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The Fan doctor - Vibrations

Out of balance fan wheel or rotor
1. Check the wheel for any dirt or foreign material, especially hard-to-see places like the backside of the wheel and the underside of the blades.  

2. Airfoil blades are usually hollow. When exposed to rain or excessive moisture, they can get water inside of them. Drilling one 3/16" drain hole in the upper surface of each blade near the trailing edge should cure the problem. Rebalancing is not usually necessary. 

3. Inspect the wheel for corrosion or erosion. Usually wheel erosion will occur at the leading edge of the blade. On a paddle wheel type fan the outer blade tip may also be worn. An airfoil wheel exposed to sand or abrasive dust can actually develop pin holes in the leading edge of the blades. Do all possible to eliminate these damaging conditions and then rebalance the wheel. If the wheel is seriously damaged, it will have to be replaced..
Improper or loose mounting
1. Foundation bolts and bearing mounting bolts can loosen themselves. Make sure they are tight.
Loose set screws that hold the wheel to the shaft
1. Again, tighten the screws, but first be certain the wheel hasn't shifted on the shaft or is rubbing on the inlet cone or drive side of the housing.
Bent fan shafts
1. First, check the shaft with a dial indicator. If bent, it should be replaced as soon as possible to avoid replacing the entire fan
Misaligned V-belt drive, another very common problem on belt driven fans
1. Realign assembly so fan and motor shaft are parallel and faces of sheaves (pulleys) are flush to a straight edge. We've found that a taut string will work fine.
Fan wheel turbulence due to the rotor running backwards
1. Since blade angles and shapes vary greatly, it is easy to misread rotor direction. Check for correct wheel rotation, clockwise or counter clockwise, as seen from the drive side. We illustrate the correct rotation for various wheel types elsewhere on this site.
Air pulsation
1. Fan may be operating in the stall area of its performance curve. That means it is oversized for your particular system or the system resistance is higher than intended. You can lower the system resistance by cleaning the filters or opening the dampers.

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