Lightweight on the brakes

  • 09-Feb-2011 09:35 EST
2010-01-1705.jpg

Cast iron brake drum and an aluminum metal matrix composite version of the same brake drum, showing weight savings in pounds.

Aluminum MMCs have been in the industry since the 1950s. MMCs have been used commercially in fiber-reinforced pistons, aluminum crank cases with strengthened cylinder surfaces, and particle-strengthened brake disks. Stir-cast MMC materials such as Duralcan are commercially available today and can be used to completely cast a component with aluminum MMC.

Stir-cast MMCs offer increased strength, temperature resistance, elastic modulus, and thermal stability due to the reinforcement of the aluminum matrix. Typically, these materials use up to 20% silicon carbide particles, and they have been used successfully for light-duty brake rotors and other components. Since these components are 100% reinforced with ceramic material, they have a low ductility. In terms of machinability, a 100% reinforced part requires much greater time and higher tooling cost.

Engineers from Century Inc. have developed a selectively reinforced brake drum, rather than the 100% reinforced technology approach. By using selective reinforcement, ceramic reinforcement can be strategically placed for maximum benefit—that is, to provide a wear surface with adequate friction couple, thermal stability, and a long life. The strategically placed ceramic reinforcement leaves monolithic aluminum in other locations of the component, with the benefit of high ductility and ease of machining.

To demonstrate the value of its proprietary Ring Extruder Technology, Century designed and manufactured an aluminum MMC brake drum in the 380 x 101.6 mm (15 x 4 in) size that would cross-reference with many commercial brake drums. The goal of the brake drum design was to match or exceed cast-iron performance at the drum wear surface while improving other areas of the drum by utilizing aluminum. The MMC has high stiffness and strength, while the aluminum portion of the drum has a good combination of strength and ductility.

The thermal properties of aluminum and aluminum MMC compared to cast iron show advantages of aluminum in applications where heat generation is involved. The specific heat capacity of aluminum is almost twice that of cast iron. This means that aluminum can handle more heat than cast iron at the same mass. Since the density of cast iron is so much greater than aluminum, the volume required to obtain the same weight as cast iron is much greater. Taking advantage of increased thermal capacity, increased thermal conductivity, and lower density, the volume of the part was increased to more closely match the overall thermal capacity of a cast iron brake, and cooling fins were added. The cooling fins take advantage of the much higher thermal conductivity of aluminum—150 W/m-K compared to 50 W/m-K for cast iron—and allow the brake drum to release heat into the air and into nearby components. The aluminum MMC brake drum is able to release heat at a rate that allows the volume increase to take advantage of the specific heat capacity. Testing has not shown any issues with heat being transferred into other components.

There are limitations in the amount of volume that could be added to the brake drum. SAE J1865: Dimensional Compatibility for Commercial Vehicles Fitment was used to ensure that the increased volume brake drum design still fit within the required package space. Computer design tools were used to ensure that a casting design would be sufficient to handle the loads of a real-world braking system. Design engineers used PTC’s Pro/ENGINEER CAD and Ansys’ FEA products to develop brake drum models.

The FEA simulated load cases were developed to reflect the axle load at these test conditions: FMVSS121 Power, FMVSS121 Hot Stop, ATPD 2354 Laurel Mountain Hot Stop, ATPD 2354 Crack Resistance, and ATPD 2354 Strength testing. The ATPD testing was developed by the U.S. Army for off-vehicle testing of brake components, and the acronym stands for Automotive-Tank Purchase Description.

For each of these simulations, an S-cam braking system was modeled to apply the drum loading in a real-world manner. The pad contact was assumed to be 80%. The models also assumed that the complete heat energy was being absorbed by the brake drum. These simulations took the inputs of deceleration, torque, heating input (BTU), friction coefficient, and initial brake temperatures to determine maximum temperatures that would be generated during each braking event.

Equivalent stresses were also calculated at each of these braking events. The stresses that were predicted could then be compared to the aluminum alloy and MMC properties at the predicted temperature. The FEA model was designed to be a conservative method to compare variations in drum design coupled with various braking events. This tool allowed an iterative process to be used and created the final design.

Century says that in its automated Ring Extruder Technology, the preform material is continuously extruded, formed into a shape, and then thermally processed and machined. The process allows a wide variety of raw materials in binders and ceramics. Another benefit of this process is the versatility to mass-produce shapes for the wide range of MMC applications.

This article is based on SAE technical paper 2010-01-1705 by Matt Kero and Andrew Halonen, Century Inc.

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