AIR BRAKES

Air brakes use compressed air to make the brakes work. They provide a safe way to stop large vehicles when maintained and used correctly. The air brake system is composed of three combined braking systems: the service brake system, the parking brake system, and the emergency brake system.

The service brake system applies and releases the brakes when you use the brake pedal during normal driving. The parking brake system applies and releases the parking brakes when you use the parking brake control. The emergency brake system uses parts of the service and parking brake system to stop the vehicle in the event of a brake system failure.

NOTE: The components of the air brake system are covered in chapter 3.

Brake Drums, Shoes, and Linings

Brake drums are located on each end of the axles. The wheels are bolted to the drums, and the braking mechanisms are located inside the drum. The brake shoes and linings are pushed against the inside of the drum, and this action causes friction that slows the vehicle and brings it to a stop. This friction creates heat and the heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can stop the brakes from working properly.

S-CAM AIR BRAKES.- When the air brake pedal is pushed, air is let into each brake chamber (fig. 7-10). Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft. This action turns the S-cam that forces the brake shoes away from one another and presses them against the inside of the brake drum. When the brake pedal is released, the S-cam rotates back and a spring pulls the brake shoes away from the drum, allowing the wheels to roll freely.

WEDGE BRAKES.- On wedge brakes, the brake chamber pushrod pushes a wedge directly between the ends of two brake shoes. The wedge shoves the shoes apart and against the inside of the brake drum. Wedge brakes have either a single-brake chamber or two brake chambers that push wedges into both ends of the brake shoes.

DISC BRAKES.- The air pressure in air-operated disc brakes acts on a brake chamber that produces movement of the slack adjuster, like on the S-cam brake. But instead of the S-cam, a "power screw" is used. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper.

NOTE: Wedge and disc air brakes are less common than the S-cam brake.

Figure 7-10.-S-cam air brake.

Spring Brakes

All trucks, tractor-trailers, and buses are equipped with emergency and parking brakes. These brakes are applied by the mechanical force produced by a spring brake. When you are driving, the powerful springs are held back by air pressure. A leak in the air pressure system causes the springs to engage the brakes. Engaging the parking brake control on the dashboard releases the air holding back the springs. This action allows the springs to engage the brakes.

Tractor and straight truck spring brakes engage when the air pressure drops to a pressure ranging between 20 to 45 psi. Do not wait for the brakes to engage automatically. When the low air pressure warning light and buzzer activates, you should bring the vehicle to a safe stop immediately, while you can still control the brakes.

The braking power of the spring brakes depends on the adjustment of the brakes. If the brake adjustment is incorrect, the regular, emergency, and parking brakes cannot work correctly.

Parking Brake Controls

On newer vehicles, a diamond shaped, yellow, push-pull control knob is used to engage or disengage the parking brakes. When the knob is pulled out, the brakes are engaged. By pushing the knob in, you can release the brakes. On older vehicles, the parking brake may be controlled by a lever. You should engage the parking brake anytime the vehicle is parked.