How does the brake system work?

Answer 1

The brake system on a tractor-trailer is an "S-Cam Air Brake." It's a drum brake system. Drum brakes have a big advantage over drum brakes on a big truck--there's more friction surface on a drum brake than there is on a disc brake. All drum brakes work the same way. The tire and wheel assembly is bolted to a cast-iron drum, which looks like a bowl (and, of course, which is used as one once it wears out--every repair shop owner in the world has one of these on his desk, which he throws pens, paper clips or whatever in). and two brake shoes fit inside the drum. When you step on the brakes, or pull the parking brake handle, the shoes are forced against the inside of the drum. As they rub the drum, the kinetic energy of the rotating wheel is converted into heat energy through friction, and the vehicle stops. There are three ways to move the brake shoes. The first is with a cable. Every car has cable-actuated parking brakes. (They're officially called "emergency brakes," but the only car I know of where the "emergency brakes" will actually stop the car in an emergency is a Saab, because the brake cables on a Saab are hooked to the FRONT brakes. Everyone else hooks the brake cables to the rear brakes, and those will kinda slow you down.) The service brakes on a car are hydraulic brakes. The system is full of mineral oil, and the brake pedal is connected to a piston. When you step on the brakes, the piston pushes down on the fluid. Fluid is essentially incompressible--you CAN compress fluid, but it takes more force than most people can muster to do it--so it has to go somewhere...and we give it some little pistons to push up against. The brake shoes, or brake pads on a car with disc brakes, are attached to those pistons. In an air brake system there is no fluid. It's operated by compressed air. An air brake system has the brake drum and brake shoes. One end of the shoes is connected to an axle, so it can pivot. The other end of each shoe has a little roller on it, and the roller rides on a rotating cam. It's called an S-cam because it's shaped like an S. They stick the cam at the end of a big steel bar, which allows it to turn. Next comes the slack adjuster. It connects the S-cam rod to the brake chamber rod, and it's called an adjuster because...well, it gets longer to compensate for the brake shoes wearing down with use. (There are manual slack adjusters and automatic slack adjusters. If you have manual slack adjusters you have to go under the truck every so often and set the slack with a wrench. I don't think it's legal to sell manual slack adjusters any more--automatic ones are just safer because they're always in good adjustment.) And finally, there is the brake chamber, where the air goes in. The brake chamber has a rod sticking out of it, which connects to the slack adjuster, and either one or three devices to move it. The first device is the service brake chamber. It is connected to the service air circuits, of which you normally have two unless your truck is ancient. When you step on the brakes, air flows into the service circuits and presses against the diaphragm in the service chamber. This pulls on the rod, which pulls the slack adjuster, which turns the S-cam and makes you stop. The other two devices are parts of the spring brake system. There are never spring brakes on the front wheels--these can activate automatically, and if your front wheels lock up when you don't want them to you couldn't steer the truck. That would be very bad. The first device is a huge spring, which will pull on the rod. The other is the spring override chamber, which pushes against the spring to allow the brakes to release. If the air system pressure drops below 20psi on your tractor, or 40psi on your trailer, or you use the buttons on your tractor's dashboard to shut the air off by hand, there won't be enough air in the system to push the spring back and the spring brakes will come on. Now for some free advice from a Class A CDL holder to people getting ready to get one... You are going to take several written tests at the DMV to get your learner's permit. The one you are most likely to fail is Air Brakes, because of the way it's written. They talk about foundation brakes, service brakes, spring brakes, parking brakes and emergency brakes. "Foundation brake" means the brakes. Service brakes are the ones that come on when you step on the brake pedal. Spring brakes, parking brakes and emergency brakes are all the same brake. A spring brake stops, or holds, the truck when there's no air pressure. It becomes an emergency brake if something happens to make your truck lose air pressure. It is a parking brake if you use the control valve on the dashboard (the one for the tractor is yellow, the one for the trailer is red) to turn the air off. If you can remember you have two brake systems on your truck--the service brake, which works by stepping on the brake pedal, and the spring brake, which is called three things--spring, parking and emergency--you'll pass with flying colors. Quote 'The brake system on a tractor-trailer is an "S-Cam Air Brake." It's a drum brake system. Drum brakes have a big advantage over drum brakes on a big truck--there's more friction surface on a drum brake than there is on a disc brake'

Wrong. The disc brake is a far more effective brake under all circumstances. Simply because it offers better cooling under heavy use and its far more effective to have two brake pads grab a brake disc than to have brake shoes pushing against a brake drum. Brake drums have been known to shatter after a very short life span than a brake disc.

Most European trucks nowadays have disc brakes fitted as standard, as well as trailers.