Y U K O N AIR BRAKE AIR BRAKE M A N UA L

Transcription

1 AIR BRAKE Y U K O N AIR BRAKE M A N UA L

2 AIR BRAKE Y U K O N AIR BRAKE M A N UA L

3 Y U K O N A I R B R A K E M A N U A L FOREWORD In keeping with the classified driver licensing system and with special regard to the air brake endorsement, this manual has been compiled to assist you in your understanding of the basic functions of the air brake system. The study of this manual, along with practical instruction, is recommended to persons who are p reparing for the completion of the air brake examination as administered by the Yu k o n Department of Community and Transportation S e rvices, Registry of Motor Vehicles, or any approved training institution. Study questions are included so that you may test your understanding of the subject. This manual is presently used by a number of jurisdictions in Canada and the United States.

4 Y U K O N A I R B R A K E M A N UA L C O N T E N T S SECTION I BRAKES AND BRAKING Heat, Energy, Traction, Friction... 6 Speed, Weight, Distance... 6 How We Obtain Force... 7 Stopping Distance... 8 Section Summary Questions... 8 SECTION II THE COMPONENTS OF AN AIR BRAKE SYSTEM Compressor Reservoirs Foot Valve Brake Chambers and Slack Adjusters Wedge Type Brake Disc Brakes Section Summary Questions SECTION III HOW THE BASIC SYSTEM WORKS Simple Air Brake System One-Way Check Valve Gauges Brake Application Gauge Low Pressure Warning Wig-Wags Stop Light Switch Quick Release Valve Relay Valve Front Wheel Limiting Valve Tandem Rear Axles Parking Brakes Dual Control Valve and Reservoir System Mechanical Release Air-actuated Hydraulic Brakes Section Summary Questions SECTION IV TRAILER SYSTEMS Glad Hands Application Line Hand Operated Valves Two-Way Check Valve Tractor Protection Automatic Trailer Supply Valve System Trailer Supply Valve Tractor Protection Valve Tractor and Trailer Unit Coupled Charging the Trailer System Brake Application Foot Valve Brake Application Hand Valve Emergency Applications Service Line Rupture Supply Line Rupture Loss of Main Reservoir Air Manually Operated Trailer Supply Valves Section Summary Questions SECTION V DUAL AIR SYSTEMS A Simple Dual Circuit Air System Combination Truck and Trailer T-75 Off-Highway System, Single Unit L-75 Off-Highway System, Combination with Schedule X Trailer Schedule SX, Off-Highway Trailer with Spring Brakes Section Summary Questions SECTION VI PRE-TRIP INSPECTION Single Unit Combination Unit Brake Adjustment Air Operated Disc Brakes In Service Check Maintenance and Servicing of the Air Brakes Section Summary Questions Metric Conversion Table... 54

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6 Y U K O N A I R B R A K E M A N UA L SECTION I BRAKES AND BRAKING HEAT, ENERGY, TRACTION, FRICTION SPEED, WEIGHT, DISTANCE HOW WE OBTAIN FORCE STOPPING DISTANCE SECTION SUMMARY QUESTIONS 5

7 Y U K O N A I R B R A K E M A N U A L BRAKES AND BRAKING HEAT ENERGY TRACTION FRICTION For a vehicle to move along the highway, an internal combustion engine must convert its HEAT ENERGY into mechanical ENERGY. This mechanical ENERGY goes from the engine to the driving wheel tires by means of a system of connecting rods, shafts and gears. The final factor that moves the vehicle is the amount of TRACTION its tires have on the road surface. TRACTION is the ability of a tire to grip the road surface on which it rolls. The vehicle s acceleration rate depends on the power the engine develops and the amount of TRACTION the tires have on the road surface. FRICTION is the force which resists movement between two surfaces in contact with each other. To stop a vehicle, the brake shoe linings are forced against the machined surfaces of the brake drums, creating FRICTION. This FRICTION produces HEAT. The engine converts the ENERGY of HEAT into the ENER- GY of motion; the brakes must convert this ENERGY of motion back into the ENERGY of HEAT. The FRICTION between brake drums and linings generates HEAT while reducing the mechanical energy of the revolving brake drums and wheels. The heat produced is absorbed by the metal brake drums, which dissipate the heat by passing it off into the atmosphere. The amount of heat the brake drums can absorb depends on the thickness of the metal of which they are made. When enough FRICTION is created between the brake linings and the drums, the wheels stop turning; but the final factor that stops the vehicle is not the brakes, but the TRACTION between the tires and the road surface. If an engine of 200 horsepower accelerates a vehicle to 100 km/h in one minute, imagine the power needed to stop this same vehicle. Not only that, but the vehicle might have to be stopped in an emergency, in as little as six seconds (just 1 /10 of the time it took to reach 100 km/h). S E C T I O N I To stop the vehicle in 1 /10 of the time it took to accelerate would re q u i re a stopping force of ten times the acceleration f o rce the equivalent of approximately 2,000 horsepower. If the vehicle had six wheels, each wheel would have to p rovide 1 /6 of the braking force. If one or two of the wheels had brakes that were not p roperly adjusted, the other wheels would have to do more than their share of the braking, and that might be more than their brakes were constructed to stand. Excessive use of the brakes would then result in a buildup of heat greater than the brake drums could absorb and dissipate. Too much heat would result in brake damage and possible failure. 250 C 425 C 1100 C NORMAL MAXIMUM PANIC! Most brake linings operate best at around 250 C and should not exceed 425 C. It s important to understand that the power needed to stop generates heat which could ruin the brakes. SPEED WEIGHT DISTANCE The distance required to stop a vehicle depends on its speed and weight, in addition to the factors of energy, heat and friction. The brake force required to stop a vehicle varies directly with its weight and the square of its speed. For example, if the weight is doubled, the stopping force must be doubled to be able to stop in the same distance. If the speed is doubled, the stopping force must be i n c reased four times to be able to stop in the same distance. And, when weight and speed are both doubled, the stopping force must be increased eight times to be able to stop in the same distance. For example, a vehicle carrying a load of kgs force at 16 km/h is brought to a stop in a distance of 30 meters by normal application of the brakes. If this same vehicle carried kgs at 32 km/h, it would require eight times the braking force to stop the vehicle in 30 meters. This would be more braking force than the brakes could provide. No vehicle has enough braking force when it exceeds its limitations. 6

8 S E C T I O N I Y U K O N A I R B R A K E M A N UA L HOW WE OBTAIN FORCE A. MECHANICALLY Braking systems use devices to gain mechanical advantage. The most common device for this purpose is leverage. Look at this simple lever system: B. USE OF AIR F o rce can also be multiplied by the use of air to gain a furt h e r mechanical advantage. Everyone has felt the force of air on a windy day. Air can be compressed (squeezed) into a much smaller space than that amount of air normally would occupy. For instance, air is compressed in tires to support the weight of a vehicle. The smaller the space into which air is squeezed, the greater the air s resistance will be to being squeezed. This resistance creates pressure, which is used to gain mechanical advantage. If a constant supply of compressed air were directed thro u g h a pipe that was one inch square, and if a one inch square 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 by the air against the plug. A lever is placed on a pivot called the fulcrum. As the distance from A to C is four feet, and from C to B is one foot, the ratio is four to one (4:1). Force has been multiplied by the leverage principle. If a 100 lb. downward force is applied at point A, then the upward force at point B is 400 lbs. This is the result of the mechanical advantage of leverage. If the scale re g i s t e red ten pounds, for example, then it could be said the force was ten pounds on the one square inch surface of the plug. This would be ten pounds per square inch (10 P.S.I.) or kpa (kilopascals). The more the air in the supply tank has been compressed, the greater the force that would be exerted on the face of the plug. C. LEVERAGE & AIR PRESSURE In actual operation, pipes are round and plugs are diaphragms of flexible material acting against push rods. If compressed air of 120 P.S.I. acts on a diaphragm of 30 square inches, 3,600 lbs., of force is produced (120 x 30). Apply this force to a push rod to move a 6-inch slack adjuster operating a cam and the total force equals 21,600 inch pounds torque, (3,600 x 6) or 1,800 foot pounds torq u e (21,600 12). It requires foot pounds of torque to tighten the wheel on a car. This comparison illustrates the f o rce obtained from using mechanical leverage and air pressure combined. C o m p a re the Points A, C, B to the previous lever d i a g r a m. 7

9 Y U K O N A I R B R A K E M A N U A L S E C T I O N I STOPPING DISTANCE In addition to the above factors, a driver must understand what is meant by the term stopping distance. Stopping distance consists of three factors: Driver s reaction time + Brake lag + Braking distance. REACTION TIME: The time it takes from the moment a hazard is recognized to the time the brake is applied, approximately 3 /4 of a second. (Reaction time is often called thinking time.) BRAKE LAG: The time the air takes to travel through a properly maintained air brake system, (approximately 4 /10 of a second). BRAKING DISTANCE: The actual distance the vehicle travels after the brake is applied until the vehicle stops. (The distance depends on the ability of the lining to produce friction, the brake drums to dissipate heat and the tires to grip the road.) The professional driver never takes the brakes for granted. The braking system must be tested and the adjustment checked before placing the vehicle into service. The professional understands the braking system, realizes the capabilities and limitations, and learns to use them to the best advantage. Heavy vehicles require powerful braking systems that are obtained by use of mechanical leverage and air pressure. Brakes must be used keeping in mind the heat generated by friction. If the heat becomes too great, braking effectiveness will be lost. The heavier the load and the faster the speed, the greater the power needed to stop. Stopping distance is also affected by the driver s reaction time, brake lag, and braking distance. The pro f e s s i o n a l driver is well aware that the vehicle, even with properly adjusted brakes, will not stop as quickly as a passenger v e h i c l e. SECTION SUMMARY 1. What is the final factor that will determine if the vehicle will move? 2. What is the final factor that will determine if the vehicle will stop? 3. How is the heat dissipated that is generated by the brakes? 4. If one set of brake shoes are poorly adjusted, what eff e c t could it have on the remaining sets of brake shoes in the system? 5. What is meant by the term FRICTION? 6. If the weight of the vehicle is doubled, how many times must the stopping power be increased? 7. If both weight and speed of the vehicle a re doubled, how many times must the stopping power be increased to be able to stop in the same distance? 8. If the speed of the vehicle is doubled, how many times must the stopping power be increased to be able to stop in the same distance? 9. What is compressed air? 10. What does the abbreviation P.S.I. stand for? 11. If 40 P.S.I. is exerted against a diaphragm of 30 square inches in area, what is the total pounds of force that could abe exerted? 12. Brake lag is approximately 4/10 of a second in a wellmaintained, single circuit brake system. What is meant by brake lag? 13. What is meant by the following terms? Reaction distance Braking distance Stopping distance. N O T E S C O M PA R ATIVE STOPPING DISTA N C E S 8

10 S E C T I O N I Y U K O N A I R B R A K E M A N UA L 9 Section I of this manual has explained that it is possible to gain a mechanical advantage through the use of levers and that air under pressure can be used to gain a mechanical advantage. Section II will explain how air under pressure can be used to operate the air brakes of a vehicle. Piping diagrams have been kept simple in order to be easily understood. The piping arrangements found on vehicles in actual use on the highway might differ somewhat from the illustrations in this manual. The diagrams in this manual ARE NOT to be interpreted as regulations for, or specifications of, air brake systems.

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12 Y U K O N A I R B R A K E M A N UA L SECTION II THE COMPONENTS OF AN AIR BRAKE SYSTEM COMPRESSOR RESERVOIRS FOOT VALVE BRAKE CHAMBERS AND SLACK ADJUSTERS WEDGE TYPE BRAKES DISC BRAKES SECTION SUMMARY QUESTIONS 11

13 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I THE COMPONENTS OF AN AIR BRAKE SYSTEM A basic air brake system capable of stopping a vehicle has five main components: 1. A COMPRESSOR, to pump air 2. A RESERVOIR OR TANK, to store the compressed air 3. A FOOT VALVE, to regulate the flow of compressed air from the reservoir when it is needed for braking 4. BRAKE CHAMBERS & SLACK ADJUSTERS, the means of transferring the force exerted by the compressed air to mechanical linkages 5. BRAKE LININGS AND DRUM OR ROTORS, to create the friction required to stop the wheels It is necessary to understand how each of these components works before studying their functions in the air brake s y s t e m. THE COMPRESSOR Compressed air is the means of transmitting force to an air brake system. The source of the compressed air is the compressor. A compressor is designed to pump air. Air pumped into a tank will result in pressurizing the air, as was explained previously in this manual. A piston type compressor operates on a similar principle to that of the intake and compression strokes of an engine. I N TAKE STROKE: The downward stroke of the piston cre a t e s a lower pressure within the cylinder than the atmospheric p re s s u re outside the compre s s o r. This causes air to flow past the inlet valve (which the atmospheric pre s s u re has opened) into the cylinder. COMPRESSION STROKE: The upward travel of the piston c o m p resses the air in the cylinder. The rising pre s s u re cann o t escape past the inlet valve (which the compressed air has closed) and as the piston nears the top of the stroke, the pressurized air is forced past the discharge valve and into the discharge line leading to the reservoir. UNLOADING: When sufficient pressure has been built up, the compressor goes into an unloading stage. The unloader plunger holds the inlet valves off their seats until pressure drops to the pre-set cut in pressure which is regulated by the governor. 12 The compressor is driven by the vehicle s engine, by either belts and pulleys or shafts and gears. In vehicles which the compressor is driven by belts, the belts should be checked regularly for cracks and tension. Belt adjustment is usually checked by pressing with a finger midway between the pulleys. A greater distance between the pulleys will affect the amount of permissible slack. If the belts become too slack, they could slip, and the compressor will not achieve

14 S E C T I O N I I Y U K O N A I R B R A K E M A N UA L its maximum eff i c i e n c y. While checking the drive belts also check the compressor for broken mounting brackets or loose mounting bolts. The compressor is in constant drive with the engine. Whenever the engine is running, so is the compressor. It is usually lubricated from the engine lubrication system. Some compressors are self-lubricating and require regular checks of lubricant levels. There are periods of time when it is not necessary for the c o m p ressor to pump air. A common pre s s u re in an air brake system is from a low of 85 P.S.I. to a high of 105 P.S.I. Some systems operate between 105 P.S.I. and 125 P.S.I. Minimum p re s s u re is approximately 20 P.S.I. below maximum pre s s u re. When the pre s s u re has reached the system s maximum, the compressor goes into an unloading stage. Most compressors have two cylinders. In the unloading stage, the inlet valves are held open, allowing the air to be pumped back and forth between the two cylinders, instead of compressing it. During the unloaded stage, the compressor is able to cool. THE RESERVOIRS R e s e rvoirs or tanks hold a supply of compressed air. A safety valve protects them from being over pressurized and bursti n g. The safety valve protects the air brake system from an excessive buildup of air pressure. The valve consists of a spring loaded ball which will allow air to exhaust the reservoir pressure into the atmosphere. The valve s pressure setting is determined by the force of the spring. A safety valve is normally set at 150 P.S.I. If the pressure in the system rises to approximately 150 P.S.I., the pressure would force the ball off its seat, allowing the pressure to exhaust through the exhaust port in the spring cage. When reservoir pressure is s u fficiently reduced (to appro x- imately 135 P.S.I.), the spring will force the ball back onto its seat, sealing off the reservoir pressure. Not all safety valves have a manual release feature as shown in the diagram below. If the safety valve has to relieve the pressure, it indicates that the governor or compressor requires service or repair. This should be done by a qualified mechanic. It is most important that the air in the air brake system be kept as clean as possible. Dirt in the system can cause tro u b l e. The air from the atmosphere that enters the compressor must first pass through a filter to remove any dust particles from the air. The air filter must be cleaned regularly. A dirty filter will restrict the flow of air into the compressor, reducing its efficiency. Some vehicles have the inlet port of the compressor connected to the air cleaner of the vehicle s engine. A governor controls the minimum and maximum air pressure in the system. As the compressor is in constant drive with the engine, the governor s job is to unload the compressor when the desired pressure is reached. The governor does this by directing air pressure to the inlet valves of the compressor, holding them open when pressure in the system reaches its maximum. When the pressure in the system drops by approximately 20 P.S.I., the governor allows the inlet valves to close, returning the compressor to its pumping stage. IMPOR TANT! SAFETY VALVE ADJUSTMENTS MUST NOT BE TAMPERED WITH. ONLY QUALIFIED MECHANICS SHOULD SERVICE SAFETY VALVES. Reservoirs or tanks are also equipped with drain cocks to drain off any moisture. When air is compressed, it becomes heated. The heated air cools in the reservoir, forming condensation. Oil leaking past the piston rings of the compressor mixes with this moisture to form a sludge in the bottom of the reservoir. If allowed to accumulate, this sludge (water and oil) would enter the braking system. An excess of water in the system causes trouble with valves and other parts. In winter, water in the system may freeze, causing malfunction of valves or brake chambers. 13

15 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I To minimize the amount of water collection, all tanks must be drained daily. Under extreme conditions, tanks may have to be drained more than once a day. Park the vehicle on a level surface, set the parking brake, block the vehicle s wheels, open drain cocks fully. Allow all air pre s s u re to escape, which will then permit the moisture, collected in the tank, to drain. Some tanks have more than one compartment and each compartment has its own drain cock. Briefly opening the valve just to allow some of the air to escape DOES NOT drain the moisture! Some re s e rvoirs are equipped with automatic re s e rvoir drain valves (spitter valves). These may be drained manually by pushing up on a rod on the underside of the valve. Most vehicles are equipped with more than one reservoir. This gives the system a larger volume of main reservoir air. As the first tank collects more of the moisture and oil from the compre s s o r, it is called the wet tank. The second tank is usually further from the compressor and so collects drier or cleaner air. This tank is called the dry tank. Reservoirs vary in size. The size of the tanks to be installed on a vehicle depends on the number and size of the brake chambers. Brake chambers with a 30 square inch diaphragm will obviously require a larger volume of reservoir air than those equipped with 20 square inch diaphragms. THE FOOT VALVE The foot-operated valve is the means of applying air to operate the brakes. The amount of distance the treadle of the foot valve is depressed by the driver determines the amount of air pressure that will abe applied, but the MAX- IMUM APPLICATION WILL NOT EXCEED THE PRESSURE IN THE RESERVOIR. Releasing the foot valve treadle re l e a s e s the brakes. THE BRAKE CHAMBERS AND SLACK ADJUSTERS 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. If a leak occurs in the diaphragm, air is allowed to escape, reducing the effectiveness of the brake chamber. If the diaphragm is completely ruptured, brakes become ineffective. When the driver applies the brakes, depressing the treadle part way, the foot valve will automatically maintain the application air pressure, without the driver having to adjust the foot pressure on the treadle. Releasing the tread allows the application air to be exhausted through the exhaust ports to the atmosphere. Air treadles are spring loaded, producing a different feel from hydraulic brake application. 14 Front brake chambers are usually smaller than rear brake chambers because the front axles carry less weight than the rear axles. A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking. Air p re s s u re is fed through an inlet port. The air pushes against the diaphragm and the push rod. The push rod is connecte d by a clevis and pin to a crank arm type lever called a slack adjuster. This converts the pushing motion of the push ro d from the brake chamber to a twisting motion of the brake camshaft and S cams. When the air is exhausted, the spring in the brake chamber returns the diaphragm and push rod to the released position.

16 S E C T I O N I I Y U K O N A I R B R A K E M A N UA L As indicated by its name, the slack adjuster is also the means of adjusting the slack or free play in the linkage between the push rod and the brake shoes. This slack occurs as the brake linings wear. If the slack adjusters are not adjusted within the limitations, effective braking is reduced and brake lag time is increased. If too much slack develops, the diaphragm will eventually bottom in the brake chamber. If this condition is allowed to occur, the brake will not be effective at all. Some systems have automatic slack adjusters which adjust automatically to compensate for brake lining wear, thus maintaining the correct clearance between the brake lining and drum. Automatic slack adjusters must be checked regularly to ensure that correct adjustment is being maintained. NOTE: Detailed brake adjustment procedures are outlined in Section VI. The diagram below illustrates a common type of brake assembly used on truck rear axles and trailer axles. A front axle assembly has the brake chamber and slack adjuster mounted on the backing-plate because of the steering action. The diagram illustrates a common type of slack adjuster, showing the worm adjusting gear. When the brakes are fully applied, the angle between the push rod and the arm of the slack adjuster should be no less than 90 degrees (a right angle). On this type of slack adjuster, the adjusting w o rm bolt tis turned until the brake linings touch the dru m s and then back off, normally 1 /4 to 1 /2 a turn. A locking device, which may be a spring loaded collar over the head of the adjusting bolt, must be depressed when the wrench is slipped over the bolt head. Some slack adjusters use a spring loaded internal check ball to lock the adjustment. The more often the driver checks the slack the less probability of brake failure. Vehicles rarely lose their brakes because of loss of air; it is usually a loss of adjustment. 15

17 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I Brake lining material is attached to these shoes. The material used depends on the braking re q u i rements of the vehicle. Brake lining must give uniform output of brake effort with minimum fade at high temperature. Fading or a reduction in braking effort occurs when the heated drums expand away from the brake linings. The brake linings also lose their effectiveness with overh e a t i n g. The twisting action of the brake cam shaft and S cam f o rces the brake shoes and linings against the drums. The brake linings generate heat from friction with the brake d rum surf a c e The thickness of the drums determines the amount of heat they are able to absorb and dissipate to the atmosphere. D rums worn thin will build up heat too quickly. Dangerously undependable brake performance will result f rom distorted drums, weak re t u rn springs, improper lining, poor adjustment, or grease or dirt on the lining. Drums must never be turned or worn beyond the manufacturer s specifications. Spring brakes added to these units are of the piggy-back type. WEDGE TYPE BRAKES Here is another example of a type of brake assembly used on some air brake equipped vehicles. The action of the brake chamber push rod forces a wedge shaped push rod between the brake shoe rollers. This forces the brake shoe lining against the brake drum. DISC BRAKES The vehicle may be equipped with a single chamber or two chambers on each wheel, depending on the vehicle s size and style. These brakes may be equipped with a self-adjusting mechanism or with a manual star wheel adjuster. The star wheel adjustment is made with the vehicle jacked up, to ensure that the brake linings do not drag. Manual adjustment of wedge type brakes is usually a job for a mechanic. The air activated heavy truck disc brake is similar in principle to the disc brake used on passenger vehicles. Air pressure acts on a brake chamber and slack adjuster, activating the brakes. For light truck and passenger car braking systems, hydraulic pre s s u re is used. Instead of the cam or wedge used in conventional heavy truck drum brakes, a power screw is used. A power screw works like a C-clamp, so that the lining pads exert equal force to both sides of the disc or ro t o r. Some types if disc brakes have a built-in automatic adjuster. Disc brakes that re q u i re manual adjustment have adjustment specifications different from conventional S cam braking systems. ALWAYS CHECK M A N U FACTURER S SPECIFICATIONS BEFORE ADJUSTING. 16

18 S E C T I O N I I Y U K O N A I R B R A K E M A N UA L SECTION SUMMARY 1. What are the five basic components of an air brake system? 2. What are two standard main re s e rvoir pre s s u res for the operation of a vehicle s air brakes? 3. At what pressure should the governor cause the compressor to return to its pumping stage? 4. How is a plugged air filter likely to affect the air compressor? 5. What causes moisture to form in the air brake system? 6. When is the compressor able to accomplish most of its cooling? 7. How are most compressors lubricated? 8. How often should the reservoirs be drained? 9. Is it necessary to allow all the pressure to escape from the re s e rvoir in order to remove the moisture and sludge which may have accumulated? 10. What is the maximum pre s s u re available for a full brake application at any given time? 11. What will result if the brake drums are worn thin or turned too far? 12. If the governor valve failed to unload the compre s s o r, what would be the result? 13. What is the purpose of having more than one re s e rv o i r. 14. What are two functions of the slack adjuster? 15. Does the amount of slack in the brake linkages have any effect on the braking efficiency of the vehicle? 16. What is the advantage of keeping the brake chamber push rod travel adjusted to minimum travel? 17. What is the most common cause of loss of effective braking in an air brake system, loss of air pressure or loss of brake adjustment? 18. Do automatic slack adjusters on S cam brakes re q u i re checking? 19. Can the adjustment on air operated disc brakes differ from S cam type brakes. 20. What occurs when drum brakes become overheated? 21. What causes brake fade at high temperatures? 22. What is the main function of the foot valve? 23. Why does the feel of an operated foot valve differ from a hydraulic brake pedal? 24. On what principle does a disc brake operate? N O T E S 17

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20 Y U K O N A I R B R A K E M A N UA L SECTION III HOW THE BASIC SYSTEM WORKS SIMPLE AIR BRAKE SYSTEM ONE WAY CHECK VALVE GAUGES BRAKE APPLICATION GAUGE LOW PRESSURE WARNING WIG-WAGS STOP LIGHT SWITCH QUICK RELEASE VALVE RELAY VALVE FRONT WHEEL LIMITING VALVE TANDEM REAR AXLES PARKING BRAKES DUAL CONTROL VALVE AND RESERVOIR SYSTEM MECHANICAL RELEASE AIR-ACTUATED HYDRAULIC BRAKES 19 SECTION SUMMARY QUESTIONS

21 Y U K O N A I R B R A K E M A N U A L HOW THE BASIC SYSTEM WORKS A SIMPLE AIR BRAKE SYSTEM N O T E : ALL PIPING DIAGRAMS ARE USED TO ILLUS- TRATE BASIC BRAKING PRINCIPLES ONLY AND DO NOT ILLUSTRATE ACTUAL AIR BRAKE SYSTEMS. BASIC OPERATION PRINCIPLE Air is pumped by the compressor (1) to the reservoir (2), which is protected from over pressurization by a safety valve (3). The governor (5) controls the pre s s u re in the re s e rvoir by governing the compre s s o r. Air is available fro m the re s e rvoir to the bottom of the foot valve (6). The driver pushes down the foot valve treadle and air pre s s u re flows to the front and rear brake chambers (7,8). The brake chamber push rods move the slack adjusters. The slack adjusters ro t a t e the S cams, forcing the brake shoe against the drums. This causes friction which stops the wheels. The driver releases the foot valve treadle, and the air in the brake chambers is allowed to exhaust through the foot valve, releasing the brakes. Other valves are necessary to ensure smooth and efficient operation. S E C T I O N I I I first tank in the system collects most of the moisture, and is referred to as the wet tank. The second tank is referred to as the dry tank. Reservoirs vary in size. The size of the tank or tanks installed on a vehicle depends on the number and size of brake chambers. To prevent air from flowing back from the reservoirs to the c o m p re s s o r, a ONE-WAY CHECK VA LVE (9) is installed. This valve allows the air to flow in one direction only. The valve is spring loaded. Pressure at the inlet side overcomes the spring pressure and lifts the check valve ball, or disc, off its seat. Air passes through the valve to the outlet. When p re s s u re at the outlet becomes greater than at the inlet, t o g e t h e r with the spring pre s s u re, the check device seats, p re v e n t i n g air from flowing back through the valve. ONE-WAY CHECK VALVE In the diagram below, two reservoirs are shown. This gives the system a larger volume of main reservoir air. The first 20

22 S E C T I O N I I I Y U K O N A I R B R A K E M A N UA L GAUGES All vehicles are equipped with a pressure gauge (10) to indicate the amount of air pressure in the main reservoir system. This gauge is mounted in the cab, usually on the d a s h b o a rd. Common operating pre s s u res are 85 P.S.I. to 105 P.S.I. and 105 P.S.I. to 125 P.S.I., depending on the system. M o n i t o r i n gthe gauge will alert the driver to any unusual changes in pressure. WIG-WAGS There are two common types of wig-wag low pressure w a rning devices that may be used. Both types will drop into the driver s view when the pressure in the system drops below approximately 60 P.S.I. The automatic warning device will rise out of the driver s view when the pressure in the system rises above 60 P.S.I. The manual reset type must be placed in the out of view position manually and will not stay in place until the pressure in the system goes above 60 P.S.I. BRAKE APPLICATION GAUGE An additional gauge can be installed on the dash to indicate the application pressure when the brakes are applied. This gauge can be piped to indicate the pressure of either a foot application or of a hand application. (Hand application will be explained later in the manual.) Whichever warning system is used, buzzer, lights or wigw a g, the driver MUST STOP the vehicle and find the cause of the air loss. The air pressure remaining in the system (approximately 60 P.S.I.) is enough for brake application if the driver acts promptly. STOP LIGHT SWITCH Any driver following must be warned that speed is being reduced or the vehicle is being stopped. The STOP LIGHT SWITCH (12) is an air operated electric switch which turn s on the brake lights on the rear of the vehicle when a brake application is being made. QUICK RELEASE VALVE The application of the brakes in the simple system was described earlier. In such a basic system, when the driver releases the foot valve, it would be necessary for the air LOW PRESSURE WARNING All vehicles equipped with air brakes must have a device to warn the driver if the air pressure in the system drops to a dangerous level. Should the air pressure drop below approximately 60 P.S.I. due to overuse or leaks, the low pressure indicator switch (11) will turn on a red warning light on the dash, or cause a buzzer to sound. Some vehicles are equipped with both a l i g h t and a buzzer to warn the driver of a low air pressure condition. 21

23 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I I under pressure in the brake chambers to return back to the foot valve to release the brakes. This releasing action would be slowed in long wheel base vehicles, due to longer lines between the foot valve and the rear brake chambers. To allow the brakes to release quickly and fully, by discharg i n g the application air near the brake chambers, a quick re l e a s e valve (13) may be installed. RELAY VALVE The foot valve is usually located closer to the front wheels than to the rear wheels. The longer the distance from the foot valve to the rear chambers, the more time it will take (known as brake lag) before the rear brakes apply. To correct this condition on a long wheel base vehicle, a RELAY VA LVE (14) is installed near the rear brake chambers. A larger diameter pipe is connected between the main reservoir and the relay valve. The air line from the foot valve to the relay valve now becomes a control line. (The air in the control line dead ends at the relay valve.) When the foot valve is depressed, the air pressure in the control line acts on the top section of the relay valve, causing the re l a y v a l v e to relay reservoir air directly to the rear brake chambers through the larger diameter pipe. The pressure of the reservoir air delivered in this way will be the same as the control pre s s u re delivered by the foot valve. Releasing the foot valve exhausts the control air to the relay valve, allowing it to cut off the flow of reservoir air to the rear chambers, in turn exhausting the air in the brake chambers by the quick release feature of the relay valve. The driver places the control valve in the normal position for dry road surfaces and the front braking application pre s- s u re is normal. On a slippery road surface, the driver may flip the control valve to the slippery road position. In this position, the control valve will cause the LIMITING VALVE to operate. Application air pressure to the front wheels is then reduced to 50 percent of the application air pressure being delivered to the rear brake chambers. Some systems are equipped with an AUTOMATIC LIMITING VALVE. (17) FRONT WHEEL LIMITING VALVE For better steering control on a slippery road surface, it can be an advantage to reduce the braking effect to the f ront wheels. This can be accomplished by installing a control valve (15) in the cab, and a FRONT WHEEL LIMI- TING VALVE (16) on the front axle. This valve will hold off brake application to the front wheels from 0 to 10 P.S.I., depending on how it has been preset. Between the preset pressure and 40 P.S.I. of brake application, the reduction is approximately 50 percent. Brake applications between 40 P.S.I. and 60 P.S.I. are reduced by less than 50 percent. Brake applications over 60 P.S.I. are not reduced; full application pressure is directed to the front wheels. 22

24 S E C T I O N I I I Y U K O N A I R B R A K E M A N UA L CHECK YOUR UNDERSTANDING From the above diagram name each numbered component and briefly describe its function in the system. Write your answers on a separate sheet of paper. TANDEM REAR AXLES The air brake system discussed previously is that of a vehicle equipped with a single rear axle. The diagram illustrates an air brake system of a vehicle equipped with an automatic front wheel limiting valve (17), a quick release valve (13) and a tandem set of rear axles. Both axles of the tandem set are equipped with brakes. A relay valve (14) is used to provide quicker application pressure to the tandem rear axles. 23

25 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I I PARKING BRAKES (SPRING BRAKES) Application of the dash control valve exhausts air from the spring brake chamber, causing spring force to activate the spring brake. Spring type parking brakes may be installed on an air brake equipped vehicle to ensure a reliable parking brake system. In the service brake system, the brakes are held retracted by springs, and applied by air pressure. Spring type parking brakes are applied and remain applied WITHOUT AIR PRESSURE. The parking brake chambers are attached to the service brake chambers and operate the brakes through the same linkage. Therefore, the effectiveness of the parking brake depends on the service brake adjustment. A cont rol valve (usually a square, yellow button) in the cab allows the driver to exhaust air out of the parking brake circuit to apply the brakes, or to repressure the circuit to release them. The system can also act as an emergency brake. Loss of air from the main system may automatically apply the brakes, depending on how the system is piped. During normal driving, air pressure cages the spring, holding it ready for parking or emergency braking. PARKING BRAKE SYSTEMS The installation of parking brakes and their piping arr a n g e- ments into a vehicle air brake system will vary depending on the vehicle make. Control valves will vary, depending on the manufacturer and type of piping arrangements. During normal service brake operation, the spring brake does not apply. Air pressure keeps the spring caged. This type of spring loaded valve requires that the driver push the button to release the parking brakes. This valve cannot be left in the released position below appro x i m a t e l y 35 P.S.I. pre s s u re in the main re s e rvoir system. Any time the main reservoir pressure drops to approximately 35 P.S.I., this valve will exhaust automatically, placing the parking brakes into full application. Similar types of spring loaded valves require that the driver pull the button out to release the parking brakes. There is a single type of push-pull control valve in use that does not have an automatic release feature. To apply the parking brakes, the valve must be operated manually, even though the main reservoir pressure has been emptied. 24

26 S E C T I O N I I I Y U K O N A I R B R A K E M A N UA L USING THE PARKING BRAKES The above diagram illustrates spring parking brakes (18) added to the brake chambers of the rear axle on the single unit vehicle. A control valve (19) is mounted on the cab. A supply line of reservoir air is piped from the dry tank to the control valve. Opening the control valve admits main reservoir air pressure to the parking brake units, releasing them. Closing the control valve shuts off the supply of reservoir air pressure and exhausts the existing pressure in the parking brake units, thus applying the brakes. SINGLE CONTROL PARKING BRAKE SYSTEM The diagram below illustrates the use of a third tank with a single control, used on the rear axle of a single unit. A control valve (19) is mounted in the cab. A supply line of reservoir air is piped from the dry tank through a oneway check valve (9) into a third tank. Closing the control valve shuts off the supply of re s e rvoir air pre s s u re and exhausts the existing pressure in the parking brake units, allowing them to apply the brakes. If the main reservoir air in the system should be emptied, the parking brakes will not apply, because the one-way check valve will isolate the pressure in the third tank. If the spring (parking) brakes are to be used to stop the vehicle, the driver will have to set the parking brakes manually with the cab c o n t rol valve. 25

27 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I I DUAL CONTROL VALVE AND RESERVOIR SYSTEM Some vehicles, such as buses, may also be equipped with an EMERGENCY RELEASE TANK (2). In this system, if main reservoir pressure is lost, the spring brakes will automatically apply. If the parking brakes have applied because of a loss of main reservoir air, the driver can draw reserve air from the e m e rgency release tank to release the parking brakes. A dual parking brake control (19) is used together with the e m e rgency release tank. The driver must press on the e m e rgency release button. Releasing the emerg e n c y release button will automatically allow the parking brakes to re-apply. The emergency release would only be used to move the vehicle from an undesirable location, if the parking brakes had been applied due to a low main reservoir air condition. CAUTION: Parking brakes should be in the released position before making a service brake application. A full brake application, made when the parking brakes are applied, can compound the force exerted on the slack adjusters and linkage which could result in damage or brake failure. Compounding is the combination of two forces; the force applied by the spring brakes and the force applied by the service brake. NOTE: Spring type brakes are primarily a parking brake, but in the event of loss of main reservoir air, they can assist in stopping the vehicle. How quickly they will stop the vehicle depends on such factors as: The weight and speed of the vehicle; The steepness of the grade; The spring force of the spring brakes which have been installed; and, The adjustment of the service brakes. MECHANICAL RELEASE Some spring type parking brakes can be released mechanically by winding them off or caging them. A bolt, which runs through the centre of the chamber body, is turned to compress the spring. It may be necessary to first remove a lock plate and stud, to gain access to the head of the bolt. 26

28 S E C T I O N I I I Y U K O N A I R B R A K E M A N UA L Other types have a plug which must first be removed and a bolt inserted. In some cases, a special wrench is required. Instruction on how to cage is usually on the body of the parking brake. If all air is lost and the vehicle has to be towed, the parking brake can be released by winding them off. Always block the wheels when caging the parking brake spring. Caging means the brakes are being released. AIR-ACTUATED HYDRAULIC BRAKES An air-actuated braking system consists of a compressor, governor, air storage tanks, dual circuit foot valve and two brake chambers. The system may also include spring type parking brakes. The brake chambers are attached to two master cylinders that direct hydraulic pre s s u re to the foundation brakes. When the foot valve is depressed, air pr e s s u re flows i n t o the primary (rear brake) and secondary (front brake) circ u i t s. The air pre s s u re in each circuit acts upon that circ u i t s brake chamber which transmits force to the master cylinder. Each master cylinder in turn transmits hydraulic pressure to the wheel cylinders, actuating the front and rear axle service brakes. When the driver pushes the parking brake control valve to the released position, air pre s s u re behind a diaphragm in the spring brake chamber compresses the spring and maintains the parking brake in a released position. When the parking brake is applied by pulling the parking brake hand valve out, air in the diaphragm chamber is released and the spring extends, applying the rear brakes. If the main re s e rvoirs drop to approximately 35 psi, the parking brake valve will exhaust automatically, placing the parking brakes into full emergency/parking application. WARNING! Parking brakes should never be disassembled without first compressing the spring with a wind-off bolt. These springs are under extreme pressure and could cause serious personal injury if disassembly is attempted by anyone not experienced in servicing these units. Disassembly is a job for a mechanic. 27

29 Y U K O N A I R B R A K E M A N U A L S E C T I O N I I I SECTION SUMMARY 1. How can the driver tell how much air pressure there is in the main reservoirs? 2. What must the driver do when a low pre s s u re warn i n g system activates? 3. What is the purpose of a quick release valve? 4. What is the purpose of a relay valve? 5. What is the purpose of using a larger diameter pipe between the main reservoir tank and the relay valve? 6. If the front axle limiting valve is in the slippery ro a d position, and the foot valve is depressed to make a brake application of 30 P.S.I., how much pressure will be applied to the front brake chambers? 7. How is the re s e rvoir protected from over pre s s u r i z a t i o n? 8. What stops pressurized air from flowing back into the compressor from the dry tank? 9. At what pre s s u re should the low pre s s u re warn i n g device activate? 10. How is brake lag to the rear wheels minimized? 11. When should a driver use the front wheel limiting valve? 12. What is meant by compounding the brakes? 13. Why are spring brakes a reliable type of parking brake? 14. How are parking brakes held in the released position? 15. What are the functions of the cab-mounted parking brake control valve? 16. Will parking brakes apply automatically in all braking systems? 17. What is the reason for releasing the parking brakes before making a full brake application test? 18. What is the purpose of an emergency release reservoir tank in a parking brake system? 19. How can some types of parking brakes be released without the use of air pressure? 20. What is the danger of disassembling a parking brake unit? N O T E S 28

30 Y U K O N A I R B R A K E M A N UA L SECTION IV TRAILER SYSTEMS GLAD HANDS APPLICATION LINE HAND OPERATED VALVE TWO-WAY CHECK VALVE TRACTOR PROTECTION AUTOMATIC TRAILER SUPPLY VALVE SYSTEM TRAILER SUPPLY VALVE TRACTOR PROTECTION VALVE TRACTOR AND TRAILER UNIT COUPLED CHARGING THE TRAILER SYSTEM BRAKE APPLICATION FOOT VALVE BRAKE APPLICATION HAND VALVE EMERGENCY APPLICATIONS SERVICE LINE RUPTURE SUPPLY LINE RUPTURE LOSS OF MAIN RESERVOIR AIR MANUALLY OPERATED TRAILER SUPPLY VALVES 29 SECTION SUMMARY QUESTIONS

31 Y U K O N A I R B R A K E M A N U A L S E C T I O N I V TRAILER SYSTEMS It can be assumed that the system illustrated up to this point is the air brake system of a truck unit. If a trailer was coupled to this unit, the brakes of the trailer would have to be operated from the tractor. The following diagram shows the piping of a unit, similar to the tandem axles of the truck unit. The trailer has a single axle equipped with brake chambers. A TEE has been installed in the application line between the foot valve and the truck axle system. A line has been connected from this tee to the trailer by a set of couplers called GLAD HANDS (20). GLAD HANDS This term refers to the coupling device used to connect the s e rvice and supply lines of the trailer to the truck or tractor. These couplers have a snap-lock position, and a ru b b e r seal prevents air from escaping. Before connection is made, couplers should be clean and f ree of dirt and grit. When connecting the glad hands, start with the two seals together and the couplers at a 90 degree angle to each other. A quick snap downwards will join and lock the couplers. Vehicles equipped with dead-end couplers should have these protection plates in use whenever the vehicle is used without a trailer. This will prevent water and dirt from entering the coupler and lines. If the unit is not equipped with dead-end couplers, the glad hands of the service line can be locked to the glad hand of the supply line to keep water and dirt from entering the unused lines. The cleaner the air supply is kept, the less chance of brake problems! Glad hands and lines should also be secured to prevent the line from bouncing off the vehicle. This could seriously damage the couplers. THE APPLICATION LINE This application line is referred to as a SERVICE LINE. In this simple system used for an example, the driver depre s s e s the foot valve treadle, and application air will be delivered to the tractor brake chambers and to the trailer brake chambers. When the driver releases the foot valve treadle, the application air to the trailer brake chambers must re t u rn to the foot valve to be exhausted to the atmosphere. In this example system, there would be disadvantages: If the trailer broke away from the tractor, the trailer would have no brakes. 30