Most automobiles utilize hydraulic brakes. When you step on the brake pedal, hydraulic fluid pressure is generated in a master cylinder that is transmitted to the actuators, i.e. the wheel cylinders and caliper pistons through the brake lines thus applying the brakes. Big rigs do not use hydraulic brakes, they utilize air brakes.
Air brakes use high pressure air which is supplied by an engine driven air compressor. This air is stored in tanks on the tractor. When the brakes are applied, the air comes from the tanks at about 100 psi and is forced into an air chamber in the brake drum. A push rod moves out turning a slack adjuster which in turn rotates an "S" cam which forces the brake shoes into the drum.
Most people assume that because trucks are big, they must have the best brakes. What they fail to realize is that large trucks cannot stop nearly as quickly as a passenger vehicle. A car driving at 55 miles per hour can stop in about 225 feet with its hydraulic brakes. However, a truck traveling at the same 55 mile per hour speed will take more than 400 feet to stop.
This is because the pneumatic and mechanical lag between the time when a driver applies his brakes until the time the air travels through a 40 foot hose to reach a valve can be more than two seconds. The stopping time of a truck may be further delayed if it is fully loaded.
Because most air braking systems do not automatically adjust for wear, they must be well maintained and inspected by the operator. If not, the performance of the air braking system can rapidly deteriorate. In fact, truck inspections demonstrate that a large percentage of trucks have some form of defect in their braking systems. These defects are usually the result of air leaks or poor adjustments.
Air brakes should be adjusted in a garage with the wheels off the ground. However, drivers are trained in making adjustments on the road by observing the adjusting arm with air brake pressure.
When air brakes are applied it generates heat. A full stop from 60 miles per hour might raise the drum temperatures to 600 degrees Fahrenheit. If the brakes are not properly adjusted these temperatures could exceed 1000 degrees Fahrenheit thereby causing a potentially dangerous situation.
When brake linings get hot the friction provided by the linings decreases. Since the linings do not offer the same resistance to the rotating drums they become slick. Moreover, as the drum heats up it expands and moves away from the brake shoes. In air brakes, the distance that the shoes can move is limited. Thus, if the brakes are improperly adjusted, when they get hot it is possible that the shoes will not make good contact with the drums. This can be disastrous.
Another potential problem with big rigs is downhill braking. Some drivers fail to appreciate the severe demands put on the brakes by long downhill runs. Negotiating a 6% downgrade in elevation for 6 miles is the equivalent of trying to stop the truck from a speed of 238 miles per hour, or 16 stops from 60 miles per hour.
Today there is almost universal agreement that the appropriate braking technique to be utilized by a driver is to apply the brakes intermittently or "snub" them as opposed to making a continuous application to the brakes while proceeding downhill.
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