How Truck Brakes Work

­Let's ­begin by considering the ways that truck and car brakes are al­ike. The purpose of bra­kes on all types of vehicles is to stop them. Brakes on both trucks and cars work on the principle of friction. Both kinds of vehicles have brake drums, along with their pads and shoes, connected to the vehicles' wheel axles.

Car brakes rely on the brake fluid that flows through the system to work properly. Thus, car brakes are a hydraulic system, relying on fluid. On the other hand, truck brakes depend on compressed air. (Trains and buses also use this type of brake system.)

A major plus to using air is that it never runs out (as brake fluid can). This means that the air brake system is very reliable -- even if there's a small leak somewhere in the system, it always works.

Most of the newer heavy trucks use a dual air brake system that is not available on automobiles. A single set of brake controls works both of these separate air brake systems. If one system fails, the other will work.

The flaw in the compressed air system of trucks is brake lag. That's the time it takes for air to get through the lines and force the linings to contact the drum. When they push the brake pedal, drivers must get used to the fact that air brakes don't work at once, as they do on a car. Lag time is less than a second, so this is not a major problem.

The truck's air braking system has several tasks. First, it keeps up a steady supply of compressed air. In addition, it must direct that air's flow. Finally, it uses the energy of air pressure and changes it into mechanical force.

One, two, three! A truck's air brake system is really made up of three different brake systems. Let's investigate each of them.

­

Service brakes are used during normal driving. A seque­nce of events occurs when a driver pushes the brake pedal.

The driver activates the parking brakes by pulling out one or both of the valves on the dash. (Tractor brakes have a yellow button; trailer brakes have a red button.) The dash button releases the spring inside the brake chamber, thus beginning steps 3-6 listed above.

The emergency brake system uses parts of the other two systems to stop the truck if brake failure takes place [source: Newbie Driver].

Inside the brake chamber is a powerful spring with about 2,500 pounds of pressure behind it. That spring is held back by a steady and constant airflow in the chamber. The emergency brakes deploy automatically if there's not enough air in the system to hold the spring back. If air pressure drops below 60 pounds per square inch, the low-pressure light comes on. A buzzer may also sound.

Not everyone is happy with the way that S-Cam brakes work. A recent innovation is the use of disc brakes. Let's head down the highway to see how disc brakes work on trucks.

­

Some trucks have disc brakes instead of S-Cam brakes. Air pressure works on the brake chamber and the slack adjuster in the same way that it does in S-Cam brakes. However, a power screw replaces the S-Cam. The power screw is turned by t­he pressure on the brake chamber and the slack adjuster. Then the power screw grasps the disc or rotor between a caliper's brake lining pads.

The many benefits of air disc brakes include the following:

Still, despite all of the advantages, change isn't always embraced by the masses. In the United States, no regulation demands the use of air disc brakes, which are viewed as a high-end item because they're about twice the cost of drum brakes. Currently, about 95 percent of the U.S. heavy-duty truck market relies on drum brakes. In Europe, more than 80 percent of commercial trucks already use air disc brakes. The complete shift to air disc brakes may require 10 to 15 years.

They say that a picture is worth a thousand words. Let's cruise to the next page to get a glimpse of a diagram of a truck's braking system.

­

Now let's put the parts together to see how truck brakes work as a whole. This diagram provides both a closeup view and an example of where the brakes are located in your vehicle.