Braking system

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Automotive Brakes Prepared By : Bibhuti Bhusan Samantaray Asst. Professor, RCET 9439373223 www.godigidocs.com
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Transcript of Braking system

Automotive Brakes

Automotive BrakesPrepared By : Bibhuti Bhusan SamantarayAsst. Professor, RCET9439373223www.godigidocs.com

Braking Efficiency:Equating work & kinetic Energy Equation we find, F S = M U2 F S = W U2 / 2g (As W = M g) S = W U2 / 2 F g S = U2 / 2 g (As = F/W & W/F = 1/ )So from the Above equation Can be derived as,

= (U2 / 2 g S ) x 100Now putting the value of g = 9.81 m/s2 & converting U to km/hr find out the new Formula for brake efficiency if required.

Hydraulic BrakesThe brakes which are actuated by the hydraulic pressure (pressure of a fluid) are called hydraulic brakes. Hydraulic brakes are commonly used in the automobiles. PrincipleHydraulic brakes work on the principle of Pascals law which states that pressure at a point in a fluid is equal in all directions in space. According to this law when pressure is applied on a fluid it travels equally in all directions so that uniform braking action is applied on all four wheels.

Construction and Working of Hydraulic Brakes

When brake pedal is pressed, the force is transmitted to the brake shoes through a liquid (link). The pedal force is multiplied and transmitted to all brake shoes by a force transmission system.The previous figure shows the system of hydraulic brake of a four wheeler automobile. It consists of a master cylinder, four wheel cylinders and pipes carrying a brake fluid from master cylinder to wheel cylinder.The master cylinder is connected to all the four-wheel cylinders by tubing or piping. All cylinders and tubes are fitted with a fluid which acts as a link to transmit pedal force from master cylinder to wheel cylinders.

Brake FluidThe fluid filled in the hydraulic brake system is known as brake fluid. It is a mixture of glycerin and alcohol or caster oil and some other additives.

Master cylinder consists of a piston which is connected to peal through connecting rod. The wheel cylinder consists of two pistons between which fluid is filled.Each wheel brake consists of a cylinder brake drum. This drum is mounted on the inner side of wheel. The drum revolves with the wheel. Two brake shoes which are mounted inside the drum remain stationary. Heat and wear resistant brake linings are fitted on the surface of the brake shoes.

Application of Hydraulic BrakesWhen brake pedal is pressed to apply the brakes, the piston in the master cylinder forces the brake fluid. This increases the pressure of fluid. This pressure is transmitted in all the pipes and up to all wheel cylinders according to Pascals law.This increased pressure forces out the two pistons in the wheel cylinders. These pistons are connected to brake shoes. So, the brake shoes expand out against brake drums. Due to friction between brake linings and drum, wheels slow down and brakes are applied.

Release of BrakesWhen pedal is released, the piston of master cylinder returns to its original position due to retractor spring provided in master cylinder. Thus, fluid pressure drops to original value. The retractor spring provided in the wheel cylinders pulls the brake shoes and contact between drum and brake linings is broken. Therefore, brakes are released.

AdvantagesEqual braking action on all wheels.Increased braking force.Simple in construction.Low wear rate of brake linings.Flexibility of brake linings.Increased mechanical advantage.

Dis- AdvantageWhole braking system fails due to leakage of fluid from brake linings.Presence of air inside the tubings ruins the whole system.

Master Cylinder

Wheel Cylinder

Brake Positioning of Car With No ABS

Brake Positioning of Car with ABS

Mechanical Brakes: Types of Mechanical Brake:

Drum Brakes (Internal Expanding or External Contracting)Disc Brakes (Single or Two caliper)

Drum Brakes:

Construction: The main components of drum brakes are1. Brake drum2. Back plate3. Brake shoes4. Brake Liners5. Returning Springs6. Cam7. Brake Linkages

In this system the wheel is attached to drum. There are brake shoes used to contact the rotating drum for braking operation. The shoes provide lining on their outer surface.The cam is used to lift the brake shoes at one end, other end is connected by some method so as to make as the brake sleeve come into contact in the brake drum.The returning spring is provided for bringing the brake shoes back to its original position, after releasing the brake pedal. All these parts are fitted in the back plate and enclosed with brake drum

Real Image of Brake Shoe

A typical Drum Brake Figure

Cam-operated Drum-brakesInitially almost all the drum-brakes were cam-operated. The cam is used to force the shoes against the drum. Foot-brake shoe-expanders for cars are now mostly hydraulically operated. Today the cam-operated expander is only used in large trucks, and particularly in articulated trailers. However, cams are preferred in parking-brake mechanisms in both drum and disc layouts.The disc-type cams use a plate in the cam-type brake-shoe expander. The plate rotates about an axis perpendicular to its plane. The profile of the plate provides a reciprocating motion to its follower, the shoe, which bears against the cam edge.

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 be overcome, hence a low force ratio is needed. But when the shoes are actually pressing hard against the drum and further braking is necessary, a progressively increasing force ratio is necessary. These requirements are met through variable-movement ratio cams.

Working:When the pedal is pressed the cam moves the shoes outwards through linkages, there by coming in frictional contact with the rotating drum. As soon as the brake pedal is released the retaining springs help the brake shoes to brought back and release the brakes.

DISC BRAKES: Construction: The discs are made of gray cast Iron. The brake pressure in case of disc brakes have to be much lighter than the drum brakes.It consists of rotating disc and two friction pads which are actuated by the four hydraulic wheel pistons contain in two halves of an assembly is called a caliper.The caliper assembly is secured to the steering knuckle in a front wheel brakes. The road wheel is fashioned to the outer surface of the disc. The friction pads rides freely on each side of the discs. They are in position being the hydraulic systems.

Different Parts of Disc Brakes

Different Parts of Disc Brake

Working:When the brakes is applied hydraulic pressure is supply to the fluid inlet tube, due to which the wheel cylinder piston force the friction pads against the rotating disc. In the released piston, the spring hold the piston pads so that they maintain contact with disc surface.

Advantage of Disc BrakesMain advantage of disc brakes is their resistance to wear as the discs remain cool even after repeated brake applications.Brake pads are easily replaceable.The condition of brake pads can be checked without much dismantling of brake system.Disadvantage of Disc BrakesMore force is needed be applied as the brakes are not self emerging.Pad wear is more.Hand brakes are not effective if disc brakes are used in rear wheels also. (Hand brakes are better with mechanical brakes).

POWER BRAKESBack in the day, when most cars haddrum brakes, power brakes were not really necessary -- drum brakes naturally provide some of their own power assist. Since most cars today havedisc brakes, at least on the front wheels, they need power brakes. Without this device, a lot of drivers would have very tired legs.The Vacuum BoosterThevacuum boosteris a metal canister that contains a clever valve and a diaphragm. A rod going through the center of the canister connects to themaster cylinder's pistonon one side and to the pedal linkage on the other. Another key part of the power brakes is thecheck valve.

Vacuum Booster

Power Brake Diagram

The vacuum booster is a very simple, elegant design. The device needs avacuum sourceto operate. In gasoline-poweredcars, the engine provides a vacuum suitable for the boosters. In fact, if you hook a hose to a certain part of an engine, you can suck some of the air out of the container, producing a partial vacuum. Becausediesel enginesdon't produce a vacuum, diesel-powered vehicles must use a separate vacuum pump.On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates apartial vacuuminside the vacuum booster on both sides of the diaphragm. When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position.

Pneumatic Brakes

SERVO BRAKESIf the force applied comfortably by a driver to the footbrake is insufficient to retard the vehicle at the required rate, some form of assistance is necessary. The boosting force applied to supplement the drivers effort is called servo assistance.In the past servo assistance was provided by rotation of the brake drum (self-servo) to keep the pedal force low. Today due to introduction of powerful disc brakes, the servo assistance is provided by either pneumatic or hydraulic means. In practice, vacuum assistance is added for medium cars, hydraulic assistance for heavy cars and vehicles fitted with anti-lock braking systems, and compressed-air assistance for some light trucks and minibuses.

Vacuum Assisted ServoThis servo system is the most popular. The induction manifold depression of the spark ignition engine is used as source of servo energy in most systems. Since vacuum energy is not available at the manifold of a diesel engine, an engine-driven vacuum pump (exhauster) in this case provides the required assistance.Vacuum servos in use today are called suspended-vacuum systems, because vacuum conditions prevail on both sides of the servo piston during operation of the vehicle with the brakes off. When the brake is applied, outside air is bled in to the chamber on one side of the piston to create a pressure difference. This arrangement allows the servo to respond quickly in comparison to the older atmospheric suspended type system.

In this older system, air is present on both sides of the piston and the air is drawn out to provide assistance. The two main types of suspended-vacuum servos are the indirect and direct type.There are 2 types of Vacuum Assisted Servo- Indirect Servo System.- Direct Servo System.

Layout of A Servo System

Hydraulic Assisted ServoSince the pressure difference of a vacuum assisted servo arrangement is limited, a system to provide a much greater source of energy is necessary to stop a heavy motor car or light truck. This is achieved by incorporated a hydraulic servo, which operates on a pressure range of 5295 to 8842 kPa. The hydraulic power produced by the engine-driven pump of this system can also be used to provide for other servo needs, e.g. power-assisted steering, lifts etc.Figure below illustrates a continuous-flow hydraulic servo system. A multi-cylinder pump, driven from the engine or transmission, supplies fluid to the servo valve, mounted behind the conventional master cylinder. The brakes are in the off position in the diagram, when the fluid can easily pass between the master-cylinder piston and servo valve to a drilling connecting the reservoir.

Hydraulic Assisted Servo

Compressed-air Assisted ServoAn alternative to the hydraulic servo, used on light trucks and minibuses, is a compressed-air servo. The layout of the system is shown in Figure. This is also known as air/hydraulic (air over hydraulic) or Air Pac system as compressed air is used to boost the force applied by the driver to a hydraulically operated brake. Air pressure is generated by a engine-driven compressor, and is stored in a reservoir adjacent to the servo chamber. The servo chamber houses a piston. This piston operates another piston that controls the main hydraulic brake line. During application of the brake, a valve supplies the compressed air into the servo cylinder to boost the pedal effort.

Basic layout of air assisted hydraulic system

DESIGN OF BRAKE LINING AND BRAKE DRUM

Brake Layouts :There are two types of drum brakes, such as externally contracting and internally expanding. External contracting brakes are preferred for automatic gearboxes. The internal expanding type, along with a drum, is commonly used in braking systems of vehicles. The drum brakes, used with light vehicles, are hydraulically operated. These brakes are commonly used for the rear wheels to complement a disc system at the front. This disc/drum layout permits the front wheels to undertake more braking effort. In addition, the compatibility of a mechanical hand-brake with a drum brake makes this type an obvious option for rear brakes.

Various Brake shoe arrangements

Leading and trailing shoe (L&T) Two leading shoe (2LS) Duo-servo.

Leading and Trailing Shoe (L&T)

The layout of a leading and trailing shoe brake is presented in Figure. This arrangement uses a pair of shoes pivoted at a common anchor point. The free ends of both shoes are radially forced against the inside surface of the brake-drum using a double piston/cylinder expander.When the brake is applied with the vehicle stationary, hydraulic pressure pushes each shoe outwards and an equal force is applied by each shoe to the drum. But this applied force does not remain equal when the vehicle is moving (Fig. 28.14A).The drag of the moving drum on the friction linings causes one shoe to be applied hard and the other to be pushed towards the off position. The shoe that does more work is called the leading shoe, and the other shoe is called the trailing shoe.

The rate of lining wear of leading shoe is higher as it does more work than the trailing shoe. Therefore, leading shoe reaches its wear limit well before the trailing shoe, unless a thicker lining is used. If the direction of the drum is reversed, the leading shoe, however, is converted into a trailing shoe.

Leading- and trailing-shoe drum-brake

Leading and trailing shoe brake arrangement

Two Leading Shoe Brake (2LS)

The 2LS system was in use for front brakes of vehicles before the adoption of the disc brake system. Each shoe of the 2LS arrangement uses its own expander; therefore both shoes can have self-servo action (Figure). An interlinking pipe fitted behind the back-plate provides an equal hydraulic pressure to each single-acting cylinder. Since the cylinder housings act as shoe anchors for the floating shoes, the cylinders are rigidly fixed to the back-plate.

Two leading shoe brake

2LS shoe brake offers the following advantages over L&T brakes:Even lining wear. Because both shoes perform an equal amount of work, the brake runs cooler needing relatively less adjustment and has a long life.Equal self-servo action. Two effective shoes provide a more powerful and stable brake.Greater resistance to fade. Since both the shoes share the braking equally, the self-servo action on this shoe can be reduced so that a more progressive braking action, which is less sensitive to heat, is achieved.One disadvantage of the 2LS type is that unless a special double-acting linkage is incorporated, both shoes change to trailing shoes during backward movement of the car.

Duo-servo Brake

This brake arrangement is also known as the self-energizing brake. Although this is a very powerful brake, its effectiveness reduces severely with the decrease in the friction value. A hydraulically operated duo-servo brake is shown in Figure. The principle of operation is based on the utilization of drum energy to considerably boost the force applied on the brakes by the driver.When the leading shoe is pushed to contact the forward-moving drum, it rotates partially with the drum due to the frictional force. This shoe movement, produced by this self-wrapping action, is conveyed through a floating adjuster to the trailing shoe so that the shoe is brought into contact with the drum. The force applied by the expander is supplemented by the self-energizing action of both shoes.

To minimize the delay in application of the self-energization action, the trailing shoe is held on the anchor pin by a stronger return spring so that the expander only moves the leading shoe. In this arrangement, the leading shoe is called the primary shoe, because this shoe is made to contact the drum before the secondary shoe.

Duo-servo brake