As vehicle maintenance goes, changing foundation drum brake friction is routine: Off comes the old lining, and on goes the new. To help ensure the best performance and brake life, though – and even avoid problems during roadside inspections – it’s worth adding a few moments to that routine to examine the used brake lining. As part of its Bendix Tech Tips series, Bendix Commercial Vehicle Systems LLC and Bendix Spicer
Foundation Brake LLC (BSFB) offer insight on key pieces of information that can be gleaned from a look at a vehicle’s friction during maintenance.
In recent years, drum brake technology has continued to evolve and improve as the National Highway Traffic Safety Administration’s (NHTSA) Reduced Stopping Distance (RSD) regulations have taken effect, and the Federal Motor Carrier Safety Administration’s (FMCSA) Compliance, Safety, Accountability (CSA) program continues to impact fleets and drivers. Today’s high-performing drum brakes are larger and more powerful than earlier versions, and are equipped with friction that has been engineered to better resist fade while meeting and exceeding performance regulations.
Equipping the Right Friction
The first thing that can be gleaned from used drum friction is whether it was properly rated for the vehicle.
Signs that the friction was not up to the task include scoring on the friction or the drum;
degradation that gives the friction a porous, charcoal-like appearance; and “hot spotting” – a
leopard-spotted pattern on the drum. These indicate that the friction was overworked and
overheated during regular brake use.
If the friction has developed a smooth, polished surface – a condition known as glazing – it was likely not experiencing enough heat to properly activate its formulation and create the optimal “transfer film” on the drum. This can be the result of installing “overrated” friction, such as equipping a 20,000-pound-rated axle with 26,000-pound-rated friction. Glazing is also associated with brake squeal, which means this condition can sometimes be heard on the road before it’s seen in the garage. With the right friction in place, glazing can be prevented by simply not treating the brakes too delicately during normal operation. Today’s RSD-compliant friction is designed to work more effectively while handling higher temperatures than older drum linings.
“We regularly field questions about replacement friction for vehicles manufactured with RSD-compliant brakes. In particular, there is confusion regarding other aftermarket RSD and non-RSD available frictions in the aftermarket,” said Keith McComsey, BSFB director of marketing and customer solutions. “The most effective way to maintain the stopping distance that your vehicle was required to meet at the time of manufacture is to replace friction with ‘likefor-like’ OE friction. OE-level friction was extensively tested and certified by the truck OE, so don’t increase your stopping distances by using non-OE-certified friction.”
Reading the Damage
Pitting and cracks in used brake lining can also provide significant insight into a brake’s
potential problems. Cracks running along the edge of the lining, roughly following the arc of the shoe, are a
form of delamination. As a lining’s surface heats up during braking and contact with the drum,
the portion of the material attached to the shoe remains cooler. In weak, low-grade friction, this
temperature variation can cause these delamination cracks. Vertical edge cracks, on the other hand, tend to be caused by mismatched geometry between the friction and brake shoe. This mismatch can be the result of brake shoe distortion such as stretching or twisting, or remanufactured shoes that have not been properly coined. The effects of stress on a lining that does not properly match the shoe’s geometry can be heightened by weak friction material.
Radial surface cracks can also be an indicator of rust jacking, which occurs when rust builds up on the steel beneath the brake lining, exerting upward pressure on the friction material, lifting friction off the brake table. Avoiding rust jacking requires not just an effective protective coating on the steel shoe to prevent the corrosion, but also combining it with a high flexural strength friction. Bendix’s remanufactured brake shoes meet this requirement with the company’s new PermaGuard protective coating and high flexural strength friction options.
The American Trucking Associations’ Technology & Maintenance Council’s (TMC) Recommended Practice 627A – which Bendix endorses – places brake lining cracks and pits into two categories: out-of-service and in-service. Out-of-service lining defects constitute a fully defective brake and would place a vehicle out of service if found on a steering axle brake, or on 20 percent or more of a vehicle’s brakes. Defects categorized as in-service will provide adequate brake performance, the practice description states, but should be repaired as soon as possible.
Cracks or pits that would put a vehicle out of service include edge cracks exceeding 1/16 of an inch in width or 1.5 inches in length, cracks across the lining face that extend through the lining edges, and missing material that exposes a fastening device such as a rivet or bolt. Inservice defects include smaller cracks in the lining edge, missing segments that do not expose a fastener, and surface cracks that may extend from hole to hole, but do not reach the lining edge.
Geometry Lessons
Uneven lining wear patterns can also signal trouble with a brake shoe’s geometry. Because brake shoes are subjected to tremendous forces and drastic temperature changes during their life cycle, almost all shoes experience some degree of deformation over time.
“Greater lining wear at the ends of a friction block, for instance, indicate a stretched shoe,” McComsey said. “A twisted shoe will cause uneven wear across the surface of the lining, or at the corners, as only the highest points of the lining come in contact with the drum. Both cases result in decreased brake performance and shorter friction life.”
If the shoe is misshaped or distorted, even slightly, simply relining the brake with new friction won’t address these issues. A shoe with proper geometry – such as a reman shoe that has been coined to return it to the shape engineered by its original manufacturer – is necessary to ensure the full contact and stopping power between the shoe and drum. That is why Bendix has always coined 100 percent of its remanufactured brake shoes.
By taking the time to look at a vehicle’s friction during replacement, fleets and owneroperators can uncover important insights that will keep brakes operating safely and effectively. Information in the Bendix Tech Tips series can be found in the Bendix multimedia center at knowledge-dock.com. Further instructional videos and interactive training on foundation drum brakes and friction are available at the Bendix On-Line Brake School, www.brake-school.com.
For more information on matching friction selection to application, contact the Bendix Tech Team at 1-800-AIR-BRAKE, option 2 (1-800-247-2725, option 2).
