For those in the commercial trucking industry, understanding heavy-duty braking systems is important. This guide dives into the components, mechanics, and safety innovations of heavy-duty truck brakes.
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The air compressor is the heart of the air brake system in heavy-duty trucks. It generates the necessary air pressure that powers the entire braking mechanism. Typically driven by the truck's engine, the air compressor takes in atmospheric air, compresses it, and sends it to the air reservoirs. Maintaining air pressure is crucial because inadequate pressure can lead to brake failure, posing significant safety risks.
Air reservoirs, also known as air tanks, store the compressed air produced by the air compressor. These tanks ensure that there is a sufficient supply of air available for the brake system, even if the compressor temporarily stops working. Most trucks are equipped with multiple reservoirs to provide redundancy and to ensure the system has a backup supply of compressed air in case of a primary tank failure.
Brake chambers convert air pressure into mechanical force. When the driver applies the brake pedal, air is directed into the brake chamber, pushing a diaphragm or a piston. This movement then actuates the push rod, which applies force to the slack adjusters and, subsequently, to the brake shoes. Brake chambers are available in various sizes and specifications, each suited to different vehicle requirements.
Slack adjusters play a critical role in maintaining the correct distance between the brake shoes and the brake drum. They automatically adjust for wear and tear in the brake shoes, ensuring consistent brake performance. Properly functioning slack adjusters are essential for effective braking, as they prevent excessive slack, which can lead to reduced braking efficiency and increased stopping distances.
Brake drums are one of the most common components in heavy-duty braking systems, as opposed to disc brakes used in many smaller passenger vehicles. The brake drum is a large, drum-shaped component that rotates with the wheel. When the brake is applied, the brake shoes press against the inner surface of the drum, creating friction that slows the vehicle. The drum's material and design are crucial for effective heat dissipation, ensuring that the brakes do not overheat during prolonged use.
Brake shoes are fitted with brake linings made from high-friction materials designed to withstand high temperatures and stresses. These linings are pressed against the brake drum to create the necessary friction to slow the truck. The materials used for brake linings must balance durability, heat resistance, and friction properties to ensure optimal performance and safety.
The S-Cam and camshaft mechanism is essential in actuating the brake shoes. When the brake pedal is pressed, the S-Cam rotates, pushing the brake shoes apart and against the drum. The camshaft's precise movement ensures that the shoes are evenly pressed against the drum, providing stable and effective braking.
Air brakes in heavy-duty trucks follow a straightforward yet robust process.
Friction is the fundamental principle behind braking. However, the significant friction generated during braking also produces a considerable amount of heat. Effective heat management is crucial to prevent brake fade—a condition where brakes lose effectiveness due to excessive heat. Brake drums and linings are designed to dissipate heat efficiently, and some systems incorporate additional cooling mechanisms to maintain brake performance.
Ensuring balanced brake force distribution across all wheels is essential for safe and effective braking. Disproportionate braking can lead to skidding, loss of control, and increased wear on certain brakes. Modern braking systems often include devices like load-sensing valves that adjust the brake force according to the vehicle's load, ensuring even distribution and preventing wheel lockup.
ABS is a critical safety feature in modern heavy-duty trucks. It prevents wheel lockup during emergency braking by modulating brake pressure. This technology maintains traction and steering control, reducing the risk of skidding and collisions. ABS is particularly beneficial in adverse weather conditions where road grip is compromised.
EBS takes brake control a step further by incorporating electronic controls. Unlike traditional pneumatic systems, EBS uses electronic signals to manage brake application, allowing for faster and more precise brake responses. EBS systems often integrate with other vehicle systems like traction control and stability control, enhancing overall vehicle safety and performance.
Modern trucks are increasingly equipped with advanced brake monitoring systems. These systems provide real-time data on brake performance, wear levels, and potential faults. By alerting drivers and fleet managers to issues before they become critical, these systems help maintain optimal brake function and reduce the risk of brake-related accidents.
Understanding the workings of heavy-duty truck brake systems is essential for anyone in the commercial trucking industry. From the air compressor to the advanced electronic braking systems, each component plays a role in ensuring safety. By staying informed and proactive about brake system maintenance and advancements, truck operators can ensure their vehicles remain in top condition.
If you are spec’ing brakes, I’m sure I don’t need to tell you that lately, the brake market is increasingly split into two categories: the tried-and-true drum brakes, and the up-and-coming air disc brakes (ADBs). Although most heavy-duty vehicles on the road still use traditional drum brakes, which cost quite a bit less than disc brakes, ADBs are increasingly being specified due to the improved performance they provide.
“The most significant advantage of ADBs is better stopping power, reducing stopping distance by 10% compared with drum brakes,” said Jon Morrison, WABCO’s president for the Americas. “Because of their design, ADBs are exposed to outside air, which works to constantly cool the rotor, greatly reducing their tendency to overheat and nearly eliminating brake fade. Other ADB advantages are the extension of service intervals up to two times versus drum brakes and the ability to exchange the brake pads of four ADB wheel ends in the time that is needed to exchange the linings of one drum wheel end.”
Brake friction often changes when making a switch from one type of brakes to the other, and it’s important for fleet managers to know exactly what these changes will mean.
“The growing adoption of air disc brakes has prompted friction suppliers to ensure that their customers are properly trained and informed of the benefits of disc brakes, such as increased performance, life and ease of change,” said David Inman, field technician / technical sales for TMD Friction.
Greg Dvorchak, engineering supervisor for Hendrickson Trailer Commercial Vehicle Systems, weighed in on the changes from an engineering perspective, “Air disc brakes in the commercial vehicle market are semi-metallic, meaning they contain more than 50% ferrous (iron). The majority of today’s commercial vehicle drum brakes are non-asbestos organic [NAOs]. These formulas contain very little if any ferrous compounds. The increase in use of air disc brakes has forced friction material companies to develop materials that are different from field-established drum brake materials.”
One upcoming change that brake and friction suppliers are monitoring closely is the upcoming implementation of the Better Brakes Law, which has been enacted in Washington and in California (where it is named the Brake Friction Material Law). The law mandates that vehicle brake pads manufactured after must contain less than 5% copper, and by , must contain less than 0.5% copper.
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Joe Kay, director of brake engineering with Meritor, Inc., said that this law is one of the biggest challenges that brake companies such as his are preparing for.
“Copper is one of the elements used to manage thermal energy,” he began. “Since disc brakes operate at higher temperatures, we want to make sure the interface temperatures don’t get too high, or damage to the pad can occur through thermal destruction. Copper also contributes to stabilizing the friction Mμ. This is important as the brake is used at various speeds, temperatures and application pressures.
“We want to make sure the next-generation materials will provide as good or better benefits to the users,” he continued. “It typically takes four years to develop and release a brake friction because of extensive testing involved. There is laboratory testing, FMVSS vehicle testing, fleet testing in numerous vocations and then OEM test requirements to be met.
There are also multiple goals to hit before releasing a friction, he went on. “Performance is the primary objective,” he said, “but we also need to make sure NVH is good, wear kindness on the rotor is acceptable, cost is where the market accepts it, etc.”
From an aftermarket perspective, there are some additional changes that the growing prevalence of disc brakes has forced.
“Drum brake friction requires substantially more investment to reline brake shoes. This technology can also be used to line new shoes as well. However, as the disc brake market continues to increase, businesses that reline drum brakes should evaluate the cost of investing in a mature product line,” advised Dan Dunkleberger, national sales/friction product manager for Haldex.
There are two important differences between the makeup of disc and drum brakes that result in different friction needs for your fleet. To understand these differences and what they mean, it’s time to talk a little science.
The first major difference between disc and drum pads from a friction standpoint is heat. Disc brakes operate, and can withstand, much higher temperatures than their drum counterparts. Drum brakes will typically operate between ambient and 300°F in service (depending on the vocation), while disc brakes in the same application would operate between ambient and 600°F, Meritor’s Kay noted.
Why is this? Application pressures are more than four times higher on disc brake pads as compared to drum brake shoes, explained Roger Jansen, product manager of trailer suspension systems (Americas) with SAF-Holland Inc. “This means that disc brake friction material has to be designed to perform at these high temperatures,” he said.
“Both drum and air disc brakes stop the vehicle by creating friction, which also generates heat,” WABCO’s Morrison explained. “Once the brake components become saturated with heat, it reduces their ability to halt a vehicle. As the name implies, drum brakes are housed inside a drum, which in high-braking conditions can result in too much heat build-up within the drum.
“Air disc brakes also rely on friction, but instead of housing the major components within a metal drum, disc brakes use calipers and pads to create friction on rotors to stop the vehicle,” he continued. “The rotor used in disc brakes is exposed to outside air, which works to constantly cool the rotor. Because ADBs are more efficient at displacing heat than drum brakes, they have significantly less tendency to overheat or fade.”
This helps to fight brake torque loss often caused by brake fade.
The second major difference between the two is that surface area of disc pads is roughly one-half to one-third of equivalent drum brakes—but, Hendrickson’s Dvorchak noted, the energy required to stop the vehicle is the same.
“This means disc pad friction material must be formulated to withstand two to three times the energy per square inch,” he added. “It also means that brake temperatures will spike hotter on an air disc brake than on an equivalent drum brake.”
“When applying the brakes, the energy per square area in contact with the rotor is smaller than the square area in contact with a drum,” Meritor’s Kay said. “Therefore, the smaller area will have a higher thermal result. It’s physics!”
This, he continued, means that disc brake frictions need to be designed to handle higher temperatures, and need higher strength materials used to hold them together during braking and thermal cycling.
Many fleets trying to save a few dollars may decide to mix friction brands or quality between the tractor and trailer brakes; that is, to use one type of friction on the tractor and another type on the trailer, or vice versa. We polled brake experts to see whether this practice is advisable. The answer, in short: a resounding “no.”
“Friction material is a safety-related product and should be properly spec’d for the application for which it will be used,” pointed out Haldex’s Dunkleberger. “This practice applies for both drum brake and disc braking systems.”
Here are five of the reasons given for avoiding mixing and matching:
In general, the advice was clear: change the friction on the complete tandem and avoid one-wheel brake jobs. When it’s time to replace the friction on one wheel, it’s time to replace it on all of them. It may seem like an unnecessary expense at the time, but it will save you money in the long run.
So how does a fleet know when it’s time to replace brake friction? When the friction is running low, the driver will often hear a high-pitched screeching sound and feel that the brakes are not as responsive as they should be or that the truck pulls to one side when the brakes are applied. Of course, routine inspections and maintenance should detect friction wear before the driver ever notices that the pads need replacing. WABCO’s Morrison recommended that fleets visually inspect or measure the brake pad thickness with wheels on every six months and do the same with the wheels off every 12 months.
To that end, most lining and pads will have visual indicators that will provide guidance as to when the friction needs replacing.
“For a brake lining, the minimum lining thickness is ¼ in., and air disc pads should be replaced when the friction wears down to 3 mm above the backing plate,” TMD’s Inman said. “Fleets should stick with the rated axle load when choosing lining. Installing 23K lining on a 20K axle is not improving wear or brake balance. Line haul 23K axle should be a different material than 23K refuse hauler.”
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