Vibration Testing - How is it Conducted?

In the realm of industrial machinery and vehicles, ensuring optimal performance, longevity, and safety is paramount. One crucial aspect of this endeavour is vibration testing, specifically through the utilisation of Fast Fourier Transform (FFT) analysis. This article explores the significance of FFT vibration testing, particularly in the context of machinery and equipment installed on anti-vibration mountings, such as engines, pumps and industrial vehicles. The focus is on conducting measurements above and below the mounting, calculating percentage reductions, and exploring additional testing points for a comprehensive evaluation of machinery health and performance of the anti-vibration mounting system.

Anti-Vibration Mountings and their Role:

Anti-vibration mountings play a crucial role in mitigating the impact of vibrations generated by machinery during operation. These mountings are designed to absorb and isolate vibrations, often using rubber, whilst preventing disturbing vibration from propagating to surrounding structures. While these components significantly enhance the overall stability of the machinery, it is beneficial to verify their effectiveness through rigorous testing.

FFT Vibration Testing Methodology:

FFT vibration testing is a powerful tool for assessing the vibrational characteristics of machinery, especially those installed on anti-vibration mountings. The primary objective is to measure vibration levels directly above and directly below the mounting to quantify the reduction achieved by the anti-vibration system. In addition, the predominant frequencies can be identified to allow fine-tuning of the mounting system to ensure the best performance is achieved.

Location of Measurement Points:

Measurements are typically taken directly above and directly below each anti-vibration mounting to capture the overall vibration levels, as well as the predominant frequency of the equipment.

These measurements provide insights into the efficiency of the anti-vibration system in isolating vibrations transmitted through the machinery, and if necessary, changes to the anti-vibration mountings design can be made to improve the reduction in transmitted vibration.

Calculation of Percentage Reduction:

The FFT analysis allows for the identification of prominent frequencies and amplitudes in the vibration spectrum.

By comparing the vibration levels above and below the anti-vibration mounting, engineers can calculate the percentage reduction in vibration transmission. As a general rule of thumb, for most applications, you should expect a minimum of 70% vertical isolation.

Expanding Testing Points for Comprehensive Evaluation:

While measuring vibrations above and below the anti-vibration mounting provides crucial data, a comprehensive assessment involves testing at various external points on the machinery may be required. This could include measuring external points, such as flexible connections to ensure excessive movement is not experienced, or measuring surrounding structures and equipment to ensure disturbance is not experienced.

Prominent Vibration Frequencies:

Identify and measure prominent frequencies in different areas of the machinery.


Understanding these frequencies aids in targeted mitigation efforts, such as adjusting anti-vibration mountings natural frequencies or changing the rubber damping characteristics. Measuring prominent frequencies can also aid in identifying the source of vibration, such as the misfire of an engine cylinder.

Displacement Measurements:

Measure displacements at external points to evaluate the amplitude and direction of motion. Not only can vibration testing allow for the amount of movement to measured, but with the use of multi-channel FFT equipment (i..e using 2 or more accelerometers) allows the engineer to detect direction of movement (i.e. vertical, lateral, longitudinal, pitch, roll, yaw)

This data helps in optimising anti-vibration mountings and identifying potential areas of concern.

Benefits of Testing on Running Machinery:

Incorporating FFT vibration testing into the development and reliability stage for machinery design on anti-vibration mountings is instrumental in ensuring optimal performance. By measuring vibrations above and below the mountings and exploring other critical points, engineers can quantify the effectiveness of the anti-vibration system, identify potential issues, and enhance the overall reliability and lifespan of industrial machinery.

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