air brake course

1BC Transit Air Brake Course

PresentedBy:Safety,Security&TrainingDepartment

RevisedDecember2010

2BC Transit Air Brake CourseLearningObjectives:

1.

AirBrakesIntroduction2.

Definitions&Concepts3.

FiveMainComponents4.

BasicAirBrakeSystem5.

OtherAirBrakeSystemComponents6.

SingleCircuit7.

DualCircuit8.

Trailers9.

OffHighwayUnits10.

Summary,Questions,WrapUp


1. AirBrakesIntroduction:

a)

Resources

b)

Homework

c)

Quizzes

d)

ICBCAirBrakeTheoryExam

e)

AirBrakeEndorsement

4BC Transit Air Brake Course1.

AirBrakesIntroduction:

a) Resources

ICBCDrivingCommercialVehiclesGuide(Chapters1,2,8,9&10 seepage2)

RichmondPublicLibraryPracticeTesthttp://www.yourlibrary.ca/driving/

BCTransitWebsite(Beingdeveloped seeBCTransit AirBrakeCourseHandoutin

Interim)


1. AirBrakesIntroduction:

b) Homework

Day1:Chapter1&2(Read&Questions),plus

Chapter8(Read)

Day2:Chapter8(Questions),Chapter9&10(Read&Questions)

c) Quizzes

Day1:AirBrakeCourse

Chapters1&2 Review

Day2:AirBrakeCourse FinalExam

6BC Transit Air Brake Course1.

AirBrakesIntroduction:

d) ICBCAirBrakeTheoryExam

Costis$15 LetterofVerificationrequired(bringthistoICBConexamday) MultipleChoiceExamtechnique(readeachquestiontwicethoroughly,

onlyanswerifcompletelyclear otherwiseskipquestion)

e) AirBrakeEndorsement

AftersuccessfullycompletingtheICBCAirBrakeTheoryExam,a

practicalevaluationwillneedtobecompleted(thisisperformedduring

theICBCRoadTest&PreTrip mustbecompletedwithin1year

of

completionofTheoryExam,orTheoryExamwouldhavetoberedone)


2. Definitions&Concepts:

a)

Traction,Heat,Friction

b)

Weight&Speed

StoppingDistance

c)

ForceMultiplication

d)

CompressedAir

e)

CombiningCompressedAirwithForceMultiplication



a)

Traction,Heat,Friction

9The final factor that will determineif a vehicle will move is traction.

The final factor that will determine if a vehicle will stop is traction.

Traction: the ability of a tire to grip the road surface over which it rolls.Friction: force that resists movement between two surfaces in contact with each other.

The engine of this truck converts the energy of heat into the energy of motion. The brakes of this truck must convert the energy of motion into the energy of heat. The friction between the the brake linings and drums generate heat while reducing the mechanical energy of the revolving drums and wheels. This energy is absorbed by the drums and dissipated as heat to the atmosphere.

10

BC Transit Air Brake Course


b)

Weight&Speed

StoppingDistance

11

Weight Weight -- Speed Speed -- DistanceDistance

When weight is doubled, the braking power to stop in the same distance, must double!

When speed is doubled, the braking power required to stop in the same distance must increase 4 times!

When weight and speed are doubled, the braking power to stop in the same distance must increase 8 times!

A vehicle carrying a load of 14,000 kgs at 16 km/h is brought to a stop in 30 meters under normal braking. If this vehicles weight increased to 28,000 kgs., and the speed was increased to 32 km/h, it would require 8 times the braking power to stop in the same 30 meter distance. Remember, this is under ideal conditions. If traction were reduced by poor road conditions, what effect would this have on overall stopping distance?

12

Stopping DistanceStopping Distance

Stopping distance consists of four factors...

1) Perception Time: (also described as see-think time) = the time it takes the brain to recognize a hazard. Usually 3/4 second or 13 meters travel.

2) Reaction time: (also described as Do Time) = it is the time required for your brain to tell your foot to apply the brakes. Usually 3/4 of a second or 13 meters travelled.

3) Brake Lag: the time required for air to travel through a properly maintained air brake system and actually begin applying the brakes. Usually 4/10 of a second.

4) Braking Distance: the time or distance a vehicle travels before it stops after the brakes have been applied.

13

Perception Time (SeePerception Time (See--Think)Think)

Perception Time, generally, is 3/4 second. This is the time it takes for us to perceive that there might be a need to stop or take some other defensive action. At 50 km/h, your vehicle will travel 13 meters (or one bus length) before your brain even decides it would be a good idea to do something about the hazard. What types of techniques can you think of that would help us reduce or compensate for this 3/4 second delay?

14

Reaction Time (Do)Reaction Time (Do)

Reaction time, also generally about 3/4 second. This is the time it takes for us to react to a need to stop or take some other defensive action. Again, at 50 km/h, your vehicle will travel 13 meters (or one bus length) over the road before the brakes even begin to stop the vehicle. Here is where defensive driving techniques can make for a whole lot less accidents.

15

Brake Lag Brake Lag (mechanical lag)(mechanical lag)

4/10 sec.Brake lag is usually 4/10 second, in a properly maintained system. It doesnt sound like much, but consider once again at 50 km/h your vehicle will travel nearly 10 meters before the brake linings come into contact with the drums.

16

Braking DistanceBraking Distance

This distance is dependent on many factors. The condition of the brake components, brake adjustment, road, operator, speed and load of the vehicle will all determine how much asphalt disappears under your bumper before coming to a stop.The professional driver realizes the limitations of his/her vehicles brakes and adjusts their driving accordingly.

17

Stopping DistanceStopping Distance(total stopping time)(total stopping time)

is the is the total distance travelled before your vehicle stops after factoritotal distance travelled before your vehicle stops after factoring in ng in perception time, reaction time, brake lag time and braking distaperception time, reaction time, brake lag time and braking distance.nce.

Other factors such as Other factors such as poor road conditionpoor road condition due to inclement due to inclement weather,weather, driver driver fatiguefatigue and and vehicle conditionvehicle condition will add length to overall stopping distance. will add length to overall stopping distance.

As a As a professional driver,professional driver, it is incumbent upon you to prepare yourself for any it is incumbent upon you to prepare yourself for any eventuality and govern your driving habits accordingly.eventuality and govern your driving habits accordingly.

Techniques such as Techniques such as brake coverage, eye lead time, effective scanning and brake coverage, eye lead time, effective scanning and comprehensive vehicle precomprehensive vehicle pre--trip inspectionstrip inspections can do a lot to even the odds.can do a lot to even the odds.

Your passengers, other road users and your own safety depend onYour passengers, other road users and your own safety depend on your attitude. your attitude. Think professional, act professional and professional you will bThink professional, act professional and professional you will be!e!

18

RoadSenseRoadSense TipTip

Total Stopping Time will be even longer if brakes are incorrectlTotal Stopping Time will be even longer if brakes are incorrectly adjust or y adjust or vehicle is on a downgrade.vehicle is on a downgrade.

The most common air brake system defect found during a commerciaThe most common air brake system defect found during a commercial vehicle l vehicle inspection is brakes that are out of adjustment.inspection is brakes that are out of adjustment.

19



c)

ForceMultiplication

20

Force MultiplicationForce Multiplication (Mechanical Advantage)(Mechanical Advantage)

give me a lever long enough and a place to rest it, and I will lift the world

4 1100100

400400

Braking systems use devices to gain a mechanical advantage. The most common device for this purpose is leverage. In this simple example, a lever is placed on a pivot called a fulcrum. The distance from A to C is 4 feet, and the distance from C to B is 1 foot. Therefore the ratio is 4:1. If a 100 lb. downward force is exerted against point A, an upward force of 400 lbs. is achieved at point B.

AA BBCC

21

AA

BCC

Apply the principal from the previous slide to the example belowApply the principal from the previous slide to the example below::

A represents the slack adjuster in the foundation brakes. As pressure is applied, the slack adjuster works as a lever, on shaft D. As the S cam attached to the shaft rotates, a mechanical advantage is achieved at points B and C. If the slack adjuster were 6 inches in length, and the S cams were 1 inch in length, the ratio would be 6:1.

DD

22



d)

CompressedAir

23

Using Compressed Air to Our Advantage...Using Compressed Air to Our Advantage...

Atmospheric Pressure

15 lbs./sq. in.

Suppose the walls of the room you are sitting in now, were to begin to squeeze together evenly in all directions. Assume the room is sealed and no air can escape.

What would happen to the atmospheric pressure as the size of the room began to shrink?

Would the pressure in the room begin to increase?

Would the rate of increase be proportional to the reduction in the size of the room?

What would the pressure be like in the red room?

24

Compressed air is air that has been forced into a space Compressed air is air that has been forced into a space smaller than it would normally occupy.smaller than it would normally occupy.

As the compressor pumps air (forces air into the cylinder), the pressure within the cylinder (reservoir) begins to increase. That pressure is exerted against all the surfaces of the reservoir at the same time, and at the same pressure.This compressed air can then be used to mechanical advantage in the operation of the vehicles brakes.

Rather than squeezing the room smaller, suppose we instead forced air into a chamber or container...

25



e)

CombiningCompressedAirwithForceMultiplication

26

Obtaining the mechanical advantage...Obtaining the mechanical advantage...

If a constant supply of compressed air were directed through a pipe one inch in diameter, and a one inch plug were placed in the pipe, the compressed air would push against the plug. Holding a scale against the plug would register how many pounds of force were being exerted against the plug.

0 5 10 25 30 35 40

The reservoir below, contains compressed air at 10 pounds per square inch (psi). (atmospheric pressure is considered to be 0.)

pounds per sq. in.made in Canada

1 inch dia.

27

Combining mechanical advantage to achieve force multiplication.Combining mechanical advantage to achieve force multiplication.

100 psi

100 psi is exerted against a diaphragm 30 sq. ins. in area inside the brake chamber pictured here. 30 X 100 = 3000 lbs. of force! Times that by the length of the slack adjuster (lever) and the total force is 18,000 ft-lbs. of force.

Brake chamber - 30 sq. in. diameter

Slack adjuster - 6 ins. in length

Air pressure at 100 psi.

3000 lbs of force

S cam 1 inch in length

18,000 ft.-lbs. of force

here!

28



PuttingItAllTogether Review

29

As the driver applies the brake pedal, the brake linings are forced against the inside surface of the brake drum. The spinning wheel produces friction as the linings make contact. This friction causes heat which is then dissipated to the atmosphere. The amount of heat the drums can absorb, depends on the thickness of the metal. If the drums are turned or worn too thin, they will be unable to absorb the heat produced by braking, and brake fade will occur.

If one set of brakes was poorly adjusted, the rest of the brakes would have to absorb more heat energy than they were initially designed for.

Speed is critical when descending a steep grade. To avoid overtaxing the brakes, follow this simple rule: Never descend a hill at a speed greater than the vehicle is capable of climbing the hill.

30

Review of Section TwoReview of Section Two

In this section we have studied the concepts of:

Heat, Energy, Traction and Friction

Speed, Weight and Distance

How we obtain a Mechanical Advantage to achieve Force Multiplication

Compressing air, and combining this with Force Multiplication

Looked at Stopping Distance and how it can be effected by adverse conditions.

The next section will begin our exploration of the components of the air brake system.

Lets Review...

31

Review Questions: Section 1Review Questions: Section 11) What is the final factor that will determine if a vehicle will move?

2) What is the final factor that will determine if a vehicle will stop?

3) How is the heat dissipated that is generated by the brakes?

4) If one set of brakes were poorly adjusted, what effect would this have on the remaining sets of brakes?

5) What is meant by the term friction?

6) What are the four components of stopping distance?

7) If the weight of your vehicle were doubled, how many times must the braking power be increased in order to stop in the same distance? If speed doubles? If both weight and speed were doubled?

8) Brakes stop the vehicle. Is this true or false?1) Traction2) Traction3)by the brake drums to the atmosphere4)the other brakes would have to do more than their share of braking5)the force resisting movement between two surfaces in contact6)perception time, reaction time; brake lag;

braking distance7)two times; four times; eight times8) False. Traction determines if the vehicles brakes will be effective.

Left click for answers

32


3. FiveMainComponents:

a)

Compressor&Governor

b)

AirLines

c)

Reservoirs

d)

FootValve

e)

FoundationBrakes

33



a)

Compressor&Governor

34

The Compressor & GovernorThe Compressor & Governor

The compressor has two adjacent pistons attached to its crankshaft which is in is in constant drive with the engine. When the compressor is loaded or pumping, air is forced through the main discharge line to the reservoirs. As pressure builds to one of two main standard operating pressures (85 - 105 or 115 115 - 135) the unloading device in the top of the compressor, holds the inlet valves open and air is pumped between the pistons, allowing the compressor to compressor to cool. This is the unloaded stage. The compressor is controlled controlled by the governor which is usually mounted to the side of the compressor. A typical two-flow compressor is pictured here.

We need to know...

The compressor output can be effected by a dirty air filter, loose belts, or worn rings.

It is usually lubricated by the engine lubrication system. (some have their own)

During the pre-trip inspection, the compressor is checked visually before engine start-up, start-up, for belt condition and tension, mounting security, and evidence of oil leaks.

During the pre-trip inspection, the compressor output is checked. It must be able to pump pump from 50 to 90 psi. in 3 minutes or less.

35

The GovernorThe GovernorThe governor may be considered to be the brain of the air brake system. It tells the compressor when to load and unload based on the amount of air pressure in the reservoirs. Two standard main reservoir operating pressures are:

85 psi. to 105 psi. and 115 psi to 135 psi.

Some systems operate at 85 psi. to 135 psi.

The spread between loading and unloading pressures must not be less than 20 psi.

During the pre-trip inspection, the cut-in (load) and cut-out (unload) pressures of the governor are checked.

next slide

36

The Governor (continued)

1 2

34

(1) Air flowing from the compressor moves down the main discharge line to the wet tank.

(2) From the wet tank, it passes through a one-way check valve to the dry tank

(3) Reservoir pressure arrives at the governor, telling it the pressure in the dry tank

(4) When sufficient pressure is developed, the governor signals the compressor to unload.

37

OneOne--way Check Valveway Check Valve

The one-way check valve is situated between the wet tank and the dry tank. This is the first defence against a catastrophic air loss. If the main discharge line ruptured or the wet tank were punctured, the one-way check valve will not allow the air pressure to flow backwards out of the dry tank.When draining the reservoirs, always open the drain valve of the wet tank first. If there is still pressure in the dry tank, you know the one-way check valve worked. The check valve is a common valve in the air brake system and is used at many other locations and for different applications.

38



b)

AirLines

39

Air Lines

Air lines link compressed air from one component to another. Air flowing from the Compressor moves down the Air Line or main

discharge line to the wet tank. Air lines are made of a durable, tensile material they can withstand

pressure of at least 150 PSI. Application air lines (also known as service or control lines) have a

narrow diameter Supply air lines (also known as delivery lines) have a larger diameter.

40


3. BasicComponents:

c)

Reservoirs(AirTanks)

41

The ReservoirsThe Reservoirs

The reservoirs, or tanks, are made of steel and serve to contain the compressed air delivered by the compressor. Air from the compressor is hot. When it comes into contact with the cold steel, condensation occurs. This moisture must not be allowed to accumulate in the tanks. If it did, it would reduce the amount of air the tank would be able to hold, and thus, reduce the volume of air available for the operation of the vehicles brakes. A safety valve is fitted to allow the air to escape if over-pressurisation occurs. (140 to 150 psi.) A one way check valve between the reservoirs disallows the back flow of air from the dry tank to the wet tank. A low air warning device will warn the operator of low air pressure when the pressure in the reservoirs drops to 60 psi. This warning is either audible or visual.

Wet tank Dry Tank

Safety Valve

Drain ValveDrain Valve

One-way check valve Low air warning device

We need to knowWe need to knowmore reservoirs, more volume; they must be drained at least once per day; they must be drained completely in order to allow all the moisture and sludge to escape; the wet tank should be drained first so the function of the one-way check valve can be tested.

42

Low Air Warning Device & Safety ValveLow Air Warning Device & Safety Valve

The low air warning device is a simple spring loaded electrical switch that triggers a warning in the drivers compartment. This device must activate before the reservoir air pressure drops to 60 psi. The warning can be either audible or visual. This device is checked during the pre-trip inspection. Some systems, usually trucking applications, use a wig-wag This is a metal flag that drops from above the drivers windshield into his/her line of sight.

The safety valve is fitted to the wet tank. Its purpose is to release excess pressure should the governor fail to unload the compressor due to a governor failure or a problem with the unloader mechanism in the compressor head. The safety valve should release when pressure reaches 140 to 150 psi.

Low air warn.

safety valve

Wig-Wag

43

105

Air GaugesAir Gauges

120

100

80

110

907060

50

403020

10bye-bye

85

135

Air gauges come in many shapes and sizes, but they all have one thing in common. Information.

A reservoir gauge will advise of the amount of air present in the tank(s) at any given time. It is used during pre-trip inspections to determine:

governor cut-in and cut-out pressureslow air warning device function (above 60 psi.)at what pressure the spring brakes apply automatically.pressure drop when a full foot valve application is made to test for brake adjustment.check for air leaks (maximum 3 psi./min. loss single units buses or tractors, 4 psi./min. tractor & trailer, 6 psi./min. tractor & tandem trailer)an application gauge will tell you how much air is being delivered to the brake chambers.

115

44

Air GaugesAir Gauges

NOTE: Gauges on Double Decker and Dart read in KPA with a scale from 0 - 11.

Operating range is outside of the red bar

45



d)

FootValve

46

The Foot ValveThe Foot Valve

Two typical foot valves are pictured here. The uppermost one is used primarily in buses. The lower example is suspended from the firewall and is used mostly in trucking applications. They operate in precisely the same way, being spring loaded and self-balancing.

We need to know...We need to know...

The foot valve is considered to be the most important valve in the system, because without it, we could not control brake application pressure.

It is a self-balancing device, meaning that even if a small leak occurred in the system during a brake application, the foot valve would regulate the application air such that the brakes would remain applied.

The foot valve is spring loaded, so the operator feels only the spring pressure, not the pressure of the brake application, as in a hydraulic brake system.

Maximum brake application pressure available at any time is only that which is present in the systems reservoirs. Application pressure cannot exceed reservoir pressure.

47

The Foot ValveThe Foot Valve

The foot valve controls application pressure.Reservoir air is available at the base of the foot valve. (dark green)Application air, as demanded by the operator, is delivered to the brake chambers. (light green and light red)The further down the operator depresses the treadle, the more air pressure is delivered to the brake chambers.When the treadle is released, the brake application is released.

Front Rear

Reservoir air

Application airApplication air

48



e)

FoundationBrakes(BrakeChambers,SlackAdjusters,BrakeLinings&

BrakeDrums)

49

The Foundation Brakes: Brake Chamber, Slack Adjusters, The Foundation Brakes: Brake Chamber, Slack Adjusters, and Brake Linings /Drumsand Brake Linings /Drums

A brake chamber is a circular container divided in the middle by a flexible diaphragm. Air pressure pushing against the diaphragm causes it to move away from the pressure, forcing the push rod outward against the slack adjuster. The force exerted by this motion depends on air pressure and diaphragm size.

The Slack Adjuster has two functions in the operation of the vehicle's brakes. First, it converts the pushing motion of the push rod into a twisting motion at the S cam. Second, as the name suggests, the slack adjuster provides a means of reducing free play or slack in the foundation brake linkages.

50

There are two types of slack adjusters currently in use with drum type brakes:

manual adjustmentautomatic slack adjustment

Manual adjustment must be made every day at the start of the shift. If conditions warrant, adjustment may be required more often.

Automatic slack adjusters can be adjusted by simply making a full brake application with the foot valve. Upon release of the foot valve, the slack adjuster will automatically adjust your brakes to optimum push rod travel.

Pre-Trip Inspection:Set up brakes before the start of each days shift.Maximum push rod travel under a full foot valve application must not exceed 1 3/4 inches.Maximum push rod travel under pry bar pull must not exceed 3/4 of an inch.When making a full foot valve application with engine off and park brake released, look for any large drops in air pressure as indicated on the pressure gauge.

Two Types of Slack AdjustersTwo Types of Slack Adjusters

51

Pre-Trip Inspection:Set up brakes before the start of each days shift.Maximum push rod travel with a type 30 brake chamber under a full foot valve application must not exceed 2 inches - using the pry bar method, there should be no more than of an inch of push rod travel. When making a full foot valve application with engine off and park brake released, look for any large drops in air pressure as indicated on the pressure gauge.

Automatic Slack AdjustersAutomatic Slack Adjusters

52

Automatic Slack Adjusters With Hexagonal Adjusting Bolts:

If the slack adjuster has a hexagonal adjusting bolt, the brakes are adjusted by turning the adjusting bolt in a clockwise direction until the lining contacts the drum.Baking off the adjusting bolt by 1/2 turn should restore running clearance (backing off may take considerable force and a ratcheting sound and feel will occur this is normal).

Automatic Slack AdjustersAutomatic Slack Adjusters

Automatic Slack Adjusters With Square Adjusting Bolts:

If the slack adjuster has a square adjusting bolt located at the bottom end of the body, do not attempt adjusting until a spring-loaded pawl that meshes with internal teeth is disengaged.If the slack adjuster has a square adjusting bolt, the brakes are adjusted by turning the adjusting bolt in a counter-clockwise direction until the lining contacts the drum.Baking off the adjusting bolt by 1/2 turn should restore running clearance. Release the button or re-install the spring and pawl if they were removed.

53

Brake Chambers and Slack AdjustersBrake Chambers and Slack Adjusters

As the linings wear, the distance between the linings and the drums begins to enlarge.

If this condition were to deteriorate, braking could be lost completely!

Loss of brake adjustment is the leading cause of brake failure in the transportation industry!

54

Brake Linings and Drums (or Rotors)Brake Linings and Drums (or Rotors)

The assembly pictured to the left, is the foundation brake. It is composed of the brake drum, linings, shoes, S cam, slack adjuster, push rod, and brake chamber.The drum dissipates the heat generated by the brakes.The linings are made of composite material and wear according to use. (load, speed, frequency of braking)The distance between the lining and the drum when the brake is released will become larger as the linings wear.The slack adjuster provides a means of reducing this distance.

RememberRememberThe most common cause of brake failure in the commercial transpoThe most common cause of brake failure in the commercial transportation industry, is rtation industry, is lack of proper adjustment! Always check brake adjustment, or setlack of proper adjustment! Always check brake adjustment, or set up your brakes, up your brakes, during your initial preduring your initial pre--trip inspection.trip inspection.

55



Review

56

Review of Section ThreeReview of Section ThreeIn this section we learnedThe 5 main components of the air brake system

Compressor to pump air, with a governor to control the compressorAir Lines to allow the pressurized air to flow between the brake system componentsReservoirs to store the compressed airFoot Valve (usually called a brake pedal ) to apply the brakes by directing compressed air from the reservoir to the brakesFoundation brakes, including brake chambers, slack adjusters, brake linings and drums or rotors, to transfer the force generated by the compressed air through mechanical linkage to apply the brakes

We know the purpose of each of these components within the air brake system, and we achieved a basic understanding of how each one works.

In the next section we will begin to put these components together to build a basic system.

Lets review...

57

Review Questions: Section ThreeReview Questions: Section Three

1) What are the five main components of the air brake system?2) How is a plugged air filter likely to effect the compressor?3) What causes moisture to form in the air brake system?4) When is the compressor able to accomplish most of its cooling?5) How often must reservoirs be drained?6) What is the maximum air pressure available for a brake application at any given time?7) Is it necessary to allow all the air to escape from the reservoirs? Why?8) What would result if the brake drums were allowed to wear thin?9) What are two functions of a slack adjuster?10) How does the amount of slack effect the operation of the brakes?11) What is the maximum allowable push rod travel under a full brake application? With a pry bar? 12) What is the most common cause of loss of effective braking on air brake equipped vehicles?13) What causes brake fade at high brake temperatures?

1) compressor/governor; air lines; reservoirs; foot valve; foundation brakes2) restrict air flow reducing compressor output4) during the unloaded stage6) that which is present in the reservoir7)Yes. Otherwise the sludge will not run out because the air under pressure will push it to the sides 8) they would overheat resulting in loss of braking9) convert a pushing motion to a twisting motion; allow a means of adjusting the brakes

10) increased brake lag time; possible complete loss of effective braking11) one and three quarter inches; three quarters of an inch12) brakes out of adjustment13) drums worn thin or turned to far

Left click for answers

5) daily or more often if conditions warrant3) condensation

58


4. BasicAirBrakeSystem:

a)

ReviewHow5MainComponentsWorkTogether

b)

QuickReleaseValveFunction

c)

RelayValveFunction

d)

ParkBrakeSystem

Please note: the piping diagrams that follow, do not accurately represent the actual valves and components in an air brake system. Our purpose is to provide you with a basic, theoretical understanding of air brake systems sufficient for you to obtain an air brake endorsement on your license, and perform a pre-trip inspection so as to diagnose problems in the system before going on the road.

59



a)

ReviewHow5MainComponentsWorkTogether

60

No. 1No. 1COMPRESSOR: pumps air; 50 to 90 psi. in less than 3 minutes; lubricated by engine oil; belt driven (some have their own lubrication and drive system); has an air filter, that if plugged, would reduce efficiency.

Left click for answer

61

No. 2No. 2

GOVERNOR: tells compressor when to load and unload (85- 105 or 115 to 135 psi.)


62

No. 3No. 3

FOOT VALVE: most important valve in the system; spring loaded and self balancing; has a different feel from hydraulic brakes


63

No. 4No. 4

AIR LINES: link air from one component to another.


RESERVOIRS: contain compressed air; must be drained daily and completely; more than one reservoir to provide a greater volume of air.

64

BRAKE DRUM: absorbs and dissipates heat generated by braking.

No. 10


65

BRAKE SHOES AND LININGS: shoes contain linings; linings make contact with inside surface of brake drum, creating friction required to provide braking.

No. 11


66

S CAM: forces shoes and linings against drum

No. 12


67

SLACK ADJUSTER: provides a means of reducing the distance between linings and drums (slack); converts pushing motion into twisting motion

No. 13


68

PUSH ROD: transmits force from brake chamber; must not move more than 1 3/4 inches under full application test: 3/4 inch under pry bar pull

No. 14


69

BRAKE CHAMBER: part of force multiplication process; drives push rod

No. 15


70



b)

QuickReleaseValveFunction

71

Quick release valve

Quick Release Valve and Stop Light SwitchQuick Release Valve and Stop Light Switch

The Quick Release Valve, situated on the front axle, provides a quicker release of the front brakes by allowing the air to exhaust at the centre point between the brake chambers. The Stop Light Switch can be activated with as little as 4 psi brake application pressure.

Stop Light Switch

72



c)

RelayValveFunction

73

The Relay ValveThe Relay Valve

The relay valve provides quicker application and release of the rear brakes.The relay valve function is used in other applications within the air brake system, which will be explored later.In order to understand the need for this valve, lets look again at brake lag time next slide

74

Brake Lag TimeBrake Lag Time

...is the time it takes for air to travel through a properly maintained system.

Lag time is dependent, however, on the inside diameter of the brake line through which the air is travelling.

In the two examples pictured here, the air will travel faster through the smaller diameter line and slower through the larger diameter line.

However, the larger diameter line is capable of delivering a much greater volume of air, as opposed to the smaller line which delivers a much smaller volume of air.

high volume high volume -- slow movingslow moving

low volume low volume -- fast movingfast moving

Click for animation (once for each pipe)

75

In this diagram, the dark green line is a LARGER diameter than the bright green line. The dark green line delivers reservoir air to the foot valve AND to the relay valve at the rear axle. When a brake application is made, the foot valve delivers the requested amount of air to the relay valve, through the bright green line, known as a control line.The air delivered by the foot valve dead-ends at the relay valve. This causes the relay valve to open and it delivers the requested amount of application air from its own supply of reservoir air.

Reservoir air

Control line

...next slide

76

From Reservoir

Control line from foot valve

Application air from the foot valve arrives at the top of the relay valve, via the control line (bright green.) It contacts a diaphragm inside the relay valve, and dead ends there.This opens the relay valve internally, and air from the reservoir is metered to the brake chambers.

Relay Valve OperationRelay Valve Operation

To rear brake chambers

...next slide

Air dead ends here!

77

Because the relay valve accesses the reservoir air directly, and the application air acts only as a control device, the brake lag that would normally occur between the foot valve and the rear brakes, is greatly reduced.

This concept of relaying a brake application is used in other valves in the system, so its important to confirm your understanding at this point.

78

If the relay valve were not installed in the system, the front brakes would be applying before the rear brakes. (The foot valve is closer to the front brakes, than the rear brakes.)

Since most braking is accomplished by the rear brakes of a long wheel base vehicle, this would not be a desirable situation.

79



d)

ParkBrakeSystem

80

By the end of this section the student will understand:

How a park brake works.

Why a spring brake is a reliable type of park brake.

How to release different types of park brakes when the air system cannot supply sufficient pressure for release.

Section - Parking Brakes

81

A spring type park brake serves three important functions:

Its primary function is to provide a reliable means of securing an air brake equipped vehicle. (parking brake)

It can serve as an emergency brake if an air loss occurred during vehicle operation.

It can be used in place of the air operated primary service brakes in a dual air equipped vehicle. (We will study dual air systems later in the air brake course.)

82

The park brake control valve, located in the drivers compartment, is standardised throughout the commercial transport industry.

The valve knob will always be diamond shaped and yellow in colour.

83

However, some valves may be designed to be pushed to apply the brake, while others may be designed to be pulled out to apply the brake.

Buses usually incorporate the first type where the valve is pushed to apply the park brake.

84

The Parts of the Spring Type Parking BrakeThe Parts of the Spring Type Parking Brake

Slack Adjuster

Push Rod

Service brake diaphragm

Spring brake diaphragm

Spring brake chamber

Service brake chamber

Park brake actuating spring

Access for caging bolt

Return springs

Note: on Maxi- brake type park brakes there is no access for caging. An emergency release reservoir is provided instead.

85

Park Brake and Service Brake ReleasedPark Brake and Service Brake Released

Service brake is released (no air present in service brake chamber)

System air pressure is present in spring brake chamber.

The park brake spring is held compressed by the system air pressure. The park brake is released, but ready for emergency braking if required.

86

Park Brake Released Park Brake Released -- Service Brake ApplicationService Brake Application

System air is still present in the park brake chamber. The park brake spring is compressed so the park brake is released.

Application air, as delivered by the foot valve, is present in the service brake chamber. The service brakes are applied.

87

Park Brake Applied Park Brake Applied -- Service Brake Released (no compounding)Service Brake Released (no compounding)

No air in service brake chamber

No system air pressure in park brake chamber

The park brake spring has expanded, pushing against the push rod, which has moved the slack adjuster away from the brake chamber, and the brakes are applied.

88

Mechanical Release of the Park Brake (Caging the Spring)Mechanical Release of the Park Brake (Caging the Spring)

On piggy-back park brake systems such as the ones weve looked at, it may be necessary to release the park brake after an emergency application due to some sort of air system failure. Here, the caging bolt has been inserted into the park brake chamber through the access hole previously described. The operator would simply turn the bolt until the park brake spring is compressed and the brakes are released.

.

Caging Bolt inserted into Caging Bolt inserted into spring brake chamberspring brake chamber

89

Mechanical Release of the Park Brake (Caging the Spring)Mechanical Release of the Park Brake (Caging the Spring)

The vehicle must be blocked before beginning the caging operation. The service brake and the park brake will not function because the caging bolt renders the push rod immovable.

.Note: Maxi-Brake park brakes used on buses (vehicles with air suspensions) utilise an Emergency Release reservoir to release the park brake, and cannot be caged in this manner.

Caging Bolt inserted into spring brake chamberCaging Bolt inserted into spring brake chamber

90

Maxi-Brake type Park Brake

The Maxi-Brake parking brake system is somewhat different in design. A typical maxi-brake chamber is pictured here.

Note there is no access hole in the end of the chamber to allow caging of the brake.

Rather, the maxi-brake uses a either a single control valve that would not permit an emergency release of the park brake, or a dual control valve that allows access to an Emergency Release Reservoir.

91

Maxi-Brake type Park Brake

The emergency release reservoir serves the same purpose as caging.

It will allow the operator to move the vehicle a short distance (usually to the side of the road) in the event a failure of the main system caused an emergency application of the park brake.

Apart from these differences, the maxi- brake works in exactly the same way as the piggy-back brakes seen earlier.

92

Maxi-Brake Operation (single control valve.)

When the park brake control valve is opened, the system air (green) flows through to the park brake chambers compressing the springs. The park brake is released.

System air

Park brake chambers

Left click to begin animationLeft click to begin animation

93

MaxiMaxi--BrakeBrake Operation (single control valve)Operation (single control valve)

When the park brake control valve is closed, this exhausts the air from the spring brake chambers, and the park brake is applied.

Park brake chambers

Left click to begin animation

94

Park brake systems with double control valves...

Some systems utilize an extra reservoir that allows the release of the park brake even if the main reservoir were completely emptied due to failure of the system.

Some systems, usually on trucking applications, would require the operator to physically crawl under the unit and cage the spring brakes if the vehicle needed to be moved to a safer location. (This procedure will be demonstrated by your instructor during this course.)

95

Park brake systems with double control valves...

Buses use an air suspension system, so crawling under the vehicle would not be safe due to the possible sudden loss of air that could occur.

The next slide depicts a system into which an isolated reservoir has been piped.

The purpose of this Emergency Release Reservoir is to allow the operator to move the vehicle to a safer location until repairs could be affected.

96

MaxiMaxi--BrakeBrake Operation (Emergency Release Reservoir)Operation (Emergency Release Reservoir)

Emergency release reservoir

Double control valves

If a loss of system air occurs, and the operator needs to move the vehicle a short distance to safety, opening the park brake control valve and the emergency release reservoir valve at the same time, will allow the air in the emergency release reservoir to flow through to the spring brakes, holding them in release.

Air loss from main system

Foot valve / service brakes NOT operational. Maxi must be re-applied to stop the vehicle


97

Park Brake Emergency Functions...

1. The park brake control valve will close automatically whenever system air pressure drops to between 45 and 20 PSI. (At BC Transit, most apply around 50-60 p.s.i.)

2. Some systems use a piping arrangement that requires the operator to close the valve manually if system air were lost. It will not close and apply the spring brakes automatically.

3. When the spring brakes apply automatically while the vehicle is in motion, the operator can expect the rear wheels of the vehicle to virtually lock-up. If road conditions were less than ideal (frost, snow, ice etc.) a skid to loss of control can be anticipated.

98

Park Brake Emergency Functions...

4. When operating a vehicle where there is no emergency release reservoir (most tractor-trailers and trucks) never go under the vehicle to wind off (cage) the spring brakes without first verifying that the vehicle cannot roll away when the brakes release. Block the vehicle!

5. The effectiveness of the spring brakes is entirely dependent upon brake adjustment. If your brakes are not properly adjusted, your park brake may be unreliable or even useless. Remember to check for slack brakes and set them up... OFTEN!!!

6. Applying the service brake when the park brake is applied can damage the brake components. (push rods, slack adjusters.) Therefore, never make full service applications when the park brake is applied. This is known as compounding the brakes.

99



Review

100

No. 5No. 5

ONE - WAY CHECK VALVE: allows flow of air in one direction only


101

No. 6No. 6

LOW AIR WARNING DEVICE: activates by 60 psi.; needs to be audible or visual


102

No. 7No. 7

SAFETY VALVE: releases excess pressure in the reservoir due to governor failure or compressor unloader mechanism failure. Releases at 140 to 150 psi.


103

No. 8No. 8

RELAY VALVE: provides quicker application and release of the rear brakes.


104

Review of Section FourReview of Section Four

1) Why are springs brakes a reliable type of parking brake?

2)What is meant by compounding the brakes?

3) How are spring brakes held in the released position?

4) What are the functions of the cab- mounted park brake control valve?

5) Will park brakes apply automatically in ALL braking systems? 1) They are applied by spring pressure, not

by air pressure.2) Application of service brakes and park brake at the same time

3) By system air pressure4) apply and release the park brake

5) No. Some types require the operator to apply the park brake.

Left click for answers.

105

Review of Section FourReview of Section Four

6) Why is it important to release the park brake before making a full brake application test?

7) What is the purpose of an emergency release reservoir in a parking brake system?

8) How can some types of spring brakes be released without the use of air pressure?

9) Why should a spring brake be disassembled by qualified personnel only?

6) So as to not compound the brakes7)to allow the operator to release the park brake when insufficient air is available for the task.8) by caging or winding them off by hand9) The spring is under extreme pressure and could fly out of the chamber causing severe injury or even death.


106


5. OtherAirBrakeSystemComponents:

a)

AirDryers,AlcoholEvaporators&Injectors

b)

AutomaticDrainValve(SpitterValve)

c)

FrontWheelLimitingSystems

d)

ABS(AntiLockBraking)&ATC(AutomaticTractionControl)

e)

LongStrokePushrods

f)

DiscBrakes

g)

WedgeBrakes



a)

AirDryers,AlcoholEvaporators&Injectors

108

Air Dryers, Alcohol Evaporators & Alcohol InjectorsAir Dryers, Alcohol Evaporators & Alcohol InjectorsAir dryers are optional devices that are installed in the compressor discharge line between the compressor and the first reservoir. They are designed to remove any water vapour, oil mist and carbon particles from the air before it is delivered to the first reservoir.Alcohol evaporators and alcohol injectors are optional devices that introduce a small amount of alcohol vapour into the air system in effect, they lower the freezing point of any moisture that has collected in the air system.

Front Rear

Reservoir air

Air Dryer



b)

AutomaticDrainValve(SpitterValve)

110

Automatic Drain Valve (Automatic Drain Valve (SpitterSpitter Valve)Valve)

The Automatic Drain Valves (Spitter Valve) are optional devices which aid in reducing the amount of moisture and contaminants that collect in the air brake system. These valves are installed on some or all of the reservoirs, or connected to the compressor/governor, or the foot valve on some air brake systems.

Most are self-contained and momentarily open each time reservoir pressure falls (typically following a brake application) or each time the compressor cycles.

Some Automatic Drain Valves are equipped with an electric heating element to prevent freezing in cold weather.

Wet tank Dry Tank

Safety Valve

Drain ValveDrain Valve

Low air warning device

Automatic Drain Valve



c)

FrontWheelLimitingSystems

112

Front wheel limiting valve

Front Wheel Limiting ValveFront Wheel Limiting Valve

The Front Wheel Limiting Valve allows the operator to reduce brake pressure to the front brake chambers by 50% (some automatic types use a graduated reduction that provides no limiting after 60 psi. of application air is reached).

Cab switch



d)

ABS(AntiLockBraking)&ATC(AutomaticTractionControl)

114

ABS are typically made up of 3 main sections: speed sensing, decision-making, and brake releasing or modulation.

As the wheels rotate, vehicle speed is detected by the sensors which delivers a pulsating current monitored by an ECU (Electronic Control Unit).

With normal braking, the sensors will detect a graduated decrease in pulsating current, with no change to braking.

If the ECU detects a sudden change (wheel lockup) in the pulsating current, the ABS system will activate, thereby signaling a release of air from the brakes (modulation).

As the brakes begin to release, the wheels will regain traction, the pulsating current will be restored, and the ECU will allow the brakes to re-apply.

If the lockup re-occurs the apply-and-release cycle will repeat as often as necessary most systems are capable of cycling the brakes up to five times per second.

ABS (Anti Lock Air Brake Systems)

115

Traction control systems are designed to prevent wheel spin in the power mode.

Traction control electronics are integrated into the ABS ECU (Electronic Control Unit).

Traction control attempts to regain traction by braking the spinning wheels, and sometimes throttling back engine power.

Unlike an ABS, traction control can automatically apply the brakes the driver does not need to depress the brake pedal for traction control to engage.

As wheel speed balance is regained, traction is stabilized, preventing spin or jackknifing.

Traction control is especially valuable when a light drive wheel load might allow the wheels to spin under power, or when a tractor is pulling multiple trailers.

ATC (Automatic Traction Control)



e)

LongStrokePushrods

117

Long Stroke Push RodsLong Stroke Push Rods

Many new air brake systems are equipped with long stroke brake chambers. As the name implies, a long stroke chamber design has a longer pushrod stroke than the pushrod of a standard brake chamber.

The key advantage of a longer stroke is that it keeps brakes in adjustment longer (it does not create more braking force). Note: long stroke push rods should not be used with regular brake chambers this could cause poor brake balance and timing.

Long stroke brake chambers can usually be identified by square-shaped inlet ports or a name tag on a clamp bolt.



f)

DiscBrakes

119

Disc BrakesDisc Brakes

operate differently from drum brakes. Rather than using shoes and linings that move outward toward the inner drum surface, disc brakes use a calliper or C clamp principal. In the representation pictured here, the callipers are at the top of the rotor. When the operator applies the brake, a power screw device forces the callipers together, sandwiching the rotor between them. The rotor is connected to the wheel and tires, and so braking occurs.The adjustment of this type of brake is quite different from that used for drum brakes. Check manufacturers instructions if adjusting disc brakes.

Callipers

Rotor



g)

WedgeBrakes

121

Wedge BrakesWedge Brakes

This type of brake uses one or two small air chambers with wedge-shaped pushrods. Once quite common on drive and trailer axles, wedge brakes are now usually found only on steering axles.

When the brakes are applied, air pressure in the brake chamber pushes the wedge part of the pushrod between two rollers, forcing the brake linings out to contact the brake drum.

Unlike conventional s-cam braking systems, drivers cannot easily check the wedge brake adjustment. While most wedge brakes have internal automatic adjusters, if the wedge brakes need to be checked for adjustment, or repairs are needed, only a qualified mechanic should carry this out.


6. SingleCircuitTractorTrailerSystems:

123

Single Circuit Tractor Trailer SystemsSingle Circuit Tractor Trailer SystemsSection SixSection Six

By the end of this section the student will be able to understand:

How the tractor and trailer brakes connect and work together

The operation of the various valves and systems

The operation of the safety devices incorporated into the system

Emergency functions of the system

How to diagnose some faults associated with the connection and functioning of the trailer brakes

Note: Single circuit systems only are covered here. Dual air systems will be dealt with later in the course. As such, the trailers depicted in this section are NOT equipped with spring brakes.

124

A typical tractorA typical tractor--trailer unit connected and ready for work.trailer unit connected and ready for work.

The unit is called a semi-trailer because the towing vehicle, or tractor, actually carries part of the load. The trailer has no front axle and would fall on its nose if it disconnected from the tractor.

The tractor pictured here is a tandem axle. (It has two sets of driving axles.) Some have only one and are called single axle tractors.

125

12

Parts of the Trailer (van style)

1) Wheel blocks: trailers not equipped with spring brakes, must be blocked whenever they are to be left alone for more than 15 minutes. After that time, it can be assumed all air has bled away, and no brakes are working.

2) Crank: allows the trailer to be raised or lowered to meet the height requirements of the towing tractor. It has two speeds or gears. Push or pull the handle to change gears.

3) King Pin: a steel pin that fits into the fifth wheel of the tractor. It connects the two units and the trailer swivels around it.

3

4) The air line connectors and cable to carry power to the lights and turn signals.

45) Landing gear: supports the trailer when disconnected. 5

126

The Parts of the TractorThe Parts of the Tractor

1

2

34

(1) The frame rails or chassis

(2) the tandem drive axles,

(3) the Fifth Wheel which bears the load of the trailer

(4) the air lines and power cable

(5) mud flaps (must always be attached to the tractor frame when bobtailing- running without a trailer connected to the tractor)

5

127

The The Fifth WheelFifth Wheel

The fifth wheel is located on the centre of the frame rails directly behind the cab of the tractor.

The release handle is used to open the locking jaws prior to connecting to or disconnecting from the trailer. The coupler arm helps the king pin of the trailer to guide itself into the locking jaws.

The pivot point allows the fifth wheel to move up and down with the trailer as the coupled unit moves over uneven roadways and hills.

128

Preparing to Connect Tractor and Trailer (Step 1)Positioning the tractor correctly is important in order to align the king pin of the trailer with the jaws of the fifth wheel. Back straight up to the trailer, not at an angle. Bring the tractor to the point where the fifth wheel just makes contact with the apron of the trailer. Then stop, set the park brake and step out to make a visual inspection.

Trailer apronFifth wheel

129

Hooking Up (Step 2)A visual inspection by the operator before backing under the the trailer will confirm...

1) The trailer height matches that of the fifth wheel

2) The fifth wheel release handle is open(jaws are open to accept the king pin.)

3) The trailer is blocked and secure.Once these steps have been verified, then its time to connect and charge the trailer air system. DO NOT attempt to back under the trailer until the air connections have been made. Even when blocked, the jolt may be enough to move the trailer over the wheel blocks and set it rolling.

130

Connecting the Trailer Air Lines and Electrical CableConnecting the Trailer Air Lines and Electrical Cable(Step 3)(Step 3)

Glad Hand couplers are used to connect the air lines of the tractor and the trailer.

They are identical in design with no left or right. (they may be coloured red and green)

They are firmly attached to the front of the trailer and are at the ends of the air lines leading away from the rear of the tractor behind the cab.

A rubber seal provides an air tight fit (it should be checked for wear)

A screen inside the opening prevents grit from entering the system. (check it, too.)

To join them together, hold them face to face at a 90 degree angle to each other. Snap down and turn them until they lock together.

When not in use (bobtailing) they should be coupled to the dead end couplers on the back of the tractor so they remain clean.

Remember to plug in the power cable for the lights and signals on the trailer.

131

Charging the Trailer Brake Reservoir (Step 4)Charging the Trailer Brake Reservoir (Step 4)

TRAILER

SUPPLY

The Trailer Supply Valve mounted on the dash board of the tractor next to the park brake control valve, allows the operator to charge the trailer reservoir. By pulling out or pushing in this valve, depending upon the design, the system air pressure in the tractor will flow through to the trailer thus supplying the trailer brake system with air pressure equal to the tractors.

SYSTEM

PARK

132

Completing the Coupling Procedure (Step 5)Completing the Coupling Procedure (Step 5)The air lines have been connected. (red: supply/delivery; green: service/control) The blue line is the electrical connection supplying power for the trailer lights and turn signals.

The operator has opened the trailer supply valve and the trailer reservoir has been filled with system air.

The operator now closes the trailer supply valve which dynamites (full on) the trailer brakes.

The operator may now select a reverse gear and slowly back under the trailer untilthe king pin locks securely into the jaws of the fifth wheel.

With the landing gear of the trailer still supporting the front of the trailer and the trailer brakes still dynamited, a tug test is performed to make sure the jaws of the fifth wheel locked around the king pin of the trailer.

The unit is now ready for its pre- trip inspection.

133

The SemiThe Semi--Trailer Ready for its PreTrailer Ready for its Pre--trip Inspectiontrip InspectionThe publication Driving Commercial Vehicles - A Guide For Professional DriversChapter 10 Page 227, details the pre-trip inspection required for a semi-trailer. Some key points:

Ensure all lights and signals are working.

Visually inspect fifth wheel for security.

Raise landing gear before moving the unit.

Check trailer brakes are working correctly by applying hand valve while rolling slowly.

Perform the three tests required to be sure trailer brakes dynamite when required.

134

Bobtail Proportioning SystemsBobtail Proportioning SystemsBecause a bobtail tractor has very little weight over the rear drive axles, it is very easy to lock up the rear brakes, even with a light brake application. To help prevent this unwanted lockup, and to increase control, some tractors are fitted with a bobtail proportioning system:

The system consists of two valves one controls the steering axle brakes and the other the drive axle brakes.

When the tractor has a trailer attached, the tractor brakes work normally.

When bobtailing, the braking pressure to the drive brakes is reduced by as much as 75%, preventing the drive axle brakes from locking.

At the same time, the steering axle brakes receive full application pressure.

A tractor fitted with a bobtail proportioning system will stop in a shorter distance and control will be increased, especially on wet or slippery road conditions.

135

TractorTractor--Trailer Air Brake SystemsTrailer Air Brake SystemsThe following slides will walk you through the air system, valve by valve and line by line. Please remember, these diagrams are not truly representative of the actual air piping arrangements. They are, however, taken from the British Columbia Air Brake Manual so that they will, perhaps, look somewhat familiar. So, lets get started!

136

PrePre--1975 Tractor Trailer Air System (no front brakes)1975 Tractor Trailer Air System (no front brakes)

This diagram depicts a single circuit air brake system in a typical tractor. There are no front brakes pictured since they were not required prior to 1975. Since the Canadian Motor Vehicle Safety Standard (dual air) was introduced, all vehicles built since 1975 must be equipped with front brakes.

137

Introduction to Valves and ComponentsIntroduction to Valves and Components

The main components of the air system have been addressed earlier in the course. We will begin by describing the function of the valves and components, some of which have already been mentioned during the trailer connection procedures.

138

The Trailer Supply ValveThe Trailer Supply Valve

The Trailer Supply Valve is a red, octagonal shaped knob located on the dash board of the tractor. When opened (pushed or pulled depending on the design) allows tractor system air to flow through to the trailer reservoir. In this diagram, if the trailer supply were opened, the air would be free to flow out to the atmosphere, as no trailer is currently connected. Once the air bled down to between 45 psi and 20 psi., the valve would close automatically.

TRAILER

SUPPLY

Some older trailer supply valves do not close automatically. In an emergency, the operator must close this type of valve manually.

Left click to demo air loss

139

The Hand ValveThe Hand Valve

The Hand Valve is mounted on the steering column on the right, the hand valve allows independent application of the trailer brakes. Should the trailer begin to slip to the right or left as the unit moves along a roadway, the operator can apply greater braking effort to the trailer wheels which helps to keep the unit straight.

Hand valves are spring loaded and will return to the fully released position on their own. NOT to be used for parking!

140

The Two-Way Check Valve

The two-way check valve will deliver application air either from the hand valve or the foot valve to the trailer brakes. Which source delivered, depends on which source has the higher pressure. If 20 psi were delivered from the hand valve, but only 10 psi were delivered from the foot valve, the 20 psi from the hand valve would ultimately be delivered to the trailer brakes. However, if 20 psi were sent from the foot valve, and only 10 psi from the hand valve, the tractor and trailer would receive the same 20 psi brake application.

Left click for animation

20 psi.

10 psi.

141

The Tractor Protection Valve

The Tractor Protection Valve is mounted on the rear of the tractor. This valve protects the tractor from air loss when the tractor is travelling without a trailer. Any foot or hand valve application made by the operator dead endsat the top of the valve and thereby cannot escape to the atmosphere. It also functions as a relay valve when a trailer is connected and a service brake application to the trailer is made. To remember its function, simply recall the tractor protection valve, protects tractor air.

142

Foot Valve Application (no trailer connected)

When a foot valve application is made with no trailer connected, the tractor brakes respond normally. Because the Tractor Protection Valve is a type of relay valve, this application air from the foot valve, will be dead-ended at the top of the valve. Therefore, the application air cannot escape from the tractor. Tractor Protection Valve- Protects Tractor Air.

Left click to start animation

143

Accidental Hand Valve Application (no trailer connected)

In this case, the operator accidentally applied the hand valve when no trailer was connected. The application air travelled to the 2-way check valve where it was directed to the tractor protection valve. (no foot valve application) The air dead ended at the tractor protection valve, and no air loss occurred.


144

Adding the Trailer System

Here, the trailer has been connected to the tractor, however, the trailer brakes are not charged. (no air in the trailer reservoir.)

We will continue to walk through the system describing the valves and lines as we go.

Later, we will put the system into operation and examine the functions of the valves and lines we learned about.

145

The Trailer Supply /Delivery Line

The Supply Line connects the tractor protection valve to the emergency relay valve near the trailer reservoir. Under normal operation the supply line holds whatever air pressure may be present in the system at any given time. (reservoir air)

Supply/Delivery Line

146

The Trailer Service /Control Line

The Service Line connects the tractor protection valve with the emergency relay valve in much the same way as the supply line. However, the service line carries only application air as delivered by the hand or the foot valve. If no brake application is being made, there is no air the service line. The service line is a type of control line.

Service/Control Line

147

The Emergency Relay ValveThe Emergency Relay Valve is located near the trailer reservoir. It is the last valve the reservoir air flows through prior to entering the trailer reservoir. Therefore, this valve is considered to be the valve that supplies air to the trailer reservoir. The emergency relay valve has three important functions:

1) dynamites the trailer brakes if the trailer breaks away from the tractor or whenever the supply line ruptures or loses its air.

2) relay valve function (quicker application and release of trailer brakes)

3) one-way check valve prevents loss of air from trailer reservoir if supply line ruptures.

148

Charging the Trailer Reservoir

When trailer supply valve is opened, reservoir air from the tractor is allowed to flow through a line into the tractor protection valve. The air flows freely through the tractor protection valve, along the supply line, through the one-way check valve in the emergency relay valve and into the trailer reservoir. At this point the trailer brakes are released, and the unit is ready to go.


149

Foot Valve Application (trailer connected)


Application of the foot valve will apply all the brakes on the tractor and the trailer to the same extent. (provided the brakes are properly adjusted.) As before, application air from the foot valve (control line air) travels to the two-way check valve. Because no hand valve application is being made, the air continues to the top of the tractor protection valve. (dead- ending there.) Air from the supply line is then relayed by the tractor protection valve, down the service line, dead-ending at the emergency relay valve. Again, the application air is relayed. Air is taken from the trailer reservoir and applied to the trailer brakes in accordance with the amount requested by the foot valve.

150

Hand Valve Application (trailer connected)This time, the operator has elected to apply the hand valve only. (independent application of the trailer brakes.) As before, the application air dead-ends at the tractor protection valve. The tractor protection valve relays the request by sending the requested air pressure from the supply line, down the service line. Again, the air dead-ends at the emergency relay valve, which relays the required pressure from the trailer reservoir to the trailer brake chambers.


151

Emergency FunctionsThe following slides will walk you through the functions of the tractor- trailer braking system when things go wrong. For example:

The trailer could break away from the tractor if, for example, the king pin or fifth wheel failed.

A sudden air loss due to a rupture of the supply line will cause the trailer to dynamite.

A rupture to the service line would render the trailer brakes inoperative.

The supply and service lines could be inadvertently connected incorrectly (crossed)

Some of these scenarios could be corrected by the operator (glad hands simply came apart). Others, would require the services of a mechanic.

152

Trailer Break-AwayIn this case the trailer has completely parted company from the tractor:1) The Emergency Relay Valve will sense the loss of air in the supply line due to the trailers separation from the tractor. It will immediately dynamite the trailer brakes, meaning that all air in the trailer reservoir will be dumped onto the trailer brakes. The one-way check valve prevents any loss of air through the severed supply line.

2)The Trailer Supply valve will remain open, allowing a loss of air from the tractor until the reservoir pressure reaches 45 to 20 psi, or the operator closes the valve manually.

3) The operator makes a foot valve application (green) to bring the tractor to a stop. That application air travelled to the tractor brakes, but dead-endedat the tractor protection valve preventing an air loss there.


153

Supply Line Rupture (trailer intact)

The supply line could rupture without the trailer breaking away:

1) The glad hand simply was not properly connected and fell apart due to vibration.

2) The hose itself ruptured due to chafing against a frame rail or for some other reason was punctured.

3) The glad hand was torn lose from the supply line and lost on the road someplace.

4) Any other irreparable failure you can think of. next slide...

154

Supply Line Rupture (continued)When the supply line ruptures:

1) The emergency relay valve immediately dynamites the trailer brakes. (full on!)

2) The trailer supply valve will close at between 45 and 20 psi (unless the operator closes it manually)

3)The operator is looking for a place to pull over, since the trailer brakes are fully applied and the unit is coming to a stop real quick!

4) The tractor brakes operate normally. (next slide)

Left click to start animation.

155

Supply Line Rupture (handling the situation)

When the supply line has ruptured and the trailer has dynamited, the unit will come to an abrupt stop. To get back into service:

1) If the glad hands simply came apart, reconnect them properly and recharge the trailer reservoir by opening the trailer supply valve. The trailer brakes will release and you are on your way.

2) If the supply line glad hands are damaged beyond service, or if the supply line cannot be repaired, you can drain the trailer reservoir in order to release the trailer brakes. This will allow movement of the connected unit, but only for the purposes of relocating the vehicle to a safer location. (out of a tunnel, for example.)

(1)(2)

Left click for animation (1) and again for (2)

156

Service Line Rupture

The service line may rupture for the same reasons as the supply line may rupture. When the service line ruptures:

1) Nothing will occur until a service brake application is made with either the foot valve or the hand valve.

2) As soon as a service brake application is made, the trailer brakes will dynamite... next slide...

157

Service Line Rupture (continued)

1) Because the air to apply the trailer brakes is taken from the supply line by the relay function of the tractor protection valve, this causes an immediate reduction in pressure in the supply line.

2) The emergency relay valve senses this loss of pressure and interprets it as a severed supply line, and so, dynamites the trailer brakes. The one-way check valve in the emergency relay valve closes to protect the remaining air in the trailer reservoir.

Air loss from service line upon service brake

application


158

Service Line Rupture (handling the situation)

As with a supply line rupture, if the glad hands simply fell apart, they can be reconnected and the system recharged by opening the trailer supply valve. This will immediately release the trailer brakes and youre on your way.

If the service line is damaged beyond repair, you need not leave the cab in order to move the vehicle to a safe location. Just recharge the trailer reservoir, the trailer brakes will release and remain so until a service brake application is made again.

If, for whatever reason, the above procedures cannot be carried out, simply drain the trailer reservoir and the trailer brakes will release.


159

Loss of Main Reservoir AirIn this case weve suffered a major failure in the main discharge line leading from the compressor to the wet tank. The low air warning devices activated by 60 psi. The one-way check valve between the wet and dry tanks prevented any loss of air from the dry tank. There is sufficient reservoir air left to make a brake application and bring the unit to a stop.The trailer brakes did not dynamite because the dry tank is still supplying reservoir air to the trailer reservoir.


160

Crossed Lines (service and supply)

Two things can happen if the supply and service lines are crossed.

1. Trailer reservoir with no air: The law requires that a trailer reservoir hold its air for 15 minutes only. After that, the reservoir may be considered to be empty, leaving the trailer with no brakes. No air, no brakes.

2. Trailer reservoir with air: The trailer has been parked for only a short time (less than 15 minutes) and there is air remaining in the reservoir.

The following slides will depict both scenarios. Whatever the case, performing a check for proper trailer brake function before going into service is imperative.

161

Supply & Service Lines Crossed (air in trailer reservoir)


With air in the trailer reservoir and the lines crossed, reservoir pressure from the trailer supply valve will travel to the relay portion of the emergency relay valve on the trailer.

The emergency relay valve will try to make a brake application on the trailer brakes in accordance with that delivered pressure. The result is, the trailer brakes will not release. This is a dead give away that the lines are crossed.

162

Supply & Service Lines Crossed (no air in trailer reservoir)


With no air in the trailer reservoir, the trailer brakes are disabled. Now with the lines crossed, the air from the trailer supply valve travels to the relay portion of the emergency relay valve. Again, the emergency relay valve tries to comply with the request for a brake application. However, the reservoir is empty, or contains very little air pressure, so the trailer brakes now will not apply. A tug test would confirm the lines were crossed since the trailer supply valve is open, the trailer is charged, but the brakes dont work.

163

Three Tests to Confirm Emergency Relay Valve Function

Test 1


Open the trailer supply valve to charge the trailer reservoir. Manually close the trailer supply valve at which time the trailer

brakes should dynamite immediately.

164


Test 2


Set the tractor park brake. Open the trailer supply valve to charge the trailer reservoir. Step out of the cab and manually separate the supply line glad hands. The trailer brakes should dynamite immediately. (Air

will flow out of the broken supply line leading from the tractor until the supply valve closes at between 45 and 20 psi.)

165


Test 3


Set the tractor park brake. Open the trailer supply valve to charge the trailer reservoir. Step out of the cab and manually separate the service line. Nothing should occur. Re-enter the cab and apply the hand or foot valve. The trailer brakes should dynamite immediately upon brake application.

166


6. SingleCircuitTractorTrailerSystems:

Review

167

Review Questions Section Six (Tractor-Trailer)

1) Why is a tractor-trailer unit sometimes referred to as a semi-trailer?

2) What is the purpose of a two-way check valve?

3) Why should the glad hands be protected when not in use?

4) How can an operator control the trailer brakes independently from the tractor brakes?

5) Can the application pressure delivered to the tractor brakes exceed the pressure delivered to the trailer brakes?

6) Can the application pressure to the trailer brakes exceed the application pressure delivered to the tractor brakes?

7) What are three ways of testing the emergency function of the emergency relay valve?


1) because the some of the weight of the trailer is carried by the tractor2) delivers higher pressure derived from two sources3) to keep them clean and free of debris and damage4) by use of the hand valve5) No. The two-way check valve will not allow this

6) Yes. This can be advantages at times of poor road conditions7) close the supply valve; break the supply line; break the service line and make a service brake application: trailer should dynamite in all cases

168

Review Questions Section Six (Tractor-Trailer)

8) What is the main purpose of the trailer supply valve?

9) What valve supplies air to the trailer reservoir?

10) What will occur if the supply line ruptures?

11) What will occur if the service line ruptures?

12) What will occur if a service brake application is made when the service line is ruptured?

13) What are the three functions of the emergency relay valve?

14) If the foot and hand valve were applied at the same time, can the application pressure be greater than the reservoir pressure?


8) provides a means of charging the trailer reservoir9) the emergency relay valve: its the last valve the air flows through before arriving at the trailer reservoir10) the trailer will dynamite immediately

11) Nothing. Until a service brake application is made, at which time the trailer will dynamite.12) the trailer will dynamite

13) dynamites trailer brakes; quicker application and release of trailer brakes; one-way check valve prevents loss of trailer reservoir air

14) No. Application pressure can never exceed that which is present in the reservoirs.

169


7. DualCircuitSystems:

170

Section Seven Dual Air

The Canadian Motor Vehicle Safety Standard (CMVSS) was introduced in 1975.

All vehicles must be equipped with front brakes

All vehicles will incorporate a dual air brake system.

By the end of this session you will be familiar with:

The reasons for bringing in the CMVSS

The differences between single circuit and dual circuit systems.

The extra benefits of incorporating a dual circuit air brake system.

Dual circuit systems in semi-trailer applications.

171

The Purpose of Dual Air Systems

Dual circuit air brake systems provide the following advantages:

1) provide a shorter stopping distance.

2) reduce brake failures due to mechanical faults.

3) delays the automatic application of the spring brakes in an emergency, keeping control of the vehicle in the hands of the operator, longer.

4) allows the operation of one part of the system even though the other part has failed.

172

Identifying Primary and Secondary Systems

The Compressor, Governor, and Supply Reservoir appear in blue. The safety valve is above the wet tank.

Air from the supply tank, flows into TWO new reservoirs. The GREEN reservoir will be supplying air to the REAR brakes. The RED reservoir will be supplying air to the FRONT brakes.

The REAR brakes will be referred to as the PRIMARY brakes.

The FRONT brakes will be referred to as the SECONDARY brakes.

There is a very good reason for making this distinction next slide...

173

Identifying Primary and Secondary Circuits (continued)

On short wheel base vehicles, such as cars motorbikes and small trucks and vans, when a brake application is made, there is a transfer of weight onto the front wheels. (Most of these vehicles will be equipped with disc brakes at the the front and drum brakes at the rear.) This further indicates that the majority of braking is accomplished by the front brakes.

The arrows indicate forces applied during braking...

174

Identifying Primary and Secondary Systems (continued)

On a long wheel base vehicle such as a semi-trailer or other larger commercial vehicle, a bus for instance, there is no such transfer of weight onto the front axle. During braking, even heavy braking, the weight of the vehicle remains over the drive axles and the trailer axles (if equipped.) Therefore, the majority of braking effort is concentrated at the rear axle or axles. next slide...

175

Primary and Secondary Systems (continued)

Primary Brakes

Primary BrakesSecondary Brakes

Secondary Brakes

Trailer Brakes

Short wheel base vehicle

Long wheel base

vehicle

176

The Simple Dual Circuit SystemIn this diagram, the blue lines indicate supply air, the red lines indicate secondary or front brake air and the green lines indicate primary or rear brake air. A dual circuit foot valve has been installed. This foot valve works in exactly the same way as a single circuit foot valve. However, the dual circuit foot valve is split internally into two parts. The upper part delivers application air to the front brakes, while the lower part delivers application air to the rear brakes.

Compressor

Supply Reservoir

Primary Reservoir

Secondary Reservoir

Dual Circuit Foot Valve


177

The Simple Dual Circuit SystemOne-way check valves have been added at the inlets to the primary and secondary reservoirs. Should an air loss occur at the supply reservoir these one-way check valves will prevent the air from the primary and secondary reservoirs from back-flowing into the supply reservoir.


178

Secondary Air Loss in a Simple Dual Circuit SystemIn this example, an air loss will occur in the secondary system. The one-way check valve at the inlet of the primary reservoir will close, preserving the primary air. The low air warning devices will activate due to the air loss (either reservoir will trigger them.) As the pressure drops, the governor will load the compressor, but that air will simply be pumped to atmosphere due to the faulty secondary system.


179

Secondary Air Loss (Foot Valve Application)

The operator now makes a foot valve application. Because there is no reservoir air in the secondary tank, the front brakes will not work. The rear brakes, however, are still functional because the one- way check valve at the primary reservoir closed. Since the rear brakes are the primary brakes, the vehicle can be brought to a safe stop with little difficulty. Of course, continuing to operate the vehicle under these circumstances would be unsafe, if not irresponsible.


180

Primary Air Loss in a Simple Dual Circuit System


This time the air loss occurred in the primary system. The one-way check valve at the inlet to the secondary reservoir will close to protect that air. The low air warning devices will activate by 60 psi., and the governor will load the compressor at 85 psi or better. Again, air from the compressor is being pumped to atmosphere. The front brakes are the only brakes working. Since this is a long

air brake course

Documents

energy of heat

brake linings

speed speed

braking power

energy of motion

mechanical energy

normal braking

distancedistancewhen