Section 3: Onsite Analysis - Measurement Menu
47
Fault Types and their Phase Relationships
Misalignment - high amplitude peaks will appear at 1X, 2X and
sometimes 3X the running speed. Look for a 180° (± 30°) phase
change across the coupling in the axial direction (for angular
misalignment) or radial direction (for parallel (offset) misalignment).
Bearing misalignment (cocked bearing) will cause a twisting motion
in the axial direction. Look for a 180° (± 30°) phase change from the
top to the bottom of the bearing housing or from one side to the
other.
Imbalance - a high amplitude peak will appear at 1X the running
speed. Confirm that the phase difference from horizontal to vertical
on the bearing is close to 90° (± 40°). The phase difference between
left and right horizontal locations should be within 30° of the phase
difference between left and right vertical locations. If the phase is
unstable then there may be another reason for a large running speed
vibration e.g. looseness or misalignment.
The degree of couple imbalance versus force imbalance can be
investigated by looking at the phase difference between the two
sides of the machine - if there is little couple imbalance then only a
single plane correction may be required to balance the rotor.
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,
blade pass frequency etc). 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.
Resonance (variable speed machines) - to investigate resonance
you can vary the machine speed. A phase shift of 180° will occur as
machine speed is increased from below a resonance to above a
resonance. 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.
Resonance (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 well be on the way towards its 90° phase change so
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