Vibraon Measurement Instruments
18
9.1 Resonance
A resonance can easily be found when a flexible machine is running up or
down its speed. The resonance frequencies are located at the RPM’s, where
the vibration has a local maximum level.
To understand a resonance you can compare with the string of a guitar. The
string has its natural basic tone that will ring as soon as the string is struck.
The actual frequency of the tune depends on the stiffness and the distributed
mass of the string.
All machines have similar built in ”tones” with corresponding properties
consisting of stiffness and a mass in the form of mechanical strings such as
shafts, beams, floors and in all mechanical parts. If any natural excitation
(=alternating force) in the machine has the same or nearly the same frequen
-
cy as a resonance frequency the vibration will be amplified in this machine
part, and a much higher vibration level will occur.
To identify, measure the vibration levels in three perpendicular directions at
the bearings. If you find a measurement with at least three times higher level
than in the other directions, consider resonance as a likely possibility. The
resonance is amplifying the mechanical force and thus gives a high vibration
in that direction. The resonance makes the machine unnecessarily sensitive
to mechanical forces.
Actions to handle a resonance are different depending on its location, opera-
ting conditions etc. It will normally require experience to alter the situation.
One reason is that the modification can affect the basic mechanical design
of the machine and normally require the competence of a machine designer.
We recommend however to consider such modifications since the change
of
the resonance frequency normally is cheap compared to the high maintenan-
ce cost that will follow any attempt to run a machine under the influence of
a resonance.
Sometimes it´s possible to change the speed of machines, which often is a
simple solution.
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