In vibration control engineering, two concepts underpin the performance of every anti-vibration mount: natural frequency and resonance. Understanding these two principles helps determine whether a mounting system successfully isolates vibration or unintentionally amplifies it.
Anti-vibration mounts are designed to reduce the transmission of vibration between machinery and its supporting structure. However, if the mount stiffness, load capacity or damping characteristics are incorrectly specified, the system may operate close to its natural frequency. When this occurs, the system enters resonance, a condition where vibration amplitude increases significantly rather than decreases.
For this reason, understanding natural frequency and resonance is essential when selecting, designing and testing anti-vibration mounting systems. Correctly engineered mounts can reduce vibration transmission by more than 70–95%.
Within this article, we will discuss both natural frequency and resonance in further detail and also how our specialist engineering solutions ensure your equipment’s vibration is kept to a minimum.
What is Natural Frequency?
Natural frequency is the frequency at which a mechanical system wants to vibrate on its own without any interruption or external force.
In vibration isolation systems, the supported equipment and its mounts behave as a mass-spring system. When displaced, the system oscillates at a frequency determined by the relationship between the mass of the equipment and the stiffness of the mounts - the natural frequency.
The natural frequency of a mounting system can be approximated using the equation:
fₙ = natural frequency
k = stiffness of the mount
m = supported mass
Using this equation we can understand that:
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Increasing the mount stiffness increases the natural frequency
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Increasing equipment mass reduces natural frequency
Most machinery operates at specific excitation frequencies. Therefore it is best advised to select anti-vibration mounts whose natural frequency is well below the operating frequency of the machine.
What is Excitation Frequency?
Excitation frequency refers to the rate at which an external force is applied to the system. It is independent of the system’s properties. In practical terms, it is the frequency produced by rotating or reciprocating components, such as engines, compressors, pumps or electric motors. However, it could also be environmental forces such as wind, road and track irregularities, earthquakes etc.
At AV Industrial Products Ltd, we identify excitation frequencies through vibration testing and analysis techniques such as FFT (Fast Fourier Transform) testing, which converts vibration signals into their frequency components for detailed analysis.
What is Resonance?
Resonance occurs when the natural frequency equals the excitation frequency. When this happens, energy is added to the system in sync with its natural oscillation, causing energy to accumulate rather than dissipate.
Resonance can be particularly dangerous and damaging, often leading to:
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Excessive vibration amplitude
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Structural fatigue
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Accelerated wear of mechanical components
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Loosening of fasteners
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Increased noise levels
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Reduced operator comfort
The Importance of Frequency Ratio
The effectiveness of an anti-vibration mount is determined by the frequency ratio, this is defined as:
By working out the frequency ratio, you can determine
Amplification Region
When the frequency ratio approaches 1, the excitation frequency is close to the natural frequency. This is the resonance zone where vibration amplification occurs.
Transition Region
As the excitation frequency increases beyond the natural frequency, transmissibility begins to decrease and vibration reduction starts to occur.
Isolation Region
When the frequency ratio exceeds approximately 1.4 (√2), the system enters the isolation region and transmitted vibration begins to reduce significantly.
Static Deflection and Its Effect on Natural Frequency
Static deflection is one of the most important parameters in anti-vibration mount selection. It refers to the amount a mount compresses under the weight of your machinery. For instance if your mount is compressed from 50mm to 45mm when bearing weight, the static deflection is 5mm.
A higher static deflection generally results in lower natural frequency, improving isolation performance.
As static deflection increases, natural frequency decreases. This allows the mounting system to achieve a higher frequency ratio and therefore better vibration isolation.
However, excessive deflection may compromise the stability of your system, therefore it’s important to balance:
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Vibration isolation performance
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Load carrying capacity
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Movement of the equipment
The Role of Damping in Anti-Vibration Mounts
While stiffness determines natural frequency, damping controls the severity of resonance.
Damping refers to the system’s ability to dissipate vibrational energy. In anti-vibration mounts this is typically achieved through:
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internal friction within rubber materials
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fluid movement in hydraulic mounts
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mechanical friction in spring-based systems
Rubber mounts provide damping through a property known as hysteresis, where energy is lost as heat during deformation.
Higher damping reduces resonance amplification. However, excessive damping can reduce isolation efficiency at higher frequencies, so the damping characteristics of the mount must be carefully selected.
Isolation Efficiency of Anti-Vibration Mounts
The goal of any anti-vibration mounting system is to minimise vibration transmission from machinery to the surrounding structure. Isolation efficiency increases as the excitation frequency becomes significantly greater than the natural frequency.
Once the system operates well above resonance, vibration transmission drops rapidly and effective isolation is achieved.
In most industrial applications, engineers aim for a frequency ratio between 3 and 5, which typically provides isolation efficiencies between 70% and 95%, depending on mount damping characteristics.
How Real-World Applications Affects Frequencies
Natural frequency considerations vary depending on the type of equipment being isolated. Across various industries, the change in excitation frequencies creates more considerations during the selection process.
Rail & Mass Transit Systems
Rail vehicles generate significant low-frequency vibration due to wheel to rail interaction and bogie dynamics. Isolation systems must therefore be designed with sufficiently low natural frequencies to avoid resonance within carriage structures.
Industrial Machinery
Industrial machines such as compressors, pumps and generators often operate at fixed rotational speeds. Anti-vibration mounts must therefore be selected so that the natural frequency remains well below the machine’s operating frequency.
Heavy Equipment
Construction machinery and off-highway vehicles experience variable excitation frequencies caused by terrain and operational loads. Hydraulic or multi-axis mounts are often required to manage these dynamic conditions.
Why Incorrect Mount Selection Causes Problems
Incorrectly selected mounts can lead to significant vibration issues. If mounts are too stiff, natural frequency increases and resonance becomes more likely. If mounts are too soft, excessive movement may occur, potentially compromising system stability.
Proper mount selection requires consideration of several key factors:
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equipment weight and load distribution
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operating speed and excitation frequency
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environmental conditions
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shock loads and transient forces
Engineering analysis is often required to determine the correct balance between stiffness, damping and load capacity.
How AV Industrial Products Can Help
At AV Industrial Products Ltd, we combine engineering expertise with advanced vibration analysis to ensure your anti-vibration mounts perform as intended.
As well as designing, manufacturing and supplying anti-vibration mounts, our team also specialise in vibration testing, computer modelling, FFT Analysis, system simulation and more.
By understanding the relationship between natural frequency, resonance and vibration isolation, we help our customers achieve reliable equipment performance and extended service life.
For more information on selecting the correct anti-vibration mounts for your application, speak with our engineering team.
