Changing the friction on your foundation drum brakes is routine maintenance: The new lining goes on, the old ones come off. But add a few minutes to that routine to examine the used brake lining and you can go a long way toward ensuring the best performance and brake life, and avoiding problems during roadside inspections.
The era of the National Highway Traffic Safety Administration’s Reduced Stopping Distance (RSD) mandate and the Federal Motor Carrier Safety Administration’s Compliance, Safety, Accountability (CSA) program has been characterized by ever-improving drum brake technology. Higher-performing drum brakes are larger and more powerful than their predecessors, and friction has been engineered to be more fade resistant while meeting and exceeding the new regulations.
The Right Friction?
The first thing you can learn from your used friction is whether it’s properly rated for the vehicle.
Scoring on the friction and drum, degraded friction that takes on a porous, charcoal-like appearance and “hot spotting” – a leopard-spotted pattern on the drum – are signs that the friction was not up to the task. If a friction has hot spotting, it was overworked and overheated during regular brake use.
If, on the other hand, you notice that 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 happen when “over-rated” friction is installed, such as putting a 26,000-pound-rated friction on a 20,000-pound axle.
Glazing is also associated with brake squeal, which means you may hear the signs of this condition before you see it.
With the right friction in place on your wheel end, glazing can be prevented by simply not treating the brakes too delicately while driving. Today’s RSD-compliant friction is designed to work more effectively while handling higher temperatures than in the past.
For a friction to be RSD compliant, it must not only be dynamometer certified to Federal Motor Vehicle Safety Standard 121, it must also pass a more recently mandated vehicle test. Bendix recommends asking your replacement supplier for a letter certifying this compliance.
Looking in the Cracks
You may notice cracks and pitting in your used brake lining. These can also say a lot about a brake’s potential problems.
Cracks that run along the lining edge, roughly following the arc of the shoe, are a form of delamination. As the surface of the lining heats up during braking, its base – attached to the steel shoe – remains cooler. This temperature variation can cause cracks in weak, low-grade friction.
Vertical edge cracks are often caused by mismatched geometry between the friction and shoe. This mismatch is usually a result of an uncoined shoe and is also amplified by weak friction. However, surface cracks can also indicate a problem with the shoe itself, such as rust jacking.
Rust jacking happens when rust builds up on the steel beneath the brake lining, exerting upward pressure on the friction. To avoid this problem, it’s crucial to not only have an effective protective coating on the steel shoe, but also friction with high flexural strength.
The Technology & Maintenance Council Recommended Practice 627A, Brake Lining Structural Defects – 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, 627A states, but should be repaired as soon as possible.
Out-of-service cracks or pits include:
- Edge cracks exceeding 1/16" in width or 1.5” in length.
- Cracks across the lining face that extend through the lining edges.
- Missing material that exposes a fastening device such as a rivet or bolt.
In-service defects include:
- Smaller cracks in the lining edge.
- Missing segments that do not expose a fastener.
- Surface cracks that may extend from hole to hole but do not reach the lining edge.
Shoe distortion, such as stretching or twisting, can also cause lining cracks, since the friction no longer matches the geometry of the shoe. Remanufactured shoes that have not been properly coined can contribute to this.
Watching the Wear
Trouble with a brake shoe’s geometry can also be signaled by uneven lining wear patterns.
Brake shoes are subjected to tremendous forces and drastic temperature changes during their life cycle, which results in deformation over time. 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. Greater lining wear at the ends of the block indicates a stretched shoe. In both cases, brake performance is decreased and the life of the friction is shortened.
Simply relining the brake with new friction won’t address these issues: You must have 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.
So take some time to look at the friction you’re replacing. Its stopping days may be over, but it can still offer important clues to keeping your brakes operating safely and effectively.
Gary Ganaway is director of OE and technical sales, and Frank Gilboy is product manager for remanufactured wheel end products, for Bendix Commercial Vehicle Systems (www.bendix.com). The company, a member of the Knorr-Bremse Group, develops and supplies leading-edge active safety technologies, energy management solutions and air brake charging and control systems and components under the Bendix brand name for medium and heavy duty trucks, tractors, trailers, buses and other commercial vehicles throughout North America.
