Common Sources Of Vibration In Centrifugal Fans

One of the significant risks to the performance and life-cycle of a fan system is vibration of the equipment. The vibration level of a centrifugal fan will determine its mechanical condition. If the vibration is too intense then it can become serious threat to fan operation causing forced shutdown. There could be many reasons for a centrifugal fan to have vibration issues, from a simple mechanical defect such as mass unbalance to more complex shaft misalignment, bearing failures or resonance problems. A fan system requires regular checking to determine the right acceptance limits of vibration. If the vibration is above acceptance level, then the fan system needs an analysis to look into the problem for rectification.

Rotating devices such as fan systems suffer unequal distribution of mass due to their mechanical design, installation and operating conditions. Centrifugal fans can have some minor imperfections that can cause vibration to the equipment. Even small changes such as debris buildup, rotating part distortion or blockage can induce imbalance in the system leading to problems in fan components. Vibration is always present as a result of one or more exciting forces. If there are no exciting forces, there will be no vibration. The cause of vibration in a rotating equipment may be due to : 1) Unbalance 2) Looseness of parts 3) Misalignment 4) Bend shaft 5) Bend bearing, gear or belts 6) Weak support.

Mechanically loose connections
Centrifugal fans can have loose mechanical connections between bearing housing & pedestals or between the fan rotor & shaft. When centrifugal fans have higher rotation speeds, they exert force on the bearings due to unequal distribution of weight. In other cases, the fan may experience loosening of the setscrews due to fretting or corrosion. This will make the hub and the impeller to become displaced relative to the axis of rotation, resulting in extreme fan imbalance. To avoid this condition and ensure proper fan functioning, an interference fit is typically used between hub and shaft connections.

Cracks in shaft or impeller
A shaft or impeller crack is very common in centrifugal fans. The typical reasons for cracking of mechanical parts is the age and fatigue. When crack occurs in shaft or rotor, it can propagate and can cause imbalance in the components causing threat to fan operation. Fan systems that are suspected of having a crack in shaft must be treated in a timely manner before they propagate and become intense.

Rotor mass imbalance
Imbalance in rotor occurs when the center of rotor mass does not coincide with the center of rotation. The uneven distribution of rotor mass or the deterioration of fan rotor due to extreme operating conditions can cause misalignment resulting in fan imbalance and vibration. One way to reduce the level of vibration is by adding a weight to outer diameter of the rotor.

Motor shaft misalignment
Misalignment between a drive motor shaft and a fan shaft typically occurs during new fan installation or when shaft/rotor assembly is replaced with a new equipment. When shafts are not straight, it can cause an unbalance and may force the bearings to twist in an oscillatory manner. This can lead to bearing defects, shaft breaking or cracking, coupling failures or excessive vibrations. Shaft misalignment can deteriorate the operating life span of a fan system. Hence it is important to have regular checks on alignment before startup of the fan and subsequently few months after installation.

Critical speed
There exists for any rotating shaft, rotor and bearing combination, a certain speed at which resonant vibration develops. The slightest unbalance will cause the shaft to bend, increasing the unbalance and bending without limit except for damping effects and loss of exact resonance. This is a dangerous condition and causes intense vibration so that the critical speed must never be held for any length of time.

Defective bearings
Bearings are the critical components in the fan operation. The bearings that are used in fan system should be selected properly. Any flaw or defect in bearing can result in system imbalance affecting the fan performance. There are many reasons that can reduce the life span of a bearing. If the bearing mounting surfaces are not flat, then it can cause a twist or distort in bearings. Similarly, if bearings are forced to pass through non-symmetric surface then it can cause premature failure resulting in system vibration.

These are few common reasons that can cause vibration in a centrifugal fan system. Sometimes, small sources of vibrations from individual components can have cumulative effect resulting in larger movements. It is important to investigate into each of these isolated issues before they become intense problems resulting in system failure. The different types of vibration can be differentiated by their frequency or signature analysis.