Maintworld 3/2017

CONDITION MONITORING
Figure 2.
vibration trend. If it is stable and has a
permissible value, then the machine is in
good condition and there is nothing else
to do. If the value gradually increases and
the warning threshold is exceeded, the
second step of the evaluation must be
carried out.
The aim of the second step of the
evaluation is to find the cause of the increased
vibration. I will now describe the
procedure of deeper analysis.
Bearing Condition
Needs Acceleration
INCREASED ACCELERATION VALUE
If the acceleration value has increased
and the increase is only in one radial
location, then it is easy. The problem is
the poor condition of the roller bearing
at this point. If the gears are measured,
then the acceleration values can be increased
in more places and it shows a
problem with the gearing.
Imbalance, misalignment and
looseness need velocity
IMBALANCE OR LOOSENESS
The values are significantly increased on
only one part (either the motor OR fan).
If the increases in both radial directions
are similar, then it is most probably an
imbalance. If you have a signal spectrum
at your disposal, you can find the significant
value only on the speed frequency.
If the increase differs significantly in
both radial directions, or there is only a
vertical increase, then it is most probably
due to looseness. You should measure
each machine foot. You would probably
find significantly higher values on one of
them.
Electrical defect
generates vibrations
ELECTRICAL DEFECT OF THE MOTOR
When the electric motor vibration looks
like imbalance is the problem, then you
should always also consider electrical
defect. The electric motor may have
winding defects, and despite this the
vibration behaviour indicates an imbalance.
Therefore a switch-off test on the
motors should always be carried out.
After switching off the power, one of two
situations will occur.
1) The velocity decreases slowly along
with the rpm drop. This is a true imbalance.
2) Immediately after switching off,
the velocity increases for a very short
time (1 sec), by a multiple and then drops
to a very low value where it remains
until the machine stops. This is an electromagnetic
problem. The force field is
not uniform and shifts the rotor off its
mechanical centre of gravity. In the vibrations
it will manifest as an imbalance.
After switching off, the force is instantly
lost and the rotor jumps back to the
mechanical centre of gravity. This shock
causes an increase in the value. Then the
rotor starts spinning normally and the
vibrations disappear.
Mechanical imbalance
Electrical fault
MISALIGNMENT
If the velocity in the axial direction increases
(usually it is higher than in the
radial), always check the coupling and
the alignment. It is misalignment that
causes vibrations in the axial direction. If
you have a spectrum, you can find higher
values on the speed frequency and several
harmonics.
RESONANCE
The velocity significantly increases on
both parts (motor and fan) and only
in the vertical directions. Take measurements
across the frame below the
machine. If there is a low value where
the frame is supported, and high values
between the supports then there is a
resonance problem.
A coast down measurement or
gradual reduction of speed (frequency
changer) will help. In case of resonance,
the vibrations will decrease dramatically
with a small speed change. If the
standard operating speed cannot be
changed, the frame must be additionally
reinforced.
Those who do Nothing,
Make no Mistakes
A lot of maintenance staff are unnecessarily
nervous about carrying out vibration
diagnostics. Simple devices are developed
just for those who have no deep
knowledge. If you take regular measurements,
you will find that you have a
much better overview of the condition
of your machines. You will also certainly
notice a decrease in the number of unexpected
temporary shutdowns.
EVEN WITH A SIMPLE VIBROMETER, 90 PERCENT
OF DEFECTS CAN BE ACCURATELY DETERMINED.
20 maintworld 3/2017