VMI VIBER X5 MKIII, Инструкция по эксплуатации инструмента

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6.2. Resonance
All machines have similar built in "tunes" with corresponding properties consisting of
stiffness and a mass in the form of mechanical strings such as shafts, beams, floors and
other mechanical parts. If any natural excitation (alternating force) in the machine has the
same or nearly the same frequency as a resonance frequency the vibration will be
amplified in this machine part, and a lot higher vibration level will occur.
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. (The applications Coast Up and Coast Down are useful tools when looking
for resonances).
To identify a resonance, 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 you should 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.
A natural excitation force is for example unbalance at the running speed, misalignment on
mainly twice the speed, gear mesh forces etc.
One common resonance frequency is the critical speed of a shaft which depends on the
stiffness and mass of the shaft, but resonances exist in all machine parts as well as in
supporting beams and concrete floors.
THE BASIC RULE IS THAT THE RESONANCE’S OF ANY PART IN THE MACHINE SHOULD NOT
COINCIDE WITH ANY NATURAL IMPULSE IN THE MACHINE.
Actions to handle a resonance are different depending on its location, operating
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 maintenance cost that will follow any attempt to run a machine in long term
operation under the influence of a resonance.
A TEMPORARY AND SOMETIMES PERMANENT SOLUTION TO A RESONANCE PROBLEM IS
TO CHANGE THE SHAFT SPEED OF THE MACHINE, IF POSSIBLE.
Existence of resonance at a frequency where the machine is running should be
rectified as early as possible, since it will significantly affect the economic operation of
the machine.