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Technical data are subject to change without notice.
ISO 9001 certified. © Copyright SPM 1996-9. 71411.B
SPM Instrument AB • Box 4 • S-645 21 Strängnäs • Sweden
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Fig. 23
Fig. 21
Fig. 22
With the help of the lowest reading, (V
2
= 3.2) one can
predict the likely result of the balancing work before
the weight is welded to the rotor.
Draw an arc with radius V
2
= 3.2 from point 2 so that it
intersects the other arcs.
If machine vibration is mainly due to out-of-balance,
all three arcs will intersect at the same point. This
means that the calculated weight and position of the
correction mass are sufficiently accurate to reduce
vibration to a value considerably lower than V
0
.
Other Causes of Vibrations
If a significant part of the measured vibration is due to
other mechanical faults, the arcs will have no common
point of intersection. They will either form a triangle
or, in some cases, not intersect at all.
The smaller the triangle between the arcs, the better
the result of the balancing works. A large triangle
means that one has to find and remove other possible
causes of vibration before achieving accurate balanc-
ing results.
Measuring Angles
For rotors that will need balancing at least once every
year, it is well worth taking time to prepare mounting
positions for the weights or at least to marks angles
on the rotor when the machine is next stripped down
for repair or overhaul.
If only a small part of the rotor is accessible through a
hatch or cover, a tape measure can be used to mark
off the angles. An arc of 120° has a length of 2.1 x
radius. 1° is an arc of 0.0175 x radius.
If is also possible to mark ”trial points” at 120° inter-
vals on the end of a shaft, a hub or some other
accessible part which turns with the rotor. Align the
first ”trial point” on the shaft end with a fixed point
on the bearing housing or casing and attach the trial
weight to the rotor. For the second and third meas-
urements align the other ”trial points” on the shaft
end with the same fixed mark and attach the trial
weight through the same access point, hatch etc.
Checking Accuracy
Out - of - balance
plus
other vibration
Mainly
out - of - balance
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