Internal Expanding Brake

I nternal Expandin g Br akes akes

Internal Expanding Brakes 1

INTRODUCTION Internal expanding brakes are used almost exclusively as wheel brakes, but can

be found on some cranes. c ranes. This type t ype 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. 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. When force from the operating mechanism is applied to the unattached end of the shoe, the shoe expands and brakes the wheel. A retracting spring returns the shoe to the original position when braking action is no longer required.

Dept. of Mechanical Engineering

1

I nternal Expandin g Br akes

2

DISC BRAKES The disc brake has a metal disc (rotor) and a pair of flat brake pads instead of a

drum and curved brakeshoes.

Figure.-Internal expanding brake.

Figure-Sectional view of a disc brake. Figure shows a sectional view of a disc brake assembly. The two flat pads are on the two sides of the disc. The assembly in which the flat pads are held is the caliper assembly. In operation, the pads are forced against the two sides of the disc by the movement of the pistons in the caliper assembly. The pistons are actuated by hydraulic pressure from the master cylinder. The effect is to clamp the rotating disc between the stationary pads, as shown in figure. Dept. of Mechanical Engineering

2


3

MECHANICAL PARKING BRAKE In most vehicles, a hand lever or foot pedal engages the parking brake.

The parking brake has its own system and can be either an external contracting brake bands on the drive shaft or a mechanical linkage that works the rear wheel brakes .

4

HYDRAULIC BRAKE SYSTEM A hydraulic brake system is primarily a liquid connection or coupling between

the brake pedal and the individual brake shoes and drums, as shown in figure

Figure-Parking brake configurations. The system consists of one master cylinder connected by pipes and flexible tubing to the wheel cylinders. The wheel cylinders control the movement of the brake shoes at each wheel. When the brake pedal is depressed, the hydraulic fluid forces the pistons in the wheel cylinder against the brake shoes, forcing the shoes against the brake drum or brake discs stopping the wheels.


3


Hydraulic brakes are self-equalizing brakes. If the actuating pistons were all the same size, each brake in the hydraulic system would receive an identical hydraulic force when the brakes were applied, because a force exerted at any point upon a closed liquid is distributed equally through the liquid in all directions at the same time. All brake systems have larger wheel cylinders in the front than in the rear. When you stop a vehicle, more weight is automatically shifted forward due to inertia, so more front-wheel braking is required. The master cylinder is a reservoir for the brake fluid and contains pistons and valves which change mechanical force to hydraulic pressure when the brake pedal is depressed, as shown in figure 3-43. The pressure on the brake pedal moves the piston within the master cylinder to force the brake fluid from the master cylinder through tubing and flexible hoses to the wheel cylinders. As pressure on the pedal is increased, greater hydraulic pressure is built up within the brake cylinders, and thus greater force is exerted against the ends of the brake shoes. When pressure on the pedal is released, the retracting springs on the brake shoes return the wheel cylinder pistons to their released positions. This action forces the brake fluid back through the flexible hose and tubing to the master cylinder.

Figure-Hydraulic brake system. Dept. of Mechanical Engineering

4


The operation of a dual system master cylinder is basically the same as a single master cylinder. However, the dual system master cylinder has two separate hydraulic pressure systems. One of the hydraulic systems normally is connected to the front brakes and the other system to the rear brakes. If either the front or rear hydraulic system fails, the other system remains operational. The master cylinder, like other parts in the brake system, is subject to wear, leaks, and deposits or corrosion on the cylinder wall and piston. Part of your prestart operation is to check the cylinder reservoir fluid level and add clean brake fluid to maintain the manufacturer's specifications. The brake lines transmit fluid and pressure from the master cylinder to the wheel cylinders, which are mounted on the brake-backing plate, and change the hydraulic pressure into mechanical force. Inside each cylinder are two pistons that move in opposite directions by hydraulic pressure which pushes the brake shoes against the brake drum or disc. The brake shoes are made of steel that transmits force to the lining which is attached to the face of the shoe and makes contact with the brake drums or discs. During contact with one another, the lining and the drum or disc create the frictional surface that gives the braking effect.


5


5

INTERNAL EXPANDING BRAKE

An internal expanding brake consists of two shoes S1 and S2. The outer surface of the shoes are lined with some friction material (usually with Ferodo) to increase the coefficient of friction and to prevent wearing away of the metal. Each shoe is pivoted at one end about a fixed fulcrum O1and O2 and made to contact a cam at the other end. When the cam rotates, the shoes are pushed outwards against the rim of the drum. The friction between the shoes and the drum produces the braking torque and hence reduces the speed of the drum. The shoes are normally held in off position by a spring . The drum encloses the entire mechanism to keep out dust and moisture. This type of brake is commonly used in motor cars and light trucks.

We shall now consider the forces acting on such a brake, when the drum rotates in the anticlockwise direction. It may be noted that for the anticlockwise direction, the left hand shoe is known as leading or primary shoe while the right hand shoe is known as trailing or secondary shoe.


6


Consider a small element of the brake lining AC subtending an angle δθ at the centre. Let OA makes an angle θ with OO1. It is assumed that the pressure distribution on the shoe is nearly uniform, however the friction lining wears out more at the free end. Since the shoe turns about O1, therefore the rate of wear of the shoe lining at A will be proportional to the radial displacement of that point. The rate of wear of the shoe lining varies directly as the perpendicular distance from O1to OA, i.e. O1B. From the geometry of the figure,

and normal pressure at A


7


6

ADVANTAGES OF INTERNAL EXPANDING BRAKE 1. It has simple construction with small number of parts. It is cheaper compared with other types of brakes. 2. It is more reliable due to small number of parts. 3. It requires little maintenance. 4. In an internal expanding brake, a small actuating force can produce a large braking

torque.

It offers protection against entry of foreign particles.

7

DISADVANTAGES

OF

INTERNAL

EXPANDING

BRAKE 1. It has relatively poor heat dissipating capacity. 2. It becomes self-locking due to wear, if not properly designed.


8


8

CONCLUSION The function of brakes is to slow down or stop the vehicle as and when

necessary. (Footbrake) They must also operate to hold the vehicle stationary and prevent it moving off accidentally from rest. (Handbrake). The road wheels of a vehicle may be braked directly by brakes acting on a drum attached to the wheel, or indirectly by brake acting upon transmission. (Transmission brake, normally only used as a handbrake) or via drive shafts (in board brakes). The control of brakes is affected in several ways. One system is to use a form of linkage consisting of shafts, levers, rods or cables which connect to a foot or hand operated pedal or lever. In addition there are hydraulic operations, vacuum servo assisted and pressure operated systems.


9


9

REFERENCES

http://sounak4u.weebly.com/internal-expanding-brake.html http://www.uniquecarsandparts.com.au/auto_terms_mechanical_brakes.htm http://sounak4u.weebly.com/internal-expanding-brake.html http://enginemechanics.tpub.com/14081/css/14081_86.htm


10


10 TABLE OF CONTENTS 1

INTRODUCTION .................................................................................................. 1

2

DISC BRAKES ...................................................................................................... 2

3

MECHANICAL PARKING BRAKE .................................................................... 3

4

HYDRAULIC BRAKE SYSTEM ......................................................................... 3

5

INTERNAL EXPANDING BRAKE ..................................................................... 6

6

ADVANTAGES OF INTERNAL EXPANDING BRAKE ................................... 8

7

DISADVANTAGES OF INTERNAL EXPANDING BRAKE ............................ 8

8

CONCLUSION ...................................................................................................... 9

9

REFERENCES ..................................................................................................... 10


11

Internal Expanding Brake

Recommend Documents