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Machinery Reliability Audits arid Reviews 183
AGMA STANDARD PRACTICE FOR
HIGH SPEED HELICAL AND HERRINGBONE GEAR UNITS

SUGGESTED GEAR AND PINION HARDNESS COMBINATIONS

Bhn
R

I 2 3 4 5 6 7 B 9 IO II
GEAR RATIO - mG

Figure 3-73. Strength factors for high-speed gear units. (Courtesy AGMA, No. 427.06.)

C,: The overload factor allows for momentary torques and overloads imposed by
the driver or driven load. Though best established by field experience, it can
be estimated from Table 3-17. Since this factor can derate gear life by about
2,000 times, care should be taken in selecting C,. Uniformity and frequency of
torque fluctuation, particularly as related to the tooth mesh frequency, are
important. If the rate is low and not a specific harmonic, a low value of C, can
be used safely.
C,: The dynamic factor has to do with the action between mating teeth. Its magni-
tude depends on spacing accuracy, pitch-line velocity, inertia, and stiffness of
connected masses. Lubricant viscosity comes into play at pitch-line velocities
of about 20,000 fpm and higher. Until being questioned in the mid 1970s,