Truck brake friction material fit, form and function

Regardless of whether a fleet's vehicles utilize air disc brakes (ADBs), drum brakes, or a combination of both, there is one crucial element that is common to all types: friction material. The friction material, commonly known as brake lining, plays a vital role in halting the vehicle, regardless of the specific braking system used. By understanding the fleet's requirements and the maintenance needs associated with brake lining, operators can optimize their operations and extend the longevity of both the friction material and vehicle uptime.

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Friction Function

"Brake linings are a vital component for stopping a truck," explains Abe Aon, business leader for Fleet Solutions at ZF Group Commercial Vehicle Control Systems. "In trucks with drum brake assemblies, the brake linings are pushed against the drum to decelerate the vehicle. In contrast, trucks equipped with disc brakes utilize a clamping motion from the brake caliper to press friction pads against a rotor from both sides to slow down the vehicle."

When it comes to stopping or reducing the speed of a vehicle, brakes must convert kinetic energy, says Keith McComsey, director of air disc brake systems at Bendix. The frictional force generated by the braking system resists motion, thereby creating heat. "It slows that spinning part down, which in turn slows the vehicle," McComsey further explains. "It doesn’t matter if you have air disc brakes or drum brakes; both systems work on similar principles by applying mechanical force that generates friction against a spinning mass, such as the drum or rotor, effectively slowing the vehicle."

Friction Form

The composition of friction material can vary widely. "Friction materials can comprise five distinct components," notes Eric Coffman, senior product manager at Meritor. "Typically, the structural package is made of fiberglass, alongside resin as the adhesive, a filler to occupy space, and various lubricants and abrasives known collectively as friction modifiers."

Coffman clarifies that these friction modifiers come in various grades, shapes, and sizes like copper, steel, fiber, and graphite which determine the characteristics of the friction material. "How you blend these materials together, along with their specific quantities and ratios, determines traits like high brake torque, extended brake life, or noise resistance," he adds. The quality of resin also plays a critical role, as higher-quality resins can endure elevated temperatures more effectively.

Upon finalizing a blended formula, manufacturers subject brake linings to rigorous testing to evaluate their performance characteristics. At Meritor, these friction materials typically undergo 50 to 80 dynamometer tests to validate performance across different brake sizes and weight classifications, measuring parameters such as brake torque, fade, and noise.

Friction Fit

Regulations requiring reduced stopping distances (RSD) have compelled brake lining and component manufacturers, along with OEMs, to introduce various friction material specifications aimed at achieving the mandated reduction in stopping distance. Different methods have been employed while maintaining the same overall objective.

"Bendix has achieved reduced stopping distances by allocating more brake torque to the steer axle," McComsey explains. "We transitioned from organic materials to semi-metallic materials on the steer brake." Other manufacturers have modified shoe sizes without altering the lining material, with each company taking a unique approach.

In past scenarios, the steer axle undertook less work than drive axles in terms of halting the vehicle. "The RSD formula involves getting a five-inch steer axle brake to perform like a seven-inch brake," says Coffman, "and this is accomplished by utilizing more aggressive friction materials and making certain system adjustments."

While different linings are employed for various axles, it's important to realize that the steer axle generally exert less brake force and is rated for a lighter load than drive axles. Hence, a less aggressive friction material may be appropriate. The functionality of drive and trailer axles greatly influences the choice of friction material, which is tailored to match the axle's ratings.

But choosing the right friction material involves more than meeting a specific stopping distance. "Regional route vehicles will typically apply brakes more frequently due to cycling needs compared to long-haul vehicles," Aon points out. "Consequently, regional route vehicles require linings designed to withstand the higher temperatures generated by increased brake usage."

Various applications, vocations, and duty cycles significantly affect the selection of suitable friction materials, also influencing their lifespan and operational efficiency. "A notable example," Coffman illustrates, "is if a refuse fleet employs a low-end brake lining rated for 20,000 pounds, it will experience rapid wear due to that material's inability to perform efficiently at high wheel-end temperatures. Conversely, a fleet running a van trailer across Kansas, primarily hauling lightweight packages, may manage satisfactorily with lower-grade materials due to infrequent braking needs."

To aid in selecting replacement friction materials, brake manufacturers offer tools and resources to assist fleets in identifying the most suitable options. Meritor, for instance, provides MeritorPartsXpress.com, a user-friendly online platform that supports fleets using drum brakes in finding the appropriate linings for their brake shoes. After inputting the manufacturer, key measurements, and vocation, users receive a list of compatible linings.

For air disc brakes, sourcing replacements is comparatively straightforward since variations emphasize the braking duty cycle rather than geometry. McComsey recommends sticking with like-for-like replacements to ensure consistent performance designed and validated by the OEM, mitigating potential issues.

For additional guidance on selecting friction materials, visit heavy duty truck brake lining bulk.

Management and Maintenance

Acquiring the appropriate friction material is just one aspect; maximizing performance and extending its lifespan while minimizing downtime is a different challenge. A recommended best practice involves tracking and documenting brake lining wear. The most effective scenario involves trucks that operate under similar conditions regarding mileage, routes, loads, duty cycles, and operators. This approach reduces variables and establishes a clear correlation between mileage and lining wear patterns.

Systematic recording of lining thickness measurements at predetermined intervals generates valuable data. Over time, this data reveals the correlation between mileage and wear. When analyzed alongside established maintenance schedules and preventive maintenance (PM) inspections, this approach enables fleets to synchronize brake lining services with other maintenance needs, preventing unexpected service disruptions.

McComsey further emphasizes the importance of replacing like-for-like materials, warning that any changes in friction specification could result in losing all previously acquired wear data, requiring fleets to start anew.

Most friction materials are equipped with wear indicators, which serve to guide fleets in scheduling replacements based on the available material.

Bendix has developed a Pad Wear Sensing system for their ADB22X Air Disc Brakes, designed to monitor brake pad thickness without the need for technicians to physically measure the material. "When the friction material reaches a certain thickness, it generates a signal detected by telematics via the ABS or the ECU in the chassis," McComsey explains.

The incorporation of such technology allows fleets to anticipate maintenance needs before the brake lining reaches end-of-life status, enabling more effective service scheduling in conjunction with other maintenance tasks.

Maintaining regular inspections is crucial. "Periodic wheel-off inspections are essential," Aon states, noting that wheel-on measurements may only provide insights into the outer pad. It is recommended to evaluate the surface of the friction material beyond thickness, as glossy or cracked surfaces could indicate further issues.

Additional maintenance best practices contribute to optimal brake lining performance. McComsey suggests cleaning contact points of the brake shoes during service, as these areas can accumulate contaminants over time due to road exposure. Moreover, he recommends lubricating all brake system components whenever an opportunity arises to prolong effective performance.

Coffman echoes this sentiment, underscoring the importance of monitoring cam shafts and bushings for wear, as excessive play can lead to various friction-related complications.

Meticulous maintenance practices and proactive wear monitoring keep technicians well-informed, effectively preventing impending failures. By correctly assessing fleet needs and specifying the right friction material, coupled with diligent wear tracking, operators can enhance operational efficiency and meet performance benchmarks. Coordinating all elements ultimately contributes to maximizing brake lining longevity and optimizing maintenance schedules to ensure minimal uptime.

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