The geometric aspects—such as height, thickness, and diameter—need to adhere to specified standards. They must all be within a precise range, known as tolerance limits. Additionally, we don’t merely inspect one disc. Our quality control system randomly selects discs for examination to ensure compliance with geometric parameters. This approach enables us to scrutinize each dimension and, notably, axial runout.
The sensor indicator positioned near the disc’s edge should not shift more than a tiny margin, specifically 0.05 mm during full disc rotation. This measurement is critical not just for comfort but also for braking safety. Excessive runout can manifest as vibrations felt through the brake pedal, potentially impacting response time and increasing braking distance.
Stopping distance = response distance + braking distance
(going at the maximum allowed speed on a highway, in one second a car covers a distance of nearly 40 meters)
We also check the roughness of the friction surface as too many irregularities may reduce the contact surface between the pad and the disc and, consequently, reduce the maximum friction force.
Friction force = (friction surface area) x (contact force) x (friction coefficient)
As a driver, you have control over the force applied to the brake pedal during braking. However, it’s important to note that pressing the pedal fully only signifies that the hydraulic system is ready to transmit maximum pressure to the piston surface, limiting contact force. Therefore, the composition and manufacturing process of the brake disc are crucial factors to consider.
Metallographic structure and chemical composition of a brake disc
What is expected from a brake disc?
- mechanical resistance – suddenly applying the brake must not cause deformation of the disc structure,
- wear resistance – frictional force is not without consequences. During it’s operation the disk’s thickness diminishes, impacting both the dissipation of thermal energy and the ventilation of the friction pair (pad versus disc),
- resistance to thermal shock – in extreme cases, rapid cooling of the disc can cause deformation, potentially resulting in exceeding acceptable axial runout limits and causing cracks on the friction surface,
- corrosion resistance – during extended periods of inactivity, particularly in winter, the exposed surface of the disc is susceptible to oxidation (corrosion). Moisture and salt notably accelerate this deterioration process.
Hence, in the laboratory, we meticulously analyze the composition of the disc, examining its constituent materials and the thermo-chemical processes involved in its production. To achieve this, we extract sections for structural examination.
This investigative process is referred to as processing.
We delicately extract a sample, ensuring a slow and careful approach to prevent temperature increases caused by friction, as elevated temperatures can alter the metal’s structure. The sample is then encased in resin within a mold.
Subsequently, we grind, lap, polish, and refine its surface.
Following the etching process, the metallographic structure is unveiled, allowing us to scrutinize for irregularities, inclusions, or any other anomalies.
Errors occurring during the casting process and thermo-chemical treatment can compromise the strength properties of the disc. Inhomogeneities within the structure are areas where mechanical and thermal stresses may gather, potentially resulting in microcracks and material degradation under extreme conditions. Additionally, impurities within the structure and casting defects, such as voids, adversely affect the strength stability of the disc.
Testing in an environmental chamber provides a comprehensive assessment of how brake discs perform under unfavorable conditions. Exposed to heightened moisture and salinity levels—conditions far surpassing standard parameters—the discs undergo accelerated wear and tear. This testing methodology enables us to verify whether they adhere to the declared resistance specifications.
Following these rigorous tests, a final report on the production batch’s quality is generated.
