Braking efficiency High Low Medium


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10.6 DISC BRAKES

The disc or callipered brake has a metal disc instead of a drum and a pair of

pads or flat shoes instead of the curved shoes used with the drum brakes.

1. Swinging caliper disc brake:

In this type the two shoes are located on each side o the disc. When the

driver depresses the brake pedal, the fluid under pressure is sent from the

master cylinder through the connecting tube into the caliper unit behind the

pistons as shown in figure a below. The shoes are now forced against the

disc by the hydraulic pressure. The shoes in effect grip the disc. The friction

between the pads and the revolving disc then provides the braking action.

The following sketch shows a sectional view of the swinging caliper disc

brake.


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2. Sliding Caliper Disc brake

In case of sliding caliper type, figure b, the caliper adjusts itselfautomatically by sliding when the brakes are applied. On braking, the fluid flows

under pressure and pushes the pistons A and B apart. The piston A presses the

nearby friction pad against the disc, while piston B moves rightwards and causes

sliding in the caliper in this way presses upon the other pad against the disc.


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10.7 Drum brakes versus Disc brakes:

Description Drumbrakes Discbrakes

1. Life of friction material. Reasonable (about 15,000

kms)

Reasonable (about

200000 kms)

2.Visibility of wear from

outside

Not possible Can be seen at a glance.

3. Heat dissipation. Inferior. Better.

4.Replacement of friction

material

Troublesome and time

consuming.Easier and rapid

5. Weight. Bulky. Lighter (by 25%).

6. Cooling. Very slow. Efficient due to exposure

to atmosphere.

7. Wear and dirt

collection.

Accumulates. Self cleaning.

8. Braking effect. Generally inconsistent. Consistent

9. Temperature effect. Drum expands, tends to

separate out from linings.

Unaffected.

10. Self energizing action. Exists. Absent.

11.Brake pedal effort Higher Proportional to

retardation.

12. Behavior of hydraulic

system in released state.

Some pressure always

exists.

No hydraulic pressure on

piston

13. Resistance to fade. Fair Excellent

14.Force needed to apply

brakes

Comparatively less.More, since brakes are

not self energizing.

15.Nature of wear Non uniform. Uniform.

16.Shape of lining Curved Straight.


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10.8 Hydraulically Operated Braking System:

The system is so designed that even when the brakes are in the released

position, a small pressure of about 50 kPa is maintained in the pipe lines to ensure

that the cups of the wheel cylinder are kept expanded. This prevents the air from

entering the wheel cylinders when the brakes are released. Besides this pressure alsoserves the following purposes :

(I) It keeps the free travel of the pedal minimum by opposing the brake shoe

retraction springs.

(ii) During bleeding, it does not allow the fluid pumped into the line toreturn, thusquickly purging air from the system.


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10.9 Master cylinder and wheel cylinder

The hydraulic braking system consists of a brake pedal, brake master cylinder,

braking mechanism, brake line, wheel cylinder and braking device.

10.10 Tandem master cylinder:

Some large cars and commercial vehicles have split hydraulic system with two

separate cylinders and reservoirs in the master cylinder. The tandem master

cylinder was devised to avoid the possibility of all the brakes of a vehicle being

put out of action by a fracture in the pipe line leading to one brake cylinder.

The details of a tandem master cylinder can be seen in the figure. There are two

pistons in the master cylinder, in line with each other. There are two compression

springs, one between the two pistons and the other between a piston and master

cylinder cover. These two springs keep the pistons apart as shown in the figure.

There are two oil inlets and two oil outlets, one pair to each reservoir as shown in

the figure.


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During operation if the line A fails, the piston A bottoms against the end of the

cylinder while the piston B continues to develop pressure in the line B and thus

applies brakes to one set of wheels. If the line B fails, the piston B comes up

against the piston A by compressing the spring between them and thereby builds

up pressure in the line A. piston B is connected through linkages to the brake


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pedal. The piston A is floating in the cylinder, being held in position by two

springs.

Tandem master cylinder: (figure type 2)


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10.11 Pneumatic Brake System:

An air operated brake system or pneumatic brake system is employed

predominantly in medium and heavy duty trucks because of the following

advantages.

1. The pressure of the pre-compressed air allows practically any force

required for braking to be developed with a very small effort applied by the

driver to the brake pedal.

2. The compressed air can also be used to inflate the tires, drive the

windshield wiper, actuate steering gear booster, open and close doors of

busses etc.

3. An air operated brake is the most dependable and convenient device for

braking full trailers and semi trailers.

The simplest air brake system consists of an air compressor, a

brake valve, series of brake chambers at the wheels, un-loader valve, a

pressure gauge and a safety valve and air reservoir. These are all

connected by tubes


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The figure above shows the layout of an air brake system for a bus or a truck.

The operation is as follows:

The air compressor operated by the engine forces air at a pressure of 9-10

kgf/cm2, through the water and oil separator to the air separator to the air

reservoir. The air pressure in the reservoir is indicated by a pressure gauge. The

reservoir contains enough compressed air for several braking operations.

From the reservoir the air is supplied to the brake valve. As long as the brake

pedal is not depressed, brake valves prevent the passage of air to brake

chambers and there is no braking effect.

When the brake pedal is released, the supply of compressed air is cut off from

the brake chambers and they are connected to the atmosphere. The pressure in

the chambers drops, the brake shoes are returned to their initial position and

wheels run free.

The brake valve is equipped with a servomechanism which ensures that the

braking force on the shoes is proportional to the force applied to the pedal.


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In addition the valve imparts a relative reaction to the movement of the

pedal so that the driver can sense the degree of brake application. A un-loader

valve is mounted in the air pressure system between the compressor and

reservoir to control the pressure of air in the reservoir. The un-loader valve

relieves the compressor of its pumping load. Once the un-loader cut out pressure

is obtained and seals the reservoir when the compressor is built up a pressure

depending upon the setting of the adjusting screw. The un-loader then delivers

the air delivered by the compressor to the atmosphere thus allowing the

compressor to run light whilst the reservoir contains an adequate supply of air.

Air filters are used in the air pressure system to prevent particles of foreign

matter from entering the operating system airlines or equipment. These are

mounted on the chassis and have a drain plug to allow the condensate to be

easily removed. The function of the air reservoir is to store the compressed air so

that there will always be an ample supply available for immediate use in brake

operation. It provides storage of sufficient compressed air to permit several brake

applications even after the engine has stopped and just restarted. It also provides

a place where the air, heated during compression may cool, and oil ands water

vapors condense.


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10.12 Vacuum Brakes

In this system, the operation of brakes utilizes the power of partial vacuum

existing in the inlet manifold of a running engine. This vacuum, via the vacuum

reservoir, is applied on one side of a large piston whose other side is exposed to

the atmospheric pressure. Since atmospheric pressure is 1.0332 kgf/cm2 (100

kPa) and the partial vacuum is less than this pressure, therefore difference in the


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pressure on two sides of the piston causes it to move within a cylinder. This

movement of the piston through a suitable mechanism can be used to apply the

brakes. An air breather port is provided in the cylinder through which the

atmospheric air escapes out when the air brakes are released.

It is not much popular as an independent unit i.e. as a purely vacuum unit.

It is there fore being used in the combination of other types of brakes. The

vacuum unit can be used with a mechanical and with a hydraulic braking system

by interposing suitably between the brake pedal and the linkage. Now a days,

the vacuum units are almost universally adopted on power brakes.

10.13 Combined Vacuum and Hydraulic power brake

When a moving vehicle is to be braked, the driver applies a touching

brake effort on the pedal. It actuates the push rod which opens the air valve

through an arrangement. The atmospheric air rushes through the air valve and

exerts pressure on the lower piston. Consequently, the power piston moves

towards the vacuum chamber. The push rod also moves with the power piston

and compresses on the piston return spring.

.


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Since it is connected to the master cylinder piston, therefore the brake piston,

therefore the brake oil is pumped to the brake lines in usual manner. The oil now

acts on wheel cylinders in a conventional way.

When vacuum brakes fail: The check valve prevents loss of vacuum in thebrakes in case of failure of engine vacuum. However, if at all the vacuum fails the

brake system will not fail instantaneously. In that case, the power brakes can be

utilized till the vacuum retained inside the vacuum reservoir lasts

Brake fade:

When the brake drum and linings heat due to braking, friction between them

becomes less. Also the brake drum expands away from the shoes. This causes

brake fade, deterioration of brake drum after a quick succession of stops or slow

downs.

Temporary loss of effectiveness of the brake during prolonged braking such as

during a long descent is called brake fade. Normally the brake regains its

efficiency when it cools again.

Difficulty in effectively cooling the shoes and the linings may cause distortion of

the brake drum due to heat. Differential thermal expansion impairs good contact

between the drum and the linings. These may result in local high pressures and

temperatures and may cause temporary loss of friction in the linings i. e. brake

fade.

If one brake goes harder than others it may fade sooner. This may cause uneven

braking and lead to a skid.


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10.14 Anti-lock or Anti-skid device:

The vehicle will stop more quickly if the brakes are applied just hard

enough to get maximum static friction between the tires and the road. If the

brakes are applied harder than this then the wheels will lock, the tires will skid orslide on the road and the lesser kinetic friction will result. Then braking the

vehicle is much less effective.

To prevent skidding and thus provide maximum effective braking several

devices have been proposed. Mostly skid control of the rear wheels only is

provided. Some others provide control at all the four wheels. As long as the

wheels are rotating the, anti-skid device permits normal application of the brakes.

But if the brakes are applied so hard that the wheels tend to stop turning and

thus a skid starts to develop the device comes into operation and partly releases

the brakes so that the wheels continue to rotate. However intermittent braking

continues. But it is held to just below the point where a skid would start. The

result is maximum braking effect.


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10.1Problems on BRAKES

c.g.

RFRF

RRRR

b

l

Wgf

WSin0

h

WCos0

W

0

Figure above shows the vehicle moving down a gradient inclined at an

angle to the horizontal. Retardation takes place when brakes are applied.

To bring the whole system in equilibrium the inertia force which is also

known as reverse effective force, is included with the system of forces

actually existing.

Let W = weight of the vehicle

RF and RR = total normal reaction between the ground and the front and

rear wheels respectively.

= coefficient of adhesion between the tyre and the road

surface

b = wheelbase

h = height of C. G. From the road surface


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l = perpendicular distance of C. G. from rear axle

f = retardation produced by the breaking force

W

fg = reverse effective force, i.e. inertia force.

Brakes may be applied (a) to the rear wheels only, (b) to the front wheels,

(c) to all the four wheels. All the three cases are treated separately.

(a) Brakes applied to the rear wheels. Referring to the figure, FR is RR.

The whole system is in equilibrium under the influence of coplanar forces.

Therefore,

RF + RR = W cos

FR =RR= W Sin +W

fg

( )


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chapter10 brakes

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