If you’ve ever seen a giant 18-wheeler barreling down the road, especially going down a hill, the thought of “how on earth is that thing going to stop?!” might have crossed your mind. Semi-trucks do have brakes, like other cars and trucks.
But, just what kind of brakes do semi-trucks use? Semi-trucks use air brakes. The air brakes system is comprised of three different types of brakes.
- Air brakes
- Parking brakes
- Emergency brakes
Of course, it’s a little bit more complicated than simple air stopping the truck and trailer attached. Each of these brakes serves a different role in semi-truck operations. But all are equally important. Keep reading to find out what each brake is used for on a semi-truck, as well as how they all work together to help a semi stop safely.
What Are Air Brakes?
How does air stop a semi-truck?! It’s really hard to imagine a puff of air being strong enough to stop all eighteen wheels of a semi-truck going seventy miles an hour down the road. But it is! Air is responsible for stopping all 35,000 to 80,000+ pounds of a semi.
Air brakes are used with heavy vehicles, like semi-trucks, because they’re efficient, and they run on air. There’s an unlimited supply of air, so that makes them extra reliable.
However, that’s not just regular air puffing around under the hood of a semi-truck. The air that’s used in air brakes is compressed air. That’s why you’ll sometimes hear these brakes referred to as “a compressed air brake system” if someone is calling them by their formal name.
How Do Air Brakes Work?
In a nutshell, air brakes work when friction is applied to the brake pad. The friction comes in the form of compressed air that’s been stored in a tank that’s part of the brake system itself. When the brake is activated, the compressed air shoots out and pushes against a piston. The piston is what then puts the pressure on the brake pad…and stop! You’ve got air brakes.
The foot valve, similar to what we use to stop our own vehicles, is what determines how much air will come out. If the foot valve (or treadle valve) is given just a gentle tap, then there won’t be a lot of air. However, if the brake pedal is pushed really hard, then there will be more compressed air released.
You know that loud noise you hear as a truck is stopping? The one that sounds almost like the tea in your kettle is done? That’s the air in the air brakes compressing and doing all the stopping of the truck.
For a visual brake breakdown, check out this Airbrakes Explained Simply video.
Air Brake Components
The list below includes parts for an air brake system that’s used on semi-trucks:
● Air compressor
● Air compressor governor
● Air reservoir tanks
● Drain valves
● Brake pedal
● Brake chambers
● Pushrod
● Slack adjusters
● Brake S-cam
● Brake shoe
● Return
spring
Parking Brakes
We all forget to put on our parking brake from time to time. But parking brakes are very important in large, heavy vehicles, such as semi-trucks or buses. Nobody wants one of those rolling away while the driver is out of the vehicle.
Parking brakes are an additional part of the air brake system. When the parking brake is on, there are springs that then apply the brakes. If the truck is moving, the compressed air is flowing and keeping the springs from releasing.
Both the tractor and the trailer have parking brakes:
- The tractor is the front part of the semi-truck, with the cab where the driver sits and operates the vehicle. The engine is also part of the tractor.
- The trailer is the back part of the semi-truck that carries the cargo. You may see flatbed trailers that have cargo stacked on top of the covered trailers that carry things inside.
Because both of these are heavy, there are different parking brakes for each. However, there is an additional valve that makes all of these work together and make for a system capable of stopping these very heavy vehicles and their loads.
Emergency Brakes
These are incredibly important to semi-trucks. If the regular brakes lose air pressure, the emergency brakes will engage. This can save the driver and other drivers sharing the road from potentially dangerous situations.
Some trucks are equipped with emergency brakes that will automatically engage. Others need to be manually done. However, if the brakes are properly maintained, the emergency brakes can be operated efficiently and prevent serious accidents from occurring.
Do Truck Drivers Have to Train to Use Air Brakes?
Truck Drivers need a Commercial Driver’s License or CDL. In order to the 160-200 hours recommended for training, truck drivers need to pass a total of up to 8 tests.
- 1 general knowledge tests
- 6 possible specific endorsements:
- Passenger Transport
- Tank Vehicle (Tanker)
- Hazardous Materials (HAZMAT)
- School Bus/Passenger Transport Combo
- Double/Triples
- Tanker/HAZMAT Combo
- 1 air brakes test (for a full, Class A license)
Air brakes are important enough to require their own required test that CDL drivers must pass. For an idea of what drivers are tested on, take a look at this study guide from a department of motor vehicles.
Why Do CDL Drivers Need to Pass the Air Brakes Endorsement?
In many states, if you don’t pass the air brakes test, you’ll have a restriction on your license. This means you will not be able to drive trucks with air brakes. Most heavy trucks and buses are equipped with air brakes, so without this endorsement, your employability is limited.
Once a driver passes the air brakes test, he or she is free to drive any vehicle in the class their license is valid for.
Why Do CDL Drivers Attain Multiple Endorsements?
Just as the air brakes test allows drivers to operate a wide variety of vehicles, having multiple endorsements opens up the road (quite literally) to more job opportunities. The more types of trucks a driver can operate, and the more types of load he can carry expand career opportunities.
The Bureau of Labor Statistics expects the number of driving job openings to increase by 5% between 2014 and 2024. The average salary in the field averages about $43,000, but many companies offer signing bonuses and additional monetary incentives to attract the most versatile drivers.
In Conclusion
Semi-trucks have a specialized air brake system designed to bring an average of 40,000 pounds to a halt safely.
This specialized system requires CDL drivers to complete training and take a test to ensure they are able to operate the air brakes properly. Without passing this test, a driver is limited in what vehicles he or she is allowed to operate. With mastery of the air brakes system, CDL drivers increase their job opportunities and, often, their potential salary.