Page 228 - Gear Technology Solutions

microns. The pitch variation required to realize the surface scattering on both
flanks of one slot is in the range of +/-5 microns. These errors reduce the gear
quality by several classes and show a negative influence on the load carrying ca-
pacity of a gearset.

The sample gearset of Figure 7 showed reduced harmonic levels in an
FFT analysis and indeed it was quieter in the vehicle, rated by test drivers
and vehicle owners. This means that the tonality was reduced, and the
subjective human noise sensation did not detect the objectionable sound
from the axles without the flank form scattering.

16.7 The Practical Obstacles with Flank Form Scattering

Applying spiral angle changes as well as pressure angle changes by using modi-
fied machine settings leads to tooth thickness and tooth indexing errors. In case of
flank form corrections which are different from tooth to tooth and in case that both
flanks of one slot are ground at the same time (completing) it becomes exceeding-
ly difficult or even impossible to control tooth thickness and indexing errors. It is
also a disadvantage that a complete set of machine settings (basic settings) has to
be transferred for each slot to the machine and compiled to a part program. This
requires 11 times the amount of data processing (for a 11-tooth pinion for exam-
ple) and data storage and it makes closed loop feedback from a coordinate meas-
uring machine more complex, because the closed loop feedback has to be applied
to 11 sets of basic setting for one single 11 tooth pinion.

The past psychoacoustic optimized gearsets show the mentioned tooth thickness
and indexing errors. These errors reduce the gear quality by several classes and
reduce the effective contact ratio in operation, which also reduces the load carry-
ing capacity of a gearset.

The flank form scattering of the psychoacoustic optimized gearsets also show de-
viations of the flank surface corner points in the +/- 5 to +/-10 microns range be-
tween the teeth of one pinion or gear. Flank form deviations in this magnitude are
not acceptable to most gear manufacturers because of contact patterns which
change from tooth pair to tooth pair which brings the risk of tooth corner load con-
centrations.

Surface scattering can reduce the harmonic frequency levels in a single flank test
and create side bands in the frequency spectrum between the harmonic frequen-
cies [1, 2, 3, 4]. As a result, a gearset with targeted surface scattering will be less
recognizable at the noise critical speeds and loads in a vehicle by the human driv-
er. The surface scattering should not reduce the ISO and AGMA gear quality level
regarding indexing and runout errors, and it should also not cause individual flank

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