How Air Brakes Work

Understanding Brakes

Before 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.