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CHAPTER 1 INTRODUCTION A brake is defined as a mechanical device, which is used to absorb the energy possessed by a moving system or mechanism by means of friction. In other words, a brake is a device by means of which artificial frictional resistance is applied to a moving machine member, in order to retard or stop the motion of a machine. The primary purpose of the brake is to slow down or completely stop the motion of a moving system, such as a rotating drum, machine or vehicle. It is also used to hold the parts of the system in position at rest. An automobile brake is used either to reduce the speed of the car or to bring it to rest. It is also used to keep the car stationary on the downhill road. The energy absorbed by the brake can be either kinetic or potential or both. In automobile application, the brake absorbs the kinetic energy of the moving vehicle. In hoists and elevators, the brake absorbs the potential energy released by the objects during the braking period. The energy absorbed by the brake is converted into heat energy and dissipated to the surroundings. This dissipation of in the surrounding air (or water which is circulated through the passages in the brake drum) so that excessive heating of the brake lining does not take place. Heat dissipation is a serious problem in brake applications. 1.1 Types of Brakes Brakes are classified into the following three groups: i.)

Mechanical brakes – Mechanical brakes are operated by mechanical means such as levers, springs and pedals. Depending upon the shape of the friction material, the mechanical brakes are classified as block brakes, internal or external shoes brakes, disk brakes and band brakes. Brakes are also classified into two groups according to the direction of the actuating force, namely, radial brakes and axial brakes. (a) Radial brakes – In these brakes, the force acting on the brake drum is in radial direction. The radial brakes may be sub-divided into external brakes and internal brakes. According to the shape of the friction element, these brakes may be block or shoe brakes and band brakes. (b) Axial brakes – In these brakes, the force acting on the brake drum is in axial direction. The axial brakes may be disk brakes and cone brakes.

ii.)

Hydraulic and pneumatic brakes – These types of brakes are operated by fluid pressure such as oil pressure or air pressure.

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iii.)

Electrical brakes – These brakes are operated by magnetic forces and which include magnetic particle brakes, hysteresis brakes and eddy current brakes.

NOTE: The hydraulic and electric brakes cannot bring the member to rest and are mostly used where large amounts of energy are to be transformed while the brake is retarding the load such as in laboratory dynamometers, highway trucks and electric locomotives. These brakes are also used for retarding or controlling the speed of a vehicle for down-hill travel Brake capacity depends upon the following factors: a.)

The unit pressure between braking surfaces

b.)

The contacting area of braking surface

c.)

The radius of the brake drum

d.)

The coefficient of friction

e.)

The ability of the brake to dissipate heat that is equivalent to the energy being absorbed.

1.2 Internal Expanding Brake Internal expanding brakes are used almost exclusively as wheel brakes but can be found on some cranes. This type of brake permits a more compact and economical construction. The brake shoes and brake-operating mechanism are supported on a backing plate or brake shield attached to the vehicle axle, as shown in figure 1.1 An external contracting brakes. The brake band is anchored opposite the point where the pressure is applied. In addition to supporting the band, The anchor allows adjustment of the brake lining clearance. Other adjusting

screws

and

bolts

provided at the ends of the band.

Fig. 1.1 External contracting transmission parking brake 2

are

The brake drum, attached to the rotating wheel, acts as a cover for the shoe and operating mechanism and furnishes a frictional surface for the brake shoes. The brake shoe of an internal expanding brake is forced outward against the drum to produce the braking action. One end of the shoe is hinged to the backing plate by an anchor pin, while the other end is unattached and can be moved in its support by the operating mechanism. Fig. 1.2 Internal expanding brake When force from the operating mechanism is applied to the unattached end of the shoe, the shoe expands and stops the wheel. A retracting spring returns the shoe to the original position when braking action is no longer required. The internal expanding brake shown in Fig. 1.3 consists essentially of three elements: the mating frictional surface, the means of transmitting the torque to and from the surfaces, and the actuating mechanism. The expanding-ring brake is often used

in

textile

machinery,

excavators, and machine tools where the brake may be located within

the

driving

pulley.

Expanding brakes benefit from centrifugal effects; transmit high torque, even at low speeds; and require both positive engagement and ample release force. Fig. 1.3 An internal expanding centrifugal-acting rim clutch. The centrifugal brake is used mostly for automatic operation. If no spring is used, the torque transmitted is proportional to the square of the speed. This is particularly useful for electricmotor drives where, during starting, the driven machine comes up to speed without shock. 3

Springs can also be used to prevent engagement until a certain motor speed is reached, but some shock may occur. Magnetic brakes are particularly useful for automatic and remote-control systems. Such clutches are also useful in drives subject to complex load cycles. Hydraulic and pneumatic brakes are also useful in drives having complex loading cycles and in automatic machinery, or in robots. Here the fluid flow can be controlled remotely using solenoid valves. These brakes are also available as disk, cone, and multiple-plate brakes. In braking systems, the internal-shoe or drum brake is used mostly for automotive applications. To analyse an internal-shoe device, refer to Fig. 1.4, which shows a shoe pivoted at point A, with the actuating force acting at the other end of the shoe. Since the shoe is long, we cannot assume that the distribution of normal forces is uniform. The mechanical arrangement permits no pressure to be applied at the heel, and we will therefore assume the pressure at this point to be zero. It is the usual practice to omit the friction material for a short distance away from the heel (point A). This eliminates interference, and the material would contribute little to the performance anyway, as will be shown. In some designs the hinge pin is made movable to provide additional heel pressure. This gives the effect of a floating shoe. (Floating shoes are not discussed in this paper, although their design follows the same general principles.)

Fig. 1.4 Internal friction shoe geometry

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CHAPTER 2 LITERATURE SURVEY 

M. Waller et al. [1] discussed wear, squeal, and fade and further developments in automatic adjustment are indicated, and a plea is made for improved oil and grease retention for hubs and axles to prevent contamination of brake linings. He considered brake testing from first principles, and a routine for road testing is suggested. He recommends that the investigation of temperature effects to be confined.



Kalpesh P. Baleja et al. [2] use different process for take less time of production and get maximum efficiency. Based on joining process of brake liner with shoe during production. This brake liner is used in internal expanding brake which is used in automobile vehicle. Various processes used for less time of production and maximum efficiency are nut and bolt assembly process, pneumatic process, hydraulic process, spring assembly process and material of construction.



Y Fuji et al. [3] describes the first attempt to utilize a dynamic friction component model in drivetrain simulations. Specifically, a dynamic band brake model is implemented to predict the up-shift behavior of a four-speed AT system under various operating conditions. Simulation results are qualitatively validated with experimental data obtained from a dynamometer test stand. The dynamic band brake model enhances the shift predictability of a drivetrain model and potentially allows analytical evaluation of shift quality and control strategy.

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CHAPTER 3 VARIOUS PHENOMENA DURING BRAKING 3.1 Energy absorbed by a brake The energy absorbed by a brake depends upon the type of motion of the moving body. The motion of a body may be either pure translation or pure rotation or a combination of both translation and rotation. The energy corresponding to these motions is kinetic energy. Let us consider these motions as follows: 1. When the motion of the body is pure translation Consider a body of mass (m) moving with a velocity reduced to

v 2 m /s

v 1 m/s . Let its velocity is

by applying the brake. Therefore, the change in kinetic energy of

the translating body or kinetic energy of translation, 1 E1= m[ v21−v 22 ] 2 This energy must be absorbed by the brake. If the moving body is stopped after applying the brakes, then

v 2=0 , and

1 E1= m (v 1 )2 2 2. When the motion of the body is pure rotation Consider a body of mass moment of inertia I (about a given axis) is rotating about that axis with an angular velocity ω2 rad / s

ω1 rad / s . Let its angular velocity is reduced to

after applying the brake. Therefore, the change in kinetic energy of the

rotating body or kinetic energy of rotation, 1 E2= I [ (ω1 ) 2 −( ω2 ) 2 ] 2 This energy must be absorbed by the brake. If the rotating body is stopped after applying the brakes, then

ω2 =0 , and

1 E2= I ( ω1 )2 2

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Fig. 3.1 Additional information of a braking system 3. When the motion of the body is a combination of translation and rotation Consider a body having both linear and angular motions, e.g. in the locomotive driving wheels and wheels of a moving car. In such cases, the total kinetic energy of the body is equal to the sum of the kinetic energies of translation and rotation. ∴ Total kinetic energy to be absorbed by the brake, E=E 1+ E2 Sometimes, the brake has to absorb the potential energy given up by objects being lowered by hoists, elevators etc. Consider a body of mass m is being lowered from a height

h1

to

h2

by applying the brake. Therefore, the change in potential

energy, E3=mg(h1−h 2) If

v1

and

v 2 m/s

are the velocities of the mass before and after the brake is

applied, then the change in potential energy is given by E3=mg Where

( v +2 v )t=mgvt 1

2

v =mean velocity=

v 1+v 2 , and 2

t=time of brakeapplication . Thus, the total energy to be absorbed by the brake, 7

E=E 1+ E2+ E3

Ft =Tangential braking force∨frictional forceacting

Let

tangentially at the contact surface of the brake drum

d=Diameter of the brakedrum , N 1=Speed of the brake drum before thebrake is applied , N 2=Speed of the brake drum after the brake is applied ,∧¿

N=meanspeed of the brake drum=

N 1+N 2 2

We know that the work done by the braking or frictional force in time t seconds ¿ Ft × π × dN ×t Since the total energy to be absorbed by the brake must be equal to the work done by the frictional force, therefore E=F t × π × dN × t

Ft =

Or The magnitude of

Ft

E π × dN ×t

depends upon the final velocity ( v 2 ) and on the braking time ( t ).

Its value is maximum when

v 2=0 , i.e. when the load comes to rest finally.

We know that the torque which must be absorbed by the brake, M t=F t ×r = F t × Where

d 2 r=radius of the brake drum

3.2 Heat be dissipated during braking The energy absorbed by the brake and transformed into heat must be dissipated to the surrounding air in order to avoid excessive temperature rise of the brake lining. The temperature rise depends upon the mass of the brake drum, the braking time and the heat

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dissipation capacity of the brake. The highest permissible temperatures recommended for different brake lining materials are given as follows: 1. For leather, fibre and wood facing =

65 – 70 °C

2. For asbestos and metal surfaces that are slightly lubricated = 3. For automobile brakes with asbestos block lining =

90 – 105 ° C

180 – 225 °C

Since the energy absorbed (or heat generated) and the rate of wear of the brake lining at a particular speed are dependent on the normal pressure between the braking surfaces, therefore it is an important factor in the design of brakes. The permissible normal pressure between the braking surfaces depends upon the material of the brake lining, the coefficient of friction and the maximum rate at which the energy is to be absorbed. The energy absorbed or the heat generated is given by E=H g=μ ∙ RN ∙ v =μ∙ p ∙ A ∙ v (¿ J /s∨watt ) Where

μ=Coefficient of friction , RN =Normal force acting at the contact surfaces ,∈newtons p=Normal pressure between the braking surfaces∈ N /m

2

A=Projected areaof the contact surfaces ∈m2 ¿ v =Peripheral velocity of the brake drum∈m/s . The heat generated may also be obtained by considering the amount of kinetic or potential energies which is being absorbed. In other words, H g =Ek + E p

Where

Ek =totalkinetic energy absorbed ,∧¿ E p=total potential energy absorbed

The heat dissipated ( H d ) may be estimated by H d =C ( t 1−t 2) A r

Where

C=heat dissipation factor∨coefficient of heat transfer ∈W /m2 /℃ t1 −t2=temperature difference between the exposed radiating surface

¿ the surrounding air ∈°C ,∧¿ 9

A r =Area of radiating surface ∈m 2 . The value of C may be of the order of

2

29.5 W /m /° C

for a temperature difference of

40 ° C and increase up to 44 W /m 2 /° C for a temperature difference of

200 °C .

The expressions for the heat dissipated are quite approximate and should serve only as an indication of the capacity of the brake to dissipate heat. The exact performance of the brake should be determined by test. It has been found that 10 to 25 per cent of the heat generated is immediately dissipated to the surrounding air while the remaining heat is absorbed by the brake drum causing its temperature to rise. The rise in temperature of the brake drum is given by ∆ t=

Hg m.c

∆ t=temperature rise of the brakedrum∈°C

Where

H g =h eat generated by the brake∈ joules , m=Mass of thebrake drum ∈kg , ∧¿ c=Specific heat for the material of thebrake drum∈J /kg ° C .

In brakes, it is very difficult to precisely calculate the temperature rise. In preliminary design analysis, the product

p∙ v is considered in place of temperature rise. The experience has also

shown that if the product

p∙ v

is high, the rate of wear of brake lining will be high and the

brake life will be low. Thus, the value of

p∙ v should be lower than the upper limit value for

the brake lining to have reasonable wear life. The following table shows the recommended values of

p∙ v as suggested by various designers for different types of service.

Table 3.1 Recommended values of Sl.No.

p∙ v Recommended value of p∙ v

Types of service

N−m / m 1.

Continuous application of load as

2

of projected area per second 0.98 ×106

in lowering operations and poor 2.

dissipation of heat. Intermittent application of load

1.93 × 10

with comparatively long periods of rest and poor dissipation of

10

in

6

3.

heat. For continuous application of load

2.9 ×10

6

and good dissipation of heat as in an oil bath. NOTE: When the temperature increases, the coefficient of friction decreases which adversely

affect the torque capacity of the brake. At high temperature, there is a rapid wear of friction lining, which reduces the life of lining. Therefore, the temperature rise should be kept within the permissible range. 3.3 Materials for brake lining The material used for the brake lining should have the following characteristics: 1. It should have high coefficient of friction with minimum fading. In other words, the coefficient of friction should remain constant over the entire surface with change in temperature. 2. It should have low wear rate. 3. It should have high heat resistance. 4. It should have high heat dissipation capacity. 5. It should have low coefficient of thermal expansion. 6. It should have adequate mechanical strength. 7. It should not be affected by moisture and oil.

Fig. 3.2 Different components of a brake The materials commonly used for facing or lining of brakes and their properties are shown in the following table. Table 3.2 Properties of material for brake lining

Cast iron on cast iron Bronze on cast iron Steel on cast iron Wood on cast iron Fibre on metal Cork on metal Leather on metal Wire asbestos on metal Asbestos blocks on metal Asbestos on metal

Coefficient of friction (μ) Dry Greasy Lubricated 0.15-0.2 0.06-0.10 0.05-0.10 0.05-0.10 0.05-0.10 0.20-0.30 0.07-0.12 0.06-0.10 0.20-0.35 0.08-0.12 0.10-0.20 0.35 0.25-0.30 0.22-0.25 0.3-0.5 0.15-0.20 0.12-0.15 0.35-0.5 0.25-0.30 0.20-0.25 0.40-0.48 0.25-0.30 0.20-0.25

N /m m2 1.0-1.75 0.56-0.84 0.84-1.4 0.40-0.62 0.07-0.28 0.05-0.10 0.07-0.28 0.20-0.55 0.28-1.1 1.4-2.1

(short action) Metal on cast iron

-

1.4-2.1

Material for braking lining

-

0.05-0.10

Allowable pressure (p)

(short action)

CHAPTER 4 CONSTRUCTION AND DESIGN OF INTERNAL EXPANDING BRAKE 12

4.1 Construction of internal expanding brake The construction of an internal expanding brake is shown in Fig. 4.1. It consists of a shoe, which is pivoted at one end and subjected to an actuating force P at the other end. A friction lining is fixed on the shoe and the complete assembly of shoe, lining and pivot is placed inside the brake drum. Internal shoe brakes, with two symmetrical shoes, are used on all automobile vehicles. The actuating force is usually provided by means of a hydraulic cylinder or a cam mechanism. The analysis of the internal shoe brake is based on the following assumptions: i.)

The intensity of normal pressure between the friction lining and the brake drum at any point is proportional to its vertical distance from the pivot.

ii.)

The brake drum and the shoe are rigid.

iii.)

The centrifugal force acting on the shoe is negligible.

iv.)

The coefficient of friction is constant.

Fig. 4.1 Internal Expanding Brake

4.2 Design of internal expanding brake The free-body diagram of forces acting on an element on the surface of the drum and the surface of friction lining is sh...