What is an Envelope Spectrum? Demodulated Signal Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is an Envelope Spectrum? Demodulated Signal Analysis • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is an Envelope Spectrum? Demodulated Signal Analysis

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Understanding Envelope Spectrum

Definition: What is an Envelope Spectrum?

Envelope spectrum is the frequency spectrum obtained by computing the FFT of the envelope (amplitude-demodulated) signal from envelope analysis. It reveals the repetition rates of impacts or modulations in high-frequency vibration, making it the most powerful technique for detecting rolling element bearing defects. While standard FFT shows the carrier frequencies (resonances excited by impacts), envelope spectrum shows the modulating frequencies (the rate at which impacts occur), corresponding directly to bearing fault frequencies like BPFO, BPFI, BSF, and FTF.

The envelope spectrum is to bearing diagnostics what standard FFT is to unbalance and misalignment detection—the primary diagnostic tool that makes early fault detection possible by extracting diagnostic frequencies from the high-frequency “noise” that standard FFT cannot effectively analyze.

How Envelope Spectrum is Generated

Envelope Analysis Process

  1. Band-Pass Filter: Filter raw vibration signal to high-frequency band (e.g., 1-10 kHz)
  2. Envelope Detection (Demodulation): Extract amplitude envelope of filtered signal
  3. Low-Pass Filter: Smooth envelope (remove residual carrier)
  4. FFT: Compute frequency spectrum of envelope signal
  5. Result: Envelope spectrum showing impact repetition rates

What the Envelope Represents

  • High-frequency vibration (carrier) modulated by bearing impacts (modulation)
  • Envelope extracts modulation (repetitive impact pattern)
  • FFT of envelope shows modulation frequencies
  • Modulation frequencies = bearing fault frequencies

Reading Envelope Spectra

Healthy Bearing

  • Low overall envelope level
  • Flat or gently sloping spectrum
  • No distinct peaks
  • Noise floor at or below instrument sensitivity

Defective Bearing

  • Primary Peak: At bearing fault frequency (BPFO, BPFI, BSF, or FTF)
  • Harmonics: 2×, 3×, 4× of fault frequency as defect worsens
  • Sidebands: Around fault frequency at cage (FTF) or running speed (1×) spacing
  • Elevated Floor: Overall noise floor rises with bearing deterioration

Severity Indicators

  • Peak Amplitude: Higher amplitude = more severe defect
  • Number of Harmonics: More harmonics = more advanced damage
  • Sideband Complexity: Extensive sidebands indicate modulation, often severity
  • Noise Floor Rise: Elevated floor suggests widespread surface deterioration

Advantages Over Standard FFT

Early Detection

  • Detects bearing defects 6-18 months earlier than standard FFT
  • Sensitive to micro-spalls invisible in velocity spectrum
  • Provides maximum lead time for maintenance planning

Clear Fault Signatures

  • Bearing fault frequencies prominent and easy to identify
  • Less interference from unbalance, misalignment (filtered out)
  • Sideband patterns clearly visible
  • Diagnostic clarity superior to standard spectrum

Low-Energy Event Detection

  • Small bearing impacts create little energy in standard FFT
  • But excite high-frequency resonances effectively
  • Envelope analysis amplifies these weak diagnostic signals

Interpretation Guidelines

Fault Identification

  1. Calculate Bearing Frequencies: BPFO, BPFI, BSF, FTF for installed bearing
  2. Search Envelope Spectrum: Look for peaks at calculated frequencies (±5% tolerance)
  3. Match Peak: Which calculated frequency matches observed peak?
  4. Verify with Harmonics: Check for 2×, 3× harmonics
  5. Check Sidebands: Sideband spacing provides additional confirmation
  6. Diagnose: Peak at BPFO = outer race; BPFI = inner race; BSF = rolling element; FTF = cage

Severity Assessment

  • Incipient (0.5-1g envelope): Small peak, no harmonics, monitor monthly
  • Early (1-3g): Clear peak, 1-2 harmonics, monitor weekly, plan replacement months
  • Moderate (3-10g): Strong peak, multiple harmonics, sidebands, plan replacement weeks
  • Advanced (>10g): Very high amplitude, many harmonics, elevated floor, replace urgently

Practical Application

Routine Monitoring

  • Include envelope spectrum in condition monitoring routes
  • Trend envelope amplitude at bearing fault frequencies
  • Earlier detection than overall vibration trending
  • Specific fault identification vs. general vibration increase

Troubleshooting

  • When overall vibration elevated but standard FFT unclear
  • Suspected bearing problems
  • Verifying bearing replacement necessity
  • Identifying which bearing in multi-bearing machine

Relationship to Envelope Analysis

  • Envelope Analysis: Complete process (filtering + demodulation + FFT)
  • Envelope Spectrum: The resulting frequency plot from envelope analysis
  • Envelope Signal: Time-domain demodulated signal (intermediate step)
  • Terms: Often used interchangeably but technically envelope spectrum is the output

Envelope spectrum is the gold standard for bearing defect detection in vibration analysis. Its ability to reveal bearing fault frequencies months before they become visible in standard spectra, combined with clear diagnostic patterns and specific fault identification capability, makes envelope spectrum analysis an indispensable tool in predictive maintenance programs for rotating equipment.

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