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The fan manufacturers are ususally “vibration experts” and can 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 addition,
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, 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, 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, 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, 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, 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, time-consuming effort that can have severe negative consequences to a manufacturing shop’s production effort because of its unpredictability.
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