Abstract
This paper describes the experimental analysis and control techniques for brake squeal originated in the medium duty bus with air over hydraulic drum brake system.
The brake noise occurrence was analyzed, giving heat history on the virgin brake lining of the test vehicle, in relation to the change of frictional characteristics between brake drum and lining, and to the change of the brake assembly's vibration damping performance (damping factor) through the heat history. Then the heat history is given by applying braking in such condition as a city bus running in the town. All of the tests were carried out, fixing the medium duty bus, which tends to radiate brake noise with a frequency of approximately 1550Hz, on the brake noise dynamometer. The damping factors of brake assembly are obtained by measuring decay time of band pass filtered vibration caused by hammering the components in the stationary condition that the brake is applied a line pressure of 3M Pa for usual braking deceleration. The damping factor of brake assembly proved to decrease and brake noise also proved to occur in accordance with the increment of test cycles, as running in the town. Namely, the brake assembly with lower damping factor proved to tend to radiate brake noise. Through the heat history, the change of braking torque during a constant pressure braking were observed. The brake shoe was found to vibrate, while braking torque, proportional to a coefficient of friction between brake lining and drum, is larger than a certain level of braking torque. Accordingly, high coefficient of friction between drum and lining tends to cause the brake noise occurrence. Moreover, vibration analysis was carried out, focusing analytical concerns on the behavior of a brake shoe and drum, detecting the rotational angle of the drum by fitting a magnetic pick-up on the brake unit. The brake drum's and shoe's vibration begins to expand, when the phase difference between drum's and shoe's vibration in the radial direction begins to appear at the early stage of brake noise occurrence. This phenomenon means that the fluctuating pressure at the interface between drum and lining is caused by the vibrations of brake shoe (or lining) and drum. In addition, the pressure causes the forces on the surfaces of the brake drum and shoes due to the rotation of drum, consequently the so called sprag -slip motion is caused by the force. Based on the above mentioned analysis results, following ideas may be effective for suppressing brake noise.
Namely,
I) brake lining with lowered frictional coefficient (with difficulty of keeping braking performance)
2) brake lining included high damping material (with difficulty of protecting damping material from heat)
3) dynamic dampers on brake component
The author tried to suppress brake noise, fitting dynamic dampers with high damping elastic material on the edges of brake shoes, where big displacements appear at the vibration mode of brake assembly during brake noise occurrence. With this treatment. the damping factor of the brake assembly is increased with the dampers. Consequently, brake noise proved to be controlled by fitting or removing the dynamic dampers on the shoes.
This technique is applied to many of their production vehicles.