Braking Systems Flashcards
What are the 3 needs for brakes?
- The vehicles own kinetic energy will keep it moving considerable distances
- The driver must be able to slow or stop the SAE in a controlled and safe manner
- The driver operator must be able to hold the vehicle in a parked or stationary position.
What is kinetic energy mainly converted in to in a braking system?
Heat Energy
What controls the rate of deceleration/retardation?
The speed of energy conversion
What causes the conversion of kinetic energy to heat energy?
The contact of a non rotating object against a rotating object
How many ways does there need to be to stop a car?
2 ways
What does a mechanical braking system provide the operator with?
- The means to safely slow down and stop ASE under controlled conditions
- The ability to prevent ASE movement when parking
What do the adjustable rods do in a mechanical braking system?
Link the cross shaft to the brake operating levers
What does the cross shaft do in a mechanical braking system?
Balances the application of the brakes to all four wheels.
What are put in place in a mechanical braking system to take up any unequal adjustment?
Compensators
What does a fully compensated brake system ensure?
- Each brake is applied at the same time
- Each brake receives its equal share of the braking effort
How many compensators does a mechanical braking system need to achieve full compensation and where are they?
- 3
- Front
- Rear
- Centre (to equalise the front and rear)
What are the purposes of a back plate?
- Acts as a mount for everything
- Acts as a seal to stop the ingress of debris, dirt and water
What is the purpose of the brake shoe anchor?
- To prevent the brake shoes from rotating
- Provides the leverage point for the shoe to act against
- Houses an integral adjuster to adjust the shoe drum clearance
What is the purpose of an adjustor in a drum brake?
To maintain a small gap as the brakes wear to provide instant braking
What does a cam expander do?
- Provides a locating point for the brake shoes
- Provides the leverage to force the brake shoes against the brake drum
What two methods are friction lining attached to the web?
- Bonded
- Riveted
What are steady posts also known as?
Brake shoe hold downs
What are the purpose of steady posts?
To ensure maximum contact and surface area of the lining against the drum
What two types of steady posts are there?
- Spring stead post (hold on)
- Adjustable steady post (hold off)
What is the major disadvantage of drum and shoe brakes?
- Brake fade
What are the purpose of return springs?
- To return the brakes to the off position
Why are return springs different colours?
To differentiate between different strengths
What mechanism does a basic handbrake use to lock it in place?
A pawl and ratchet
What are the two types of handbrake designs?
- Rear wheel
- Transmission brake
What is self energizing or ‘self wrapping’?
Where the shoes come into contact with the drum that enhances brake effort disproportionally to the effort exerted by the driver at the pedal
What two arrangements of brake shoe are there?
- Leading shoe
- Trailing shoe
how do you identify a leading shoe?
It is the first shoe after the expander in the direction of rotation
Wat is the main advantage of a leading and trailing shoe set up?
It gives equal braking effort in reverse, as the shoes change roles
How can the braking effort be increased even further? And what is it called?
Make both the shoes leading so that are both self energizing.
Called twin leading shoes
How can you achieve a twin leading shoe set up?
Place another cam expander at the bottom to make the trailing shoes contact point lower
What is the disadvantage of twin leading shoes?
In reverse the twin leading shoes becoming twin trailing shoes and reduces brake effort
What is the best brake set up for a 4 wheeled ASE?
Twin leading on the front axle and leading trailing on the rear axle
What problems does a hydraulic braking system overcome?
Routeing rods and linkages around moving parts of suspension and steering systems
What properties must brake fluid have?
- Low viscosity
- Compatibility with rubber components
- Other fluid compatibility
- Resistance to chemical ageing
- High boiling point
- Low freezing point
What is the disadvantage of brake fluid?
It is hygroscopic (absorbs moisture from the air)
When should brake fluid be changed?
- When laid down in the AESP
- When found to be contaminated
To avoid severe damage to seals, what type of brake fluid must be used and what shouldnt?
- Mineral based should not be used
- Vegetable based should be
What is the most common brake fluid used in the RAF?
OX8
Why must there be free play in the brake pedal?
To ensure the full release of the brake
How many seals are mounted on the piston in a hydraulic brake system? And what are they called?
- 2
- Primary cup and secondary cup (primary in front of the piston)
What is the check valve for on a hydraulic cylinder?
- To allow fluid flow in both directions
- Maintain a pressure in the lines to aid in bleeding
What causes fluid to move in to the front chamber when the brake is off?
Atmospheric pressure in the reservoir is higher than the pressure in the front chamber
What is the major disadvantage of a single piston barrel type master cylinder?
If there is a break in the brake pipe the braking ability will be lost
How was the issue with a single piston barrel type master cylinder overcome?
A system was designed called a divided-line braking circuit where the braking system is split into two circuits
What type of cylinder does a divided-line braking circuit use?
A double acting/tandem master cylinder
What two reservoir options are there in a double acting master cylinder?
- 2 reservoirs
- 1 reservoir with 2 chambers
What 3 configurations of divided line circuits are there?
- Front/Rear split
- X split
- L split
What is the pressure differential warning indicator used for?
Signals when the pressure between the two lines of a split system is unequal
What are characteristics of brake pipes?
- Narrow bore
- Usually made of steel
- Seamless
Why are brake hoses made of rubber?
To absorb steering and suspension movement
What 2 common types of wheel cylinder are there?
- Double acting
- Single acting
What 3 types of manual adjuster are there?
- Snail cam
- Wedge and tappet
- Screwed plug type
What is the name of the peg that is attached to the back of the brake shoe?
- Striker pin
What are the two variants of automatic adjuster?
- Foot brake adjuster
- Handbrake adjuster
Where are hydraulic brake systems most commonly used?
On light vehicles
What are the advantages of a hydraulic braking system?
- Fully compensated (pressure is passed to each brake evenly)
- High efficiency
- They allow for suspension and steering movement
- Distribution of brake pressure from master cylinder can be changed
What two types of rotor are there in a disc and pad configuration?
- Solid Rotor
- Vented Rotor
What is the purpose of a groove in a brake pad?
To aid in the removal of dust
What designs of calliper are there?
- Opposed twin piston brake calliper
- Single feed 4 piston
- Twin feed 4 piston L split system
- Floating calliper
What retracts the piston in a calliper when the brakes are released?
The seal between the piston and the cylinder
What are the 3 purposes of the seal in a calliper?
- Prevents the leakage of brake fluid
- Returns the pads to there original position
- Acts as a self adjuster when pads wear
What is the main purpose for having a single feed 4 piston calliper?
Space saving
What is the main purpose for using a twin feed 4 cylinder over a single feed cylinder?
If one feed fails there is another feed to compensate
What are the advantages of disc brakes?
- Improved heat dissipation and greater resistance to brake fade
- Braking torque proportional to the applied force
- Weight saving
Because there’s no self energizing in disc brakes, what else is in place?
Brake servo
What gives you greater assistance in a servo?
A larger diaphragm
What component in a brake servo allows the driver to feel the load applied to the brake?
The reaction disc
What makes the brake effort proportional to the vehicles weight?
Load Sense Valve (LSV) / Load apportioning valve
What types of vehicle are load sense valves most commonly fitted to?
Light commercial vehicles and estates
If the braking system incorporates a diagonal split line arrangement, should there be a LSV in place?
There should be two to stabilise each line
In an LSV, what senses the load on the rear wheels?
A tension/compression spring
How does an LSV work?
In the unladen position the fluid enters the valve and due to the ball not being lifted from its seat by much, less fluid is delivered to the brakes.
When laden the piston and the vale set moves down and there is a greater gap for the brake fluid to travel through.
What faults are there on an LSV?
- Possible leakage of the seals
- Fracture of the external spring causing the valve to reduce the pressure
- When the valve is defective, the complete unit is usually replaced
Without an LSV, what may happen when braking?
The brakes may lock up due to maximum braking effort at all times
What component controls the brake effort to the rear brakes when weight transfer occurs on non cargo carrying vehicles?
Brake pressure proportioning valve
What two types of brake pressure control valves are there?
- Mechanical
- Inertia
What is used to maintain control of a trailer when braking?
Overrun brakes
What two types of overrun brakes are there?
- Mechanical
- Hydraulic actuator
What prevents slipping/skidding whilst braking?
ABS (anti-lock braking system)
What is cadence braking?
Applying the brake and releasing continuously
How does ABS apply cadence braking?
In 3 stages
- Pressure maintenance
- Pressure decrease
- Pressure increase
What sub systems can an ABS system be?
- Hydraulic
- Electronic
What components do a hydraulic ABS system consist of?
- Master cylinder
- Brake lines
- High pressure pump
- Brake callipers/wheel cylinders
- Accumulator
How many lines are there in an ABS system?
4 lines
What components are fitted to an electric ABS system?
- ABS ECU
- ABs modulator
- Solenoid valves
- Brake switch
- Wheel speed sensors
What electronic component determines when a vehicle is about to skid?
The ABS ECU
How many signals does the ECU monitor every second?
8000 sensor signals per second
What type of sensor is attached to the wheel in an ABS?
- Electro-magnetic type
- Hall sensor
What does a traction control system do?
Brakes the spinning wheel to induce positive traction
What are the benefits of a TCS?
- Improved safety
- Reduced tyre wear
What does a TCS consist of?
- An electric motor driven hydraulic pump
- Traction boost unit consisting of cylinder housing, piston and poppet valve
Where is the poppet valve in normal braking?
Down
Where is the poppet valve in wheel spinning?
Up, closing the port
What is the need of air brakes?
- HGVs are heavier, needing a more powerful braking system
What are advantages of air brakes?
- Air is convenient and often is fitted to ancillary equipment on HGV’s
- Air is free and lightweight
- More power is available and components are smaller
- The system will tolerate a small amount of leakage without failing
What are the compression and storage components in an air braking system?
- Compressor
- Governor valve
- Unloader valve
- Reservoirs
- Automatic drain valve
- Air dryer
- Indication equipment
What are the control components in an air braking system?
- Foot control valve
- Differential protection valve
- Quick release valve
- Pressure protection valve
- Relay valve
- Emergency relay valve
- Load sense valve
- Hand operated valves
What are the two basic layouts of air brakes in general use?
- Dual system for rigid vehicles with 2 separate air supplies to operate rear and front brakes
- 3 line system used on articulated vehicles which provide service, emergency and secondary systems of operating brakes
What is the function of an air braking system?
To compress, store, meter and deliver a volume of air
What are the 3 main parts of an air brake system?
- Compression and storage
- Control
- Actuators
How are air brakes failsafe?
The spring brake actuators will automatically apply the brakes when there is a loss of compressed air
If air falls below a predetermined pressure in air brakes, what happens?
A warning light or buzzer is activated to warn the driver
What ways are the compressor cooled in an air braking system?
- Air cooled
- Water cooled
What type of compressor runs the air braking system?
A reciprocating piston
Either single or twin in-line
What causes the inlet valve to open in the air brake piston?
A partial vacuum above the piston head
What is the purpose of the air reservoirs?
- Storage at the desired pressure
- Maintain pressure after the engine has stopped
What factors influence the size of an air reservoir?
- Air delivery rate
- Size and number of wheel brake actuating chambers
- Frequency of brake application
- Further demands of the ancillary services
What is attached to the air reservoirs to remove water and/or oil?
A manual or automatic drain valve
What 3 positions can the foot control valve of an air brake system be in?
- Brakes applied
- Brakes held on
- Brakes released
What is the alternative name of a Air brake diaphragm?
Brake chambers
What is placed between the pushrod and the expander in an air brake system?
Slack adjuster
Why might there be different length push rods for different vehicles?
Some vehicles require longer or shorter stroke lengths for correct setting up of the brakes
What are the two main purposes of the slack adjuster?
- To adjust the brake shoes
- To act as a lever and helps generate the necessary torque
Why is it important that the slack adjuster is set up correctly?
Because if it isn’t the brake effort will not be achieved
In the brakes applied position, what angle is the slack adjuster at?
90 degrees to the pushrod
Due to the important of correct adjustment, what was put in place instead of manual slack adjusters and why?
Since 1994 automatic slack adjusters have been a mandatory fitment under EC legislation
What is the name of the component in an airbrake system that acts on the cam expander?
Cross shaft or cam shaft
What are the different designs of expander?
- Basic straight cam expander
- The cam and struts expander
- The ‘S’ cam
What assembly is used to attach the slack adjuster to the push rod?
A yolk assembly, including a crevice pin
What controls the pressure in the reservoir of air brakes?
- Governor valve (air limiter valve)
- Unloader valve
Where is the governor valve situated?
In a sense line between the reservoir and compressor
What does the compressor running light mean?
When the governing valve shuts the inlet valve and the air doesn’t enter the compression chamber
What does a safety valve do in an air braking system?
Provides a safeguard against any excessive build up of air if the governor or unloading valve fails
Why are gauges fitted to vehicles to show the driver how much are is in the reservoir?
Legislation dictates the driver must know
What is in place to ensure the driver they have enough air in the reservoir?
A warning light, buzzer or combination of the both
How are stop lights activated?
When braking a small pressure of approx 5 psi under a diaphragm lifts a small piston to complete an electrical circuit
In what stages does an air dryer remove condensation from the air?
- Cooling
- Filtering
- Drying
What is the process off drying air in an air brake system called?
The charge cycle
When air in an air dryer is allowed to open a purge valve in the base of the dryer, what is this called?
The regeneration cycle
What drying technique is used to dry air in an air braking system?
Adsorption
Due to the danger of having a split in the air braking system, what is put in place?
A dual braking system
What is the major difference between a single air braking system and a dual air braking system?
Another reservoir
How is in air brakes secondary circuit operated?
By a hand lever
Because a secondary air brakes circuit was introduced, what happened to the actuator?
It became a double diaphragm
What have double diaphragm actuators been replaced with in an air braking system?
Spring brake actuators
What causes sequence charging of reservoirs in an air braking system?
Multi circuit protection valves (used to be single element protection valves)
What are the two purposes of a multi circuit protection valve?
- Allows the brake circuits to build up in a set sequence
- Isolates a failed or leaking circuit and ensures the remaining circuits continue to function
How often should a multi circuit protection valve be tested?
Annually
What are the purpose of the two check valves in a multi circuit protection valve?
To control delivery to the secondary/park and trailer reservoirs and prevent cross leakage between the front and rear reservoirs
What is the purpose of the relay valve?
Increases the application speed of the brakes
What is the purpose of the emergency relay valve?
Used on trailers as a relay valve and prevents the loss of air when the trailer is disconnected
What is the purpose of the quick release valve?
To quickly release the brakes
What is an alternative name for a differential protection valve and what is its purpose?
Anti-compound valve
To prevent the service brake and parking brake applying full force at the same time
When is a double check valve used?
- When there are two delivery sources but only one delivery pipeline
- Instead of a differential pressure valve
What is fit to an air brake system to sense the load?
Bendix Westinghouse Load Sense Valve
What is the purpose of the air at the bottom of the Bendix Westinghouse LSV?
To act as a time delay valve
How can we ensure that each pipe is correctly placed?
An international numerical standard was established
what does the first and second numbers in the numerical standard dictate?
1st - What it does
2nd - What circuit
What do the 1st numbers mean in the numerical standard?
1 - Main supply port
2 - Delivery port
3 - Main exhaust port
4 - Pilot/Signal port
What do the 2nd numbers mean in the numerical standard?
0 - Suction connection 1 - Supply (inlet) port 2 - Delivery port 3 - Exhaust (to atmosphere) 4 - Pilot or signal connection 5 - Spare 6 - Spare 7 - Antifreeze liquid connection 8 - Lubricating oil connection (compressor) 9 - Coolant connection (compressor)
How are the 3 air brake circuits differentiated?
Colours
Service brake - Yellow
Secondary brake - Blue
Park emergency - Red
What air brake system do we run now?
A two line system
service and emergency/park
What are trailer connecting hoses also known as?
Suzie hoses
What two types of connectors are fitted to suzie hoses?
- Bayonet
- Palm
What was the reason for introducing EBS?
It is fast and efficient, and is attached to the air brake system
What are the components of an EBS system?
- Potentiometer
- ECU
- Electro-pneumatic relay valves
What are the 3 functions of the electro-pneumatic relay valves?
- Isolates the slower acting pneumatic back up control
- Admits air rapidly at a regulated pressure
- Releases pressure and restores back up control when brakes aren’t needed
Why do we need to test brakes?
To comply with:
- Department of transport regulations, codes of practice and EC regulations
- Service directives
What are the methods of testing brakes?
- Roller brake test
- Decelerometer: Turnkey BrakeSafe and Churchill
- Functional
- Gradient test (park brake only on a 1:6:25 incline
When should brakes be tested?
- Annually (after first 3 of life)
- Scheduled maintenance
- Scheduled safety inspections
- After a repair or adjustment
- After brakes are unserviceable
- After an accident
- After long term storage
What don’t we RBT?
- Motorcycles
- AVGAS bowsers (unless cleaned, degassed and purged)
- Some specialist equipment
What are the possible faults whilst diagnosing brakes with an RBT?
- Bind
- Grab
- Judder
- Efficiency drops when held
- Ovality
- Imbalance
- Brakes apply and release unequally
- Maximum/minimum braking effort
What is brake bind in an RBT and what causes this?
- When rollers are rotating and gauges are showing a reading when no brakes are applied
- Anything that creates drag on the rollers
- Over adjustment of the brake shoes
- Under inflation of the tyres
- Transmission drag
What is brake grab in an RBT and what causes this?
- The needle leaps violently on braking
- Contaminated shoes
- Cold shoes
What is judder in an RBT and what causes this?
- Needle vibrates intensely
- Glazed or baked brake linings
What could cause the brakes to drop in effeciency?
- Air in the system
- Hydraulic leak
What may cause ovality in an RBT?
- Eccentric brake drums
- Warped disc
What may cause imbalance in an RBT?
- Faulty wheel cylinder
- Wrongly adjusted slack adjuster
What may cause the brakes to apply and release unequally?
- Sticky wheel cylinder
- Air in the system
What may cause incorrect max/min braking effort?
- Faulty load sense valve
- Faulty power assist
