Drivers of vehicles with air brakes must be familiar with the function and appearance of the various types of foundation brake assemblies, and be able to inspect the components for safety defects. This chapter explains the components of common foundation brakes and how they work.

Types of foundation brakes

The brake-assembly components at the wheels of a vehicle are generally called the foundation components because they form the basis on which the rest of the system is built. Foundation components are the mechanical parts contained in or around the wheels that are operated by the air brake system. An air brake system can be designed to work with several designs of foundation brake subsystems - even on the same vehicle.

There are three types of foundation brake systems - “S” cam brakes, disc brakes and wedge brakes.

"S" cam brakes

The “S” cam brake is the most common type of foundation brake used on commercial vehicles with air brake systems. This is a drum brake that uses air brake chambers and linkage to press the brake shoes against the surface of the brake drum. The stroke of the air brake chamber pushrod acts on the slack adjuster attached to the end of the camshaft. Pushrod stroke causes the camshaft to rotate. The “S” shape on one end of the camshaft forces the brake shoes apart and against the brake drum.

Many of the parts of an “S” cam brake are located in the wheel. Characteristic of the “S” cam brake design is the exposed pushrod. This allows easy access to the brake linkage to check adjustment. The components within the wheel are difficult to see, particularly when a dust shield or backing plate is used to protect the components.

The brake shoes and drum are located within the wheel, along with the hardware to hold these parts in position.

Other internal parts may include some or all of the following:

  • A brake spider that serves as the main mounting base for the brake shoes
  • Single or dual anchor pins to attach one end of the brake shoes
  • Anchor springs to hold the brake shoes in position at the anchor end
  • The head of the "S" camshaft
  • Camshaft rollers that rotate with the camshaft while moving the brake shoes inward and outward
  • A return spring to pull the brake shoes in to the released position and keep the camshaft rollers engaged with the camshaft head

The external components are much easier to see and identify. They include:

  • Brake chamber and mounting brackets
  • Slack adjuster
  • Pushrod
  • The shaft portion of the "S" camshaft
  • Support brackets and bushings for the camshaft
  • Dust shields or backing plates
  • Brake drums

Disc brakes

The disc air brake system also uses both external and internal components, but there are fewer involved. All disc air brakes use calipers and rotors. The brake rotor is only partially visible because the wheel, caliper and dust shields usually cover parts of it. Disc brake designs use brake chambers that may have exposed linkage and a slack adjuster. Brake chamber pushrod stroke presses the brake pads against the rotor.

Wedge brakes

The wedge air brake system is a type of drum brake that includes brake drums and shoes with no exposed brake linkage. Air brake chambers are mounted so that their pushrods face inward toward the brake shoes and drum. The stroke of the brake chamber pushrod slides a wedge between the brake shoes, forcing them outward against the drum. Wedge air brakes are designed to be self-adjusting.

Key points to remember

  • Components of “S” cam air brakes include brake drums, brake shoes and linings, camshafts, pushrods, cam support brackets, spiders, rollers, return springs, dust shields and backing plates, slack adjusters and brake chambers.
  • Components of disc air brakes include rotors, calipers, brake pads, slack adjusters and brake chambers.
  • Components of wedge air brakes include brake drums, brake shoes and linings, spiders, dust shields and brake chambers.