In the last two decades, air brakes have become standard equipment for semi trucks and trailers. Air brakes have a very high safety record, which is why they are standard on motor carrier vehicles. Improvements in technology and the increased numbers of fail-safes on air brakes have made them nearly fail-safe. The only failing on an airbrake system in this day and age is brake fade, which is an uncommon occurrence for most professional drivers.
In order to pass a CDL, drivers have to pass an airbrake course. In the 1970’s commercial driver education classes taught that airbrakes were dangerous. This is no longer true but some people still think that they are unsafe. With advanced technology, modern air brake systems are more or less bulletproof. Air brake performance is solid and consistent; in-depth training of the inner workings of an air brake system is no longer required.
Here are 10 safety features of air brake systems today.
- Air Tanks (Reservoirs)
The air tanks on air brakes systems are capable of holding enough air for 6 to 12 full brake applications. If the entire system shuts down there will still be enough volume of air in the air tanks to make 6 go 12 full brake applications and to stop a fully-loaded commercial vehicle equipped with air brakes. Furthermore, you don’t need a full brake application to bring the vehicle to a stop. If the compressor falls of the motor or the whole system shuts down, there will still be enough air to bring the truck to a full stop.
- One-Way Check Valves
The one-way check valves at the entrance to the primary and secondary systems are the next failsafe on the system. These prevent air from bleeding out of the system in the event there is a system failure or a break in the line between the compressor and the air tanks. The air can only move one way through the system, so it’s unlikely that if the compressor stops working or the main discharge line is broken, the air is going to bleed out of the system because of those one-way check valves which prevent the air from coming back through the system. The air will only go forward in the system.
- Dual Air Brake Systems
All braking systems are divided into two independent sub-systems. If one system fails the other will continue to work normally so long as the air compressor continues to pump air. It is rare that the air compressor will fail. Even on a hydraulic system on your passenger vehicle, if you open the cover off of the master cylinder, you will see two chambers. Since the late 1960’s all hydraulic braking systems have been divided into two independent sub-systems, one for the front and one for the rear. An air brake system is the same; it is divided into two independent sub-systems, the secondary and the primary. The secondary runs the steer axles and the primary everything behind the driver. This is the most prominent fail-safe system of an air brake system.
- Air Compressor
The air compressor itself is another fail-safe on the system. The air compressors pump more air than could possibly be required. The air compressor system on modern systems pumps enough air to run all of the air accessories on the vehicle. One of the most demanding systems that uses air pressure is the air-ride suspension, because, unlike a passenger vehicle that has airbags and it is just a balloon that you pump up and it just pushes down as if you were going to squish a balloon. On a tractor-trailer there is air constantly being evacuated out of the airbag system. When the vehicle goes over a bump it actually releases some of the air and fills back up. The compressor is constantly pumping air to fill the air ride suspension as well as other accessories. The compressor is able to not only run the air brakes but also all the accessories on the vehicle.
- Pressure Protection Valve
There is a pressure protection valve on the system. If the system pressure starts to drop between 60 and 90 lbs the pressure protection valve will shut off air to the accessories and direct air into the brakes only.
- Low Air Warning
One of the most prominent fail-safe on a modern air brake system is the low air warning device. The low air warning device includes an audible and visual notification. While it is an annoying sound, it is an important warning mechanism. The warning device comes on at 60 lbs because at that level there isn’t enough air pressure in the system to hold the gigantic springs off in the caged position, which activate the parking brakes and also work as the emergency brake.
All of the air pressure on an air brake system is pumped through the dash. Even over all of the sound of road noise, the diesel engine and other ambient sounds, the driver will be able to hear air leaks. Even a minor air leak will be noticeable enough to be heard.
- Spring Brakes
Another important fail-safe are the spring brakes. Most of the time the spring brakes are used for parking, but in the event of an emergency, and there’s a loss of air, the spring brakes are going to activate. This is because while the vehicle is going up and down on the road, those giant springs are held in the released position by air pressure. If you lose all the air pressure in the system, the springs expand and engage the brakes. In the event of a catastrophic air loss in the system, the spring brakes will engage and the vehicle will come to a screeching halt. This will often occur between 20 and 45 pounds per square inch (psi).
- Tractor Protection System
The next fail-safe on a truck designed to pull trailers with air brakes is the tractor protection system. The tractor-protection system protects the tractor’s air supply in the event that the trailer falls off the back of the truck. The tractor-protection system consists of two valves: 1.) the trailer air supply valve on the dash and 2.) the tractor protection valve. The tractor protection system in the back is like the guard; it monitors air pressure into the trailer. If it detects catastrophic air loss in the trailer it will signal the air supply valve on the dash to shut off air to the trailer to protect the air supply in the truck.
- Automatic Slack Adjuster
In the past, air brake systems had manual slack adjusters that had to be adjusted by the driver to keep the system adjustment and working properly. Up until the mid 1990’s and early 2000’s, 75-80 percent of trucks were put out of service at truck check points due to the brakes being out of adjustment. Today, if a truck goes into the shop for its annual inspection and is found to have manual slack adjusters (which is against regulations) they have to be swapped out for the vehicle to pass its annual inspection. Automatic slack adjusters are on virtually all vehicles today. Automatic slack adjusters are reliable and work well to keep the brakes in adjustment.
Automatic slack adjusters were the last weakness in the older air brake systems. By designing a fail-safe for the slack adjusters, modern air brake systems have become almost 100 percent foolproof. Never underestimate the importance of your braking system; it is one of the most important pieces of your truck equipment.
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