The Fan Doctor is In
This checklist outlines major symptoms of fan problems as well as the more common causes and suggested remedies. Because this is only a guide and not a technical service manual, more complex causes and remedies are not included. Some ailments may have more than one cause.
VibrationIf your fan is experiencing excessive vibration, there are several possible causes.
Out-of-Balance Fan Wheel or Rotor.
Check the wheel for any dirt or foreign material, especially in hard-to-see places like the back side of the wheel and the underside of the blades.
Airfoil blades usually are hollow. When exposed to rain or excessive moisture, water can get inside. Drilling a 3I16" drain hole in the upper surface of each blade near the trailing edge should cure the problem. Rebalancing usually is not necessary.
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 also may be worn. An airfoil wheel ex-posed to sand or abrasive dust actually can develop pin holes in the leading edge of the blades. Do whatever is possible to eliminate these damaging conditions, then rebalance the wheel. If the wheel is ser-iously damaged, it will have to be re-placed.
Improper or Loose Mounting.
Foun-dation bolts and bearing mounting bolts can loosen themselves. Make sure they are tight.
Loose Set Screws that Hold the Wheel to the Shaft.
Again, tighten the screws, but first be certain the wheel has not shifted on the shaft or is rubbing on the inlet cone or drive side of the housing.
Bent Fan Shaft.
Check the shaft with a dial indicator. If it is bent, it should be replaced to avoid replacing the entire fan. A rule of thumb is not exceed 0.001 per inch in diameter of shaft.
Misaligned V-Belt Drive.
Another common problem on belt-driven fans is a misaligned drive. To solve it, realign the assembly so the fan and motor shaft are parallel and the faces of the sheaves (pulleys) are flush to a straight edge. Often, a taut string will work just fine for this.
Fan Blade Turbulence due to the Rotor Running Backwards.
Because blade angles and shapes vary greatly, it is easy to misread rotor direction. Check for correct wheel rotation, clockwise or counterclockwise, as seen from the drive side.
The 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 system resistance by cleaning the filters or opening the dampers.
NoiseAnother common complaint about fans is excessive noise.
Foreign Material in the Fan Housing.
This could be anything from a loosened bolt to somebody's lunch bag. Inspect the wheel and fan housing and clean thoroughly.
Squealing is a sign that the belts are loose or misaligned. If belts show wear, you are better off to replace them.
Worn Ball or Roller Bearings.
If you hear howling, screeching or clicking, the problem may be worn ball or roller bearings. Change the bearings immediately before they cause additional damage. Failing bearings tend to wear the shaft, so you will want to be absolutely certain the shaft is full size before installing new bearings. Using a micrometer, measure the shaft both under the bearing and next to it, and compare the two readings. If they do not match, replace the shaft. New bearings installed on a worn shaft will not last long.
Bearing Seal Misaligned.
If you hear a high pitch squeal, the bearing seal may be misaligned. Realign the face of the bearing so it is perpendicular to the shaft.
If the fan housing has a metal shaft seal, it could be misaligned and rubbing on the shaft. Loosen the seal plate bolts, recenter the seal on the fan shaft and tighten the bolts. If the seal is fiberglass, cork or rubber, be sure the metal backing plate does not touch the shaft.
Running a rotor in the wrong rotation will create a loud noise and excessive vibration.
Overheated BearingsBall or roller bearings tend to heat up when they have been overgreased and will cool down to their normal running level when the excess grease oozes out. You can remove excess grease from split roller bearings by lifting the top half of the pillow block. The normal operating temperature of a bearing may be well above 140°F (60°C), but anything above 180°F (82°C) should be questioned. If you place a drop of water on the bearing and it sizzles, the bearing is in distress and should be changed before it seizes and ruins the shaft.
Use a multipurpose lithium-soap-based, channeling-type grease. Do not use high temperature or general purpose grease.
Slippage and Friction.
Loose V-belts may cause belt slippage and friction heating, resulting in hot bearings, shafts or sheaves. To solve this problem, tighten the belt to proper tension. You should be able to de-press the belt the same distance as its width.
Poor Air PerformanceIf your problem is poor air performance, here are some possible causes.
Fan Rotation Incorrect.
It is easy to misread fan blade orientation. One way to change rotation on most three-phase motors is to reverse any two motor leads.
Poor Duct Design.
Abrupt turns in the duct close to the fan discharge can cause pressure loss, or ductwork elbows at the inlet can cause air pre-spin. Install turning vanes or elbow splitters in the duct. If air performance still is inadequate, the discharge position may have to be changed.
Improperly Installed Control.
If fan has an inlet volume control (IVC), check to be sure it is installed with pre-spin of the air in direction of wheel rotation when the IVC is partially closed.
This can occur on double-width, double-inlet fans. Center the fan between the inlet cones to avoid overloading one side of the wheel while starving the other. On single-inlet fans, too much gap and overlap of the wheel to inlet cone will cause lost performance.
Fan Horsepower Unexpectedly Low.
Correct one or more of the following: air pre-spin into the fan inlet; fan drive sheaves set for too low a fan speed; or resistance to airflow, such as that caused by a closed damper, is much higher than calculated.
Fan Horsepower Unexpectedly High.
There are three possible causes: The fan may be operating without ductwork at low resistance, so too much air is flowing; the fan may be handling ambient air when it was intended for hot, less dense air; the fan may be running backwards.