An analysis of disc brake noise using holographic interferometry.
A predominantly experimental approach using the whole body visual technique of
holographic interferometry is employed to investigate the mechanisms involved during a
noisy brake application.
Following the modal analysis of component parts, dynamic trials include the
development of the holographic technique, making use of mirrors to permit three
orthogonal views of the brake to be recorded simultaneously with smaller inset mirrors
allowing for additional areas, such as the ends of the piston pad, to be observed at the
same time. These dynamic experiments take the form of changing the operating
parameters of the brake through variations in speed, pressure and temperature and
through changes in the system geometry by adjustment of pad abutment and pad centre
of pressure loading. The tests show that pad abutment plays an important role in the
propensity of the system to generate noise and that a relationship between pad
abutment, pad material coefficient of friction and interface coefficient of friction
between pad-end and calliper-support finger exists which results in an offset in the pad
centre of pressure with the spragging angle being satisfied and resulting noise. This is
supported by basic theory. Additionally it is shown that the disc/pad interface
relationship is complicated and that it is not reasonable to assume mechanical integrity
of the pair and as a consequence the use of an "equivalent mass" is not appropriate for
high aspect ratio pads.
Advancements in the laser triggering process allow for holograms to be taken at specific
stages over and along a cycle of excitation by delaying the laser triggering initiation to
give variable time delays. The variety of techniques available are used to show that pad
excitation plays an important role in the generation of noise and that the piston pad in
particular is seen as the initiator leading to system excitation. Mechanical coupling of
the component parts is also seen to be fundamental, but not essential, to the generation
of noise. The techniques also show that, when complete coupling exists, the disc holds a
diametral mode of vibration which travels around the disc at a speed related to the
excitation frequency divided by the disc mode order. Results from the application of the
techniques also allows component parts to be analysed over a typical cycle of excitation
when it is shown that symmetrical components such as the pad are not necessarily
excited in a symmetrical manner. Phase relationship between the component parts may
also be determined by comparison of related holograms.
Holographic interpretations are confirmed and validated by mechanical measurements
when it is also demonstrated that noise is often preceded by, or accompanied by, a high
frequency excitation which is experienced by the complete brake.