Comparison of the Surfaces and Coefficients of Friction of Truck Brake Pads Tested on a Flywheel Test Bench and a Scaled Tester – A Case Study

Mechanical brakes have been used and studied in many areas of mechanical engineering and automotive technology for decades. However, the processes in the contact zone have not yet been described with sufficient accuracy. One reason for this is the inability to observe the contact zone during contact. Studies in which one contact partner is replaced by an optically transparent material influence the processes in the contact area. Researchers are therefore forced to examine the surfaces of the contact partners after the actual contact has taken place. Various measuring systems, such as microscopes or laser sensors for surface structure measurement, are used to measure the surfaces of the brake pads. Due to shorter measuring times, mainly only smaller samples from measurements using pin-on-disc tribometers are examined for this purpose. However, various studies show that the friction and wear values of pin-on-disc tribometers cannot be transferred to real brake systems. For this reason, surface measurements are performed on full-size truck brake pads in this publication. For this purpose, the SN6 truck brake from Knorr and the corresponding brake pads were subjected to defined load scenarios. The load scenarios comprise four different surface pressures of 0.8, 1.7, 2.5, and 3.3 MPa, as well as three different starting speeds of 10, 20, and 40 km/h. The rotational masses used in the flywheel test bench correspond to an equivalent translational mass of approximately 9 t. For each combination, 450 braking test cycles were performed. Following each measurement cycle, the surfaces of the brake pads are photographed and measured using a laser triangulation sensor. In addition, the contact surface between the brake pads and brake disc is examined using pressure measurement foil. In addition to a qualitative and quantitative evaluation of the surfaces of the truck brake pad from the truck brake, a comparison is made with a partial lining sample of the same truck brake pad, which has been tested on a partial lining test bench under comparable test scenarios.