Air Brake Systems - How They Work
Air brakes are used in trucks, buses, trailers and semi-trailers. This is the preferred type of braking system for these vehicles for several reasons. First, the use of air allows multiple vehicle units to be coupled so that all units have braking capability and so that all of those units' brakes may be controlled from the cab. Coupling would be infeasible if a liquid were used as the mode of transmission of force, as it is in hydraulic brakes. In addition, the use of an air brake system allows for the incorporation of an emergency braking system that utilizes parts of the service brake and parking brake systems. Emergency braking systems are required on all semi-trailers by CFR 49 393.43, as it states "Every motor vehicle, if used to tow a trailer equipped with brakes, shall be equipped with a means for providing that in the case of a breakaway of the trailer, the service brakes on the towing vehicle will be capable of stopping the towing vehicle."
Air brake systems are three braking systems combined:
- The service brake system applies and releases the brakes when one uses the brake pedal during normal driving.
- The parking brake system applies and releases the parking brakes when one uses the parking brake control.
- The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.
The air brake system:
An air brake system uses air as a way to transmit pressure from the driver's control to the service brake. It also includes an air-over-hydraulic brake system.
Air compressor and governor:
The air compressor pumps air into the air storage tanks (reservoirs). The air compressor is connected to the engine through gears or a V-belt. The compressor may be air cooled or may be cooled by the engine cooling system. It may have its own oil supply, or be lubricated by engine oil. If the compressor has its own oil supply it has to be checked regularly. The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the "cut-out" level (no higher than 135 pounds per square inch, or psi or 900 kPa), the governor stops the compressor from pumping air. When the tank pressure falls to the "cut-in" pressure (no lower than 85 psi or 590 kPa), the governor allows the compressor to start pumping again.
Today, most vehicles that use air brakes are equipped with an air dryer. An air dryer is used to take the moisture out of the air, so that water condensation will not build up in the air storage tanks and cause the brakes to fail, such as when the water freezes in the winter.
Air Storage and Drain Tanks:
Air storage tanks are used to hold compressed air. The number and size of air tanks varies among vehicles. The tanks will hold enough air to allow the brakes to be used several times even if the compressor stops working. Compressed air usually has some water and some compressor oil in it. This is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Each air tank is equipped with a drain valve in the bottom. There are two types:
- Manually operated by turning a quarter turn, or by pulling a cable. One must drain the tanks his or herself at the end of each day of driving. Automatic. The water and oil are automatically expelled from the valve (these valves are equipped for manual draining as well).
- Spit valve. The water and oil are automatically expelled from a spit valve. The type of valve "spits" out water and air each time the governor cycles. The automatic types are available with electric heating devices. These help prevent freeze up of the automatic drain in cold weather.
Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps to reduce the risk of ice in air brake storage tanks, valves, and other parts during cold weather. Ice inside the system can cause brake failure.
A safety relief valve is installed in the first tank the air compressor pumps air into. The safety valve protects the tank and the rest of the system from too much air pressure. The valve is usually set to open at 150 p.s.i. (1030 kPa)
The brakes are applied by pushing down the brake pedal (also called the foot valve or treadle valve). The harder you push down on the pedal, the more air pressure is applied from the storage tanks into the brake chambers. Letting up on the brake pedal exhausts the air pressure from the brake chambers and releases the brakes. The air pressure used to apply the brakes must be built up in the reservoirs by the compressor. Pressing and releasing the pedal (fanning) can unnecessarily let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work. When the brake pedal is pushed down, two forces push back against the driver's foot. One force comes from a spring in the valve. The second force comes from the air pressure going to the brake chambers. This lets the driver feel how much air pressure is being applied to the brake chambers. However, this "feel" does not tell the driver how much force is being applied to the brakes because that depends on brake adjustment.
Drum brakes (foundation brakes) may be used at each wheel. The most common type is the S-cam drum brake (so called because of the mechanism that applies force to the brake shoes). Brake drums are located on each end of the vehicle's axles. The wheels are bolted to the drums. The braking mechanism is inside the drum. To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction which slows the vehicle (and creates heat). The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can cause brake failure.
When one pushes the brake pedal, air is let into each brake chamber (Figure 5-2). Air pressure pushes the push rod out, moving the slack adjuster, thus twisting the brake cam shaft. This turns the S-cam. The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When one releases the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again. It is a common use in heavy trucks over 15 tonnes of GVW. MAN, Mercedes-Benz, Scania and Volvo is using in their construction trucks.
The CamLaster brake has two key design differences over traditional S-cam brakes. One feature is a completely internal adjustment system which is designed to continually keep the brake in proper adjustment. S-cam brakes, on the other hand, require an external slack adjuster. The second feature is a unique cam design that applies the brake shoe. Unlike a standard drum brake that has either a single or double anchor-pin brake, CamLaster slides the shoes down an inclined ramp on a cam to evenly contact the brake drum.
The brake chamber push rod pushes a wedge directly between the ends of two brake shoes. This shoves them apart and against the inside of the brake drum. Wedge brakes may have a single brake chamber, or two brake chambers, pushing wedges in at both ends of the brake shoes. Wedge type brakes may be self-adjusting or may require manual adjustment.
In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. But instead of the S-cam, a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large C-clamp.
One-way check valve:
This device allows air to flow in one direction only. All air tanks on air-braked vehicles must have a check valve located between the air compressor and the first reservoir. The check valve keeps air from going out if the air compressor develops a leak.
Air supply pressure gauge:
All air-braked vehicles have an air supply pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system or, sometimes, a single gauge with two needles. Dual systems will be discussed later. These gauges tell you how much pressure is in the air tanks.
Application pressure gauge:
This gauge shows how much air pressure you are applying to the brakes (some vehicles do not have this gauge). When going down steep grades, increasing brake pressure to hold the same speed means the brakes are fading. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.
Low air pressure warning gauge:
A low air pressure warning device is required on vehicles with air brakes. A warning device which you can see must come on when the air supply pressure drops below 60 psi (414 kPa) or one half the compressor governor cut-out pressure on older vehicles. The warning is usually a red light. A buzzer may also come on. Another type of warning is the "wig wag." This device drops a mechanical arm into your view when the pressure in the system drops below 60 psi. An automatic wig wag will rise out of your view when the pressure in the system goes above 60 psi. The manual reset type must be placed in the "out of view" position manually. It will not stay in place until the pressure in the system is above 60 psi. On large buses, it is common for the low pressure warning devices to signal at 80–85 psi.
Stop light switch:
Drivers behind a vehicle must be warned when that vehicle's brakes are being applied. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when pressure is applied to the brake pedal.
Front brake limiting valve:
Some vehicles made before 1975 have a front brake limiting valve and a control in the cab. The control is usually marked "normal" and "slippery." When you put the control in the slippery position, the limiting valve cuts the normal air pressure to the front brakes by half. Limiting valves are used to reduce the chance of the front wheels skidding on slippery surfaces. However, they also reduce the stopping power of the vehicle. Front wheel braking is good under all conditions. Tests have shown front wheel skids from braking are not likely even on ice. Make sure the control is in the normal position to have normal stopping power. Many vehicles have automatic front wheel limiting valves. They reduce the air to the front brakes except when the brakes are stepped on very hard (60 psi or 410 kPa or more application pressure). These valves cannot be controlled by the driver. (NOTE: Some vehicles are manufactured with no front brakes.)
All trucks, truck tractors, and buses using air pressure to apply the service brakes must be equipped with emergency brakes and parking brakes. The parking brake must be held on by mechanical force (because air pressure can eventually leak away). Spring brakes are usually used to meet the emergency and parking brake requirements. When driving, powerful springs are held back by air pressure. If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put on the brakes. A leak in the air brake system will generally cause the springs to put on the brakes. Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi or 140 to 310 kPa (typically 20 to 30 psi or 140 to 210 kPa ). Do not wait for the brakes to come on automatically. When the low air pressure warning light and buzzer first come on, bring the vehicle to a safe stop right away while you can still control the brakes. The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted, neither the regular brakes nor the emergency/parking brakes will work correctly.
Parking brake controls:
In newer vehicles with air brakes, the parking brakes are set using a diamond-shaped, yellow, push-pull control knob. Pulling the knob out sets the parking brakes (spring brakes), and pushing it in releases them. On older vehicles, the parking brakes may be controlled by a lever.
Some vehicles are equipped with an anti-compound system. This keeps the brakes from being damaged by the compounded forces of the spring and the air, if the driver were to push on the brake pedal with the parking brakes applied.
Modulating control valves:
In some vehicles, a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on. They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device.
Dual parking control valve:
When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes. This is so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. The other valve is spring loaded in the "out" position. When you push the control in, air from the separate air tank releases the spring brakes so you can move. When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times.
Dual air brake systems:
Most newer heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer, if there is one. The first system is called the primary system and the other is called the secondary system. Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi (690 kPa) pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge). The low air pressure warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi or 410 kPa. The warning system devices should come on before the air pressure drops below 60 psi in either system. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop and have the air brake system fixed.
Combination vehicle air brakes:
The trailer hand valve (also called the trolley valve or Johnson bar) works the trailer brakes. The trailer hand valve is used only to test the trailer brakes. It is not used in driving because of the danger of making the trailer skid. The foot brake sends air to all of the brakes on the vehicle including the trailers. There is much less danger of causing a skid or jackknife when using just the foot brake.
Tractor protection valve:
The tractor protection valve keeps air in the tractor or truck if the trailer breaks away or develops a bad leak. The tractor protection valve is controlled by the trailer air supply control valve in the cab. The control valve allows you to open and shut the tractor protection valve. It will close automatically if air pressure is low (in the range of 20 to 45 psi or 140 to 310 kPa). When the valve closes, it stops any air from escaping and lets the air out of the trailer emergency line which causes the trailer emergency brakes to come on. (Emergency brakes are covered later.)
Trailer air supply control:
The trailer air supply control on newer vehicles is a red 8-sided knob which controls the tractor protection valve. Pushing it in supplies the trailer with air, and pulling it out shuts the air off and puts on the trailer emergency brakes. The valve will pop out and close the tractor protection valve when the air pressure drops into the range 20 to 45 psi. Emergency valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The normal position is used for pulling a trailer. The emergency position is used to shut the air off and put on the trailer emergency brakes.
Trailer air lines:
Every combination vehicle has two air lines—the service line and the emergency line. They run between each vehicle (tractor to trailer, trailer to dolly, dolly to second trailer, etc.).
Service air line (normally blue):
The service line (also called the control line or signal line) carries air which is controlled by the foot brake or the trailer hand brake. The pressure in the service line will similarly change depending on how hard you press the foot brake or hand valve. The service line is connected to a relay valve on the trailer to apply more or less pressure to the trailer brakes. The relay valve connects the trailer air tanks to the trailer air brakes. As pressure builds up in the service line, the relay valve opens and sends air pressure from the trailer air tank to the trailer brake chambers, putting on the trailer brakes.
Emergency air line (normally red):
The emergency line has two purposes. First, it supplies air to the trailer air tanks and secondly, the emergency line controls the emergency brakes on combination vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, tearing apart the emergency air hose. It could also be caused by a hose, metal tubing, or other part which breaks, letting the air out. When the emergency line loses pressure, it causes the tractor protection valve to close (the air supply knob will pop out).
Hose couplers (glad hands):
Glad hands are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The couplers have a rubber seal which prevents air from escaping. To connect the glad hands, the driver presses the two seals together with the couplers at a 90° angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers. It is very important to keep the air supply clean. To keep the air supply clean, some vehicles have "dead end" or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines. To avoid mistakes, metal tags are sometimes attached to the lines with the words service or emergency stamped on them. Sometimes colors are used. Blue is used for the service lines and red for the emergency lines. If the driver were to cross the air lines, supply air would be sent to the service line instead of going to charge the trailer air tanks. Air would not be available to release the trailer spring brakes (parking brakes). If the spring brakes don't release when the trailer air supply control is pushed in, the air line connections may be reversed. Older trailers do not have spring brakes. If the air supply in the trailer air tank has leaked away, there will be no emergency brakes and the trailer wheels will turn freely. If the air lines were crossed, the vehicle could be driven away, but when air is supplied to the emergency line when the pedal is depressed (because the lines are crossed), the trailer air tank would begin to fill with air, and eventually, the brakes would work. However, this would take at least several seconds and would be very dangerous.
Trailer air tanks:
Each trailer and converter dolly has one or more air tanks. They are filled by the emergency supply line from the tractor and they provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves. The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal and the trailer hand brake. It is important that water and oil are not allowed to build up in the air tanks. If they do, the brakes may not work. Each tank has a drain valve on it, and must be drained every day. If the tanks have automatic drains, they will keep most moisture out. However, the drains should still be opened manually to check for moisture.
Shut-off valves (also called cut-out cocks) are used in the service and supply air lines at the back of the trailers used to tow other trailers. These valves permit closing the air lines when no other trailer is being towed. Drivers must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.
Service, spring, and emergency brakes:
Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. These have parking brakes which work from the air stored in the trailer air tank. These trailers have no emergency brake. The parking brakes come on whenever air pressure is supplied to the trailer air tank. The brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away and then there will be no brakes. A major leak in the emergency line will cause the |tractor protection valve to close and the trailer emergency brakes to come on. One may not notice a major leak in the service line until he tries to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.
Essentially, the air brake operates, at its most basic level, similar to the piston in an internal combustion engine would inside its cylinder. Air storage tanks are filled with compressed air by the compressor, which is powered by the engine. When the driver steps on the brake pedal, or treadle valve, this allows air from the storage tanks to flow into the cylinder, pushing the piston down the cylinder. This is the service portion of the system. This cylinder is called the brake chamber. The piston is attached to the push rod, which turns the slack adjuster. The slack adjuster connects the push rod to another rod, which then rotates. This rod is connected to the S-cam. There are several types of air brakes, including S-cam and wedge type brakes. The S-cam are the most widely used type. It is the S-cam that turns and pushes the brake shoes against the lining of the drum brake. The air brakes on a semi-trailer are connected to a tractor with two lines. One line is called the supply line or the emergency line. It is usually larger and is red or has red fittings. The emergency line provides air pressure to fill the semi-trailer's reservoir tank and the pistons that activate the brakes. The other line is called the service line. It is usually smaller and is blue or has blue fittings. In normal braking, pressing the brake pedal pressurizes the service line. This activates a valve in the trailer which directs air from the reservoir and the emergency line to the brake cylinders where it moves the piston that activates the brakes. When the pedal is let up, the service line pressure is decreased. When the service line pressure drops, it causes the valve in the trailer to block the air supply from the reservoir while releasing the pressure in the brake cylinder and the brakes are released. The system is a form of servo or amplifier. If the pressure in the emergency line drops, due to the activation of a valve in the cab, the disconnection of the emergency line coupling, or a break in the emergency line, the spring brakes will come on, because there is no more air pressure in the trailer air tank to hold them back.