Doc.Ref.No.:mVMT/om/101
masibus
Issue No. :00
Model : VM908
Operator’s Manual VM-908 Vibration meter
Page 9 of 11
02/05/16
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.
A natural excitation force is for example unbalance at the running speed, misalignment on mainly twice the
speed etc.
THE BASIC RULE IS THAT THE RESONANCE’S OF ANY PART IN THE MACHINE SHOULD NOT
COINCIDE WITH ANY NATURAL IMPULSE IN THE MACHINE.
A broad band Vibration-meter can not recognize the frequencies of the vibration but as a rule
most high vibrations are caused by only one mechanical problem.
To identify the presence of 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 a resonance 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.
It is possible to locate the resonance peak while the speed of the machine is changing. The resonance
frequency is located at that rpm where the vibration has a local maximum.
The proper action against a resonance is very different depending on its location, operating conditions etc.
It will normally require good experience to alter the situation.
One reason is that the modification affects the basic mechanical design of the machine and where you
normally require the competence of the machine designer.
We recommend you however not to hesitate 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.
What is a bearing condition value?
The bearing condition value in VM-908 is the sum average value, RMS value, of all high frequency vibrations
between 1000 Hz to 5000 Hz.
This value is an acceleration average with the unit "
g
" because high frequencies give a large signal if it is
measured in acceleration. When the balls or rollers rotate inside the bearing a wide-band noise and
vibration arises. This noise and vibration are increased if the bearing is poorly lubricated, overloaded due to
misalignment or has a damaged surface.
