Fan Resonance and Balancing

Mechanical components, such as transmission gear and bearings, generate a mechanical loss that reduces the power transferred from the motor shaft to a pump or fan impeller. Industrial or dynamic balancing of these system components can assist with the efficient running of any rotating equipment, and thus improve mechanical efficiency.

There is also another issue that can sometimes lead to problems in industrial fan operation, broadly termed “resonance”. Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others.

In Fan Impeller design consideration needs to be made in relation to the ability of the blade to withstand vibration caused by factors such as machine vibration, turbulence and the like. Here are a couple of examples.

blade Failure due to resonance
Blade failure due to resonance

This first impeller shown has unsupported blades and as such a simple “dong test”, a smart tap on the blade, reveals that it is acting like a tuning fork, likely a cause or contributing cause to the blade failure shown.

fan impeller cracking
Fan impeller cracking

Analysis of a second impellor revealed a fractured edge highlighting a typical fatigue crack. Torsional vibration is rare but can also occur. In this case the problem was shown to be the waveform from the Variable Speed Drive (VSD). It also had a devastating effect on the impeller, as can be seen below. Significant cracking can be seen along the weld of the blade and around the rod (heal ring). This proved to be a particularly difficult case, as the stiffer the impeller the quicker it cracked!

In a practical sense fans can be designed to minimize or eliminate the tuning fork resonance effect, but this torsional vibration example highlights that you can only design for what is known. In this case it took many months, and several broken impellers, to finally track down the source of the problem (the VSD waveform).

Precision Balancing Pty Ltd often gets requested to conduct industrial investigations of this nature. Precision Balancing has been offering in house and on site dynamic balancing and vibration analysis services since 1989 and have the equipment, skills and experience to satisfy your industrial balancing requirements. Precision Balancing strictly adhere to the requirements of the relevant ISO standard, and all jobs are tracked through our system and are issued with a certificate of compliance.

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