Sensor-Position Domain Shift in Gearbox Fault Diagnosis: Healthy-Reference Calibration and Frequency-Localized Transfer-Path Sensitivity

Sensor relocation can induce structural domain shift because gearbox vibration depends on fault excitation and the transmission path. This study evaluates transferability using 180 source recordings representing ten states, six accelerometer positions, and three coupled operating conditions. In the independently reproduced 170-feature ExtraTrees evaluation, raw nine-fault balanced accuracy was 43.8% with a held-out sensor position versus 84.6% with a held-out operating condition; local healthy referencing increased cross-position accuracy to 61.7% but requires a target-position healthy baseline. Fault-by-position interaction increased from 9.7% below 2 kHz to 25.3% at 5–10 kHz. The original non-nested bandwidth sweep found a raw gear-fault model-mean maximum of 84.4% below 1 kHz (62.6% for 0–10 kHz). New nested source-position-only cutoff selection yielded 82.2% for gear faults versus 62.6% over the full band, selecting cutoffs between 1 and 2 kHz across folds and classifiers. Raw bearing-fault accuracy did not benefit consistently from nested cutoff selection. The findings establish a within-rig baseline for frequency-localized structural portability rather than a universal frequency limit or new classifier; independent machine-level validation remains necessary.