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Structural vibration: What is it and how to control it

What are issues caused by high levels of vibration, how to control it, the different types of vibration, what is involved in a structural vibration analysis? Image: MIDFIX

Here, the design engineering team at MIDFIX discusses the issues caused by vibration, the various types, and what can be done to control structural vibration in the first instance

Mechanical contractors will know that if rotating and reciprocating machinery are used, these types of machinery tend to vibrate.

In this type of environment, companies are susceptible to both brackets and support failure causing reputational damage and leading to high rectification costs.

What issues do high levels of vibration cause?

Structural vibration occurs when forces generated by machines such as compressors, pumps, chillers and air handling equipment, causes the structural steels to vibrate.



Consequently, this can prompt equipment failure, noise transfer and most importantly, safety concerns.

The vibration is due to the structure being mechanically resonant. The term ‘resonance’ is the sound produced by an object when it vibrates at the same rate as the sound waves from another object. It occurs when dynamic forces coincide with the natural frequencies of the supporting structure.

At resonance, these forces are amplified and cause structural elements to vibrate above safe operating limits.

Structural resonance can also occur with smaller machinery, such as reciprocating pumps or compressors.

Once a facility is built, it is very costly to modify any structural substructures to fix a vibration problem. The good news is that structural vibration can be alleviated if it is addressed early at the design stage.

How can we control high levels of vibration?

In the design phase, a structural vibration analysis of specific areas of the deck or platform should be undertaken.

The objective here is to understand the effect on the structure, consequently, engineers can then look to control or modify the vibration or isolate it from the structure to minimise structural response.

What are the different types of vibration?

It is important to understand the different types of vibration so that we can recommend the best-placed solution. Vibration can be divided into the following categories:

Noise vibration causes an issue within a factory or industrial environment, it is important to control noise with steps taken to reduce the level of noise.

Options could be enclosing the machine to reduce noise, investigate the use of the machine and ensure effective management to avoid overuse. You could also offer ear defenders for employees and limit the time spent in noisy areas.

Free vibration is the natural response of a structure to some impact or displacement. The response can often be determined by the properties of the structure, and its vibration can be understood by examining the structure’s mechanical properties.

Similar to plucking a guitar string to see the effect. When you strike the string, it vibrates at the tuned frequency and gives you the desired tone.

Forced vibration is when one object forces to another adjoining or interconnected object into vibrational motion. In forced vibration, the vibrations occur if a system is continuously driven by an external force. There is a relationship between the amplitude of the forcing function and the corresponding vibration level. The relationship is dictated by the properties of the structure.

Sinusoidal vibration is quite uncommon. In this scenario, the structure is affected by forcing with a pure acceleration and one frequency. This is great as an engineering tool, as it enables us to understand complex vibrations by breaking them down into simple, one-tone vibrations.

Random vibration is very common. One nice example would be the vibration you feel when driving a car, which is usually a result from a combination of the road surface, engine vibration, wind buffeting or resistance on the car’s exterior, etc. They are often described by using statistical parameters, and another consideration is that future behaviour cannot always be precisely predicted.

Rotating imbalance is another common source of vibration. The rotation of an unbalanced machine part can cause the entire structure to vibrate. The imbalance generates the forcing function that affects the structure. Rotation vibration is often unwanted, and the goal is to eliminate or minimise it by properly balancing the machine. For example, washing machine, steam or gas turbines.

The most common factor in all these different types of vibration is that the structure responds with some repetitive motion that affects its mechanical properties.
Through understanding some basic structural models, measures and analysis techniques, it is possible to successfully characterise and treat vibration in structures.

Structural vibration testing and analysis is so important and will contribute to the progress in many industries, including aerospace, automobile manufacturing, tool manufacturing, electronics, transportation and more!

What is involved in a structural vibration analysis?

This complex process entails:

* Dedicated Finite Element Analysis software to analyse complex dynamic load conditions
* Identify the natural frequencies and mode shapes
* Performing the vibration analysis to determine the structural vibration responses and confirm if the structure will be satisfactory to meet the requirements
* Double check the analysis results including acceleration, velocities, displacement and stresses to ensure that fatigue failure is not an issue

What makes analysing these types of vibration even more challenging is the fact that the machinery, its equipment and whatever it is mounted on cannot be one size fits all approach.

They will interact with the foundation, or the platform and the only way to know the magnitude of this interaction is to conduct a structural dynamic analysis that includes the foundation. This is a serious consideration which, if overlooked, can considerably reduce the reliability of the machine and might even cause safety concerns.

Recommendations

Be proactive, rather than reactive, as the chances are, if you are supporting or providing bracketry for any reciprocating or rotating machinery there will be consequential high levels of vibration.

The analysis is cost-effective and the recommendations from this process will lead to an increase in equipment reliability, bracket failure risk management and ultimately cost savings in the long term.

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