Effect of Thermal Behavior of Friction Materials on Brake Squeal 2014-01-2514
Brake noise is an emerging concern in Indian Auto-industry; with brake squeal being the most evident form of brake NVH. Squeal noise generation attributes to many parameters including kinematics of braking parts during pressure application, structural dynamic behavior which in turn depends on coupling at resonant frequencies of different parts of a brake assembly, material of brake parts, operating clearances in the mating parts etc. The genesis of brake squeal lies in the generation of unstable frictional forces during braking event. These frictional forces induce uncontrolled amplification of brake parts vibration, which in turn tend to produce perceivable sound or noise. The magnitude this vibration induced squeal depends on co-efficient of friction, braking pressure, speed and temperature of friction material. It is known from typical squeal evaluations on dynamometer that at different temperatures of friction, the ability/ occurrence, of squeal is different, typically in the range of 50°C to 200°C. This could be linked to the thermo-elastic behavior of friction, as it is a heterogeneous material comprising thermally sensitive raw contents as compared to other parts of brake assembly which are largely a base of iron and less sensitive to change elasticity in above temperature range. The change in elastic behavior of friction can shift the resonant frequencies of pads/shoes and hence affect propensity of the squeal for a given brake. This behavior of friction needs an objective judgment, especially with different types of fiction materials like NAO, Low Metal, Semi-Metallic used in automotive brake applications. This paper focuses on comparative analysis carried out on two identified brakes viz. a disc brake (of an SUV) and a drum brake (of a small car) using 3 different friction parts per brake for NAO and low metal type. The thermal behavior of friction in terms of shift in resonant frequencies of pads/shoes and associated change in modal loss factor, as a measure of structural damping at identified temperatures, have been objectively compared between different grades for a given brake type. Its significance towards brake noise has been discussed.