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Technical data are subject to change without notice.
ISO 9001 certified. © Copyright SPM 1996-9. 71519.Z
SPM Instrument AB
• Box 4 • S-645 21 Strängnäs • Sweden
Tel +46 152 22500 • Fax +46 152 15075 • [email protected] • www.spminstrument.se
Measuring Points
Vibration at the measuring point should be repre-
sentative of the overall vibration pattern of the ma-
chine. The forces involved are usually transmitted
through the bearings and their housings to the ma-
chine foundation. Consequently measuring points
should be located on or near the bearing housings.
Machine guards, cover panels and other parts which
are considerably less stiff than the main structure are
not suitable as measuring points.
Generally speaking, the more measuring points cho-
sen, the easier it is to locate a specific mechanical
problem. Consider a fan, belt driven from an electric
motor. Measurements taken on the fan bearing (3)
will primarily give information on fan balance. If out of
balance is the only problem to guard against, measur-
ing on that bearing will be sufficient. To be able to
make an adequate assessment of the mechanical state
of the whole machine, one should also measure on
the drive end bearing (2) and the motor (1).
The direction of measurement is very important. Out
of balance forces rotate with the shaft and cause
radial vibration acting in all directions within the plane
of rotation.
Axial vibration, along the line of the shaft, is normally
caused by faulty alignment, i.e. badly assembled cou-
plings or bent shafts.
Normal practice is to take vibration readings in three
directions at each measuring point: vertical (V), hori-
zontal (H) and axial (A). Of the two radial measure-
ments, a reading in the vertical direction tends to give
information about structural weakness, whereas the
horizontal reading is most representative of balance
conditions.
The measuring point, meaning the exact spot on the
machine where the transducer is placed, should be
clearly marked and used each time a reading is taken.
Relatively small changes in the measuring point can
cause misleading changes in the measured value and
trend analysis difficult.
Axial vibration
Radial vibration
