Drum Brake Squeal
Drum brake squeal is a common discomfort. The main goal of this thesis is to investigate the mechanisms that generate drum brake squeal and to create a base for the search for solutions. A series of steps towards the understanding of these mechanisms are taken.
Firstly, a simple model for friction-induced vibration is presented. The solutions to this model might be unstable in spite of a constant coefficient of friction. The essential thing observed is that a coupling between different modes is necessary to form instabilities. The coupling is shown to be between two translational degrees of freedom.
Secondly, the mathematical model for friction-induced vibration is refined to take the flexibility of the drum, shoes and linings into account. This model is used to study the mechanisms that couple modes. The analysis shows that there are four mechanisms present in generating drum brake squeal. These mechanisms all occur owing to lining deformations, and the instability type is given the name "Lining Deformation Induced Instabilities". The mechanisms create waves that move in different directions. In a squealing brake, all the waves are superposed, which leads the solution towards a standing wave. A standing wave or synchronous vibration is always stable, and if a pure synchronous vibration is created, the noise would be eliminated at the source.
The experimental partt of the investigation concerns the measurement of the vibration of a drum brake. The deflection shape is measured in operation, i.e. while the vehicle is running on a test ground. The deflection shape covers radial vibrations of the drum as well as radial and tangential vibrations of the leading shoe. The accelerometers are distributed both axially and tangentially on both the shoe and the drum.
Finally, a finite element model for friction-induced vibration and noise generation is used to study the influence of different self-excitations on drum brake squeal. The model is semi-three-dimensional, with a two-dimensional drum and three-dimensional shoes. A contact element is derived that takes the friction variation into account as well as follower forces and negative .my.-velocity slope.
For the brake analyzed, the results are:
"Stick-Slip" is impossible for vehicle speeds over 0.04 km/h.
The destabilizing effect of the "Negative .my.-Velocity Slope" is less than the stabilizing effect of the material damping of the parts.
"Self-Locking" is shown to be impossible.
The "Lining Deformation Induced Instability" type results in great squeal propensity.
The instabilities from "Follower Forces" are negligible.
The rotation of the drum gives frequency shifts that stabilize the brake.
The frequencies and mode shapes generated from the model with the lining deformation induced instability type show very good agreement with the measured ones.
With the knowledge of the mechanisms behind and characteristics of drum brake squeal, a set of solution classes is outlined. A series of examples is given to show the usefulness of such classes as an aid in the creative process of generating solution principles.