Maintworld 3/2017

CONDITION MONITORING
measured, but we will not be dealing
with them. They typically need many
more measurement points than the
five points that will be enough for our
measurement. The machine has four
roller bearings and we should select four
measuring points as close as possible to
these bearings. These four points must
be radial, i.e. perpendicular to the shaft.
Do not worry about whether to measure
vertically or horizontally. You can
choose any direction between these two
directions. The last fifth point will be axial,
i.e. parallel to the shaft. Put it on the
coupling and it doesn’t matter whether
it is on the engine or the fan. This fifth
point is therefore perpendicular to the
previous four.
approximately +/- 5%. If the same test is
carried out without a pad, the results will
vary by +/- 50%. (See Figure 1)
We deliberately did not mention
measuring with a sensor that has no
magnetic base, and that is just pushed
onto the machine by hand. This method
is unrepeatable. Unfortunately, it is
sometimes used in maintenance and
so the results are disappointing. Sometimes
the whole vibration diagnostics
programme is rejected, the only reason
being unprepared measuring points.
THE USUAL OPINION
AMONG MAINTENANCE
PEOPLE IS THAT A
POWERFUL ANALYZER
IS NEEDED FOR A REAL
DIAGNOSIS; THIS IS A
MYTH. IN MY OPINION
EVEN WITH A SIMPLE
VIBROMETER, 90 PERCENT
OF DEFECTS CAN BE
ACCURATELY DETERMINED.
Once the measuring points have been
chosen, they need to be prepared for
the measurement. It is not possible to
simply take the sensor with a magnetic
base and put it on the uneven surface of
the machine. Measuring pads must be
stuck on the selected locations before
measurements are taken. They have a
flat surface. In addition, they guarantee
that you will always measure at the same
machine location. The basic rule for taking
measurements is to make sure the
measurement conditions are 100 percent
repeatable. That is exactly what the
measuring pads guarantee. Let’s try, for
example, 10 repeated measurements in
one place i.e. put the sensor on the pad,
measure it and then remove it from the
pad. You will find that the measurements
are almost identical. They will vary by
Figure 1. Measurement pad glued onto the
machine surface (1), magnetic base (2),
acceleration sensor (3).
How to Find Warning and
Alert Vibration Levels
The first measurement has already been
taken and the results obtained. But what
do the numbers mean? Are the vibration
readings low or high? With what should
the results be compared? The easiest
way is to use ISO 10816, but the limits
given here have one significant defect.
They apply to machines with speeds of
600-3000 RPM. Let’s suppose the fan is
unbalanced. The centrifugal force that
causes vibration will vary significantly
for 600 RPM and 3000 RPM. The dependence
of the force on the speed is
quadratic, i.e. 2x higher speed means 4x
higher force. Therefore, the weight of
the heavy point on the rotor may not create
a problem at 600 RPM but will cause
the fan to destruct at 3000 RPM. The
warning and danger limit values should
depend on the speed. (See Figure 2)
If several similar machines are
measured, then the situation is simpler
because we can compare the values from
all machines. If we get results equal to
1.8, 2.1, 1.9 and 4.5 from the same point,
then it is obvious that 2.0 means good
machine condition. A machine condition
with a value of 4.5 should be investigated
further.
The first step deals with regular
measurements and monitoring the
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