How do Truck Brakes Work?

How do Truck Brakes Work?

“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.

Truck Brake Types

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

  • Air moves into a brake chamber through airlines.
  • The air forces out a pushrod.
  • The pushrod pushes the slack adjuster.
  • The camshaft turns.
  • The turning of the camshaft twists the S-Cam. (You guessed it — it’s called an S-Cam because it’s in the shape of an S.)
  • The brake linings are forced to contact the brake drum.
  • 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.


Big Disc Brakes for 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 calliper’s brake lining pads.

Compared to drum brakes, air disc brakes reduce stopping distances by almost 40 percent. Because pressure is continually applied, it’s easier for the vehicle to come to a complete stop.

Air disc brakes almost completely stop brake fade.

Air disc brakes don’t increase the wear of brake linings on a trailer or disc pads on a tractor.


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.


h/t to howstuffworks.com for this info!


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