FOCUS ON: BRAKES
Advanced engineering and materials for foundation brake performance and longevity By John Campo As an accompaniment to Jerry Thompson’s article “Advantages in Brake Design” in the January 2015 edition of BUSRide Maintenance, this article addresses the drum and rotor side of the friction-couple equation. In his article, Thompson highlighted some of ProTec Friction Group’s state-of-the-art friction materials and how they manage inevitable heat generated during braking cycles. He described the significance of the surface conditions and the friction materials used when it comes to directing “energy trajectories” and reducing brake component wear. Similarly extreme demands are placed on brake rotors and drums.
uring the past decade, significant technological advances have found their way into the light, medium and heavyduty brake markets. More recent innovations like engine brakes, transmission retarders and regenerative brakes are assisted by electronic and computer-based systems to improve brake performance and safety. A field once dominated by mechanical engineers now shares responsibilities with electrical and software engineers, who are developing advances in antilock braking systems (ABS), stability control, collision mitigation and adaptive cruise control, to name just a few. Today, stopping distance is sometimes dependent as much on lines of code as on the characteristics of disc pads clamping to a rotor. And while such smart innovations may greatly improve the brain of a braking system, its backbone remains the foundation brakes – namely the drums, shoes, rotors and disc pads. Perhaps these same innovations help foster the prevailing “conventional wisdom” that foundation brake materials are about as good as they are ever going to be. Indeed, competing manufacturers offer an array of choices when it comes to varying designs and chemistries in friction materials, rotors and drums which are largely distinguished by their price point. However, the general perception is that, for the most part, these different materials have plateaued and operate within a narrow spectrum of conventional friction and mating surface technology. Like so much conventional wisdom, nothing could be further from the truth. Two companies, ProTec Friction Group and Power Brake, are collaborating on the manufacturing and marketing of the bestperforming and longest-lasting foundation brakes ever seen. Of course, when it comes to slowing or stopping a moving vehicle, no one is suggesting that ProTec/Power Brake have discovered a way to defy the physical laws of conservation of energy and momentum. It just seems that way. In a typical closed braking system where the energy of motion is exchanged for heat energy, how this heat is managed is the crucial element determining the brake’s life and performance. ProTec/Power Brake’s specially-engineered rotors and drums consist of materials that optimize the forces and energies present at the friction couple interface during and after a brake cycle.
Simply stated, when the vehicle’s brakes are applied, the disc pads or brake shoes clamp against a rotor or drum. The friction resisting the motion of the rotating wheel converts kinetic energy into thermal energy which is then conducted primarily through the cast iron rotor or drum. In many cases specially designed rotor and drum fins, vanes, grooves, holes or vents generate air movement to finish the job by dissipating the heat via the process of convection. Any remaining heat is transferred through the friction surfaces and other components in the brake system. Through a proprietary manufacturing process using extreme force, hardening alloys are introduced into the surface of the cast iron rotor or drum. It is not a coating or plating. These elements actually coalesce with the cast iron, reducing latent stresses and creating a much more “friction friendly” environment… without sacrificing brake torque. The cast iron becomes less of a “heat sink” and more of a “heat mirror,” thereby reducing the residual heat gain in the wheel end. Because of this, rotors and drums are less subjected to warping, scoring, heat-checking, hot-spotting and cracking. Additionally, because they are stronger (higher yield and tensile strength per ASTM test E8) and harder (Vickers hardness test), rotors and drums retain their original configuration, allowing for a more complete and uniform contact of friction to cast iron. Because cast iron fretting corrosion is minimized in a ProTec/Power Brake drum or rotor, when brake pressure is released the uncoupling is clean, with no temperature-increasing drag. It cannot be overstated that the quality and condition of the mating materials of friction to cast iron is a key component to promoting an efficient brake. Employing advanced materials allow for a greater surface contact of the materials at the friction couple. Optimum brake torque can be achieved with the least amount of pressure. Therefore an efficient brake is a higher-performing, cooler-running and longer-lasting brake. PowerBrake longevity drum, rotor and friction products for private coach, school bus and paratransit fleets are available from ProTec Friction Group. John Campo is a heavy-duty brake and suspension professional for Power Brake LLC, St. Petersburg, FL. Visit the company online at www.powerbrake1.com. Visit ProTec Friction at www.protecfriction.com and www.protec-bus-com.
busridemaintenance.com | BUSRIDE MAINTENANCE
The exclusive maintenance resource for the motorcoach and transit bus industries.