Gentelmen I would like to start off by wishing every one a Happy New Year. I have a problem with a vertically mounted Strobic Air Fan with the squirell cage directly mounted to the shaft of the motor. A little history of the fan. This is a new building commisioned in April 2008. I did not take the baseline readings for the commisioning. When i was asked to start taking vibration readings I found that the strobic fan in question had two bolts very loose (nuts hat fallen off)the other two bolts, one was loose but the bottom bolt was still very tight. Basically being held by just one bolt. All the shims had fallen off the feet of the motor on the loose bolts. The manufactorer or the people who was contracted to install the fan were recalled to place the shims back. They have found that due to the location of the fan and the very tight opening it is very difficult to replace all the shims back to the bolts. The vibration readings that I am seeing is a High one times turning speed in the vertical direction on both bearings, higher on the top bearing. Nothing in the horizontal plane but higher than normal axial reading. Normally this data would tell me that we were out of alignment. But due to the fans configuration I'm not sure what is happening. Also the time waveform is sinusodial. If someone could help me I would appreciate it
STROBIC_AIR_FAN_DATA.doc (33 Kb, 34 downloads)
If the motor isn't shimmed properly you could have a frame distortion issue. But since this is your first reading it might also be some unbalance. I would check and correct for soft foot and if that doesn't bring the 1X down, try trim balancing. Since the fan is overhung, the axial vibration could be a product of some couple unbalance, the difference between H & V could be stiffness differences.
I checked Strobic's Web site to see what these fans are all about. Its interesting that the installation manual calls for "paper shims" to correct any motor base soft foot. See link:
I would check into these possible problems:
1) Proper motor base shims (should be stainless steel)
2) Motor-fan shaft alignment (align to precision tolerances)
3) Fan rotor balance (include abnornal blade pitch issues)
4) Structural natural frequency near shaft speed (conduct impact test)
5) Improperly installed vibration isolator mounts (see manual and inspect)
If this is a new installation and it cannot be restored to proper low vibration, then it should be exchanged for a new unit.
Happy New Year,
Can you verify speed? The plots say 29.66 Hz, but it seems to actually be 14.83???
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Bill has a good observation. Either the fan is operating at 1/2 speed or the vibration frequency is at 0.5xSS. If the later case is correct, then looseness or a rub, possibly combined with a resonant structure, could be the problem. If the speed were actually 14.83 Hz then unbalance and possibly resonant structure could be at fault.
I apologize I never changed the speed in the set up. The running speed is 14.83HZ.
Walt: shouldn't the horizontal reading be a lot higher if unbalance is the problem? But yet the horizontal reading is very low (Vertical reading.3757 IPS versus .0417 IPS horizontal). Do to the vertical installation of this fan, there is more flexebility horizontaly then there is vertically.
Is your "vertical" direction actually a shaft axial direction with a vertical shaft axis? If yes, then you would normally expect higher vibrations in the horizontal direction, however according to the info on the Web site the unit is on vibration isolators. So the driving force can be from shaft misalignment or fan unbalance with amplification from a resonant structure. I would test for resonant structure and measure/correct shaft alignment and rotor balance. I have seen plenty of situations with an overhung fan rotor where unbalance caused high axial vibrations.
Gentelmen thank you for your input. I will try to keep everyone up to date on the results. Thanks again
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