BRAKE SYSTEM

Brake System

Types of Brakes: The two main types of friction brake are:• Drum Brakes– Hydraulic operated drum brakes– Cam operated drum brakes

• Disc Brakes

Drum Brake System

Drum Brake System• This internal expanding type of brake contains two shoes that

are attached to a back-plate (aka “torque plate”) and are fixed to a stub axle.

• A friction lining is riveted or bonded to the outer face of the shoe.

• A mechanism is fitted at the one end of the shoe so that the shoe expands when the brake pedal is applied.

• The shoe anchor is rigidly attached to the back-plate and takes the form of large pin that passes through the shoes, or housing. The shoe butts against the anchor.

• Strong springs are generally used, which pull the brake shoes on the back plate and also return the shoes to the “off” position after the brake has been applied and released.

Drum Brake System• The inner cylindrical surface of the (cast iron) drum is made

smooth onto which the brake linings rub.• The drum is generally fixed to the hub flange using studs and

secured by the wheel nuts. • It is necessary to adjust the clearance between friction facing

and drum internal face. This is carried out either manually adjusting the brakes periodically or having an automatic adjuster.

Drum Brake Layouts

Drum BrakeHydraulic-operated Drum-brake System

• The drum-brake arrangement is shown in fig.

• To expand the shoes hydraulically, two separate pistons are fitted in a wheel cylinder fixed rigidly to the back-plate.

• An anchor pin, rigidly mounted on the back-plate, arrest the rotation of the with drum and resists the full braking torque.

• A strong spring, secured on the back-plate side of the shoe, returns the shoes (so also fluid) when brake is released.

Drum BrakeCam-operated Drum-brake System

• The cam is used to force the shoes against drum.• Initially almost drum brakes were cam operated, but now are

only preferred in parking brake mechanisms.• The shoe-expander provides a suitable force ratio between the

input effort and the output brake shoe load. • During initial period of braking, only the tension of the pull-

off springs and friction in the mechanism are required to overcome, hence a low force ratio is required.

• When shoes are actually pressing hard against the wall of drum and further braking is necessary. (i.e. progressively increasing force ratio is required)

Drum BrakeCam-operated Drum-brake System

Types of (variable movement ratio) cams used are:1. Flat and oval cams2. S cam shoe expander3. Strut and cam brake shoe expander (fixed and floating type)

S cam shoe expanderFlat and oval cams

Working of S cam shoe expander

Working of Strut and cam brake shoe expander

Disc Brakes• The disc-brake basically consists of

a rotating circular plate disc attached to and rotated by wheel hub and a bridge member known as caliper.

• The caliper straddles the disc and is attached to the suspension carrier, stub axle or axle casing.

• The caliper contains a pair of pistons and friction pads which, when brakes are applied clamp the rotating disc.

• Reduction in speed proportional to the hydraulic pressure acting on each piston produced by the pedal effort is achieved.

Disc Brake

Disc Brake• A disc brake has a rotating cast iron disc, bolted to the wheel hub

and a stationary caliper unit.• The caliper, made of cast-iron in two halves, astride the disc and

is bolted to the stub-axle.• Each half of the caliper forms a separate cylinder block with the

cylinder axis perpendicular to the disc.• The two cylinders are connected together by drillings at the

pressure faces of the two caliper halves near to the inlet port.• Each cylinder uses a rubber sealing in the form of a ring located

in a groove in the body and dust-cover protects the hollow piston.• A friction pad in the form of a segment is bonded to a steel plate

and is sandwiched between each piston and disc face. These pads fit into slots formed in each half of the caliper housing. Held in position by retaining pins or spring plates

Disc Brake• The application of the brake pedal causes hydraulic pressure to be

transmitted in all the directions.• The caliper pistons and friction pads apply equal and opposite

forces on the rotating disc in direct proportion to the applied effort.

• Once the brake (force) are released, the hydraulic pressure collapses and the distorted rubber seal retracts the piston and pad to clear the faces from frictional contact.

• Since most of the frictional contact surface is exposed to the atmosphere, dissipation of heat is effective as compared with the drum-brake arrangement.

Hydraulic Braking System

Hydraulic single-line braking system

Hydraulic Braking System

Dual- or Split-line braking system• Dual- or Split-line braking systems are used on all cars and

vans to continue to provide some degree of braking if one of the two hydraulic circuits fail.

• A tandem master cylinder is incorporated in the dual-line braking system, which has two separate master cylinder circuits placed together end on so that it can be operated by a common push rod and foot pedal.

• If there is fault in one of the hydraulic circuits, the other pipe will be unaffected and therefore will still actuate the (caliper/drum) brake cylinders.

Dual- or Split-line braking system

1. Front to rear brake line split:• With this arrangement, the two

separate hydraulic pipe lines of the tandem master cylinder are in circuit with either both the front or rear caliper or shoe expander cylinders.

• Roughly 2/3rd of the braking power is designed to be absorbed by the front calipers, and 1/3rd by the rear brakes.

Dual- or Split-line braking system3. Triangular Front to rear brake

split:• This hydraulic pipe line system uses

front calipers which have two independent pairs of cylinders, and at the rear conventional calipers/drum brakes.

• Each fluid pipe line circuit supplies half of each front caliper and one rear caliper/drum brake cylinder.

• Thus a leakage in one or the other hydraulic circuits will cause the two pairs of caliper cylinders and one rear brake cylinder to provide braking equal to about 80% of that which is possible when both the circuits are operating.

Dual- or Split-line braking system

2. Diagonally Front to rear brake split:

• To enable the braking effort to be more equally shared between each cylinder circuit (if fault occur in one of these lines), the one front and one diagonally opposite rear wheel are connected together.

• Each hydraulic circuit therefore has the same amount of braking capacity and the ratio front to rear braking proportions do not influence the ability to stop.

Components of Hydraulic Drum Brake

Wheel Cylinder The wheel cylinder consists of a

number of components as illustrated. One wheel cylinder is used for each

wheel. Two pistons operate the shoes, one at each end of the wheel cylinder.

When hydraulic pressure from the master cylinder acts upon the piston cup, the pistons are pushed toward the shoes, forcing them against the drum.

When the brakes are not being applied, the piston is returned to its original position by force of the brake shoe return spring.

Components of Hydraulic brake System

Master Cylinder:• The master cylinder converts the motion of the brake

pedal into hydraulic pressure. • It consists of the reservoir tank, which contains the

brake fluid; and the piston and the cylinder which generate the hydraulic pressure.

• The reservoir tank is made mainly of synthetic resin, while the cylinders are made of cast iron or aluminum alloy.

Components of Hydraulic brake System

Master Cylinder Construction:

• The master cylinder has a single bore separated into two separate chambers by the primary and secondary pistons.

• On the front of the master cylinder primary piston is a rubber piston cup, which seals the primary circuit of the cylinder. Another piston cup is also fitted at the rear of the primary piston to prevent the brake fluid from leaking out of the rear of the cylinder.

• At the front of the secondary piston is a piston cup which seals the secondary circuit. At the rear of the secondary piston the other piston cup seals the secondary cylinder from the primary cylinder. The primary piston is linked to the brake pedal via a push rod.

Components of Hydraulic brake System

Master Cylinder

Normal Operation:• When the brakes are not applied, the piston cups of

the primary and secondary pistons are positioned between the Inlet port and the Compensating port. This provides the passage between the cylinder and the reservoir tank.

• The secondary piston is pushed to the right by the force of secondary return spring, but prevented from going any further by a stopper bolt.

Master Cylinder

• When the brakes are depressed, the primary piston moves to the left. The piston cup seals the compensating port blocking the passage between the primary pressure chamber and the reservoir tank.

• As the piston is pushed farther, it builds hydraulic pressure inside the cylinder & is applied/transmitted to the wheel cylinder in that circuit.

• The same hydraulic pressure is also applied to the secondary piston. Hydraulic pressure in the primary chamber moves the secondary piston to the left also.

• After the compensating port of the secondary chamber is closed, fluid pressure builds and is applied to the secondary circuit.

Master Cylinder

Master Cylinder

Brake release:• When the brake pedal is released, the pistons are returned to

their original position by the force of the return springs. • However, because the brake fluid doesn’t return to the

master cylinder immediately, the hydraulic pressure inside the cylinder drops momentarily.

• As a result, the brake fluid inside the reservoir tank flows into the cylinder via inlet port, through small holes provided at the front of the piston, and around the piston cup.

• This design prevents vacuum from developing and allowing air to enter at the wheel cylinder.

Master Cylinder

Brake Fluid

• Brake fluid is specifically designed to be compatible with its environment of high heat, high pressure and moving parts.

• Standards for brake fluid have been established by SAE and DOT ( Dept. of Transportation). Requirements of fluid used in automotive brake application must :– Remain viscous– Have high boiling point– Act as lubricant for moving parts.

• The Federal Motor Vehicle Safety Standard (FMVSS) states that by law, brake fluid must be compatible regardless of manufacturer.

Brake Fluid

Brake fluid types: Two types of brake fluids are used in automotive brake applications.

1. Polyglycol: amber in color Most common brake fluid used in the industry It is a solvent and will immediately begin to dissolve paint. Hygroscopic ( -ve characteristic) i.e. it has propensity to water.

2. Silicone: Purple in color Not hygroscopic and therefore has virtually no rust and corrosion

problems. High boiling point and can be used in higher heat applications. It will not harm paint when it comes in contact with it. Silicone has greater affinity for air than Polyglycol (so more

difficult to bleed air).

Pad and Shoe Linings

There are two friction surfaces in any brake assembly. The brake pad or shoe lining, which are made of a mixture of heat resistant materials, and the cast iron rotor or drum. • Characteristics of Pad and Shoe Linings: Materials used must have about the same coefficient of

friction when either hot or cold. They must also resist fading at high temperatures. Brake lining materials must also be able to stop when the

linings are wet, and recover quickly when dried out. They must stop the vehicle smoothly and quietly, and, Lasts for tens of thousands of miles and not cause

excessive wear to the rotor/drum.

Pad and Shoe Linings

Lining Materials:• Brake linings use various metals and high temperature synthetic

fibers such as Kevlar and other heat resistant compounds.• Some vehicles use linings made from organic or completely

non-metallic materials.• Non-metallic lining materials are quieter (less prone to

squeaking) and do not damage the cast iron drums or rotors.• But, they provide lowest co-efficient of friction. (therefore least

amount of braking power)• Most brake linings are semi-metallic linings.• Semi-metal lic materials are made from a combination of non-

metallic materials and iron, mixed and molded into the proper shape.

Pad and Shoe Linings

• Semi-metallic linings are harder and last longer. • They are also more fade resistant than completely non-metallic

linings.• However, semi-metallic linings increase brake pedal effort,

may squeak on application, and• Cause some wear to the rotors and drums. • Metallic brake linings are used on high performance vehicles.

They are made from sintered metal.

• Disc brake linings are bonded

(glued) or riveted to a

metal backing pad.

• The attachment method is

not an indicator of quality, since

both methods are widely

used.

• Drum brake linings are also

bonded or riveted to a metal

shoe

Factors Affecting Operation of Brakes

• Applied pressure: The more pressure applied to the brake friction mem

bers, the more they resist movement, and the more friction is developed. More friction means more braking action.

Pressure is created by a combination of mechanical lever age and hydraulic pressure, plus the action of the power assist unit.

Factors Affecting Operation of Brakes

• Friction material temperature: The temperature of the friction materials has a great effect

on the amount of friction developed. As the friction material gets hotter, its ability to stop the vehicle is reduced.

Not only must friction materials be designed to operate under greatly varying temperatures, they must have roughly the same coefficient of friction, both cold and hot.

Too much variation means the brake pedal feel and required pressure would change drastically as the brakes become heated.

Factors Affecting Operation of Brakes

• Friction material contact area: Although a small braking surface could produce as much

friction as a larger surface by being applied harder, it would quickly overheat and become useless.

Brake friction members must be large enough to absorb and spread the frictional heat out.

• Friction material finish: The finish, or smoothness of the friction materials has an effect

on the vehicle's braking ability. A rough brake surface would have a higher coefficient of

friction, but would grab and wear quickly. Modern brake lining mate rial develops a smooth finish as it is

used. As the top layer of material wears away, the underlying surface maintains the smooth finish.