Measurement of Gearbox Surface Frequency Response Functions for HUMS Algorithm Improvement

Vibration diagnostic algorithms used to identify incipient bearing faults are installed in Digital Source Collectors (DSC) on nearly 2000 US Army rotorcraft. These algorithms depend on the structural vibration characteristics of the bearings, gearboxes, and mounting brackets to transmit the signatures associated with bearing abnormalities and faults. Two distinct algorithm families exist to measure changes in these signatures: the broadband spectral energy family and the demodulation family. Each of these uses the unique vibration signatures associated with the natural frequencies and transfer paths of the monitored bearings. The Aviation Engineering Directorate has estimated the Frequency Response Function for each of the monitored bearings from 5 to 50 kHz using a piezo-exciter mounted directly to the gearboxes at the bearing load zones and measured by the appropriate DSC accelerometer. The testing for this effort was completed on six airframes: the Apache AH-64D, the Black Hawk UH-60L and MH-60M, the Chinook MH-47G and CH-47D, and the Kiowa Warrior OH-58D. This paper details the three-phase effort showing the entire process including the validity of the measurements and samples of the FRF estimates.