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Friction fit, form, and function

Dec. 4, 2020
Understanding all aspects involved in spec’ing, maintaining, and monitoring brake lining material can lead to maximizing performance and uptime.

Whether a fleet’s assets are configured with air disc brakes (ADBs), drum brakes, or both, one aspect bridges across all specifications – friction material. The friction material, or brake lining, serves a particular role no matter the braking system configuration – to bring the vehicle to a stop. Understanding the needs of the fleet and the maintenance considerations associated with brake lining will help a fleet establish an efficient operation, and maximize the life of the friction material as well as vehicle uptime.

Friction function

“Brake linings are one of the essential elements required in stopping a truck,” says Abe Aon, business leader – Fleet Solutions, North America, ZF Group Commercial Vehicle Control Systems. “When a truck is utilizing a drum brake assembly, the brake linings are forced against the drum to slow the vehicle. On a truck utilizing disc brakes … the friction pad material is pressed against a rotor from both sides using a clamping motion from the brake caliper to slow the vehicle.”

In order to stop or slow a vehicle, the brakes must expend kinetic energy, says Keith McComsey, director, air disc brake and systems, Bendix. The frictional force of a braking system resists motion, which in turn generates heat.

“It slows that spinning part down, which then slows the vehicle down,” McComsey explains. “Whether you have air disc brakes or drum brakes, it doesn’t matter. They both, in similar principles, apply mechanical force that applies frictional material against a spinning mass, AKA the drum or the rotor, which then generates that frictional force and slows the vehicle.”

Friction form

Friction material varies in composition.

“Friction materials may have five different components,” says Eric Coffman, senior product manager at Meritor. “Your first one is typically a structural package; a lot of times that is fiberglass. Then you have your resin, which is your glue. Then you have a filler, which basically takes up space in the material. There are lubricants and abrasives, which collectively are called friction modifiers.”

Coffman explains that the friction modifiers such as different grades, shapes, and sizes of copper, steel, fiber, graphite, steel powder, aluminum oxide, among others, determine the friction material’s characteristics.

“How you take these materials and blend them together, and in what quantities and ratios, will give you those characteristics of either high brake torque, or maybe … long brake life, or resistance to noise, et cetera,” Coffman continues. Further, Coffman says that the quality of resin can affect the friction material as higher quality resin will handle higher temperatures.

After finalizing a blended formula, brake lining manufacturers put the material through rigorous testing to determine its performance capacities. In the case of Meritor, Coffman explains that friction material typically goes through 50 to 80 dynometer tests in order to be validated at different brake sizes and weight ratings. During these tests, brake torque, fade, noise, and other parameters are measured and assessed.

“Then it will go on a fleet test for 18 to 24 months,” Coffman says of tested brake linings. “Typically, we try to get two winters in a fleet test, [as well as] track testing certifications.”

Friction fit

Reduced stopping distance (RSD) regulations have made it necessary for brake lining and component manufacturers and OEMs to introduce various friction material spec’ing options in order to achieve the required reduction in stopping distance. Different methods were used though the goal was the same.

“Bendix managed getting that reduced stopping distance by pushing more of the brake torque on the steer axle,” Bendix’s McComsey says. “We went from an organic material to a semi-metallic material on the steer brake. Other manufacturers managed that by changing the size of their shoe without changing the lining material. Each brake manufacturer approached it a little differently.”

“In the past, the steer axle did less work than the drives in terms of bringing the [combination vehicle] to a stop,” Meritor’s Coffman relates. “The secret sauce of RSD is getting that five-inch steer axle brake to perform like a seven-inch brake. That’s done through having a more aggressive friction material, as well as some other system changes.”

“Different linings are used between axles,” ZF’s Aon explains. “The steer axle, in general, will not create as much brake force and will usually have a lighter axle rating than the drive axles. As a result, a less aggressive friction material could be used. Drive and trailer axles do most of the brake ‘work’ and the friction material is based on the axle ratings of those axles.”

But there is more to keep in mind than just meeting a certain stopping distance when considering friction material.

“Regional route vehicles generally will use their brakes more frequently based on the nature of their routes compared to long haul vehicles,” Aon says. “As a result, regional route vehicles will create more heat due to the frequency of them using their brakes and should use a lining that is designed to regularly reach those higher temperatures. Off-road or refuse vehicles will also need to match their higher axle ratings with a higher temperature rating for their application. In some cases, these applications will use a form of metallic in their linings.”

Different vocations, applications, and duty cycles play a major role in determining the appropriate friction material, as well as impact the life and performance of that friction material.

“This is the most extreme example, but I like using it,” Coffman says. “If you’re running a refuse fleet and you put a low-end 23- or 20,000-pound brake lining on it, you’re going to see super accelerated wear because that material does not wear well at a high wheel-end temperature. On the flip side, if you’re the guy that runs a van trailer across Kansas and you’re hauling packages that are mostly air, you might be able to get away with a lower-end material and not really have that big of a deal because you don’t hit the brakes that often.”

When it comes time to select replacement friction material, brake manufacturers have tools, resources, and recommendations to assist fleets in finding what fits best for their operations.

Meritor offers MeritorPartsXpress.com as an online tool to help fleets running drum brakes through the challenge of finding a lining that fits their brake shoes. Coffman explains that after entering details such as manufacturer, a few measurements in critical dimensions, and even vocation, MeritorPartsXpress.com will display a list of applicable linings that are available and suitable.

“On the air disc brake side, it’s a little more simplistic because the variations are focused on more of the brake duty cycle rather than geometry and some of the other things that impact things on the drum side,” McComsey says of sourcing appropriate friction material replacement. He also recommends replacing like-for-like when it comes time to replace the brake lining.

“Whatever came on the vehicle already has a part number associated with it, and you can get that from the OEM or request that through various distributors,” McComsey says. “And that is really the best way to ensure a couple of things. One, it ensures that you maintain the proper brake performance as was designed and tested by the OEM. That [also] ensures proper fit and consistent performance.”

McComsey warns that should a fleet change friction away from like-for-like, it may have unintended consequences. A driver that has gotten used to how a vehicle performs may experience changes in braking performance, or the stopping distance may be different than it was with the OEM-supplied friction material.

Manage, monitor, and maintain

Getting the right friction material on the asset is one thing; getting the desired performance and maximizing its life while minimizing unnecessary downtime is another matter entirely. A best practice is to manage brake lining performance through tracking and recording wear. The ideal setting to track and record wear happens when a truck operates with similar mileage, routes, loads, duty cycles, and vehicle operator. With as few variables included as possible, a correlation between mileage and lining wear should present itself.

Recording lining thickness measurements at specific intervals will create a data set. Over time, this data set can provide insight into how many miles cause how much wear. Analyzing this data and cross referencing it to established maintenance schedules and practices, particularly preventive maintenance (PM) inspections, can allow fleets to align brake lining services with other necessary maintenance in order to avoid bringing in an asset and racking up downtime for singular services.

Bendix’s McComsey further advises replacing like-for-like by reminding that if a fleet has been tracking wear and associating it with mileage and then chooses a different friction material, “all that data goes out the window and you’re starting from square one.”

Most friction material is manufactured with a wear indicator; utilizing the wear indicator should allow fleets to coordinate replacement intervals after understanding the status of remaining material available.

To assist in wear measuring efforts, Bendix developed Pad Wear Sensing for the company’s ADB22X Air Disc Brakes. Pad Wear Sensing monitors the brake pad thickness and provides a warning without technicians having to physically measure the friction material.

“The system has sensors built into the pad friction [material],” McComsey says. “When the pad friction gets down to a certain thickness, it creates a signal that ultimately gets picked up by telematics through ABS [the anti-lock braking system], or through an ECU [electronic control unit] in the chassis system.”

With such technology, fleets can expect when the brake lining end-of-life is approaching while there is still material remaining, allowing the fleet to effectively schedule service, McComsey says. “The fleet can effectively schedule their maintenance to be more efficient, and when they’re bringing their vehicle in for maintenance, maybe they can combine that with other things at the same time,” he says.

Regular lining wear is not the only aspect to be noted.

“Periodic wheel-off inspection is still required because the wheel-on measurement only checks the outer pad,” ZF’s Aon says.

He further recommends examining the surface of the friction material beyond just its thickness, as glossy or cracking surfaces may be indicative of other issues.

“The lining will often tell a story of other components that may need your attention,” Aon says. “For instance, if a technician notices uneven wear on the pads when he or she is replacing them, they should look into the drum or rotor and replace them if necessary. A technician should check for abnormal wear of the lining to the drum or rotor. In addition, it’s recommended to take a close look at all the boots and make sure the caliper moves freely while the pads are out.”

Other best practices in maintenance will assist in brake lining performance and longevity. McComsey recommends any areas where brake shoes make contact be cleaned when in for service.

“Those things get bombarded with contaminants, as far as road wash and spray, road grime, [et cetera], and over time as things heat up and cool down, there are paths for contamination to get into those components,” he says.

McComsey advises fleets lubricate brake system components any time they have the opportunity.

“When you’re lubricating them, you’re basically purging the old grease out, so with the old grease comes the contaminants,” he explains. “You’re basically breathing new life into those components so that they last longer and continue to operate properly. Get any kind of contamination off that might interfere with how the shoe sits in the rollers, in the cam pocket, and take the opportunity to check the S cam for play. Look at the seals for any kind of grease leakage.”

Meritor’s Coffman seconds McComsey’s recommendations, emphasizing monitoring the cam shaft and bushings, along with the slack adjustment.

“Axial and radial cam play is either a worn camshaft or worn cam bushing, and those can manifest themselves in different friction issues,” Coffman explains. “Axial cam play can give you some noise. Radial cam play can mess with your brake stroke and mess with how your shoe ends up contacting the drum if you get too much movement there. Check your slack as well as your foundation brake setup to make sure that you’ve got enough running clearance. Some excessive wear can be indicative of something that was set up too tight initially. If you’ve got it set up too tight, you don’t have enough running clearance and you can get some brake drag and get some wear there.”

Coffman warns about the manner in which the camshaft tolerances are measured. If axial and radial play measurements are taken with the slack attached and/or after greasing the camshaft, the technician can get a false reading showing the system is tighter and in better health than it actually is.

Deliberate maintenance practices and diligent monitoring of wear will keep technicians and the fleet informed and ahead of impending failures. With the needs of the fleet determined, specification is just the first step in adequately utilizing the most appropriate friction material. Tracking wear and establishing a data set will help a fleet build efficiency in their operations and meet performance expectations. Coordinating all factors will help to maximize the brake lining life and create a maintenance schedule that maximizes uptime.