No doubt the brakes are the most important component part of commercial vehicles. Air brake system on a commercial vehicle or diesel trucks as commonly referred to, work with compressed air that push the brake shoe lining onto the brake drum creating friction to slow down the wheel. It may sound difficult and it is. To make sure the air brakes are functioning safely, first of all you have to understand how the air brake works. The picture on the left is a simple explanation how the air brake system works on a semi truck. The air is pushed through the lines where it finally reaches the air brake after which brake shoe lining creates the friction and slows down the vehicle depending on how much air is pumped into the drum. First moment is the air compressor pumps air into the air tank which is located beneath the truck. That air is then pushed through the air brake lines and reaches the air brake.
On “S-Cam” brakes, when you press the truck brake pedal, air pressure moves the rod out pushing the slack adjuster. The slack adjuster is used to mainly adjust the brake system and ensure that the internal spring is working right. If the internal spring is not fully out it won't create enough friction to slow down the truck. Air travels through the nozzle all the way to the brake chamber and causes the spring to move the “S-cam.” The “S-cam” pushes the brake shoe lining away from each other and compresses them inside the brake drum. Pressure inside the drum causes the friction and pressure needed to slow down the wheel. When you take your foot from the brake pedal, the “S-cam” rotates back and causes the spring to pull the brake shoes back from the brake drum and eliminates the friction and the slowing process. Both brake shoes and brake pads wear off over time depending on how often the driver depresses the brake pedal and how heavy he depress break pedal. That's why it is important for a truck driver to properly use engine brakes. It is recommended for the commercial driver to keep their brake shoes at a 1/4 inch thickness at least.
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