If you are training to become a commercial driver in the U.S.,
one of the crucial pieces of your CDL test will be knowledge of air
brake systems on large trucks. Even if you're a competent mechanic who
knows small passenger cars and trucks inside and out, air brakes are a
technology that you will be unfamiliar with and will need to understand
before qualifying as a commercial driver.
A passenger car uses brake fluid to transmit pressure from the pedal to the brake assembly on each wheel; the fluid runs through pipe lines and pushes brake pads against the wheel in order to stop its rotation. Fluid works well for this purpose since, unlike gas, liquids are practically incompressible, and are therefore ideally suited to transfer pressure in a closed system (such as a brake system), as well as amplify it hydraulically (hydraulics are outside the scope of this article, but How Stuff Works has an excellent explanation of brake hydraulics for those interested). The fact that liquid has constant volume but no shape has also made it the superior alternative to rigid braking systems in use on early wheeled transport and automobiles.
However, fluid is an inadequate medium for the brakes of large commercial vehicles like 18-wheeler tractor-trailers. There are several reasons for this - one is that given the heat generated due to the friction involved in stopping a massive cargo trailer, the brake fluid may vaporize and render the brake system inoperable. Another reason is that tractor-trailers feature a detachable, mobile component - the trailer - which would be very problematic to connect to a tractor's braking system without creating a mess and leaking large amounts of fluid. Brake systems that operate primarily using compressed air sidestep both of these problems, but they also require the truck driver to understand the unique issues that come with using a substance that is highly compressible and bad at absorbing and transferring heat.
If you're training to receive a commercial driving license, you will be given a regular driving test, and you will also need to pass a special test on truck-grade air brakes.
Commercial driving schools, including ours, teach candidates how to use air brakes, how they work, how to test them, how to maintain them in order to avoid failure or accidents. While no substitute for hands-on training, here is a rundown on the basics of air brake functionality that you will need to master in order to qualify for a Commercial Driver's License.
Air is pumped into the braking supply system from the atmosphere by a compressor that's powered by the truck's engine. The air passes through filters and dryers which remove moisture and particles that could clog brake lines and impair their operation. The air is stored in a main reservoir tank, from which it is routed to smaller peripheral tanks that power the front and rear brakes, as well as the parking brake and other systems.
For optimal operation, the air needs to be maintained at a constant pressure, usually around 125 psi. At 150 psi, a safety valve opens and air is purged from the reservoir. The fact that pressure mounts to that level is a sign that something's wrong with the system and maintenance is required.
On the wheel end, most commercial trucks today use a so-called "S-cam" brake system, where air pressure in the brake drum turns an S-shaped shaft that, in turn, presses brake pads against the wheel to stop motion.
The parking brake is an important safety feature of an air brake system. It's a spring-actuated brake that is engaged by default and prevents the vehicle from moving. When air pressure mounts in the braking system, it pulls the spring open and disengages the brake; however, if some part of the air brake system fails and causes the loss of air pressure, the spring will automatically re-engage the brake and cause the vehicle to stop; the faster air loss occurs, the more abrupt the stop.
Brakes aren't the only part of a truck that are powered by compressed air - gearbox systems & pedals, for example, use compressed air to amplify & assist the driver's input.
This is a very simple introduction to the principles of air brake operation. The actual systems are far more complex, and take weeks of learning and training to master. Fortunately, there are many materials available to an aspiring truck driver that can help them prepare for a CT CDL test. Review online materials and apply to a reputable CDL driving school for training, and you should ace your commercial driving test on the first try!
A passenger car uses brake fluid to transmit pressure from the pedal to the brake assembly on each wheel; the fluid runs through pipe lines and pushes brake pads against the wheel in order to stop its rotation. Fluid works well for this purpose since, unlike gas, liquids are practically incompressible, and are therefore ideally suited to transfer pressure in a closed system (such as a brake system), as well as amplify it hydraulically (hydraulics are outside the scope of this article, but How Stuff Works has an excellent explanation of brake hydraulics for those interested). The fact that liquid has constant volume but no shape has also made it the superior alternative to rigid braking systems in use on early wheeled transport and automobiles.
However, fluid is an inadequate medium for the brakes of large commercial vehicles like 18-wheeler tractor-trailers. There are several reasons for this - one is that given the heat generated due to the friction involved in stopping a massive cargo trailer, the brake fluid may vaporize and render the brake system inoperable. Another reason is that tractor-trailers feature a detachable, mobile component - the trailer - which would be very problematic to connect to a tractor's braking system without creating a mess and leaking large amounts of fluid. Brake systems that operate primarily using compressed air sidestep both of these problems, but they also require the truck driver to understand the unique issues that come with using a substance that is highly compressible and bad at absorbing and transferring heat.
If you're training to receive a commercial driving license, you will be given a regular driving test, and you will also need to pass a special test on truck-grade air brakes.
Commercial driving schools, including ours, teach candidates how to use air brakes, how they work, how to test them, how to maintain them in order to avoid failure or accidents. While no substitute for hands-on training, here is a rundown on the basics of air brake functionality that you will need to master in order to qualify for a Commercial Driver's License.
Air is pumped into the braking supply system from the atmosphere by a compressor that's powered by the truck's engine. The air passes through filters and dryers which remove moisture and particles that could clog brake lines and impair their operation. The air is stored in a main reservoir tank, from which it is routed to smaller peripheral tanks that power the front and rear brakes, as well as the parking brake and other systems.
For optimal operation, the air needs to be maintained at a constant pressure, usually around 125 psi. At 150 psi, a safety valve opens and air is purged from the reservoir. The fact that pressure mounts to that level is a sign that something's wrong with the system and maintenance is required.
On the wheel end, most commercial trucks today use a so-called "S-cam" brake system, where air pressure in the brake drum turns an S-shaped shaft that, in turn, presses brake pads against the wheel to stop motion.
The parking brake is an important safety feature of an air brake system. It's a spring-actuated brake that is engaged by default and prevents the vehicle from moving. When air pressure mounts in the braking system, it pulls the spring open and disengages the brake; however, if some part of the air brake system fails and causes the loss of air pressure, the spring will automatically re-engage the brake and cause the vehicle to stop; the faster air loss occurs, the more abrupt the stop.
Brakes aren't the only part of a truck that are powered by compressed air - gearbox systems & pedals, for example, use compressed air to amplify & assist the driver's input.
This is a very simple introduction to the principles of air brake operation. The actual systems are far more complex, and take weeks of learning and training to master. Fortunately, there are many materials available to an aspiring truck driver that can help them prepare for a CT CDL test. Review online materials and apply to a reputable CDL driving school for training, and you should ace your commercial driving test on the first try!
Connecticut's Allstate Driving School offers CT Class A and B CDL training, including preparation for the air brake portion of your CDL test. Go to Allstatetraining.com to sign up for our classes today!
By Christopher Maiorano
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