Commtest Instruments vb Series Instrument Reference Manual page 57

the two phases may no longer be 90° apart (assuming that the
structure has a different natural frequency in either direction).
Eccentric belt pulley - a high amplitude peak will appear at 1X the
running speed of the pulley. Look for a phase change of either 0° or
180° between the horizontal and vertical readings, which indicates
straight line motion instead of the usual 90° difference. This symptom
is similar to bearing resonance (described above) but in this case the
direction of maximum vibration will be in line with the two belts.
Mechanical looseness - high amplitude peaks will show up at many
multiples of the running speed and sometimes even multiples of 0.5X
running speed e.g. 1.5X, 2.5X etc. Look for fluctuating phase
readings at a location from one recording to the next to the next. Also
compare the phase readings at 30° increments around a bearing
housing - the phase will be significantly different at each angle.
Mechanical foot looseness (soft foot) - a high amplitude peak will
show up at 1X the running speed. Look for a 90° to 180° phase
change between the machine foot and its concrete base. The soft
foot may also have a different phase measurement from the other
feet.
48 Section 3: Onsite Analysis - Measurement Menu
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