By John Campo
Published in BUSRide Maintenance magazine, April 2016
Note: As an accompaniment to Jerry Thompson’s article “Advantages in Brake Design” in BusRide Maintenance (http://busridemaintenance.com/2015/01/focus-on-brakes/) this article addresses the drum and rotor side of the friction-couple equation. In his article Mr. 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. New manufacturing processes that change the thermal conductive properties of cast iron drums and rotors are dramatically improving performance and longevity.
During the past decade significant technological advances have found their way into the light, medium and heavy duty brake markets. Some of the more recent innovations like engine brakes, transmission retarders and regenerative brakes of various designs and functions are now being assisted by electronic and computer-based systems created 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 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 seems to have settled on the erroneous notion 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. However the general perception is that, for the most part, these different materials have plateaued and range only within the narrow spectrum of “good, better, best” – depending primarily upon the price of the product. Like so much conventional wisdom, NOTHING COULD BE FURTHER FROM THE TRUTH.
Power Brake’s specially engineered rotors and drums consist of materials that optimize the characteristics of 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 clamp to a rotor or brake shoes apply against a brake drum. The friction resisting the motion of the rotating wheel converts kinetic energy into thermal energy, which the larger part 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 finish the job by carrying away the heat (dissipating) to the atmosphere via the process of convection. Air movement is assisted via positive pressures resulting from the drum or rotor’s centrifugal fan action. And, depending on the chemistry of the friction (ceramic materials have greater convective properties than denser semi-metallic), the smaller part of the heat energy transfers through the friction (pads or shoes), and various foundation brake parts, until it is dissipated to the atmosphere via convection.
Power Brake rotors and drums possess superior thermal properties compared to their conventional counterparts. Additionally, they are stronger (higher yield and tensile strength per ASTM test E8) and harder (Vickers hardness test). Through a proprietary manufacturing process using extreme force, certain elements are introduced into the surface of the cast iron rotor or drum. It is not a coating or a plating as these elements coalesce with the cast iron, reducing latent stresses, 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. They 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 Power Brake drum or rotor, when brake pressure is released the uncoupling is clean and complete with no temperature-increasing drag.
The unique alloy at the surface of the Power Brake drum also solves a condition known as “brake sticking”. In harsh winter environments corrosive factors cause brake linings to stick to brake drums while parked, failing to release when required. This brake lock-up problem was significant in non-powered, mid-axles on certain 60-foot high-floor articulated transit buses. Power Brake drums solve this problem and are registered with the National Highway Traffic Safety Administration (NHTSA) as the recall solution.
Life is also very good at Florida’s Palm Beach County transit, Palm Tran. Entering its ninth year under the Power Brake program, frequent brake jobs are a thing of the past. Brake maintenance costs are dramatically reduced by keeping buses out of the shop and on the road, serving customers. “It’s all about cost-per-mile” says Eugene Bitteker, Palm Tran’s Manager of Procurement and Stores.
When it comes to foundation brakes, 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 great surface contact of the materials at the friction couple. Optimum brake torque can be achieved with the least amount of pressure – so an efficient brake is a higher-performing, cooler-running and longer-lasting brake.