General Measurement Methods
Practical considerations
The rotor to be balanced should
be easily accessible, and should
have provision for mounting trial
masses at various angles around it.
The mounting points should prefera
bly be at the same radius from the
axis of rotation to simplify calcula
tion. The datum position must be
marked on the rotor in such a way
that it triggers a pick-up installed
alongside it on a stationary part of
the machine. A non-contacting type
of pick-up is recommended, be
cause this will cause a minimum of
disturbance to the rotor. Fig.2. The basic measuring chain
Vibration level can be measured
in terms of acceleration, velocity, or
displacement. However as most to the reference channel (A) of the One very useful addition to the
standards for balancing are written Phase Meter Type 2 9 7 1 . The out- measuring arrangement is the Tun-
in velocity terms, a legacy of the put from the Accelerometer Type able Band Pass Filter Type 1 6 2 1 ,
days when vibration was measured 4 3 7 0 is fed to the Vibration Meter which ensures that the vibration
by mechanical velocity sensitive Type 2511 which displays the vibra- measurements are made at the rota
transducers, usually velocity will be tion level. A signal taken from the tional frequency only, and the
the chosen parameter. Use of accel "Recorder Output" of the Vibration Phase Meter (or Stroboscope) re-
eration levels will tend to empha Meter is fed to channel B of the ceives a clean input signal. Use of
size higher frequency components, Phase Meter. the Filter is recommended for instal
while displacement will emphasize lations where the required trigger-
low frequency components. When the machine is run, a vibra- ing signal would otherwise be bu-
tion level will be displayed on the Vi- ried in noise, or where high vibra-
Instrumentation bration Meter, and an angle on the tion levels occurring at several fre-
Phase Meter, which together give a quencies cause difficulties in signal
vector representing the unbalanced tracking. As shown in Fig.5, the
The basic measuring arrange
mass and its line of action. Band Pass Filter is connected as ex
ment, shown in Fig.2, consists of
ternal filter to the Vibration Meter,
an accelerometer, a vibration me
Various modifications and addi- which is actually the same arrange-
ter, and a means of determining the
tions can be made to the instrumen- ment as in the Vibration Analyzer
angle of the unbalance relative to
tation arrangement shown in Fig.2 Type 351 3. The Filter has two band-
the datum position. The most effec
to improve sensitivity, or to take ad- widths, 3% and 23%, that can be
tive method of measuring this angle
vantage of instruments that are al- tuned continuously from 0,2 Hz to
is to use a phase meter as shown,
ready at hand. Figures 3 to 6, plus 20kHz to match the rotational
but a stroboscope can also be used
1 0 to 13 illustrate some of the pos- speed of the machine. It can also be
(see Fig.3), or the angle can be de
sibilities. employed to find the relative levels
duced from the results of several
measurements. The Magnetic Trans
ducer M M 0 0 0 2 emits a pulse each
time the High-// Disc passes, and
thus establishes a datum position
on the circumference of the rotor.
Similarly the Photoelectric Tachome
ter Probe M M 0 0 1 2 can be fixed to
scan the rotor to pick up a trigger
ing mark, for example a piece of
adhesive tape or a painted patch
with (infrared) reflectivity contrast
ing with the background. A pulse is
emitted for each pass of the mark.
The Probe must be powered from a
6 to 10 volt DC supply, such as that
available in the Tacho Unit Type
5 5 8 6 . The transducer output is fed Fig.3. Using a Stroboscope to measure angles
