Air brakes are essential components that enable trucks to transport thousands of heavy pounds of weight safely. All heavy vehicles, trucks, busses, and trailers’ braking systems consist of air brakes.
But why are air brakes used on heavy trucks? Air is free and everywhere, but brake fluid is not; fluids can leak anywhere and cause brake failures.
Since accidents on heavy vehicles can cause catastrophic damage to lives and properties, heavy brakes are equipped with air brakes to eliminate fluid leaks and reduce the rate of accidents to the nearest minimum.
Hence, bus drivers are expected to know how to use air brakes on buses and every other driver who drives an air brake-powered vehicle.
Due to the importance of air brake systems in trucks, buses, and trailers, diesel mechanics need to know air brake system components and locations. They should also know how to diagnose, service, and maintain it for optimum brake performance.
If you’re asking ‘how do air brakes work,’ this article has provided sufficient information on this subject matter and provides answers to users’ frequently asked questions. Sit back and have a 5minute read.
What Are Air Brakes?
Air brakes were not originally designed for automotive vehicles. The air brake system was first used for trains as an alternative to traditional hydraulic braking.
Why? In a hydraulic braking system, the number one danger is hydraulic brake fluid leaking and causing a driver to lose braking control of their vehicle.
The problem is a hydraulic leak is more difficult to spot the larger a vehicle gets. And, if braking is compromised, the consequences are much more dire for transportation carrying heavy cargo.
Air brakes posed a solution for this hazard, because the system is cleverly designed to mitigate the danger of leaks. Swapping out hydraulic fluid, pressurized air is kept at a constant in an air brake system.
When a driver needs to stop, the system releases pressure to slow down the vehicle. That means even if there is a leak in an air brake system, at worst, the drop in air pressure causes a vehicle to slow to a stop.
This design makes air brakes a safer alternative to traditional hydraulic brakes, especially for larger vehicles.
What A Working Principle of Air Brakes?
Air brakes are a crucial component of modern vehicles, particularly heavy-duty trucks and buses. Understanding the working principle of air brakes is essential for anyone involved in the or maintenance of these vehicles.
In this section, we will explore the three key aspects of the working principle: compressed air generation, brake activation and release, and brake balance and control.
Compressed Air Generation
Compressed air generation is the first step in the operation of air brakes. The air compressor, a vital component of the air brake system, is responsible for generating the compressed air needed to activate the brakes.
The compressor is typically driven by the engine of the vehicle, either directly or through a belt and pulley system.
The compressor draws in air from the atmosphere and compresses it to a higher pressure. This compressed air is then stored in the air reservoir, ready to be used when the brakes are activated.
The air reservoir acts as a buffer, ensuring a steady supply of compressed air even when the compressor is not running.
Brake Activation and Release
When the driver applies pressure to the brake pedal, it initiates the activation of the air brakes.
The brake pedal is connected to a valve called the brake valve, which controls the flow of compressed air to the brake chambers. The brake valve allows the driver to modulate the braking force applied to the vehicle.
Within the brake chambers, the compressed air acts upon a diaphragm, which in turn applies mechanical force to the brake shoes or pads.
This force is then transmitted to the wheels, causing them to slow down or stop depending on the amount of force applied.
When the driver releases the brake pedal, the brake valve releases the compressed air from the brake chambers, allowing the brakes to release and the vehicle to resume normal operation.
Brake Balance and Control
Achieving the right balance and control of the brakes is crucial for safe and efficient operation.
Brake balance refers to the distribution of braking force among the different wheels of the vehicle. Proper brake balance ensures that all wheels brake evenly, preventing skidding or loss of control.
One of the key responsible for brake balance is the slack adjuster. The slack adjuster is a mechanism that automatically adjusts the clearance between the brake shoes or pads and the brake drums or discs.
This ensures consistent and even braking performance across all wheels. Brake control, on the other hand, refers to the driver’s ability to modulate the braking force applied to the vehicle.
The brake valve plays a crucial role in brake control, allowing the driver to vary the braking force according to the road conditions and the weight of the vehicle. This control is essential for maintaining stability and preventing wheel lock-up.
In summary, the working principle of air brakes involves the generation of compressed air, the activation and release of the brakes, and the achievement of brake balance and control.
Understanding these aspects is essential for safe and efficient of vehicles equipped with:
- Compressed air generation is achieved through the use of an air compressor and an air reservoir.
- Brake activation and release are controlled by the brake valve, which regulates the flow of compressed air to the brake chambers.
- Brake balance is achieved through the use of slack adjusters, which ensure even braking force distribution among the wheels.
- Brake control is provided by the brake valve, allowing the driver to modulate the braking force according to the driving conditions.
By mastering the working principle of air brakes, drivers and maintenance personnel can ensure the proper functioning and safety of vehicles equipped with this braking system.
Types of Air Brakes
There are two primary types of air brakes:
- S-Cam Brakes: These brakes utilize a cam mechanism to apply the brake shoes. They are commonly found on older trucks and buses.
- Diaphragm Brakes: These brakes use a flexible diaphragm to apply the brake shoes. They are more common in modern vehicles due to their smoother operation and greater reliability.
Air Brakes in Different Types of Vehicles
#1. Air brakes in cars.
Air brake systems, while commonly associated with larger commercial vehicles like trucks and buses, are not typically found in standard passenger cars. Instead, passenger cars predominantly utilize hydraulic brake systems.
These systems operate using brake fluid to transfer force from the brake pedal to the brake pads or shoes, creating friction to slow or stop the vehicle.
#2. Air brakes in buses.
When it comes to the safety and functionality of buses, especially those of substantial size and weight, air brakes are extremely important.
Buses, like heavy trucks, commonly utilize air brake systems due to their efficiency in managing the significant braking demands posed by these vehicles.
#3. Air brakes in trucks.
Integral to the safe operation of trucks, air brakes play a crucial role in managing the substantial weight and stopping power required for these heavy vehicles.
Trucks commonly employ air brake systems due to their ability to handle the demanding braking needs imposed by their size and load capacities.
#4. Air brakes in semitrucks.
Air brake systems serve as the backbone of safety and control in semitrucks, providing the necessary braking force to manage their immense size and weight.
Semitrucks extensively rely on air brake systems, which are vital for ensuring the safety of operation, particularly when navigating highways while hauling heavy loads.
Parts of Air Brake System
The crucial components of an Air Brake System include:
#1. Air Compressor.
The air compressor’s pivotal role lies in generating and maintaining the required air pressure. Driven by the vehicle’s engine through mechanisms like belts and pulleys or shafts and gears, it pumps air into a reservoir, creating pressurized air.
The compressor remains in continuous operation while the engine runs, ensuring a steady supply of compressed air for air brakes and auxiliary systems.
#2. Reservoir.
Reservoirs function as pressure-rated tanks for storing compressed air. They house an ample air volume to accommodate multiple brake applications in scenarios such as engine stops or compressor failures.
The number and size of reservoirs vary based on factors like brake chamber count, size, and parking brake configuration.
#3. Air Dryer.
Positioned between the compressor and the wet reservoir, an air dryer aids in moisture removal from the compressed air. It might contain a moisture-absorbent desiccant and oil filter or feature baffles to separate moisture from air.
#4. Safety Valve.
Acting as a safety measure, the valve prevents reservoir over-pressurization. If the governor malfunctions and fails to unload the compressor, this spring-loaded valve releases excess air into the atmosphere, its pressure setting determined by the spring’s force.
#5. Foot Valve.
The foot valve draws compressed air from reservoirs for braking purposes. It’s operated by the driver’s foot and regulates the air delivered to the brakes based on the degree of treadle or brake pedal depression.
Releasing the valve facilitates the movement of the air from the service brakes. The foot valve automatically maintains application air pressure when the brakes are applied partially.
#6. Brake Chamber.
Brake chambers serve to convert compressed air pressure into mechanical force, engaging the vehicle’s brakes.
These circular containers house a flexible diaphragm, with air pressure causing the diaphragm to move and apply force against the slack adjuster. Mounted on the axle near the brake-equipped wheel, brake chambers transfer air pressure into mechanical motion.
#7. Brake Assembly.
The brake assembly combines the brake chamber and slack adjuster, connected to the backing plate due to steering dynamics. Air pressure moves through an inlet port, pushing the diaphragm and the pushrod.
The pushrod connects to a slack adjuster, which then transforms the motion into a twisting motion for the brake camshaft and S-cams. Upon air exhaustion, the return spring in the brake chamber restores the diaphragm and pushrod to their original positions.
How To Use Air Brakes?
Air brakes are used for normal stops, emergency stops, controlled braking, and stab braking. Here’s how to use air brakes.
Normal stops
Like hydraulic braking systems, apply air brakes on normal stops, step on the brake pedal. You need to control the pressure to bring the vehicle to a safe and smooth halt.
If you’re driving a manual transmission, do not depress the clutch until the engine RPM is low; close to the idling rate.
Emergency stops
If a semi-truck or small passenger car suddenly pulls in front of you, apply the brake immediately. However, that depends on the speed you’re traveling.
If you’re at high speed, you need to brake in a way that won’t wave your steering of the straight lane and allow you to steer off the obstacle.
Controlled braking
This method implies applying the brake as hard as possible while ensuring the wheels do not lock up. If you need to make a sharp turn, release the brake and re-engage it immediately.
Stab braking
This method is only used on vehicles with an anti-lock brake system. Apply the brake and release it as soon as the wheels lock up. Once the wheels start moving, reapply the brakes again.
Typically, it takes around 1 second for the wheels to start moving after releasing the brakes. Do not reapply the brakes before the wheels start rolling. If not, the wheels will not straighten out.
What are air brakes used for?
An air brake slows or stops a moving vehicle. They are also commonly used on railroads and other vehicles. Air brakes are also used in emergencies, such as accidents. Air brakes are usually found on passenger cars and light freight trains.
Air brakes are also referred to as pneumatic brakes. In addition, air brakes are sometimes referred to as ‘vacuum’ brakes.
There are two main types of brakes: vacuum brakes and air brakes. Vacuum brakes use the negative pressure created by the car’s movement to apply pressure to the brake shoes, and air brakes use compressed air to apply pressure to the brakes.
The purpose of air brakes is to slow or stop a vehicle. Air brakes, also called P-type brakes, are used in passenger cars and light freight trains. Air brakes are also called “air brakes” or “pneumatic brakes.”
Why Do Trucks Use Air Brakes?
Standard passenger cars use hydraulic systems for their brakes, while large vehicles like trucks use air brakes. Since trucks are heavier than standard passenger cars, they need a more significant force to stop.
If a hydraulic system is used in a truck, the driver would be required to exert more force on the brake pedal to stop the truck, which is impossible.
Even if a brake booster were to be used, designing the hydraulic system to allow a push on the pedal connected to the pneumatic valve or master cylinder and then to the brake calipers and disc brake would be challenging.
Due to the potential energy required, air brakes use compressed air to keep the engaging brakes open.
When the brake is pushed, it reduces air pressure from the brakes, engaging the calipers again. Hence, the driver does not need to put much effort into pressing the brake pedal.
Besides providing more stopping power than hydraulic brakes, air brakes are used in trucks because they are less likely to overheat and less sensitive to temperature variations.
Maintenance and Troubleshooting of Air Brakes
Air brakes are an integral part of any heavy-duty vehicle, ensuring the safety of both the driver and those on the road.
However, like any mechanical system, air brakes require regular maintenance and troubleshooting to ensure they function optimally.
In this section, we will explore the key aspects of maintaining and troubleshooting air brakes, including checking air pressure, inspecting brake lines and hoses, identifying and fixing air leaks, and adjusting brake system.
#1. Checking Air Pressure.
One of the most critical maintenance tasks for air brakes is regularly checking the air pressure. Adequate air pressure is essential for the proper functioning of the brake system. To check the air pressure, follow these steps:
- Start the vehicle’s engine and allow the air compressor to build up pressure.
- Locate the air pressure gauge, usually located on the dashboard or instrument cluster.
- Observe the gauge and ensure that the pressure reaches the recommended level, typically between 100-125 psi (pounds per square inch).
- If the pressure is too low, it may indicate a leak or a faulty air compressor. Conversely, if the pressure is too high, it can lead to brake lockup or reduced braking efficiency.
Regularly checking the air pressure not only ensures the safety of the vehicle but also helps identify potential issues early on.
#2. Inspecting Brake Lines and Hoses.
Brake lines and hoses are critical components of the air brake system. They carry compressed air from the air compressor to the brake chambers, enabling the application and release of brakes.
Regular inspection of brake lines and hoses is crucial to identify any signs of wear, damage, or leaks.
During the inspection, pay close attention to the following:
- Check for any visible signs of wear, such as cracks, abrasions, or bulges in the brake lines and hoses.
- Ensure that all connections are secure and free from leaks.
- Look for any signs of oil or moisture around the fittings, which may indicate a leaking brake line or hose.
- Inspect the rubber hoses for any signs of degradation or brittleness.
If any issues are detected during the inspection, it is essential to address them promptly. Damaged or leaking brake lines and hoses can compromise the performance of the air brake system and pose a safety risk.
#3. Identifying and Fixing Air Leaks.
Air leaks in the brake system can significantly impact braking performance and compromise safety. Identifying and fixing air leaks promptly is crucial to maintain the integrity of the air brake system. Here are some steps to follow:
- Perform a thorough visual inspection of the entire brake system, including the air compressor, reservoir, brake chambers, and all connections.
- Listen for any audible hissing or air sounds, which may indicate the presence of a leak.
- Use a soap and water solution to spray on all connections and fittings. If bubbles appear, it indicates a leak.
- Once a leak is identified, determine the source and take appropriate action to fix it. This may involve tightening loose connections, replacing faulty seals or gaskets, or repairing damaged.
Regularly inspecting for and addressing air leaks is crucial to maintain the safety and functionality of the air brake system. Ignoring or delaying repairs can lead to brake failure and potentially catastrophic accidents.
#4. Adjusting Brake System Components.
Proper adjustment of brake system components is vital for optimal braking performance. Over time, such as slack adjusters and brake pads may require adjustment to ensure they are operating within the manufacturer’s specifications. Here’s an overview of the adjustment process:
- Start by inspecting the slack adjusters, which are responsible for maintaining the correct clearance between the brake shoes and the brake drum. Ensure that they are properly lubricated and functioning correctly.
- Measure the pushrod stroke using a brake stroke measuring tool. Compare the measurement to the manufacturer’s specifications to determine if adjustment is necessary.
- If adjustment is required, follow the manufacturer’s guidelines for the specific type of slack adjuster and adjust it accordingly.
- Additionally, check the brake pads for wear and replace them if they are worn beyond the recommended thickness.
Properly adjusted brake system components ensure even braking, reduce the risk of brake fade, and extend the lifespan of the braking system.
Advantages of Air Brake System
The advantages encompass the following:
- Demonstrates enhanced effectiveness in comparison to alternative braking methods.
- Simplifies chassis design as air brake components are conveniently situated.
- Utilizes compressed air for multiple applications like tire inflation, wipers, horn, and other accessories.
- Operates solely on air as the readily available working medium.
- Facilitates convenient storage of air under high pressure.
- Yields potent braking power suitable for heavy vehicles and trucks.
- Offers superior control over braking actions.
- Reduces the distance required for a complete stop.
- Minimizes wear and tear on components.
- Incorporates flexible hose connections for added versatility.
Disadvantages of Air Brake System
The disadvantages include:
- Air braking systems involve intricate components and mechanisms, making installation, maintenance, and troubleshooting more complex.
- Air brakes can have slightly slower response times compared to hydraulic systems due to the time required for air compression and release.
- In prolonged heavy use, air brakes may experience fading due to increased heat buildup, impacting their braking efficiency.
- Regular checks for air leaks, moisture, and system integrity are essential, increasing maintenance requirements.
- Air systems can be sensitive to contaminants, affecting their performance and potentially leading to malfunctions.
FAQs.
Q: What happens when air brakes fail?
With advances in technology, it is nearly impossible for air brakes to fail. They are more or less bulletproof.
However, if the air brake system has low air pressure, it’ll trigger an alarm that indicates low air pressure in the braking system.
In any case, trucks, buses, trailers, trains, and airplanes are designed with backup emergency brakes that activate automatically when the air brake fails, making it impossible for vehicles with air brakes to fail.
Q: Are air brakes hard to use?
Air brakes are suitable and reliable braking systems for heavy vehicles with a gross weight of around 26,000 pounds and above. Twenty-six thousand pounds of weight will outweigh the hydraulic brake systems, which is why that is replaced with air brakes.
However, brake fluids cannot be compressed, and air can, it is very difficult to bring air brakes to a complete stop smoothly. When a driver applies the brake on a hydraulic braking system by stepping on the pedal, the same pressure is applied to all the wheels.
In contrast, when a driver applies a brake on the air braking system, it creates a pressure wave that travels to all the wheels simultaneously before stopping.
Q: How long should it take for a truck to build air?
Air compressors have a cut-in pressure of around 20psi or lower and a cut-out pressure of 110 psi and 130 psi. In any case, air pressure from 85 to 100 psi will take less than a minute to build up.
Q: Which is better air brakes or hydraulic brakes?
Air brakes are more or less bulletproof. They are better than hydraulic brakes. Using compressed air instead of brake fluids eliminates unnecessary brake failures.
For instance, any leak on brake lines will cause brake failures. On the other hand, minor or significant brake leaks on the air brake systems will not cause the system to fail.
Q: Why do air brakes take longer to stop?
The primary reason why air brakes take a longer time to stop is that it uses air. Air takes longer to travel to all the wheels than hydraulic.
When you depress the brake pedal on heavy vehicles with air brakes, it transports the compressed air from the storage tank to the wheels simultaneously. Hydraulics takes a shorter time compared to air.
Q: Why are disc brakes not used in trucks?
Disc brakes are still used in trucks. However, 95 percent of trucks in the US use drum brakes.
The significant reason trucks rely on air brakes is that they require less replacement than disc brakes. And are cost-effective. Since they require less maintenance, they are a better option than disc brakes.
Final words
At this juncture, we have explained how to use air brakes on trucks and other heavy vehicles.
If you have followed this post religiously to this point, you’ll no longer ask, ‘how do air brakes work.’
So when next you hear a loud hiss from a truck or bus behind you on the road, you won’t fret knowing they are reliable brake systems and are built to make such noise.