Brake wear is influenced by many factors, notes Jeff Wittlinger, business unit director for wheel-end and braking systems at Hendrickson. “Dynamics like length of haul, application, terrain, driver patterns and tractor-trailer equipment combinations are factors,” he says. “In addition, tractor to trailer balance, aerodynamics, engine size, vehicle and load weight and even the use of engine brakes are all potential variables that influence performance.”
A comprehensive brake system preventive maintenance program is driven in part by understanding all of those operating conditions and equipment factors. They are the things that will determine intervals at which critical components like brake pads should be replaced.
Brake systems manufacturers offer a range of helpful tips to pass along to technicians regarding trends in brake wear. Keith McComsey, director of marketing and customer solutions (wheel-end) at Bendix Spicer Foundation Brake, notes that after the Reduced Stopping Distance (RSD) mandate went into effect, drum brake friction was redesigned to stop vehicles in a much shorter distance. To obtain that level of performance a more aggressive friction material was needed, which resulted in shorter service intervals.
“There are comparisons showing drum friction lasting longer than air disc brake friction on combination vehicles that utilize both air disc brakes and drum systems,” McComsey says. “What is happening is that there is a work balance shift when you have portions of your vehicle using air disc brakes and other portions of the combination using drum brakes.
“For instance, when drum brakes heat up and start to fade, they are basically contributing less toward helping slow and stop the vehicle,” McComsey adds. “Air disc brakes virtually eliminate fade, so in this scenario, the brake work for the drum axles that are not contributing to braking is shifted toward the air disc brake equipped axles. That leads to some accelerated wear in air disc brakes when compared to drum brakes on the same vehicle.”
Eric Coffman, senior manager of drum brakes at Meritor, cautions against the use of brake friction materials that have only undergone a minimum of testing and are unproven in combinations of lined shoes and drums or in air disc brake pads and rotors. “Those have shown accelerated wear rates, reducing vehicle uptime,” he adds.
“Increased use by fleets of ‘white box’ lined shoes and lightweight drums without consideration of using a friction material and drum based on vehicle application, terrain and duty cycle also increases lining wear,” Coffman continues. “In that case, wheel-end temperature elevates and a lightweight cast brake drum has roughly 10% less thermal capacity to dissipate heat than a standard weight cast brake drum can achieve.”
Corrosion, or rust jacking, can result in cracks between the backing plate and the friction material.
“Corrosion often shortens the life of brake shoes and increases downtime for maintenance and replacement,” says Meritor’s Coffman. “When the lining cracks, it can lead to shorter brake shoe life and to failed inspections, and raise the risk of an out-of-service violation.”
“Issues like pads binding in the caliper and uneven brake wear are typically due to corrosion,” says Scott Lambert from the Global Brake Safety Council. “Truck brakes experience tremendous heat and high shear loads; high heat accelerates the breakdown of the adhesive and leads to the complete separation between friction material and braking plate and brake failure.”
“Brake shoes and pads need to be replaced if the corrosion has caused the friction material to separate from the brake shoe table,” Hendrickson’s Jeff Wittlinger notes. “And while heat checking is normal on both rotors and brake drums, air disc brake rotors should be inspected and replaced if cracks extend through more than 75% of the rubbing surface, and brake drums should be replaced if cracks pass through the wall of the drum.”
Some brake pad manufacturers are offering new solutions to combat corrosion. Galvanization is one of the latest options. It trades a painted backing plate for one that’s treated with a protective zinc coating. The steel backing plate is electro-galvanized using a zinc alloy to provide complete corrosion protection and improve use in proper applications.
“The fit and function of the brake pads is vastly improved with galvanization—no paint build up when installing, and no rust build up on brake pad edges that causes binding, and uneven wear in the caliper,” said Troy Hylton, vice president of engineering with NRS Brakes, who noted that the galvanized, mechanically attached brake pads can last up to four times longer than traditional painted brake pads. “It is critical that you buy brake pads that meet or exceed OEM specs; your brake pads are the most important safety component in your vehicle.”
During inspections, relates McComsey of Bendix, there are things technicians should be looking for to identify when friction should be replaced. “For drum friction, they may notice rust jacking that occurs when contaminants get between the friction and shoe table, and the metal shoe table corrodes and starts to expand,” he explains further. “This puts pressure on the friction block that may eventually cause the friction material to crack. Continued overheating of the friction material can also cause premature cracking or small chunks to break away.”
For air disc brakes, premature replacement can be needed if water gets in the caliper and causes corrosion, says Justin McCoy, senior manager of disc brakes at Meritor. “That is why frequent inspection of rubber seals, boots and plugs is important,” he adds. “Replacing a seal or boot is an inexpensive insurance policy for the performance and life of the caliper.”
