Page 225 - Gear Technology Solutions

However, the conclusion that unequally spaced gear teeth will accomplish the
same mission of reducing noise as discussed with fans and tires is incorrect
because the physical principles which are applied in both cases are quite dif-
ferent. The wheel in Figure 4 will not change its RPM nor will the vehicle speed
change as a result of the random or sinusoidal spaced tire treads. The treads
merely have an influence on dynamics and noise generation due to the chang-
ing contact frequency between rubber sections and street surface. The analo-
gy in gearing is the modulation of the contacting surfaces - not the phase loca-
tion of those surfaces. In other words, this means that the teeth have to be
equally spaced in order to assure proper function of a gear transmission but
the surface texture or the contact characteristic should be shifted within each
tooth and from one tooth to the next [8].

16.5 Noise Reduction by Scrambling

The expression “scrambling” has been used for the determined or random
change of geometrical features from tooth to tooth on gears. One example is
the introduction of a random spacing error (see Figure 5). The goal of this
“tooth position scrambling” was to reduce the fundamental tooth mesh fre-
quency which is generated by the tooth mesh impact and precisely repeats
with the timing of one pitch depending on the gear quality. It was expected that
the existing tooth impact energy would now be partially re-directed to the side
bands and therefore provide an additional masking of all harmonic amplitude
peaks.

Figure 5: Introduced spacing error

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