An Investigation into the Influence of the Contact Pressure Distribution at the Friction Pair Interface on Disc Brake Squeal

The main purpose of this thesis is to investigate the dynamic centre of pressure during a noisy brake application. A novel technique is employed to measure the centre of pressure and contact pressure distribution between the disc/pad interfaces during braking events.

The test rig was developed to study the contact pressure distribution between disc/pad interfaces. The caliper and set of pads were modified to measure both static and dynamic centre of pressure during braking events. The brake uses a 12 piston opposed caliper arranged to allow a number of the pistons to be controlled independently using 4 master cylinders. This allows the interface centre of pressure to be adjusted both along the length of the pad and radially. The tests included static pressure measurements with the sensor film between the pad friction face and the disc, the centre of pressure being adjusted using the master cylinders to provide a “system benchmark”. Once the static characteristic behaviour of the modified pad is established, the centre of pressure variation is measured under dynamic conditions. This allows the movement of the centre of pressure to be plotted against brake pressure and rotor speed.

Furthermore, a detailed finite element model of a disc brake assembly is developed. Contact analysis was performed to determine the pressure distribution, interfacial contact area and normal contact forces under both frictionless (μ=0) and frictional braking conditions. The effect of varying friction coefficients and the brake hydraulic pressure is also examined. Preliminary finite element results of contact pressure distribution between the disc/pad interfaces were compared with the experimental results, followed by a detailed modal analysis of disc brake to predict the natural frequencies and the mode shapes of disc brake. In addition, a stability analysis of brake assembly is carried out to distinguish the unstable frequencies. Structure modification of disc brake assembly was also investigated to understand the characteristics behaviour of brake system in terms of squeal noise performance.

It is established from the results that there is a strong relationship between the interface pressure distribution, the effective centre of pressure and the propensity of the brake to generate noise. It is noticed that the centre of pressure may vary both along the pad and radially during braking which adds to the complex analysis of instability.

The finite element results compared well with the experimental results. It is observed that the contact pressure distribution and the magnitude of normal contact forces are much higher towards the leading edge of the pads comparing to the trailing edge. It is also established yet again that with a leading centre of pressure the brake is more prone to noise whereas with a trailing centre of pressure the system is more likely to be stable.