Acceleration Envelope
Acceleration Envelope (Enveloped Acceleration) is a vibration analysis technique that demodulates high-frequency acceleration signals to detect incipient rolling element bearing defects. It applies a band-pass filter to isolate bearing fault frequencies, then rectifies and demodulates the waveform to extract the repetition rate of energy spikes from rolling elements impacting defects, enabling early fault detection.
In shop floor maintenance, an accelerometer is mounted near the bearing housing to capture raw high-frequency signals. Software like Digivibe MX removes low frequencies below 10× RPM, inverts the FFT, and calculates the envelope spectrum to reveal fault frequencies. In CMMS systems like ServiceGrid, Acceleration Envelope values are trended over time to trigger alerts when thresholds for bearing defect frequencies (inner race, outer race, rolling element) are exceeded. This technique identifies both lower-order problems (balance, misalignment) and higher-order problems (bearing defects, motor bar pass, gearmesh) due to its filtering capabilities.
Why use acceleration instead of velocity for enveloping?
Acceleration captures high-frequency stress waves (typically >1 kHz) generated by micro-impacts in bearings, whereas velocity is more sensitive to low-frequency rotational issues like misalignment.
How does envelope analysis differ from standard FFT?
Standard FFT displays the raw frequency spectrum where bearing defects are often hidden by noise; envelope analysis demodulates the signal to extract the repetition rate of impacts, making defect frequencies stand out clearly.
What are the typical frequency ranges analyzed?
The band-pass filter typically isolates frequencies from 10× RPM up to the bearing's natural frequency (often 1–10 kHz), where bearing defect frequencies (BPFO, BPFI) reside.
Can it detect gearbox faults?
Yes, it is effective for gearmesh frequencies and gear damage due to their repetitive impact nature, in addition to roller bearings and motor bar pass faults.
What is the minimum resolution required for diagnosis?
800 to 1600 lines of frequency resolution is advisable to correctly identify closely spaced bearing defect frequencies and avoid diagnosis errors.
Is it a replacement for oil analysis?
No; it is a complementary tool to be used alongside oil lubricant analysis, temperature monitoring, and running noise trending for a comprehensive condition-based maintenance program.
How is the 'envelope' mathematically derived?
It involves rectification (inverting negative waveform parts to positive) followed by low-pass filtering to extract the modulation envelope, effectively converting amplitude-modulated high-frequency signals into a low-frequency representation of impact rates.