Brakes

Image Gallery: Brakes Think you could move one of these bad boys in a pinch? See more brake pictures.Greg Pease/Getty ImagesImagine it's your first week on the job as a dockhand at a rundown trucking company. Everyone is running around trying to finish loading the last pallet of cargo on the back of a huge tractor-trailer truck bound for the opposite coast. Suddenly, one of the foremen tells you to move one of the trucks out of the way so another driver can back up to the loading dock. Assuming you know how to drive such a vehicle, the foreman keeps going, but you pause -- because you don't.Trying to please the higher-ups and to ignore the fact that you don't have a truck driver's license, you hop into the cab, close the door and turn the key. Before the diesel engine cranks, youre startled by a mind-numbing buzzer and flashing light on the dashboard. You fire the engine, but the buzzer and light keep grabbing your attention.Youve driven a stick shift before, so you think you have it covered. Despite the sensory overload, you push in the clutch, grab what you think is the low gear and ease out the clutch. Instead of lurching forward as you expect, youre greeted with a violent bang, the engine dies and youre almost thrown through the windshield.You restart the engine, figuring you put the truck in the wrong gear, and select what you think is the right one. Still, the buzzer and light cause havoc inside the cab. Maybe the emergency brake is still on. You dont see any brake handle or lever that you would normally see in a car, so you decide to just let the clutch out and give it another shot.Much to your embarrassment, the same thing happens. Out of the corner of your eye you see that same foreman hollering at you from the loading dock. Frustrated, you jump out of the cab and throw up your hands in bewilderment, as the scowling supervisor jogs toward you.Welcome to the world of air brakes. In this article, youll learn how air brakes and their components work, how to maintain an air-brake system and why you couldnt move that truck. Next, let's see how George Westinghouse got you into this situation

Diagram of air-brake componentsHowStuffWorksAir-brake Components in Trucks and BusesFoundation brakes are the most common air-brake systems found in trucks and buses and work the same way as in rail cars. Using the triple-valve principle, air builds up inside the brake pipes or air lines, releasing the brakes. Virtually all of the roadgoing vehicles equipped with air brakes have a graduated release system where a partial increase in pressure dictates a proportional release in brakes.The following components are exclusive to a foundation air-brake system in a truck or a bus: Air compressor: Pumps the air into storage tanks to be used in the brake system Air compressor governor: Controls the cut-in and cut-out point of the air compressor to maintain a set amount of air in the tank or tanks Air reservoir tanks: Hold compressed or pressurized air to be used by the braking system Drain valves: Release valves in the air tanks used to drain the air when the vehicle isn't in use Foot valve (brake pedal): When depressed, air is released from the reservoir tanks Brake chambers: Cylindrical container that houses a slack adjuster that moves a diaphragm or cam mechanism Push rod: A steel rod similar to a piston that connects the brake chamber to the slack adjuster. When depressed, the brakes are released. If extended, the brakes are applied. Slack adjusters: An arm connects the push rod to the brake s-cam to adjust the distance between the brake shoes Brake S-cam: An s-shaped cam that pushes brake shoes apart and against the brake drum Brake shoe: Steel mechanism with a lining that causes friction against the brake drum Return spring: A stiff spring connected to each of the brake shoes that returns the shoes to the open position when not spread by the s-cam or diaphragm.At idle (foot off the brake and vehicle's air system charged), air pressure overcomes the diaphragm or the s-cam is in the closed position, resulting in a released brake system. As soon as you depress the brake pedal, the air pressure decreases, turning the s-cam and spreading the brake shoes against the drum. The compressor refills the reservoir tanks and when you allow the pedal to retract, the air pressure increases back to the original state.Emergency air brakes complement standard air-brake systems and can be activated by pulling a button on the dash (near the one with the light that we saw in the introduction). Before you can drive a vehicle with air brakes, you must push in the emergency brake button to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. If the system has a leak, the pressure can decrease enough to engage the emergency brake. In addition, heavy trucks are often equipped with an exhaust brake that aids the braking process, but this relies on the engine, not the air-brake system.We've learned how air brakes work. Now let's look at how to maintenance can prevent brake failure in the next section.What's that sound?Have you ever wondered why trucks and buses make those funny squeaking and hissing sounds? The squeaking is the air escaping after braking and the ppssss sound is the automatic bypass safety valves at work, ensuring the air pressure remains at the correct level. Since a main advantage of air-brakes systems is their ability to use air to operate, the compressor is constantly kicking on and kicking off to refill the reservoirs with pressurized air. When the compressor builds too much air, the valves open, producing that loud hiss.Runaway Train Could Have Been AvoidedOn June 27, 1988, a commuter train slammed into a stationary train at the Gare de Lyon station in Paris, France, killing 56 people and injuring 32 more [source: AP, National Geographic]. The disaster occurred after a series of mistakes left the train with a greatly reduced braking ability. After a passenger inadvertently pulled the emergency brake upon her exit, the driver shut a brake valve, thinking the system had an air lock. After he released the air from the system, the train rolled freely, but the remaining cars that had a charged system didn't have enough stopping power. In a panic, the driver failed to activate the electric emergency braking system, and the train collided with a resting train at the station. If not for a brave driver in the stationary train that stayed until the collision, aiding in passenger evacuation, the death toll would have been much higher [source: AP, National Geographic] George Westinghouse and Air-brake HistoryAir is everywhere. Hydraulic fluid isn't. Trains, buses and tractor-trailers use air-brake systems so they dont have to rely on the hydraulic fluid in car braking systems, which can run out in the event of a leak. All of these types of transportation are weighed down by heavy passenger or cargo loads, so safety is of the utmost importance. A speeding locomotive that relied on hydraulic brakes would turn into a deadly steel bullet if the brake system suddenly busted a leak.Before air brakes, trains used a primitive brake system that required an operator, or brakeman, in each car to apply a hand brake at the signal of the train director or engineer. This inefficient manual system was replaced by direct air-brake systems, which used an air compressor to feed air through a brake pipe into air tanks on each car. When the engineer applied these brakes, the pipe filled with air and squeezed the brakes.In 1869, an engineer named George Westinghouse realized the importance of safety in the relatively new railroad industry and invented the first triple-valve air-brake system for railcar use. Westinghouses system worked the opposite way of a direct air-brake system. The triple-valve system performed three functions, thus its name. Lets take a look at those functions.1. Charging: The system must be pressurized with air before the brakes will release. At rest, the brakes remain engaged. Once the system reaches its operating pressure, the brakes are freed and ready to use.2. Applying: As the brakes are applied, air pressure decreases. As the amount of air decreases, the valve allows air back into the reservoir tanks, while the brakes move to the applied position.3. Releasing: Once the brakes are applied and the air escapes after braking, the increased pressure releases the brakes.Instead of using force or directed air to apply the brakes much like hydraulic fluid in our cars, the triple-valve system fills a supply tank and uses air pressure to release the brakes. In other words, the brakes in a triple-valve system remain fully engaged until air is pumped throughout the system. Pretty ingenious, considering if this type of system had a complete loss of air, the brakes would engage and stop the train. Think about that when you are zooming down the freeway and you hit the brake pedal. If your cars brake fluid leaked out, your brakes wouldnt work.The triple-valve system is the basic concept at work in todays air-brake systems in trains, buses and tractor-trailers. Lets switch gears and learn how air brakes in roadgoing vehicles work in the next section

Understanding BrakesBefore we learn about air brakes in road vehicles, let's look at how the brakes in your car work. Anyone who has driven a car knows when he or she pushes the brake pedal towards the floor the car slows and eventually stops. But how in the world can our foot stop a 3,000-pound (1,361-kg) car traveling down the road at high speeds?To begin with, let's discuss the different types of brakes and then we can explore the different components. Every rolling vehicle, including trains, tractor-trailer trucks, buses and cars contain one of two types of systems. Hydraulic brakes, found in light-duty trucks and passenger cars, use hydraulic fluid or oil to operate their brakes. Air brakes, which we'll break down in the next section, use air to operate their brakes. Let's look at the differences.In a hydraulic system, fluid is stored in a reservoir commonly referred to as a master cylinder. When you push the brake pedal, fluid is pumped through brake hoses or lines into pistons mounted on each wheel. These brake pistons either push against two brake shoes, which expand and cause friction inside a brake drum, or against a brake pad, which clamps down on a brake rotor. Below are the components in a hydraulic disc brake system. Brake reservoir: Contains hydraulic brake fluid Master cylinder: Device that pumps the fluid from the reservoir to brake lines that run throughout the vehicle Brake lines: Rubber or steel braided hoses that run from the master cylinder to each brake caliper Brake caliper: A steel housing that mounts on a fixed point of the brake rotor that contains a piston and brake pads Brake piston: A round rod that extends and pushes against a brake pad when hydraulic fluid is fed from the master cylinder Brake pad: A metal backing pad with a semi-metallic overlay that grips the steel rotor Brake rotor: A steel disc mounted to each wheel and hub that the pads grasp to stop the wheels from rotating[source: Brakes]Here's a look at how some of the parts fit within a disc brake.Before disc brakes, cars relied on drum brakes. The principal mechanics were the same, but drum brakes used brake shoes set inside a drum that was mounted on the hub, versus a rotor. Disc brakes increase stopping power, as they are more easily cooled and have more surface area to grasp. In addition, brake dust, which forms as the brake pads wear and decreases braking ability, is vented more easily with disc brakes than with drum brakes. For more information on disc brakes and drum brakes, read How Disc Brakes Work and How Drum Brakes Work.Now that we understand the fundamentals of brakes in trains and cars, let's talk about the big rigs and the buses.

Poor maintenance of air brakes can lead to accidents.Andy Sacks/Getty ImagesAir Brakes: Preventative MaintenanceEvery state in the U.S. has specific guidelines for operating a vehicle with air brakes. The tests to obtain a commercial drivers license are demanding, as are the steps to maintain such a vehicle. Here are some steps you'll want to take before heading out on the road: Make sure the minimum operating pressure for a vehicle air-brake systems is no less than 85 psi (pounds per square inch) for a bus and 100 psi for a truck. Check that it takes no longer than two minutes for air pressure to rise from 85 psi to 100 psi at 600 to 900 rpm. (This is called the air pressure buildup rate.) Confirm that the correct cut-out governor pressure for the air compressor is between 120 psi and 135 psi. Cut-in pressure is 20 psi to 25 psi below cut-out pressure.You'll also want to watch for water in the air-brake system, a byproduct of the condensed air. Air-brake lines dont like water, especially in colder climates where ice can block air from reaching the brake mechanism and cause the wheel to lock up. To prevent this problem, many of the modern systems have automatic drain valves installed in each air tank.Air couplers can also pose a problem. Worn rubber seals will cause air to escape. While the compressor can overcome a small leak, running compressors too hard can lead to failure. Again, as weve learned, air loss isnt necessarily a bad thing, but it will mean youre stuck. For truck drivers, getting stranded in the middle of a mountain pass probably isn't on the itinerary.Brake sensitivity, another byproduct of air brakes, can lead to accidents, especially for inexperienced drivers. Air-brake systems are designed to work on vehicles carrying heavy loads. Have you ever wondered where all those dual skid marks on the expressway come from? Thats a product of light or empty trailers locking their rear wheels. Probably the worst fear for a truck driver is jackknifing. Its never good when the back end of the trailer creeps up alongside the cab. Trucks traveling in rain and snow can easily jackknife if too much brake is applied.Most modern vehicles with air brakes use a dual system. In essence, such equipped vehicles have two systems in case one should fail. Anti-lock brakes can now be found in tractor-trailer rigs and work much the same way as ABS systems found in passenger cars.Fundamentally speaking, air brakes are efficient and reliable. However, dont hold your breath if youre hoping to find them in your car any time soon. Air-brakes systems occupy too much space and attention to be considered practical in cars. Just look at a Peterbilt truck as it saunters down the interstate. Have you seen the big tanks tucked behind the fuel tanks? Try finding a place for those under the hood of a Honda Civic.If youd like to learn more about air brakes and read some related HowStuffWorks articles, explore the links on the next page.Poor maintenance leads to runaway truckOn April 25, 1996, a 1988 Mack cement truck collided with a small Subaru sedan in Plymouth Meeting, Pa. As the driver of the cement truck approached an intersection at the end of a downhill off ramp, his brakes failed and the truck barreled into the intersection, striking the Subaru and killing its driver. The National Transportation Safety Board investigated the incident and found several problems with the truck, notably reversed brake lines and a secondary system failure. Those two issues left the truck with only an estimated 17 percent to 21 percent of its total braking capability. Unfortunately, the driver had no idea he had a brake failure. Poor maintenance resulted in a senseless death that could have been avoided. [source: NTSB]

Location of anti-lock brake components. See more pictures of brakes.Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock braking systems (ABS) take a lot of the challenge out of this sometimes nerve-wracking event. In fact, on slippery surfaces, even professional drivers can't stop as quickly without ABS as an average driver can with ABS.In this article, the last in a six-part series on brakes, we'll learn all about anti-lock braking systems -- why you need them, what's in them, how they work, some of the common types and some associated problems.

Anti-lock brake pump and valvesUp Next How Brakes Work How Drum Brakes Work How Disc Brakes Work How Power Brakes Wor Turbo.com: How to Inspect BrakesThe ABS SystemThe theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. If you have been stuck on ice, you know that if your wheels are spinning you have no traction. This is because the contact patch is sliding relative to the ice (see Brakes: How Friction Works for more). By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop.There are four main components to an ABS system: Speed sensors Pump Valves ControllerSpeed SensorsThe anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.ValvesThere is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions: In position one, the valve is open; pressure from the master cylinder is passed right through to the brake. In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder. In position three, the valve releases some of the pressure from the brake.PumpSince the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.ControllerThe controller is a computer in the car. It watches the speed sensors and controls the valves.ABS at WorkThere are many different variations and control algorithms for ABS systems. We will discuss how one of the simpler systems works.The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 kph) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.The ABS controller knows that such a rapid deceleration is impossible, so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.When the ABS system is in operation you will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. Some ABS systems can cycle up to 15 times per secondAnti-Lock Brake TypesAnti-lock braking systems use different schemes depending on the type of brakes in use. We will refer to them by the number of channels -- that is, how many valves that are individually controlled -- and the number of speed sensors.Four-channel, four-sensor ABSThis is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.Three-channel, three-sensor ABSThis scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle.This system provides individual control of the front wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.One-channel, one-sensor ABSThis system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle.This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness.This system is easy to identify. Usually there will be one brake line going through a T-fitting to both rear wheels. You can locate the speed sensor by looking for an electrical connection near the differential on the rear-axle housing.ABS QuestionsShould I pump the brake pedal when stopping in slippery conditions?You absolutely should not pump the brake pedal in a car with ABS. Pumping the brakes is a technique that is sometimes used in slippery conditions to allow the wheels to unlock so that the vehicle stays somewhat straight during a stop. In a car with ABS the wheels should never lock in the first place, so pumping the brakes will just make you take longer to stop.In an emergency stop in a car with ABS, you should apply the brake pedal firmly and hold it while the ABS does all the work. You will feel a pulsing in the pedal that may be quite violent, but this is normal so don't let off the brake.Do anti-lock brakes really work?Anti-lock brakes really do help you stop better. They prevent wheels from locking up and provide the shortest stopping distance on slippery surfaces. But do they really prevent accidents? This is the true measure of the effectiveness of ABS systems.The Insurance Institute for Highway Safety (IIHS) has conducted several studies trying to determine if cars equipped with ABS are involved in more or fewer fatal accidents. It turns out that in a 1996 study, vehicles equipped with ABS were overall no less likely to be involved in fatal accidents than vehicles without. The study actually stated that although cars with ABS were less likely to be involved in accidents fatal to the occupants of other cars, they are more likely to be involved in accidents fatal to the occupants of the ABS car, especially single-vehicle accidents.There is much speculation about the reason for this. Some people think that drivers of ABS-equipped cars use the ABS incorrectly, either by pumping the brakes or by releasing the brakes when they feel the system pulsing. Some people think that since ABS allows you to steer during a panic stop, more people run off the road and crash.Some more recent information may indicate that the accident rate for ABS cars is improving, but there is still no evidence to show that ABS improves overall safety

Anti-lock brake componentsHowStuffWorksAnti-Lock Brake DiagramNow let's put the parts together to see how anti-lock brakes work as a whole. This diagram provides both a closeup view and an example of where the brakes are located in your vehicle.For more information on anti-lock brakes and related topics, check out the links on the next page.