Experimental investigation of friction-induced noise in disc brake systems

This paper presents experimental and analytical investigations examining the influence of the interfacial forces between a rotating disc and a friction element on the generation of chatter and squeal. Due to inevitable misalignment between the element and disc surface, a kinematic constraint instability known as sprag-slip is created. The experimental measurements include time history records of normal and friction forces, time variation of the friction coefficient, and acceleration of the friction element. The time history records of interfacial forces revealed short periods of high frequency component. It is found that the friction force is non-Gaussian and that its power spectral density covers a wide frequency band. The dependence of the root mean square of the friction coefficient on the relative velocity is found to have a negative slope at lower disc speeds. Depending on the direction of disc rotation, it is found that the friction velocity curve for clockwise disc speed is completely different from counter-clockwise rotation. The associated noise is also different in pitch and frequency content. The analytical modeling emulates the dynamics of the friction element. The transverse motion of the friction element is described by a homogeneous partial differential equation with non-homogeneous boundary conditions. The analysis shows that the normal force appears as a coefficient of the stiffness term, while the friction force appears as a non-homogeneous term. Since the normal force varies randomly as observed experimentally, it acts as a parametric noise to the friction element, and results in parametric instability in the form of squeal or vibration.