Introduction to brakes and its classification

Theory Of MachinesTheory Of Machines

Introduction and types of Brakes

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Prepared byPrepared by

1. Navroz Navodia2. Kishan desai3. Rahul Vyas

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Introduction to BrakesIntroduction to Brakes

A brake is a device by means of which an artificial resistance is applied to a moving body in order to retard or stop the motion of body.

The brake is a friction device for converting the kinetic energy of the moving vehicle into heat by means of friction.

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Types of Brakes: Types of Brakes:

Main types of brakes are:

1. Mechanical brakes2. Hydraulic brakes3. Disc brakes4. Pneumatic brakes5. Vacuum brakes6. Electric brakes

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I.I. Mechanical BrakesMechanical Brakes

1. Block or shoe brakes: This brakes consist of blocks which are pressed against the surface of a rotating drum.

2. Band brakes: This brakes consists of rope, belt or flexible steel band lined with frictional material which is wrapped partly round the drum.

3. Band and block brakes: A band and block brake consist of number of wooden blocks fixed inside a flexible steel band.

4. Internal expanding brakes: They consist of two semicircular shoes pivoted at the fixed fulcrum respectively.

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Single block brakeSingle block brake

•Single block or shoe brake consists of block or shoe which is pressed against a rotating drum by means of a lever as shown in figure. This type of a brake is commonly used on railway trains and tram cars.•The friction between the block and the wheel causes a tangential braking force to act on the wheel, which retard the rotation of the wheel. The block is pressed against the wheel by a force applied to one end of a lever to which the block is rigidly fixed. The other end of the lever is pivoted on a fixed fulcrum O. Let, P = Force applied at the end of the lever, RN = Normal force pressing the brake block on the wheel, r = Radius of the wheel,

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2θ = Angle of contact surface of the block, µ = Coefficient of friction, and Ft = Tangential braking force or the frictional force acting at the contact surface of the bloc k and the wheel.

If the angle of contact is less than 60°, then it may be assumed that the normal pressure between the block and the wheel is uniform. In such cases, tangential braking force on the wheel,Ft= µ.RNand the Braking Torque, TB = Ft.r = µRN. R

Let us now consider the following three cases :


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Single block brakeSingle block brake CASE I : When the line of action of tangential braking force (Ft)

passes through the fulcrum O of the lever, and the brake wheel rotates clockwise as shown in Fig.

Then for equilibrium; taking moments about the fulcrum O, we have,

It may be noted that when the brake wheel rotates anticlockwise, then the braking torque is same.

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Case 2 : When the line of action of the tangential braking force (Ft) passes through a distance ‘a’ below the fulcrum O.

The brake wheel rotates clockwise, then for equilibrium, taking moments about the fulcrum O,


When the brake wheel rotates anticlockwise, then for equilibrium, taking moments about the fulcrum O,

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Case 3 : When the line of action of the tangential braking force passes through a distance ‘a’ above the fulcrum, and the brake wheel rotates clockwise

then for equilibrium, taking moments about the fulcrum O, we have

• When the brake wheel rotates anticlockwise, then for equilibrium, taking moments about the fulcrum O, we have


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Pivoted Block or Shoe BrakesPivoted Block or Shoe Brakes

If the angle of contact between block or shoe brake drum is more than 60° then pressure at the ends than the centre. In this case it is assumed that wear is not uniform.

To get uniform wear in direction of applied force ,block or shoe is pivoted to the lever instead of being rigidly attached as in figure.

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Double block brakeDouble block brake

When a single block brake is pressed against the rotating drum , an unbalanced load on the bearing of the shaft supporting the drum will act due to normal reaction , this unbalanced load produces bending of shaft.

It is prevented by using a double block brake having two blocks on the two sides of the drum. Upper ends of brake arms are held together using spring.

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Band BrakesBand Brakes They consists of a rope , belt or steel band lined with frictional

material which is wrapped party round the drum. When band is pressed against the external surface of drum , the

frictional force between drum and band will induce braking torque on the drum.

These brakes are of 2 types which are as follows :-1. Simple band brake.2. Differential band brake.

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Simple band brakeSimple band brake A simple band brake is shown in figure .in this one end of the

band is attached at the fulcrum of the lever while the other end is at some distance from the fulcrum.

The band brake may be lined with blocks of wood or other material. The friction between the blocks and the drum provides braking action. Let there are ‘n’ number of blocks, each subtending an angle 2 θ at the centre and the drum rotates in anticlockwise direction.

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In a differential band brake, neither end of the band is attached to the fulcrum of the lever. The two ends of band are attached to the two points on opposite side of the fulcrum as in figure.

Differential band brakeDifferential band brake

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Band and Block brakeBand and Block brake A band and block consist of number of wooden blocks fixed inside a

flexible steel band . When the brake is applied, the blocks are pressed against the drum. The friction between block and drum provides the braking action.

Consider one of the blocks (say first block). This is in equilibrium under the action of the following forces :1. Tension in the tight side (T1),2. Tension in the slack side (T1') or tension in the band between the first and second block,3. Normal reaction of the drum on the block (RN), and4. The force of friction (µ.RN).

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• Let there are ‘n’ number of blocks, each subtending an angle 2 θ at the centre and the drum rotates in anticlockwise direction.

• Resolving the forces radially, we have

Resolving the forces tangentially, we have

Dividing above equations:

• Similarly it can be proved for each of the blocks that

Band and Block brakeBand and Block brake

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• Braking torque on the drum of effective radius re,

• The advantage of providing wooden blocks is that it provides a higher coefficient of frictional and they can be easily replaced after being worn out , because of higher coefficient of friction of wooden block, effectiveness of this brake increases.

Band and Block brakeBand and Block brake

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Internal expanding shoe brake consist of two semicircular shoes pivoted at the fixed fulcrum respectively as in figure.

One end of the shoe is pivoted at fulcrum while the other end is subjected to the actuating force . Internal expanding shoe brake is used in motor cars and light trucks

Internal expanding brakeInternal expanding brake

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Hydraulic BrakeHydraulic Brake Main components of a hydraulic brake system are as

under:1. Brake pedal.2. Fluid reservoir.3. Master cylinder.4. Wheel cylinder.

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Working PrincipleWorking Principle To bring vehicles to stop, its kinetic energy must be

changed in to heat energy. When the brakes are to be applied, the driver presses

the pedal , the piston is forced into the master cylinder, this increasing the pressure equally to each hydraulic system.

The pressure is conducted instantaneously to the wheel cylinders on each of the four brakes , where it forces the wheel cylinder pistons outwards. The piston in turn, force the brake shoe out against the brake drums. Thus, the brakes are applied.

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Disc BrakesDisc Brakes

In a disc brake system, as the brake pedal is pressed or brake lever is squeezed, a pushrod exerts force on the piston in the master cylinder causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port which results in an increase in the pressure of the entire hydraulic system.

This forces fluid through the hydraulic lines toward one or more calipers where it acts upon one or two additional caliper pistons secured by one or more seated O-rings which prevent the escape of any fluid from around the piston.

The brake caliper piston then apply force to the brake pads. This causes them to be pushed against the spinning rotor, and the friction between the pads and the rotor causes a braking torque to be generated, slowing the vehicle.

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Subsequent release of the brake pedal or lever allows the spring within the master cylinder assembly to return that assembly’s piston back into position.

This relives the hydraulic pressure on the caliper allowing the brake piston in the caliper assembly to slide back into its housing and the brake pads to release the rotor.

Disc BrakesDisc Brakes

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In this type of braking system, the air pressure is converted into hydraulic pressure. Here, the air power cylinder is combined with the hydraulic master cylinder and the reservoir.

The conventional type hydraulic brakes are actuated by the air power with the help of this unit . The bore of the power cylinder is generally kept four times that of the master cylinder.

The ratio between hydraulic pressure and air pressure is generally maintained at 15:1.

In India, the commercial vehicles manufactured by m/s. Tata Engg. And Locomotive Co., have air hydraulic brakes.

Pneumatic (or air) brakesPneumatic (or air) brakes

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Working Principle:• The compressor takes air from the atmosphere through

the filter and the compressed air is sent to the reservoir through the non-return valve, which gets lifted at a predetermined reservoir pressure and relives the compressor or load.

• From the reservoir the air goes to various accessories and also to the brake chambers also called the diaphragm units at each wheel, through the brake valve. The control of the brake valve is with the driver who can control the intensity of braking according to the requirements.

• When the brakes are applied , the air pressure in the reservoir decreases.


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Thank you

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Engineering

Introduction to brakes and its classification