FFT Missed a 0.3mm Inner Race Crack for Two Weeks
Running FFT on 10kHz vibration data from a test rig bearing looked clean. Peak amplitudes at 1x and 2x shaft speed, some harmonics at the gear mesh frequency, nothing alarming. Then I switched to envelope analysis on the same signal and immediately saw a 157Hz modulation pattern — the bearing pass frequency for inner race (BPFI). The crack was 0.3mm deep when I pulled the bearing.
FFT works great when fault frequencies show up directly in the velocity spectrum. But early-stage bearing faults generate high-frequency impacts (often 5-30kHz) that get buried under low-frequency machinery noise. Envelope analysis demodulates these high-frequency bursts to extract the fault signature. The trade-off? Envelope analysis requires bandpass filtering in the right resonance band, and if you pick the wrong band, you get nothing.
This is the decision tree I use now: FFT first for speed and simplicity, envelope analysis when FFT spectra look suspiciously clean or when I know I'm hunting early-stage defects. But the details matter — sampling rate, filter design, and sensor mounting all change the answer.
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