Page 335 - Machinery Component Maintenance

Balancing of Machinery Components 317

Field Balancing Overview
Once a balanced rotor has been mounted in its housing and installed in
the field, it will not necessarily stay in balance forever. Corrosion, tem-
perature changes, build-up of process material and other factors may
cause it to go out of balance again and, thus, start to vibrate. However,
unbalance is not the only reason for vibration. Bearing wear, belt prob-
lems, misalignment, and a host of other detrimental conditions will also
cause it. In fact, experience has shown that vibration is an important indi-
cation of a machine’s mechanical condition. During normal operation,
properly functioning fans, blowcrs, motors, pumps, compressors, etc.,
emit a specific vibration signal, or “signature.” If the signature changes,
something is wrong.
Excessive vibration has a destructive effect on piping, tanks, walls,
foundations, and other structures near the vibrating equipment. Operat-
ing personnel may be influenced too. High noise levels from vibration
may exceed legal limitations and cause permanent hearing damage.
Workers may also experience loss of balance, blurred vision, fatigue, and
other discomfort when exposed to excessive vibration.
Methods of vibration detection, analysis, diagnostics, and prognosis
havc bccn described by the authors previously in detail.s A quick review
of the hardware required to perform field balancing should therefore suf-
fice.

Field Balancing Equipment
Many types of vibration indicators and measuring devices are available
for field balancing. Although these devices are sometimes called “porta-
ble balancing machines,” they never provide direct readout of amount
and location of unbalance.
Basically, field balancing equipment consists of a combination of a
suitable transducer and meter which provides an indication proportional
to the vibration magnitude. The vibration magnitude indicated may be
displacement, velocity, or acceleration, depending on the type of trans-
ducer and readout system used. The transducer can be held by an opera-
tor, or attached to the machine housing by a magnet or clamp, or perma-
nently mounted. A probe thus held against the vibrating machine is
presumed to cause the transducer output to be proportional to the vibra-
tion of the machine.
At frequencies below approximately 15 cps, it is almost impossible to
hold the transducer sufficiently still by hand to give stable readings. Fre-
quently, the results obtained depend upon the technique of the operator;
this can be shown by obtaining measurements of vibration magnitude on