Fan vibration

Noise may be regarded as the transmission of pressure waves through a fluid, usually air (and less usually through some other gas) on its way to the human (or some other animal) ear. Vibration may be seen as a similar phenomenon, but transmitted through a solid to some other part of the recipient’s anatomy.

This is a fast moving subject in which the electronics industry has become much involved. There have been numerous amalgamations of the companies concerned, whilst new ones have started up. There is however, one certainty for the author-all his descriptive material will be long out of date by the time this book is published! Modern instruments are remarkable in their versatility and ability to capture data for analysis and diagnosis. They are very much in the “black box” category, but the earlier instruments did have the capacity for displaying everything — so you thought you understood what was going on!

When describing the performance of a fan, the customer is ac­customed to specifying the volumetric flowrate, the fan pres­sure and even the noise. These are met with the supplier’s re­sponse of a fan size and model, a fan speed and motor requirements.

Just as fan noise has been added to the specification over the past 20 years, so vibration is now recognised as an important parameter. It gives an indication of how well the fan has been designed and manufactured and can also provide advanced warning of possible operational problems. The measured re­sults may be useful in determining the adequacy or otherwise of concrete foundations, or the necessary stiffness of supporting structures.

It will be realised that this chapter follows on logically from Chapter 14. Noise may be regarded as the transmission of pressure waves through a fluid, usually air (and less usually through some other gas) on its way to the human (or some other animal) ear. It can however be transmitted through a liquid, such as water, and this is used in submarine detection and for communication between whales and other sea mammals. In this progression, Vibration may be seen as a similar phenome­non, but transmitted through a solid.

Vibration measurements may be required for a number of rea­sons of which the following are but examples:

• design/development evaluations

• in-situ testing

• as baseline information for condition monitoring pro­grammes

• to inform the designers of foundations, supporting struc­tures, ducting systems etc., of the residual vibration which will be transmitted into their part of the system

• as a quality assessment at the final inspection stage.

Identification

Perhaps the most important cause of vibration is unbalance. Reference is made to the relevant Standards and recommen­dations made as to an acceptable grade. Fan unbalance mani­fests itself as a periodic vibration characterised by a sine wave. The so-called simple harmonic motion.

With the necessary instruments three properties can be directly measured:

• displacement,

• velocity,

• acceleration.

The importance of each is discussed and the relationship between them shown. The keys to the identification of the cause of a vibration are in its frequency and velocity — NOT nec­essarily its amplitude except below about 10Hz. It is therefore of value to obtain a vibration signature and the analysis of this will lead to possible sources of trouble being identified. Unbalance, misalignment, eccentricity, looseness, aerodynamic forces, bearing and electric motor problems are all discussed and the troublesome frequencies identified. Particular attention is de­voted to bearing defects and the concepts of shock pulse, spike energy and Kurtosis factor are introduced and the meters for their measurement described.

History

From the very early years of fan manufacture the problems of vibration and its reduction or isolation have given engineers many happy (?) hours of listening and analysing. The absence of vibration came to be seen as a sign of a fan’s health. Per­haps this was why the old-timers used a stethoscope to hear the odd rumblings coming from the bearings!

Over the last decade or so a completely new science has emerged for accurately measuring and identifying the causes of vibration in our modern highly stressed, high speed fans. Us­ing transducers to convert the vibrations into electric signals, these could be amplified, integrated, filtered and metered.

Sources of vibration

It is virtually impossible to avoid all vibration as this arises from the dynamic effects of out-of-balance, misalignment, clear­ances, rubbing or rolling contacts, the additive effects of toler­ances etc. Sometimes the vibrations from these sources may be small, but excite the resonant frequencies of the stationary parts such as casings or bearing pedestals. Where the fan is di­rectly driven by an electric motor, electromagnetic disturbances will also exist, these producing further vibrations.

Definitions of vibration

Vibration may be defined as the periodic motion in alternately opposite directions about a reference equilibrium position. The number of complete motion cycles which take place during unit time is called the frequency. This frequency may also be mea­sured in cycles/minute which is useful for a direct comparison with the fan revs/minute. In recent years, however, the SI unit has come into prominence and frequency is usually now given in Hertz (Hz) equivalent to cycles/second.

The motion could consist of a single frequency as with a tuning fork. With a fan however there are likely to be several motions taking place simultaneously at different frequencies. These various motions can be identified by frequency analysis — or the plotting of a graph showing vibration level against frequency.

Measuring parameters

There are three properties of a vibrating element which can be measured. Each is of value and may be recorded according to the application:

A) Displacement, or the size of the movement is of impor­tance where running clearances have to be maintained for efficient performance or where contact between stationary and rotating surfaces could take place. Most weight is given to low frequency components.

B) Velocity, which is directly proportional to a given energy level and therefore where low and high frequencies are equally weighted. The disturbing effects on people and other equipment are by experience related to velocity.

C) Acceleration, which is a measure of the forces and stresses set up within the fan and motor, or between these and the foundations. Weighted towards the higher fre­quencies and therefore should be used where such com­ponents exist.

Posted in Fans Ventilation A Practical Guide


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