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The most fundamental function of an automobile brake system is assuring stable braking effectiveness under various conditions. In a previous paper (2004-01-2765), the author et al. confirmed that the friction behavior of disc brakes during running-in depends on both the friction materials and discs’ friction-surface textures. Various friction pairs were tested by combining discs finished with roller-burnishing and grinding and five friction materials including NAO and low-steel. Some NAO material exhibited large effects on the difference in friction behaviors between the discs’ surface textures. A disc finished with roller-burnishing needed a longer running-in period than that with grinding. In another paper (2011-01-2382), a further experiment was conducted by combining eight surface textures (finished under four turning conditions with and without additional roller-burnishing), two NAO materials, and two rotational directions.

Brake judder is one of the most serious problems in automotive-brake systems, and brake discs play a significant role in judder. There are two types of brake judder: cold and hot. Hot judder is caused by the thermo-mechanical deformation of a disc rotor due to high-speed braking. There are several causes and shapes of the deformation, e.g., coning and circumferential waviness. Circumferential waviness of brake discs is typically found as a butterfly shape in a 2nd rotational-order and corrugation (or hot-spotting) around a 10th order, which are caused by thermo-mechanical buckling. The author focused on the effects of material and dimensional homogeneity on the transient and permanent wave-like deformation of ventilated discs in low rotational-orders during repetitive high-speed braking. The tested discs were in two groups that had the same design and gray-cast-iron class but were cast in two foundries by using horizontal- and vertical-molding machines, respectively.

There are various processes for finishing the friction surfaces of a brake disc, which affect the braking effectiveness of a vehicle in the early stages of use in some cases. To examine the interaction between the disc surface texture, rotational direction, and friction material, a series of experiments on a tribotester using small-scale specimens was conducted. In a previous paper (2013-01-2056), the results from the first series of experiments, which involved of thirty disc surface textures and a less aggressive non-asbestos organic (NAO) friction material in on-brake-drag conditions combining constant speed and normal-load, was reported. Disc surfaces were finished by the following finishing processes in two rotational directions: turning under four cutting conditions, roller burnishing after turning, turning with a wiper insert, and grinding with two stones. Contact-pressure dependency of friction and wear was confirmed.

The surface texture of a brake disc in some cases affects the braking effectiveness of a vehicle in the early stages of use. Brake discs usually turn in one rotational direction during their finishing process but are turn in two directions on a vehicle. This causes a difference in friction or wear between two wheels. Directional surface textures of brake discs finished by turning or roller burnishing may cause this interaction to become more severe than those finished by grinding. Full-scale tests using actual friction pairs are effective for estimating the total braking performance of a full vehicle or its corners. However, they are exposed to various factors and different brake-disc locations creating different friction and wear histories. The author, therefore, concluded that fundamental experiments using small-scale specimens are necessary to examine the details of the interaction between the disc surface texture, rotational direction, and friction material.

This paper focuses on the interaction in friction behavior between the surface texture of brake discs, rotational directions in braking operations and metal-cutting process, and friction materials with different degrees of aggressiveness. A factorial experiment for front brake discs was conducted by combining eight discs with directional surface finishes, two rotational directions, and two NAO friction materials on a brake dynamometer. The author analyzed several test results, such as the friction coefficients, disc wear, roughness, and the correlation between them. An assumed mechanism describing the friction behavior is discussed using the experimental results and by introducing the contribution of the aggressiveness and adhesiveness to the friction and confirmed by the test results.

Regarding the vibrational characteristics of a brake disc causing brake squeal, there are two factors: eigenmode alignment (or natural frequencies) and damping capacity. Focusing on the effects of dimensions on damping capacity of a brake disc, intensive CAE experiments for analyzing the effects were conducted. It was found that disc damping capacity can be increased independently of natural frequency by modifying disc dimensions. It was also found that peak accelerance obtained from a frequency-response function of a brake disc is an effective parameter for evaluating damping capacity of a brake disc.

The friction surface of a brake disc is generally finished through several machining processes such as grinding and turning. They have gains and losses in braking effectiveness, cost, etc. In some cases, effectiveness is not stable under a certain condition during running-in. This phenomenon is becoming more apparent as non-asbestos organic lining materials have come to be used more widely. In Part I of this study, brake discs finished through two processes were burnish-tested together with five lining materials that had various levels of aggressiveness. These tests show that a more aggressive lining material needed fewer burnish stops but, as expected, caused more wear. The behavior of friction and wear was found to depend both on lining materials and finishing processes. In Part II, many discs finished through various processes were tested together with one of the lining materials that showed the largest difference between the finishing processes described in Part I.