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Thursday, December 23, 2010

Braking system components

  • Park brake system
  • Brake pedal
  • Brake lines
  • Brake fluid
  • Bleeding
  • Master cylinder 
  • Divided systems
  • Tandem master cylinder
  • Power booster or brake unit
  • Hydraulic brake booster
  • Electrohydraulic braking (EHB)
  • Applying brakes 
  • Brake force
  • Brake light switch

Park brake system

Park brakes

All vehicles must have at least two independent systems.

The primary components of the park brake system are:

Park brake system
Parking brake cables

Parking brake cable
The mechanical park brake is normally hand operated by a brake handle located on the vehicle floor or under the dash. In either position, it is in easy reach for the driver to apply.

Some manufacturers have used a foot-operated lever for the driver to apply the park brake.

Disc brake caliper fitted with park brake

Disc brake caliper fitted with park brake
On vehicles fitted with 4 wheel disc brakes, the park brake is applied at the rear caliper by the cable-operated system.

Park brake lever on drum brakes

Park brake lever on drum brakes
For passenger cars fitted with drum brakes, the park brake passes through the backing plate.

Park brake on drum brake
After passing through the backing plate, the cable is connected to the park brake lever. As the driver applies the park brake, the cable will pull on this lever and force the brake shoes into contact with the brake drum.

Brake pedal

Brake pedal
The brake pedal uses leverage to transfer the effort from the driver’s foot to the master cylinder.

Different lever designs can alter the effort the driver needs to make. This is achieved by the utilization of mechanical advantage. This uses a second order level to gain its mechanical advantage or ratio.

The pedal is solidly mounted to the firewall, and works as a force-multiplying lever. If the power assist fails, the pedal's leverage is designed to allow the driver to still generate thousands of pounds of pressure at each wheel cylinder.

Brake pedals should be mounted securely and free from any excessive sideplay. As the brake pedal is attached to a master cylinder, manufactures endeavour to keep the master cylinder reservoir level higher than the horizontal plane of the brake stations to prevent/reduce the possibility of excessive drain back of braking fluid.

Brake pedals should be free to return when no pressure is being applied, allowing the master cylinder pushrod to return to its undepressed position. Binding in the pedal operation will lead to brake problems and could cause the brakes not to work efficiently under emergency stopping requirements.

Pedal height and angle is important for comfort and safety. Consideration must be given to the functionality of the driver are being able to lift their foot from the gas pedal to brake pad quickly. Correct spacing between gas, brake and clutch pedals is vital when the driver has to make a split decision to stop their vehicle.

Problems That Can Be Identified From the Pedals

Pedal pulsation, excessive pedal travel, a “soft” or “hard” pedal can be indicators of serious problems, including a leak in the hydraulic system, low fluid levels, or unevenly worn shoes or pads. Vehicles fitted with ABS brakes, it can be reassuring to feel that ABS-connected brake pedal pulsating beneath the foot during a full brake application, there is a reason to suspect a potential problem if the driver get the same pedal pulsation with a light to medium braking application.

Brake Light Switching

In vehicles fitted with ABS brakes, considerable pulsing can be felt through the pedal during a full brake application, however there is a reason to suspect a potential problem if the driver experiences the same pedal pulsation with a light to medium braking application.

Important

If there is any change in the "feel" of your brake pedal when applied in its normal mode, this could signal a potentially serious fault developing in the hydraulic system.

Brake lines

Brake lines
Brake lines carry brake fluid from the master cylinder to the brakes. They are basically the same on all brake systems.

For most of their length they are steel, and attached to the body with clips or brackets to prevent damage from vibration.

A flexible section must be included between the body and suspension to allow for steering and suspension movement.

These flexible lines are made of reinforced tubing, to protect them from objects that could be thrown by the tires.

In some vehicles, the brake lines are inside the vehicle to protect them from corrosion.

Background

For many years the tubing in automotive brake systems has been manufactured from low-carbon steel. Although coating composition has changed since the original hot-dip lead-tin coatings were used, coating flaws remain a problem. Current aluminum-zinc coatings and added polyvinylfloride coatings are still inadequate to totally protect the steel tube.

Brake Lines

The brake fluid travels from the master cylinder to the wheels through a series of steel tubes and reinforced rubber hoses. Rubber hoses are only used in places that require flexibility, such as at the front wheels, which move up and down as well as steer. The rest of the system uses non-corrosive seamless steel tubing with special fittings at all attachment points. If a steel line requires a repair, the best procedure is to replace the complete line. If this is not practical, a line can be repaired using special splice fittings that are made for brake system repair. You must never use brass "compression" fittings or copper tubing to repair a brake system. They are dangerous and illegal.

Brake fluid

Brake fluid
Brake fluid is hydraulic oil that has specific properties. These properties have been tested by governmental regulators who have graded them in accordance with their predetermined compliance data set by the Department of Transportation (DOT).

These fluids have been designed to withstand varying climatic conditions such as:
cold temperatures without thickening, as well asvery high temperatures without boiling.
NOTE: If the brake fluid should boil, it will cause the vehicle to experience a spongy pedal and the vehicles stopping ability will be compromised.

The current standard is DOT-3 which has a boiling point of 460ยบ F. But check your owner’s manual to see what your vehicle manufacturer recommends as to the rating for brake fluid.

Brake fluid must maintain a very high boiling point.Exposure to air will cause the fluid to absorb moisture which will lower that boiling point. Brake fluid is hygroscopic, which means, it absorbs water over time.

The two most common brake fluids used in the automotive industry are fluids that contain Polyalkylene Glycol Ether and fluid that contains Silicone or Silicium-based Polymer. Both Fluids are common but very different in regards to the manner in which they perform.

The most common rated brake fluid used in automobiles has been DOT 3 or DOT 4. If a substation of a DOT 3 rated fluid in an application that calls for a rated DOT 4 this could create a safety hazard.

It must be noted that, DOT 5 brake fluid is different from DOT 3 and DOT 4 in that it is silicone-based. DOT 5 is NOT recommended for any vehicle equipped with antilock brakes (ABS) - but it can provide long-lasting protection against corrosion for vehicles that are stored for long periods of time or are driven in wet environments.

To accommodate vehicles fitted with antilock systems, DOT 5.1 fluid has been introduced. The DOT 5.1 fluid contains Polyalkylene Glycol Ether the same as DOT3 and 4. The major variation comes from the ‘wet boiling point’ determinations.



Minimal Boiling Points are recorded as:

Wet Boiling Points:-
DOT 3 = 284° F (140° C) DOT 4 = 311° F (155° C) DOT 5 = 356° F (180° C) DOT 5.1 = 375° F (190.6° C)
A major drawback for the use of DOT 5.1 is its higher cost factor.



Overview
The brake fluid transmits hydraulic pressure from the master cylinder to the wheels. It is a special fluid with special properties.Most are a mixture of glycerin and alcohol, called glycol, with additives to give it the characteristics that are needed.It must have the correct viscosity for hot and cold conditions.Its boiling point must be higher than the temperature reached by the system.It must not damage seals, gaskets or hoses or cause corrosion.Glycol-based fluids meet most requirements, although they do damage paint.And they absorb moisture. Hence the warnings. This is important because as moisture is absorbed, it lowers the boiling point of the fluid.Brake fluids should not be mixed with mineral-based oils or solvents. If contamination is suspected, the braking system must be drained and flushed with a suitable solvent, and rubber components replaced.


Background Information

Brake fluid is a type of hydraulic fluid used in brake applications in automobiles and light trucks. It is used to transfer force under pressure from where it is created through hydraulic lines to the braking mechanism near the wheels. It works because liquids are not appreciably compressible. Braking applications produce a lot of heat so brake fluid must have a high boiling point to remain effective and must also not freeze under normal temperatures. These requirements eliminate most water-based solutions.

Brake fluid can come in a number of forms, standardized under the DOT standard.
DOT 2 is essentially castor oil;DOT 3, DOT 4, and DOT 5.1 are composed of various glycol esters and ethers; andDOT 5 is silicone-based.
Most cars produced in the US use DOT 3.



General Requirements

Government regulations will specify the requirements for fluids for use in hydraulic brake systems of motor vehicles, containers for these fluids, and labeling of the containers. The purpose of these regulations/legislation is to reduce failures in the automotive hydraulic braking systems which may occur because of the manufacture or use of improper or contaminated fluid. In addition, give the relative brake fluids their DOT rating.

This DOT rating is dependent on how the fluid compares to the predetermined standards set by government regulators.

The tests which are contained within the test procedure are used to determine compliance with the specified requirements. These tests are as follows:
Equilibrium Reflux Boiling Point (ERBP)Wet ERBPKinematic ViscosityPh ValueFluid StabilityCorrosionFluidity and Appearance (Low Temperature)Water ToleranceCompatibilityResistance to OxidationEffect on SBR CupsContainer Information

Bleeding

Bleeding
Bleeding means removing air from a hydraulic system.

When pressure is applied to liquid in a hydraulic system, the liquid does not compress into a smaller volume. Pressure is transmitted without loss. Gases however are compressible.

Pressure applied to air changes its volume, and some pressure is lost. That is why if air enters a hydraulic braking system, it can be dangerous.

Pressure on the brake pedal will not be transmitted in full through the system to apply the brakes. The brakes will be spongy.

Bleeding the brakes means removing this air, so that only liquid is left in the system.

Master cylinder

Drum master cylinder
The master cylinder is connected to the brake pedal via a pushrod.

This is a single master cylinder for a drum brake system. Its one piston has a primary and a secondary cup. These are also known as seals, because, when force is applied to the brake pedal, the primary cup seals the pressure in the cylinder. The secondary cup prevents loss of fluid past the end of the piston. An outlet port links the cylinder to the brake lines. An inlet port connects the reservoir with the space around the piston. A compensating port connects the reservoir to the cylinder, ahead of the primary cup.

With the brakes off, this port connects the brake system with the reservoir. It compensate for changes in volume of the fluid due to heat or wear.

The rod from the brake pedal pushes on the piston. It moves, closing off the compensating port and trapping fluid ahead of the primary cup. Any fluid trapped in the cylinder is then forced through a valve, called a residual pressure valve, into the brake lines. When the brakes are released, the master cylinder piston returns to its original position.

When the piston fully returns against its stop, the primary cup uncovers the compensating port. Fluid ahead of the primary cup can now return this way to the reservoir.

When the pedal is released quickly, the spring makes the piston return quickly. But the fluid cannot return as quickly to the cylinder. A low-pressure area develops ahead of the primary cup, which could draw air into the system. To prevent this, small holes are drilled in the piston. Fluid from the reservoir can pass through the inlet port and past the edge of the primary cup. This is called recuperation.

When the fluid in the lines returns to rest, its pressure is held above atmospheric pressure, by a valve called the residual line pressure valve. The residual pressure helps stop air from entering, at the wheel cylinder. And it keeps fluid from leaking out.

Divided systems

Disc divided systems
Modern cars use tandem master cylinders to suit divided or dual line braking systems.

A divided system is safer in the event of partial failure. Fluid loss in one half of the system still leaves the other half able to stop the vehicle, although with an increase in stopping distance.

A wheel’s braking ability depends on the load it’s carrying during braking. So the type of vehicle is a major factor in how its system should be divided.

A front-engined rear-wheel drive car has around 40% of its load on its rear wheels, so its braking system can be divided in a vertical or front-rear split. This puts the front wheels in a different system from the rear wheels. If one half of the system fails - the front or the back - there’s still enough separate braking capability left in the other half, to stop the vehicle.

This doesn’t work well for a front-wheel drive vehicle. A load of about 20% on the rear wheels can’t provide enough braking force to stop the vehicle.

Front-engined, front-wheel-drive vehicles use a braking system split in a diagonal or X. The left hand front brake unit is connected to the right hand rear unit, and the left hand rear, to the right hand front. If one system fails, a 50% braking capability is left in the other system. Dual proportioning valves maintain optimum braking in each system.

A system that partially failed would cause severe braking pull on a vehicle’s suspension. So suspension geometry is usually revised to counter this.

An alternative arrangement for front-engined, rear-wheel-drive vehicles is an L split. The front disc brake units have 4-piston calipers. 2 of the pistons on each front unit connect to the right hand rear, and the other 2 pistons of each unit connect to the left hand rear. As with the X split, if there is failure of either half of the system, it still leaves 50% braking capability.

Tandem master cylinder


Tandem master cylinder
Overview

With a basic master cylinder in the braking system, any loss of fluid, say because a component fails, could mean the whole braking system fails.

To reduce this risk, modern vehicles must have at least 2 separate hydraulic systems. That’s why the tandem master cylinder was introduced.

Like 2 single-piston cylinders end-to-end, a tandem cylinder has a primary piston and a secondary piston. Each section of the cylinder has inlet and outlet ports, and compensating ports.

There can be 2 separate reservoirs, or just one but it is divided into separate sections.

When the brake is applied, the primary piston moves, and closes its compensating port. Fluid pressure rises, and acts on the secondary piston. It moves, closing its compensating port. Pressure builds up in this circuit. Both pistons then move, and displace fluid into their separate circuits and apply the brakes.

If there is a failure in the secondary circuit, the primary system continues to operate normally, but with increased travel.

If the primary circuit fails, no pressure is generated to move the secondary piston. So a rod attached to the front of the primary piston will push the secondary piston directly so that it still operates.

A switch can warn of loss of pressure in the front or rear circuits. Or one that warns of low fluid level can be fitted to the reservoir.

The tandem master cylinder just like the single piston master cylinder can have problems with a low-pressure area developing when the piston returns quickly but the fluid lags. The tandem master cylinder overcomes this by using grooves in the side of the primary cup. These grooves allow fluid to flow from the inlet port into the low-pressure area.



Operation

In a similar way to the single brake master cylinder, the tandem master cylinder displaces hydraulic brake fluid the the brake system components when the driver applies pressure to the brake pedal. As with the single master cylinder, the tandem master cylinder reservoir contains the majority of fluid for the brake system. With the single system there was only one operational chamber for both the front and rear braking circuits. With the tandem system there is effectively an operating chamber for the front brakes and a separate operating chamber for the rear brakes, but operated from a common brake pedal.

The main reason for the use of a tandem braking master cylinder is for safety reasons. In the advent of a leak or failure in one circuit the other circuit will still have a braking potential, but with a reduced effeciency.The system utilises two sub-systems.

The tandem master cylinder is used in a split braking system. The type of split systems include:
Front to rear split. That is, the front brakes operate of one circuit while the rear brakes operate off another circuit; and alternatively Diagonially split. That is, one front brake station and the opposite rear brake station are in the same circuit and the other front and rear brake stations are operated from the other circuit.
When the brake pedal is depressed, the push rod connected to the pedal moves the 'primary' piston contained in the master cylinder forward. The primary piston now activates one of the sub-systems. The hydraulic pressure created by the primary piston and spring now moves the secondary piston forward thus activating the other sub-system.

With the forward movement of the pistons the primary cup seal and the secondary cup seal closes off the supply ports from the reservoir and creates an increasing hydraulic pressure build-up. The building pressure is transmitted to the brake stations and the brakes are applied.

As the brake pedal is released, the rearward movement of the pistons the primary cup seal and the secondary cup seal uncover the supply ports to the reservoir and decreases the hydraulic pressure build-up. Thus releasing the brakes.

Electronic Monitoring of System

Some manufacturers fit electronic sensors within the master cylinder reservoirs to monitor the hydraulic brake fluid levels. If the fluid levels drop below the sensor contacts, the driver is alerted by a light coming on in the vehicle's dash.

The more common monitoring device is an electronic switch that monitors the functionality of the sub-systems. While each sub-system is operating correctly, a pistion remains balanced between the the two chambers. If a failure occurs in either sub-system, the with the extra movement of the primary or secondary pistons an electronic switch is activated as a spring-loaded drops down and closes the electrical circuit. The driver is alerted to this malfunction when the brake light on the vehicle's dash is illiminated.

In either of these situations, the driver should have the braking system inspected and repaired as necessary by a suitably qualified automotive technician.
 

Power booster or brake unit


Power booster or brake unit
A power booster or power brake unit uses a vacuum to multiply the driver’s pedal effort, and apply that to the master cylinder. This increases the pressures available from the master cylinder.

Units on petrol/gasoline engines use the vacuum produced in the intake manifold.

Vehicles with diesel engines cannot use manifold vacuum so they are fitted with an engine-driven vacuum pump.

The most common booster now operates between the brake and master cylinder. It increases the force that acts on the master cylinder.

Whenever the pedal is depressed, the power brake unit assists the driver. The level of assistance depends on the pressure applied.



Power Brakes

Power brakes are also referred to as 'power assisted brakes' and are designed to use the power from the engines and/or battery to increase the braking power of the vehicle. There are four commonly utilised types of power boosting braking performance. These are:
Vacuum suspended, Air suspended, Hydraulic boosting, and Electro-hydraulic boosting.
The majority of cars and light vehicles opt for the use of 'vacuum suspended' units (commonly referred to as Vacuum Boosters), which employ a vacuum assisted power boosting device to provide the additional thrust to the driver's pressure to the brake pedal.

With some hydraulic booster systems, they can tap into the power steering hydraulic circuitry to use the power from the power steering to supplement the actuating pressure to the master cylinder. The electro-hydraulic system uses an electric motor to pressurize the hydraulic system which supplements the pressure to the master cylinder. This allows a braking force to exist in the advent of an engine shutdown or total loss of power.

To test if a vacuum sourced power booster is operational, shut the engine down, then pump the brake pedal until all the power source (such as the vacuum) is exhausted. The pedal should feel hard with a shorter travel. Hold your foot hard on the brake pedal, and start the engine. If the booster is operating correctly, the pedal will depress slightly as the vacuum enters the power booster chamber. This indicates that the booster is receiving a vacuum that will assist in increasing the braking performance. This assumes that the rest of the braking system is functioning correctly according to the manufacturer’s specifications.

In the vacuum type booster system, the pressure on the brake pedal pushes a pushrod connected between between the pistons within the master cylinder.

At this time:
a pushrod opens the vacuum-control valve (thus closing the vacuum port); and seals the forward half on the booster unit.
Then:
vacuum supplied from the gasoline engine creates a low-pressure within the vacuum chamber, pressure on the diaphragm forces it forward; thus supplying pressure on the master cylind pistons.
 

Hydraulic brake booster


Hydraulic brake booster
1. Brake fluid reservoir
2. Master cylinder
3. Accumulator
4. Actuator unit
5. Pressure side
6. Return side

Although not as common as a conventional brake system fitted with a vacuum booster, many vehicles are now equipped with hydraulically assisted boosters for the brakes. The system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide the power assistance required in a conventional system. This application is particularly suitable to vehicles with diesel engines as a separate vacuum source does not have to be provided for the system to operate.

Because the system uses fluid pressure from the existing power steering pump the booster uses the pressure from the fluid that is always circulating through it, as the source of pressure that applies against the master cylinder actuating piston.

The hydraulic pressure generated by the power steering pump is stored in an accumulator, which is then routed to the master cylinder by the hydraulic booster unit when the brake pedal is applied.

When applied the booster can generate pressures of between 1,200 to 2,000 psi or 8273 to13789 Kpa to the brake calipers. The systems are generally available with or without a matched the master cylinder. The systems that have an included master cylinder have a reservoir as part of the assembly.

As a safety measure part of the system includes a component to assist in the maintenance of system pressure known as an "accumulator." Some are nitrogen pressurized while others are spring loaded depending on the application. In the case where pressure is lost (such as when the engine stalls or power steering pump drive belt breaks) the systems accumulator is designed to store sufficient pressure to provide for three full-power applications. If this is insufficient, the system then resorts to manual brakes.
Operation problems can be caused by a number of things such as leaks inside the booster unit, by a worn power steering pump, slipping or broken pump drive belt, or hose connections.

A simple way to test the system is to pump the brakes five or six times with the engine off to discharge the accumulator. Press down hard on the pedal and then start the engine. Like a vacuum booster, you should feel the pedal fall slightly when the engine starts, then rise again.

The leak-down in relation to the capacity of the accumulator can be checked by pumping the brakes several times whilst the engine is running and then shutting the engine down. The vehicle should then be left for about an hour, and the brakes applied without starting the engine. In an efficient and operational system it should be possible to get 2 or 3 soft brake applications before it takes more effort to push the pedal.
 

Electrohydraulic braking (EHB)


Electrohydraulic braking components
Electrohydraulic Braking (EHB) gets rid of the vacuum booster and replaces the current modulator with one that includes a high pressure accumulator.

Like the Hydro boost system it uses an accumulator to provide the required pressure to activate the master cylinder, however, it uses electrical power to effectively “charge” the accumulator and build sufficient pressure for efficient brake operation. This system means that less power is taken away from the engine during operation as battery power is used, and that there is no chance of problems caused by things such as a worn power steering pump, slipping or broken pump drive belt, or hose connections.
 

Applying brakes


Applying brakes
When the driver moves the brake pedal pushrod, it transmits movement through the power unit to the master cylinder piston, to apply the brakes.

It also operates a control valve that admits air, at atmospheric pressure, to the rear of the unit. How it works depends on the position of the pushrod.

A hose connects the intake manifold to a vacuum check valve on the power unit. With the engine running, the vacuum in the intake manifold is used to evacuate the power unit. This valve is held off its seat and a vacuum is produced in both chambers of the unit.

The chambers are separated by a flexible rubber diaphragm attached to the diaphragm plate. It is held in the off position by a diaphragm return spring. The master cylinder pushrod and the control valve assembly are centrally located on each side of the plate.

As the brakes are applied, the pedal pushrod and plunger move forward in the diaphragm plate. This brings the control valve into contact with the vacuum port seat. It closes the vacuum port, sealing off the passage connecting the chambers. Further movement of the pushrod and plunger moves the air valve away from the control valve to open the atmospheric port. Air at atmospheric pressure comes into the air filter and passages, and enters the chamber at the rear of the diaphragm. The difference in pressure now on both sides of the diaphragm moves the diaphragm plate forward, and it takes the master cylinder pushrod with it.

Hydraulic pressure builds up in the brake system to operate the brakes. As pressure rises, a counter-force acts through the master cylinder pushrod and the reaction disc. This counter force acts against the plunger and pedal pushrod. It tends to move the plunger slightly to the rear, and it closes off the atmospheric port.

If the vacuum source is interrupted then, as the pedal is pushed down, the pedal pushrod and plunger assembly come in contact with the reaction disc. This forces the master cylinder pushrod forward, to operate the brakes. The pedal force needed then is much greater than with vacuum assistance.
 

Brake force


Brake force
During application, the reaction force against the valve plunger works against the driver to close the atmospheric port. With both the atmospheric and the vacuum ports closed, the power unit is in a holding position. It stays this way until increased pedal force re-opens the atmospheric port, or a drop in pedal force re-opens the vacuum port.

With the force on the pedal held constant, the valve returns to the holding position. But if the pedal is fully applied, the plunger moves away from the control valve to open the atmospheric port and give full power application.

When the brakes are released, vacuum returns to both sides of the diaphragm, so the spring releases the brakes.

When the engine is switched off or stops for any reason, no vacuum is available. The vacuum remaining in the booster, held by the non-return valve, will provide for at least one power-boosted application.

After this, the brakes will still operate, but without power assistance, they require more effort from the driver.
 

Brake light switch


Brake light switch
We must be able to indicate to the people driving behind our vehicle that we are slowing down or stopping.

This is the function of the stop light switch.

In most cases the stop light switch is mounted under the dash. As the driver depresses the brake pedal, the stop light switch closes.

This allows current to flow through the circuit and illuminate the stop light bulbs at the rear of the vehicle.
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