AIR BRAKESAir brakes use compressed air to make the brakeswork. They provide a safe way to stop large vehicleswhen maintained and used correctly. The air brakesystem is composed of three combined braking systems:the service brake system, the parking brake system, andthe emergency brake system.The service brake system applies and releases thebrakes when you use the brake pedal during normaldriving. The parking brake system applies and releasesthe parking brakes when you use the parking brakecontrol. The emergency brake system uses parts of theservice and parking brake system to stop the vehicle inthe event of a brake system failure.NOTE: The components of the air brake system arecovered in chapter 3.Brake Drums, Shoes, and LiningsBrake drums are located on each end of the axles.The wheels are bolted to the drums, and the brakingmechanisms are located inside the drum. The brakeshoes and linings are pushed against the inside of thedrum, and this action causes friction that slows thevehicle and brings it to a stop. This friction creates heaton how hard and how long the brakes are used. Toomuch heat can stop the brakes from working properly.S-CAM AIR BRAKES.— When the air brakepedal is pushed, air is let into each brake chamber(fig. 7-10). Air pressure pushes the rod out, moving theslack adjuster, thus twisting the brake camshaft. Thisaction turns the S-cam that forces the brake shoes awayfrom one another and presses them against the inside ofthe brake drum. When the brake pedal is released, theS-cam rotates back and a spring pulls the brake shoesaway from the drum, allowing the wheels to roll freely.WEDGE BRAKES.— On wedge brakes, the brakechamber pushrod pushes a wedge directly between theends of two brake shoes. The wedge shoves the shoesapart and against the inside of the brake drum. Wedgebrakes have either a single-brake chamber or two brakechambers that push wedges into both ends of the brakeshoes.DISC BRAKES.— The air pressure in air-operateddisc brakes acts on a brake chamber that producesmovement of the slack adjuster, like on the S-cam brake.But instead of the S-cam, a “power screw” is used. Thepressure of the brake chamber on the slack adjuster turnsthe power screw. The power screw clamps the disc orrotor between the brake lining pads of a caliper.NOTE: Wedge and disc air brakes are less commonand the heat a drum can take without damage dependsthan the S-cam brake.Figure 7-10.—S-cam air brake.7-9
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