Acceleration envelope: get to know this analysis

March 1, 2024

Acceleration envelope is one of the most commonly used analyses for fault diagnosis. This method is quite useful for evaluating the behavior of sliding components, mainly bearings, as over 22% of failures occur in these components. Naturally, the envelope has become one of the most widely used tools by vibration analysts.

What is acceleration envelope?

The acceleration spectrum obtained from a machine is filled with data. Therefore, it often becomes difficult for the analyst to understand what is happening from the entire set. Thus, the most appropriate approach is to make a cross-section of this spectrum that encompasses the main indicators useful for checking possible faults. This is, therefore, the principle of the acceleration envelope.

Acceleration envelope highlighting failures

This section allows for a more precise observation of everything that is happening in a fragment of the acceleration range. For this, the first step is to properly configure the software to allow for the attenuation of low frequency and, thus, highlight the high frequency – where faults appear. In this sense, for the analysis to function properly, it is essential to understand the machine and its spectral signature. This knowledge is essential for the configuration to occur correctly and for the envelope to frame the elements suitable for analysis.

Once configured, the analysis of envelope spectrums will allow determining the repetition rates of impacts that generate stress waves.

What does the envelope spectrum enable us to see?

Basically, the acceleration envelope helps identify micro impacts at high frequency. However, many people have doubts when they open the envelope and see frequencies from 0 to 500 Hz, for example. To understand what is happening, it is important to understand that although the problem is occurring at high frequency – which concerns the operation of the bearing, the fault is at low frequency.

Why does the envelope appear at low frequency?

That is, if there is backlash or wear, for example, this tends to “appear” in the spectrum once every complete cycle of the bearing. And the envelope allows you to look at the impact interval of this fault and measure this energy. Therefore, although the band-pass – the region of the configured filter – is at high frequency (which is the occurrence of the impact), the repetition (modulation) of this impact occurs at low frequency. And it is precisely this information that is of interest for problem identification.

Thus, the envelope is a signal demodulation and not an elimination. That is, the application of a filter in the high-frequency region, taking advantage of the natural frequency of the bearing to amplify periodic frequencies.

Do you want to learn more about Envelope Analysis? Watch (video available in Portuguese):

How to apply acceleration envelope analysis?

To apply the envelope from the acceleration spectrum, it’s possible to identify high-frequency vibration and apply the filter to it. Dynamox software allows customizing this filter with identified values, enabling the selection of that specific point, selecting it as the central element of the band-pass. For example, if the vibration is at 3 kHz, it’s possible to apply the filter from 1 kHz to 4 kHz, resulting in much higher quality data for analysis.

It’s important to remember that the envelope won’t be applied to the acceleration as a whole, but rather from a segment of it. This allows for proper framing, facilitating the analysis of fault frequencies.

Do you want to know how to identify faults in rolling bearing cages? Read here.

Case analysis: detection of a defect in conveyor tail pulley

Using the Dynamox solution, it was possible to identify an increase in vibration, which caused a defect in a conveyor tail pulley, a critical asset for the industry’s production process.

The trend graph was one of the tools used in vibration analysis to monitor the dynamic behavior evolution of the machine. It allows identifying trends of vibration increase or decrease within certain frequency ranges and amplitudes.

Trend curve
Trend curve
Data history - Monitoring progress and post-intervention
Data history – Monitoring progress and post-intervention

Through the system, a defect was detected in the outer race of the BPFO bearing located on the return pulley of the conveyor belt.

Using the bearing marker (2224) with the acceleration envelope, it was possible to identify the problem accurately. The images below demonstrate the defect frequency and the modulation by rotation, indicating severity.

Waveform graph
Waveform graph
Spectral envelope highlighting the BPFO outer race failure
Spectral envelope highlighting the BPFO outer race failure

An intervention was scheduled for the asset, and conveyor tail pulley was replaced at an opportune moment, avoiding losses in the industry’s production flow, as well as a widespread equipment failure.

Do you want to know other successful detection examples? Read more cases here.

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