Appeal No. 1998-1533 Page 8
Application No. 08/411,202
[i]n order to determine if this geometrical relationship
is indicative of a real physical effect, or is merely
fortuitous, a set of friction pads 42' and 44', shown in
FIG. 5, which had previously squealed at frequencies of 2
to 15 kHz with a median frequency of 7 kHz were beveled
on both sides to reduce the footprint of the pad on the
rotor to 50% of its original value. When retested using
these pads, the brake squealed at 17 to 18 kHz with a
median frequency of 17 kHz. FIG. 6 shows that the
footprints of beveled pads 42' and 44'. Footprints 82
subtends an angle subtended by three whole adjacent nodal
diameters, for the mode having a total of ten nodal
diameters a'-j', corresponding to a natural frequency of
16.5 kHz.
These experimental results indicated correlation
exists between the footprint of the friction pads 42 and
44 on the rotor 12 and the axial mode of rotor vibration
that is excited by the pads 42 and 44. Exciting an axial
mode of rotor vibration can potentially excite any
torsional modes of rotor vibration. However, elementary
vibration theory shows that a resonant system is more
sensitive to vibration that is less than its natural
frequency rather than greater than its natural frequency.
Hence, if a higher frequency axial mode is excited, it
becomes less likely that lower frequency torsional modes
would be excited. Since the ability of the total human
population to hear squeals drops off as the frequency
increases, sufficiently high frequencies that produce a
brake squeal can be disregarded. Thus brake squeal
complaints can be expected to decrease as squeal
frequency increases. The above test indicated a
beneficial effect on brake squeal by decreasing the
friction pad footprint since higher and higher axial
modes are excited as it decreases, which in turn
decreases the possibility of exciting torsional modes.
There is, however, a serious objection to gross
reductions in friction pad area, namely that wear is
substantially increased. What is wanted is a method of
obtaining the helpful effect of area reduction, while
keeping the actual surface area as large as possible.
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