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Vibration Level of Replacement
Impellers
All replacement impellers are dynamically balanced by the manufacturer prior
to shipment. Occasionally, an impeller that has been factory-balanced will yield
poor balance/vibration results when installed and operated. This does not mean
that the impeller was incorrectly balanced at the factory. It can result from dif-
ferences between test-stand conditions and operating conditions. A factory test
stand has different bearings, bearing spans, structural response, stiffness,
mechanical impedance, and by necessity, running speeds. The test stand cannot
duplicate the actual "fan system" and its response. For these reasons, the "fan
system" vibration levels must be checked after installing a replacement
impeller. Refer to the "Balance and Vibration" section for acceptable vibration
levels.
AD-10 l Addendum