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As a result of the development of the vibration measuring
instrumentation
market Quincy
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, computer software and maintenance training 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 Malik
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, 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
addition,
may cause metal fatigue in the bearings. Excessive vibration can
cause fasteners to loosen or can cause fatigue failure of structurally loaded
components.
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, 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
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, 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, outer race, balls
or rollers tend to generate 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 Marlon
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, 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.
8) 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 Tarell
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, most of the forces mentioned above are relatively
small and cause no problems. However, as specified vibration levels are pushed
lower 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 Quincy
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, 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 Malik
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, air turbulence through the fan must be
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 Camo
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, time-consuming effort that can have severe
negative consequences to a manufacturing shop’s production effort because of its
unpredictability.

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Oleg Cthetchel Match
Time锛?72017 02:45 Wednes.
07-12-2018 02:26 PM
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