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Air Brake Manual • 11

The Components of

an Air Brake System

Section 2

12 • Air Brake Manual

Section One 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 Two will explain how air under pressure can be used to operate the air brakes of a vehicle. Piping illustrations 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 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 with a governor to control it.
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 and slack adjusters to transfer the force exerted by the compressed air to mechanical linkages.
5. Brake linings and drums or rotors to create the friction required to stop the wheels. It is necessary to understand how each of these components work before studying their functions in the air brake system.

Compressor and Governor

Compressed air is used to transmit force in an air brake system. The source of the compressed air is a compressor (1). A compressor is designed to pump air into a reservoir which results in pressurized air. The compressor is driven by the vehicle’s engine, either by belts and pulleys or shafts and gears. In vehicles where the compressor is driven by belts, they should be checked regularly for cracks and tension. Also, check the compressor for broken mounting brackets or loose bolts. The compressor is in constant drive with the engine. Whenever the engine is running, so is the compressor. When pressure in the system is adequate, anywhere from a low of 80 psi to a high of 135 psi it is not necessary for the compressor to pump air. A governor (2) controls the minimum and maximum air pressure in the system by controlling when the compressor pumps air. This is known as the “loading” or “unloading” stage. Most compressors have two cylinders similar to an engine’s cylinders. When the system pressure reaches its maximum, which is between 115 and 135 psi, the governor places the compressor in the “unloading” stage. The compressor must be able to build reservoir air pressure from 50 to 90 psi within three minutes. If unable to do so the compressor requires servicing. A compressor may not be able to build air pressure from 50 to 90 psi within three minutes if the air filter is plugged or if the belt was slipping, if these were not at fault the compressor could be faulty.

Governor

Pressure

setting

spring

Exhaust port

Unload port

Reservoir port

Exhaust port

Unload port

Reservoir port

14 • Air Brake Manual

• Compression stroke: The upward motion of the piston compresses the air in the cylinder. The rising pressure cannot escape past the inlet valve (which the compressed air has closed). 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.

Reservoirs

Reservoirs or tanks hold a supply of compressed air. The number and size of the reservoirs on a vehicle will depend on the number of brake chambers and their size, along with the parking brake configuration. Air brake vehicles are equipped with more than one reservoir. This gives the system a larger volume of main reservoir air. The first reservoir after the compressor is referred to as the supply or wet (5) reservoir. The other reservoirs are known as primary (8) and secondary (10) or dry (8)(10) reservoirs. When air is compressed, it becomes hot. The heated air cools in the reservoir, forming condensation. It is in this reservoir that most of the water is condensed from the incoming air. If oil leaks past the piston rings of the compressor and mixes with this moisture, it forms sludge, which

accumulates in the bottom of the reservoir. If allowed to accumulate, this sludge (water and oil) would enter the braking system and could cause trouble with valves and other parts. In winter, water in the system may freeze, causing the malfunction of valves or brake chambers. Reservoirs are equipped with drain valves so that any moisture or sludge that may have accumulated can be drained. If you notice sludge when draining your system, have it inspected by a mechanic. To minimize the amount of water collection, all reservoirs must be drained daily. Under extreme conditions, reservoirs may have to be drained more than once a day. To drain the reservoirs always start with the wet reservoir on the tractor. Open the drain valve fully and allow all air pressure to escape, which will also exhaust the moisture collected in the reservoir. Some reservoirs have more than one compartment and each compartment has its own drain valve, which must be drained individually. Briefly opening the valve just to allow some of the air to escape does not drain the moisture! It is not safe to assume that the wet reservoir, or the presence of an air dryer is reason to neglect the other reservoirs on the power unit, trailers or dollies. They should all be completely drained daily by opening the drain valve fully and allowing all of the air to escape. This should be done during the post–trip inspection at the end of the day. See post–trip Inspection later in this manual for more information. Some reservoirs may be equipped with automatic reservoir drain valves (spitter valves). These valves will automatically exhaust moisture from the reservoir when required, although they should be checked daily and drained periodically to ensure the mechanism is functioning properly. Any loose or disconnected wires associated with the valve heaters should be repaired immediately.

Reservoir

Intake air filter

Unload

plunger

To reservoir

Inlet valve

Discharge valve

Compressor (Compression stroke)

Piston

Air Brake Manual • 15

Air Dryer

An air dryer (3) may be installed between the compressor and the wet reservoir to help remove moisture from the compressed air. It may be partially filled with a high moisture–absorbent desiccant and an oil filter, or it may be hollow with baffles designed to assist in separating the moisture from the air. Both types of air dryers use air pressure to purge or eject the accumulated contaminants from their desiccant bed. The purge valve has a heater element, which prevents the moisture from freezing in cold climate operation. The wiring connected to the heater should be inspected for loose or disconnected wires. They are also equipped with a safety valve.

Control

port

Supply

port

Cut–off

piston

Exhaust

Purge

valve

Delivery port

One–way check valve

One–way check valve

Orifice

Desiccant bed

Control Port

Dried Air

Check valve

assembly

Delivery Port

Heater

element

Exhaust

Purge

valve

Cut–off

piston

Supply

Port

Reservoir

Compressor

Governor Sump

Air Dryer (Purge cycle)

Desiccant

Cartridge

Air Dryer (Drying cycle)

Air Dryer

Control Port

Supply Port

Oil Separator

Air Brake Manual • 17

A brake chamber (11) (14) (32) 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.

Front brake chambers (32) are usually smaller than those in the rear because front axles carry less weight. A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking. Air pressure is fed through an inlet port. The air pushes against the diaphragm and the push rod. The push rod is connected by a clevis and pin to a crank arm–type lever called a “slack adjuster”. This converts the pushing motion of the push rod from the brake chamber to a twisting motion of the brake camshaft and S–cams. When the air is exhausted, the return spring in the brake chamber returns the diaphragm and push rod to the released position. As indicated by its name, the slack adjuster adjusts 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, and the brakes will not be effective. Illustrated below are two common types of manual slack adjusters, showing the worm adjusting gear.

Push rod

Brake chamber

Diaphragm Diaphragm

return spring

Air inlet

Mounting bolts

Clevis and pin

Slack

adjuster

Manual Slack Adjusters

Ball Indent Slack Adjuster Positive Lock Slack Adjuster

Lock screw

Adjusting bolt Worm shaft

Worm gear

Locking collar

Spline Spline

Grease fitting

Adjusting bolt

Brake Chamber and Slack Adjuster (Brakes on)

18 • Air Brake Manual

On manual slack adjusters, the adjusting worm bolt is turned until the brake linings touch the drums and then backed off, normally N to M 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. This is known as a positive lock slack adjuster.

Or they could use a spring–loaded internal check ball to lock the adjustment, and it must be removed to make any adjustment. This is known as a ball indent slack adjuster. The more often the driver checks the “slack”, the less the probability of brake failure. Vehicles rarely “lose” their brakes because of air loss; it is usually because they are out of adjustment. It is the driver’s responsibility to ensure that brakes are adjusted correctly. A simple service brake application at low speed to check brake adjustment is not adequate. Braking at highway speed causes brake drum expansion due to heat, which in turn requires greater push rod travel to maintain the same braking force. If a brake is out of adjustment there would not be enough reserve stroke of the push rod travel to compensate for drum expansion. This would cause a brake fade and would greatly extend stopping distance. If travelling down a hill, this could cause complete brake loss. Note: Detailed brake adjustment procedures are outlined in Section 8.

Pushrod

Air

inlet

Slack

adjuster

Brake Chamber and Slack Adjuster (Brakes on)

Thrust washer

Clevis

Clevis pin (large)

Clevis pin (small)

Actuator rod

Hairpin clip

Boot and strap

Actuator (adjusting sleeve)

Roller (pin)

Actuator piston

Pressure relief capscrew (pull pawl)

Pawl spring

Adjusting pawl

Worm

Worm seal

Adjusting bolt

Grease groove

Grease fitting

Housing

Worm gear

Automatic Slack Adjuster

20 • Air Brake Manual

Brake lining material is attached to the shoes. The material used depends on the braking requirements of the vehicle. Brake lining must give uniform output of brake effort with minimum fade at high temperatures. Fading or reduction in braking effort occurs when the heated drums expand away from the brake linings. The brake linings also lose their effectiveness with overheating. The twisting action of the brake cam shaft and S–cam forces the brake shoes and linings against the drums. The brake linings generate heat from friction with the brake drum surface. The thickness of the drums determines the amount of heat they are able to absorb and dissipate into the atmosphere. Drums worn thin will build up heat too quickly. Dangerously undependable brake performance will result from distorted drums, weak return springs, improper lining, poor adjustment, or grease or dirt on the lining. Drums must never be machined or worn beyond the manufacturer’s specification.

Wedge Brakes

This is another example of a 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.

The vehicle may be equipped with a single or dual 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 insure that the brake linings do not drag. Manual adjustment of wedge brakes is usually done by a qualified mechanic.

Brake

chamber

Adjusting wheel

Brake shoe

roller

Push rod

Shoe return

spring

Brake lining

Brake shoe

Wedge Brake – Single Chamber

Brake

chamber

Single chamber Dual chamber

Shoe return

springs

Brake lining

Adjusting wheel

Brake

chambers

Adjusting wheel

Wedge Brakes

Air Brake Manual • 21

Disc Brakes

The air–activated heavy truck disc brake is similar in principle to that used on passenger vehicles. Air pressure acts on a brake chamber and slack adjuster, activating the brakes. 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 rotor. Some types of disc brakes have a built–in automatic adjuster. Disc brakes that require manual adjustment have adjustment specifications that differ from conventional S–cam braking systems. Always check the manufacturer’s specifications before adjusting. Disc brake assemblies may have a spring parking brake unit attached to the service brake chamber.

Air–Over–Hydraulic Brake Systems

Air–over–hydraulic brake systems were developed for medium weight vehicles because:

• diesel engines do not have a source for vacuum boosting unless they are equipped with a vacuum pump.
• medium weight vehicles do not require a full air brake system.
• it gives the option of pulling an air brake equipped trailer. These systems combine the best features of an air and hydraulic brake system. They use hydraulic brakes at each wheel with their reliable self adjusters and limited maintenance. On these systems the air is used to either actuate the hydraulic brakes or boost the hydraulic brake pressure as explained in the following.

Air–Actuated Hydraulic Brake System

(Air Brake Endorsement Required) An air–actuated system usually has the same components of a standard air supply system including a warning buzzer and light, compressor, governor, wet and dry reservoirs, and a foot valve that could be a single or dual type. These components are found usually in the same places as on a full air brake system. Also there are one or two air actuated hydraulic pressure converters depending on if the system is a single or a dual system. This system consists of an air chamber or cylinder attached to a hydraulic master cylinder. When the foot valve is depressed, the air pressure actuates the pushrod from the air unit that pushes against the master cylinder piston, producing hydraulic pressure directed through tubing to the wheel cylinders actuating the front and rear axle service brakes.

Disc Brake

Air Brake Manual • 23

This is where air from the reservoir is directed. As the pressure from the master cylinder increases, the air control section in the booster will open and begin to deliver air pressure to the rear of the air cylinder. The air cylinder pushrod transfers pressure on a piston in the hydraulic section of the booster, increasing the hydraulic pressure at the wheel cylinders.

The driver has full control of the braking force as the air control section modulates the boost pressure in proportion to the master cylinder pressure. If the vehicle was to lose all of the air pressure the brake system would lose the air assist boost, however the hydraulic system would continue to work but at reduced effectiveness. An air brake endorsement on a driver’s licence is not required to operate a vehicle with this brake system. Consult the operator’s manual for the vehicle you drive for maintenance requirements.

Air lines

Hydraulic

wheel

cylinders

Compressor

Reservoir Hydraulic master

cylinder

Brake

pedal

Booster unit Hydraulic line

Air–Boost Hydraulic Brake System

Air lines Booster unit

Hydraulic

line

Hydraulic

wheel

cylinders

24 • Air Brake Manual

Section Summary Questions

1. What are the five basic components of an air brake system?
2. At what pressure should the governor cause the compressor to return to its “loading” stage?
3. At what pressure will the governor place the compressor in the “unloading” 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 reservoir in order to remove the moisture and sludge which may have accumulated?
10. What is the maximum pressure 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 compressor, what would protect the reservoirs from becoming over pressurized and bursting?
13. What is the purpose of having more than one reservoir?
14. What are two functions of the slack adjusters?
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 within limitations?
17. What is the most common cause of loss of effective braking in an air brake system?
    18. Do automatic slack adjusters on S–cam brakes require checking?
    19. Can the adjustment on air–operated disc brakes differ from S–cam brakes?
    20. What occurs when drum brakes become overheated?
    21. What causes brake fade?
    22. What is the main function of the foot valve?
    23. Why does the “feel” of an air–operated foot valve differ from a hydraulic brake pedal?
    24. On what principle does a disc brake operate?
    25. What type of air–over–hydraulic brake system requires the operator to hold an air brake endorsement?