Section 4 — Measurement Types and their Uses
67
Resonance
Flexible
structures
High amplitude peaks will appear
where natural resonance frequencies of
the structure coincide with excitation
frequencies (e.g. 1X, other low orders of
running speed, or blade pass
frequency).
Look for a 0° phase change when both
sensors are positioned in the same
direction between two stationary
points. The phase change will be 180°
when there is a stationary point
between the sensors.
Variable speed
machines
You can vary the machine speed to
investigate resonance. A phase shift of
180° will occur as machine speed is
increased from below a resonance to
above it. A 90° phase shift will be
present at the resonant frequency. If a
machine is running close to resonance,
any small speed variation will cause
large phase shifts.
At a bearing Look for a change from the normal 90°
phase difference in the horizontal and
vertical directions at 1X. When a natural
frequency is close to 1X in either
direction, that phase angle may be
heading for its 90° phase change. This
means the two phases may no longer
be 90° apart (assuming the structure
has a different natural frequency in
either direction).
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