Envelope Analysis (Demodulation) for Early Fault Detection

Definition: What is Envelope Analysis?

Envelope analysis, also known as demodulation or high-frequency enveloping, is a powerful signal processing technique used in vibration analysis to detect early-stage faults in rolling-element bearings and gearboxes. These types of faults, such as microscopic cracks or spalls, generate a series of low-energy, high-frequency stress waves or “impacts” each time a rolling element passes over the defect. Envelope analysis is a method for extracting these repetitive impact signals from the machine’s general background vibration.

Why is Standard FFT Not Enough?

The energy from these tiny initial impacts is often too low and at too high a frequency to be seen in a standard vibration spectrum (FFT). The signal is often buried in the noise floor or overwhelmed by larger, lower-frequency vibration from sources like unbalance or misalignment. The impacts act as a *modulator* for the machine’s natural resonant frequencies. Envelope analysis is designed to demodulate this signal and reveal the underlying fault frequencies.

The Envelope Analysis Process

The technique works by isolating the high-frequency signals and then examining their repetition rate. The process involves several steps:

1. Band-Pass Filtering: The raw vibration signal is first passed through a high-pass or band-pass filter. This removes all the strong, low-frequency vibration (e.g., below 1 kHz or 5 kHz) and isolates the high-frequency ringing and stress waves associated with the impacts.
2. Rectification: The filtered high-frequency signal is then rectified, which essentially flips the negative portion of the signal to be positive. This prepares the signal for enveloping.
3. Enveloping (Low-Pass Filtering): A low-pass filter is applied to the rectified signal. This smooths out the high-frequency carrier signal, leaving behind only the “envelope”—a waveform that represents the amplitude modulation pattern, which is the repetition rate of the original impacts.
4. FFT of the Envelope: Finally, a Fast Fourier Transform (FFT) is performed on this new envelope signal. The resulting “envelope spectrum” clearly shows the frequency of the repetitive impacts.

Diagnosing Faults with the Envelope Spectrum

The peaks in the envelope spectrum correspond to the bearing’s calculated fault frequencies. By comparing the peaks in the spectrum to these known frequencies, an analyst can determine the exact location of the fault:

• BPFO (Ball Pass Frequency, Outer Race): Indicates a defect on the stationary outer race of the bearing.
• BPFI (Ball Pass Frequency, Inner Race): Indicates a defect on the rotating inner race. This peak will often have sidebands at 1x RPM because the defect is moving in and out of the load zone.
• BSF (Ball Spin Frequency): Indicates a defect on one of the rolling elements (the balls or rollers).
• FTF (Fundamental Train Frequency): The slowest frequency, indicating a fault with the bearing cage that holds the rollers in place.

Similarly, for gearboxes, the envelope spectrum can reveal peaks at the running speed of a gear that has a cracked or broken tooth, indicating repetitive impacting once per revolution.

The Power of Early Detection

The primary advantage of envelope analysis is its sensitivity. It can detect bearing and gear faults months, or even a year, before they would become apparent in a standard velocity spectrum or generate enough heat to be seen with infrared thermography. This provides an invaluable early warning, allowing maintenance to be planned and scheduled with minimal disruption, preventing catastrophic failures and secondary damage.

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