Rolls Royce Inchinnan

Mechanical contractors will be aware that if reciprocating and rotating machinery are used, these types of machinery have a tendency to vibrate. In this type of environment, companies are susceptible to both brackets and supports failure causing reputational damage and leading to high rectification costs. Here at MIDFIX, our design engineering team discusses the issues caused by vibration, the various types and what can be done to control structural vibration in the first instance.

What issues do high levels of vibration cause?
Structural vibration occurs when dynamic forces generated by compressors, pumps, chillers and air handling equipment causes the slab and structural steels to vibrate. Consequently, this prompts equipment failure, noise transfer and safety concerns. The vibration is due to the structure being mechanically resonant. The term “resonance” occurs when dynamic forces coincide with the natural frequencies of the supporting structure. At resonance, the forces are amplified and cause structural elements to vibrate above safe operating limits. Structural resonance also occur with smaller reciprocating pumps or compressors.

Once a facility is built, it is very costly to modify structural substructures to fix a vibration problem. The good news is that structural vibration can be mitigated 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 can be undertaken. The objective here is to understand the effect on the structure, consequently engineers can 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 optimum 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, offering ear defenders for employees and limiting the time spent in noisy areas.

Free vibration is the natural response of a structure to some impact or displacement. The response is completely determined by the properties of the structure and its vibration can be understood by examining the structure’s mechanical properties. E.g. plucked guitar string.

Forced vibration is the response of a structure to a repetitive forcing function that causes the structure to vibrate at the frequency of the excitation. In forced vibration, 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. E.g. child’s swing pushed on each downswing.

Sinusoidal vibration is uncommon. In this scenario, the structure is excited by a forcing function that is a pure tone with a single frequency. It provides an excellent engineering tool that enables us to understand complex vibrations by breaking them down into simple, one-tone vibrations.

Random vibration is very common. For example, the vibration you feel when driving a car is a result from a combination of the rough road surface, engine vibration, wind buffeting the car’s exterior, etc. They are often described by using statistical parameters.

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 usually unwanted and the goal is to eliminate or minimise it by properly balancing the machine. E.g. washing machine, steam or gas turbines.

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

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;
• Perform 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.

Our recommendations
Our advice is be proactive rather than reactive, 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. For advice, contact us today.

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