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As a result of the development of the vibration measuring
instrumentation
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programs have been adapted which equate “low vibration” with long life, igher
quality, and lower maintenance costs. The fan manufacturers used to be the
“vibration experts” and set the tolerance for acceptable levels of
vibration.

Forces that are not constant in amplitude or direction over
time can cause solids to move repetitiously. We call this vibration. The similar
motion imparted to the air is called sound. These phenomena are somewhat
interchangeable since sound can cause vibration and vibration can cause sound.
One difference is that sound is only dealt with when it is audible while
vibration must be addressed even when it is outside the range of human
perception. All fans must generate some vibration. They continuously rotate and,
since nothing is perfect, cyclic forces must be generated. It’s only when
vibration reaches a certain amplitude that we call it “bad.” Vibration may just
be an indicator of some problem with a mechanism, or it may be a cause of other
problems. High vibration can break down lubricants in the bearings and, in
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may cause metal fatigue in the bearings.
Excessive vibration can cause fasteners to loosen or can cause fatigue failure
of structurally loaded components. Finally, vibration can transmit into adjacent
areas and interfere with precision processes, or create an annoyance for
people.

The forces which result in vibration in fans are primarily due to
minor imperfections in the rotating components. The most common of these
imperfections is that the center of mass does not coincide with the center of
rotation. We call this “unbalance.” Unbalance is corrected by adding (or
removing) weight so as to make the two centers coincide.

There are
numerous other “imperfections” that can result in vibration. Some of the more
common are as follows:

1) The center of V-belt sheave grooves is not
concentric with center of rotation. This can be caused by a bent shaft, a bad
bushing, or an improperly machined sheave. The end result is a tugging action
between the two sheaves.
2) Misaligned sheaves will cause the belts to try to
climb the sides of the sheaves, then slip back in the groove periodically,
resulting in vibration.
3) Shafts that are not straight can cause an
unbalance and also may force the bearings to rock or twist in an oscillatory
manner.
4) Bearing flaws are a common source of vibration. Flaws on inner
race Discount
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vibration at predictable higher frequencies.
5) Setscrew mounted bearings can
trap some misalignment between the bearing bore and the shaft. Sometimes this
can be corrected by loosening one bearing, running the fan for a few seconds,
then retorquing the setscrews. Repeat on the other bearing if required.
Misalignment can also be trapped by having more than two bearings on a single
shaft, and in this case it may be difficult to correct. The bearings can be
aligned with the fan stationary, but the misalignment will reappear in
operation.
6) Imperfect fan wheel shapes can cause non-uniform pressure
generation and result in vibration.
7) Turbulent or eccentric airflow on the
fan inlet (and sometimes the discharge) can excite vibrations.
Operating
the fan at flows lower than where the peak pressure occurs can cause
instabilities that result in vibration.
9) Rotating components that make
contact with stationary parts (rubbing) can result in major noise and
vibration.
10) Coupling misalignment tends to generate forces that commonly
result in vibration in the axial direction.
11) The fan wheel impacting with
solids or liquid in material conveying systems can shock load the fan into very
high vibration.
12) Wind blowing on a roof mounted fan can excite large
amplitude vibrations due to vortex shedding.

All fans are exposed to a
variety of vibrational forces. Fortunately, most of the forces mentioned above
are relatively small and cause no problems.
However Discount
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and lower, more factors come into play. Each of these must be investigated
before they can be excluded. A common characteristic of fans is that they tend
to be large, bulky, and relatively light and flexible for their size compared to
other rotating machinery. The impact of this is that small forces can result in
large motions. Acceptance criteria are based on the magnitude of the motion, not
on the force that creates the motion. Therefore, vibration energy must
constitute a very small percentage of the total energy consumed by the
fan.

The cumulative effect of many small sources of vibration is the
creation of a background (lower limiting) vibration level. Once this background
level is achieved, finer balancing is futile. In order to consistently achieve
lower vibration levels than that typical to the fan many things may have to be
done. The precision level of all fan components must be improved. The fan
rigidity may need to be increased. The fan mounting arrangement must be very
solid.
Finally Discount
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minimized.

If a fan is already built, and the specified levels cannot be
achieved by balance, the fan vibration detective must go to work. All parts of
the fan must be examined for precision, and any possible contributor to
vibration energy must be considered and corrected if required. This is an
expensive, time-consuming effort that can have severe negative consequences to a
manufacturing shop’s production effort because of its
unpredictability.

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