F2008-03-007
Integrated Approach to Reducing Brake Judder by Using Homogeneous Brake Discs
Brake judder is one of the most serious problems in automotive brake systems. There are two types of judder, cold and hot. The various causes and mechanisms have been extensively studied. Reducing noise, vibration, and harshness (NVH) problems including judder and responding to increasing customer needs concerning braking performance depend on manufacturing process for brake discs. We therefore have been pursuing optimal brake discs to reduce judder from various aspects both in terms of product and process engineering as follows; Precise and accurate machining, Design strategy for less thermal coning, and Reducing disc thickness variations (DTV) by increasing material homogeneity. The first approach is precise and accurate machining. Geometric accuracy is the critical issue that brake disc suppliers must address to improve the NVH performance of automobiles. The higher geometric accuracy of the finished friction plate of brake discs can reduce off-brake DTV growth and resulting cold judder. We have achieved 20 micrometers in lateral runout and 10 micrometers in DTV after semi-finish turning operations. With an additional grinding process, by sustaining steady machining operations through productive maintenance based on in-line measurement data, we correct the geometric dimensions of the disc friction plates to obtain higher accuracy. As a result, we can provide brake discs with geometric accuracy of 5 micrometers in DTV. The second approach is our design strategy to reduce thermal coning. We describe our practical procedure of designing thermally robust brake discs using conventional CAE. We constructed an original brake disc design system by standardizing, automating, and speeding up each design process. It takes less than two hours for a complete finite element analysis (FEA) of a new disc design. Through the accumulation of vast numbers of FEA cases and tests for validation, we have obtained the knowledge of the effects of disc configurations, dimensions, and material properties on thermal deflections. We carried out a series of CAE experiments based on the Taguchi Method and determined the critical parameters for reducing the amount of thermal coning. We propose a design strategy for developing thermally stable brake discs with an outer-hat configuration. The thermal coning and resulting hot judder are reduced by applying this strategy to design thermally stable brake discs with an optimized geometry. The third approach is the reduction of on-brake DTV growth by increasing material homogeneity. We studied the relationship between DTV and casting material properties and clarified that the circumferential homogeneity of brake discs has significant effects on brake judder. As a solution, we established an integrated system for developing and manufacturing homogeneous brake discs to reduce judder by making the most of our integrated business process from R&D through manufacturing, including in-house foundries. The system we developed consists of the following elements; foundry engineering to obtain homogeneous discs, an original flake graphite structure index (K-FGI) to measure the homogeneity of our products, and accelerated wear tests on a brake dynamometer to estimate the potentials of on-brake DTV growth. By integrating the three above-mentioned approaches, we provide optimized brake discs to reduce judder.
Session: Vehicle Dynamics