Why Do Semi-Trucks Use Air Brakes?

Why Do Semi-Trucks Use Air Brakes?

Are you curious about why semi-trucks use air brakes instead of the traditional hydraulic brakes?

In this video, we take a deep dive into the different types of brake systems and explain the benefits of air brakes.

We compare hydraulic brakes and air brakes and their pros and cons. You'll learn how each system works, their reliability, and the fail-safe features of air brakes.

Discover why air brakes are the preferred choice for heavy vehicles like semi-trucks and why most car manufacturers stick to hydraulic brakes.

Learn about the extra costs of air brakes and how they can be used on trailers too. Don't miss out on this informative video and hit the subscribe button for more interesting content like this. "

▬▬▬ The video ▬▬▬
00:00 Start
00:57
How do hydraulic
brakes work?
01:51 How do air
brakes work?
02:54 Why Semi trucks
use Air brakes?
04:16 Conclusion

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▬▬▬ Credit ▬▬▬
Volvo

Volvo USA

Freightliner

Wolf Technicals:

▬▬▬ End ▬▬▬
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[randomguy]

CORRECTION: air brakes absolutely do fade from over heating, just like hydraulic brakes. It’s much more common of an issue with air brakes in fact, since they are more powerful they press the pads harder and heat up faster, they have to absorb a lot more energy to stop a heavy truck. If used too much the brake drum can expand enough from heat to make the S cam unable to push the pads far enough. Most run away truck situations are caused by the driver relying on the brakes too much on a downhill, instead of engine braking, and overheating the brakes to the point of failure.

As mentioned hydraulic brakes fade too from over heating, but this is mostly seen with sports cars on race tracks. It’s why a lot of race cars and super cars have exotic ceramic material brake rotors that can handle higher temperatures

[scottrayhons]

WARNING! Before you work on any air brakes. There is a large heavy spring in the brake chambers under tremendous pressure and it will kill you, if disassembled wrong!

[mongolordofdarkness]

Air brakes are used because of the trailer. The trailer has to have the ability to decouple from the tractor. The connection for fluid transfer would potentially have a leak. With a liquid being the fluid to move the brakes, you might lose all your fluid. The air compressor replenishes the air which is the fluid used for air brakes. Air leaks are bad but leaking hydraulic fluid would be more dangerous.

[rtkpX]

Wrong, when the truck has no air in its reserve, the brakes are locked, hence it's impossible for the brakes to be activated when air is injected in the chamber, it's the complete opposite, air is released from the chamber which activates the brakes.

[ChristopherBurtraw]

Another big perk of air brakes too is that if they leak a little bit, they might still work for some time. Even the tiniest leak in a hydraulic system will render that circuit inop and will quickly deplete the fluid reservoir, which can knock out all circuits. A small air leak may only cause the compressor to work more.

[Blacked_OutMedia]

THIS IS HOW AIR BRAKES ACTUALLY WORK:

In the trucking air brake systems there’s two types of brake chambers, dual and singles. Single chambers only have a service brake or foot brake. While duals have a spring or parking brake and a service brake.

The spring brake works by removing all air from the chamber to engage, if you introduce air to the system, around 40 PSI it will over power the spring and move a diaphragm holding the brakes on to release them. Keep in mind not every axle has these so most brake chambers are single.

A service brake works as follows. You push the brake pedal, and air fills the chamber. This pushes on a separate diaphragm moving a push rod. This connects to something called a slack adjuster to ensure your brakes are always adjusted properly. The slack adjusters twists something called an S-Cam which pushes the linings into the drum to engage the service brake. This is the only type of brake a single chamber has.

A dual chamber has both types of brakes and two separate diaphragms, 1 adding air to engage the foot brake, and the other removing air to engage the parking brake.

This might be a little complicated to understand so look up some diagrams on the internet if I didn’t explain it. Well Enough.

[TheDrNinjaman]

One other reason why hydraulic brakes are preferred in personal vehicles is their improved braking modulation and feel. Hydraulic fluid is incompressible, so when you engage them your foot feels the brake much more quickly and directly. Air brakes are slightly slower to respond and mushier feeling at the pedal, with a more on or off behavior.
The other thing people are missing is that hydraulic brakes do fade in a way that air brakes don't, assuming moderately competent brake pads. The first type of brake fade you get in hydraulic systems is usually brake fluid fade, which is when the fluid gets so hot it begins to boil. When the brake fluid boils, it is no longer incompressible liquid, but partially compressible gas, which significantly reduces brake pedal feel and max braking force.

[AaronSmith-kryf]

Another big advantage air brakes is that you can use that compressed air for other purposes. Obviously the brakes and trailer brakes. Compressed air is also used for the airbags on most trucks that act like springs. Some trailers also use airbag springs. The horn is obviously air powered. The seat springs/cushion is also air powered, makes the truck ride smoother cause the air the driver's seat floats on absorbs impacts. On older trucks they used compressed air to operate the windshield wipers. This was a stupid/trouble prone system, ie a slight leak makes the wiper malfunction. You can also use the air system to inflate a low tire.

[bnnttdenn]

Air brakes allow connecting and disconnecting from trailers without having the problem of getting Air in a hydraulic brake system... Down side Air brakes have a slight lag from brake pedal application to brake application about 1/3 of a second on tractor brakes a little more for the trailer...

[JohnGeorgeBauerBuis]

Air brakes were originally developed for trains in the late 1800s, and are also still used on the majority of them.

[mactep]

They do overheat, but another added benefit is that their default position is locked, eliminating the need for a parking brake.

[mndlessdrwer]

For a multitude of reasons, but the main one is that the desired failure state for a semi is stopped. Air brakes in these vehicles use air pressure to decouple the brakes from the rotor or drum and, thus, if you experience a compressor failure, brake line failure, etc. then it will fail to stopped rather than fail to a lack of ability to stop.

[rondini]

Air brakes on trucks are more accurately called spring brakes. There is a massive strong spring in each brake housing. It is applied by default. The air pressure is used to keep the brakes off, not on. When applying pressure on the pedal, the air pressure keeping the shoes off the drum is lessened, causing the spring to gain dominance. This is why you see the double skid marks veering off the road. Loss of air pressure causes the brakes to be applied full force. A technical detail Ton Clancy got wrong in one of his stories

[davidgrisez]

Another reason that air brakes are not used on automobiles is that air brakes require a sizable air tank that takes up space. On a semi truck there is plenty of room for air tanks. On a car an air tank would take up a sizable amount of trunk space.

[Codger]

Some 50 + years ago big trucks used the vacuum air brake system. This system was quite unreliable for quick stopping power. I know because I was involved in an accident that caused my truck to not get slowed down fast enough to prevent running into the back of a car that pulled out in front of me on a two lane road near Springfield, Illinois. The trailer only had 8000 pounds of household goods inside, which amounted to about half of the 32 foot trailer I was pulling at the time. I was traveling about 50 mph and this car pulled out in front of me as best I can remember about 500 to 700 feet in front of me and even that distance was not enough time to keep the tractor from running into the back of the car that was involved. A car that was driven by a 90 some year old man who obviously had clear vision of my presence before he pulled out. The only thing that explains his reckless behavior was, besides his age, was his depth perception and reaction time to existential threats, or possibly did not even look in my direction. Before running into the back of his car (I merely clipped the rear driver's side of his car before veering off to the oncoming lane on a two lane bridge, wiping out about 20 some feet of bridge guardrail before coming back onto the road surface and eventually laying the whole truck on its left side and well off the bridge which spanned the Sangamon River.

[reruddock]

I've read a lot of comments about why semi trucks use air brakes for their trailers, and some people are missing the point. The reason why is that when you couple and decouple a trailer from the tractor, there is NO way to prevent the slightest amount of air from entering a hydraulic system. A small air bubble in a hydraulic braking system greatly reduces or even totally prevents hydraulic pressure from building. This is why standard hydraulic brakes on cars need to be bleed. Think about it: the time it would take to bleed a hydraulic brake system every time a driver coupled a trailer would be too significant of time wasted. Not to mention, decoupling the trailer would result in hydraulic fluid loss. This would even be worse for the yard hostler who couples, moves, and decouples countless trailers every day.

[joshbobst]

As others are pointing out, the critical reason to use air brakes is that it takes a lot of force to apply heavy truck brakes, and an 18-wheeler has ten of them. This requires _much_ more force than could be put by a human with their leg and foot into a hydraulic system. The system is applying about a hundred pounds per square inch of air pressure to a quite large rubber diaphragm inside the brake chamber, which in turn applies it to the brake shoes. Since the diaphragm has a surface area of about 30 square inches, something like 3000 pounds of pressure is being applied to the brake shoes. This makes sense if you think about how much kinetic energy the brakes have to transmogrify into heat energy in just a few seconds.

Also, don't mess with the spring in spring brake chambers. They don't have to apply quite as much force as the service brakes, but more than enough to kill you if their energy gets released onto your body.

[RobMilstrey]

It's not about the fade -- any braking system will fade if applied sufficiently, be it disk or drum, pneumatic or hydraulic. Air brakes are on trucks, as other commenters said, for two primary reasons -- they're tolerant of poor maintenance (small leaks) and it's very convenient to provide an air supply (and signal) to (potentially multiple) trailers for braking capability. However, brake reaction time is significantly poorer than hydraulic and brake feel is virtually non-existent. These two items make air brakes undesirable in passenger car roles. Can you use electric brake controllers to provide braking control to the trailer? Yes, but that system requires a priori knowledge of the braking topology (i.e., how many trailers will be connected) (which is not a problem for modern electronics) and would be less fault tolerant than air brakes.

[tomp]

There is also the system of air actuaded hydraulic brakes, slowly becoming phased out but sometimes still around

[skellertons]

In London, the famous Routemaster bus uses mainly the Lockheed power hydraulic braking system. The mineral oil fluid is pressurised in the lines by a pump and continuously flows around and into two storage accumulators which have sliding pistons inside The fluid pushes the piston against a pre charge of air to give the storage of pressure for the system. A cut off valve which gives off a "clonk" tells you that full pressure has been reached. Also, a flag in the Driver's cab with STOP cut into it, drops into the line of sight, and as well as a warning light warns of low pressure. Handbrake is the traditional pull on mechanical type.
When the Driver depresses the brake pedal, the pressure is applied to the wheel cylinders, and a smooth application of the shoes is made.
London Transport chose this system as they found air brakes could freeze and have air tank capacity problems in heavy traffic. An alternative supplier was Clayton, who used spherical accumulators with rubber diaphragms inside rather than the cylindrical ones with sliding pistons.
On the maintenance side, occasionally a compressor called an intensifier, was connected to the Schrader valves on the accumulators to restore the air pre charge.
On the other hand, I know Engineers who detested this system, dismantling meant them getting saturated in Tellus, (the mineral oil)
Is this system used in the States, apart from possible aircraft where it was derived from?