SPM VibChecker, User Manual

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19
Vibration severity measurement
If a fan is out of balance, it will shake at its speed of rotation, i.e. move backwards and forwards once
per revolution. The number of vibrations per time unit is the vibration frequency, measured in
Hz (Hertz
= cycles per second) or CPM (cycles per minute).
The rotational speed of any piece of plant is known as its fundamental frequency. For a fan with a speed
of 1500 RPM the fundamental frequency is 25 Hz (1500 RPM ÷ 60).
In practice, machine vibration usually consists of many different frequency components. For a general
assessment of machine condition one uses wide frequency band measurements, meaning all vibrations
within a large frequency range are measured simultaneously.
Vibration parameters ACC, VEL, DISP
Cyclic movement can be measured and described in three different ways; as displacement, acceleration
and velocity.
A part that is moving from rest, speeding up, slowing down and stopping twice per cycle is obviously
accelerating and decelerating continuously.
Acceleration (ACC) is measured in m/s
2
or g (1g = 9.81 m/s
2
).
The second measuring parameter is the speed at which the object moves, the vibration velocity (VEL).
Velocity is expressed in mm/sec.
Displacement (DISP) means the actual distance the object moves, measured either from its rest posi-
tion in one direction (peak) or as the total movement in both directions (peak to peak). Displacement
is expressed in micrometers.
VibChecker always measures an acceleration signal, which is integrated to obtain the velocity, and
double-integrated to derive displacement.
Both acceleration and speed are constantly changing. One can measure a peak value of either, but a
mean value often gives a better indication of the forces involved in the movement. Most instruments
measure the
RMS value (root mean square value) of the movement and use a scaling factor to indicate
the peak levels, if they are given at all.
All three vibration parameters – acceleration (ACC), velocity (VEL), displacement (DISP) – are mathemati-
cally related. As mentioned above, the accelerometer signal can be converted, via integrating circuitry
in the measuring instrument, into a reading of vibration velocity or displacement.
The choice of displayed parameter (the instrument reading) depends on the problem to be solved and
on the cost, the complexity, and the reliability of the measuring equipment.
Experience has shown, that the RMS level of vibration velocity, measured over a frequency range of
10 to 1000 Hz , is most useful for general assessment of machine condition. The technical term used is
vibration severity, defined as above and displayed in mm/s RMS or in/s RMS on the instrument. Vibra-
tion severity is directly related to the energy level of machine vibration, and thus a good indicator of
the destructive forces acting on the machine.