Vibration in a centrifugal fan can cause a host of problems that, if left unchecked, can lead to issues ranging from periodic downtime to catastrophic failure. Here are some of the more common causes of centrifugal fan vibration:
Rotor imbalance is caused by a number of conditions. Imbalance can occur when particulates cause rotor wear or buildup on the fan wheel, and high air stream temperatures can cause uneven growth of a rotor. Periodic cleaning and field balancing will minimize vibration due to these conditions. Also, manufacturing issues can occasionally cause an uneven distribution of mass in the fan rotor.
Loose mechanical connections between bearing caps, bearing pedestals or foundations can lead to excessive vibrations levels or exacerbate an existing imbalance problem. Looseness between the rotor shaft and fan shaft can also cause vibration.
Excessive vibration from a cracked shaft or rotor can lead to catastrophic failure. Cracks can be detected through visual inspection or vibration trending analysis.
Misalignment between a drive motor shaft and a fan shaft is a common source of vibration. This can be caused by a bent shaft, improperly seated or worn bearings, or faulty installation of new equipment. Improper lubrication (using the wrong type, or lack thereof) can overheat bearings and cause warping in the shaft.
Resonance is the tendency for a vibration to occur at a particular frequency. Two types of resonance that can affect a centrifugal fan are critical speed and structural resonance. Most fans are designed to operate below critical speed, but if a fan is operated above critical speed, the vibrations that occur while passing through the resonance band can lead to severe damage to bearings, seals, etc. Similarly, operating at the structural resonance point should also be avoided. Structural resonance is somewhat difficult to predict, as it varies from unit to unit, but can be easily identified through vibration analysis.
Other conditions that can cause centrifugal fan vibration include a skewed fan wheel, dynamic forces produced by motors, gearboxes and belts, and loose rotors due to thermal expansion differential between the fan wheel hub and the shaft.
MSC is an expert in fans and motors. Please call us if you have questions regarding centrifugal fan maintenance, vibration, or vibration analysis.