In 2010, RMCI developed methods for the Army to effectively monitor epicyclic gear trains in HUMS-monitored rotorcraft. RMCI supplied formulas to correctly perform the calculations.

For current HUMS gear train diagnostics, STAs are calculated to diagnose gear faults. Gear faults threatening drivetrain health and crew member safety, such as worn or cracked teeth, can be tracked using STA signals. If a gear fault exists that experiences contact every rotation, the fault will typically manifest in the gear’s STA through anomalous behavior including a peak or increase in magnitude.

To generate the STAs, tachometer (tach) data are used to divide accelerometer time domain data into sections correlating to gear shaft rotations. Tach data is typically multiplied by gear ratios to obtain the number of tach pulses per rotation, since the tach usually does not reside on the shaft for which the STA is being calculated. The tach divided sections are averaged together to obtain average vibration signals per shaft or gear rotation.

Fixed-axis gear train STA calculation is relatively simple since, despite gear rotation, points of gear mesh contact remain fixed in space relative to the tach’s external frame of reference—the transmission housing. Fixed-axis gear rotation ratios are therefore calculated for the gear for its rotation relative to the transmission housing. However, points of contact in epicyclic gear trains are not fixed relative to the transmission housing or tach orientation, but rotate around the ring, sun, and planet gears at the rate of the planetary carrier’s angular velocity if the ring gear is fixed relative to the transmission housing. Therefore, STA gear ratios must be calculated for the sun and planets as they rotate with respect to the planetary carrier instead of the transmission housing. Otherwise, tach-divided data sections don’t accurately correlate to gear rotations and the desired gear mesh amplitudes average out.

Without correct planetary gear ratios, the Army fleet did not possess epicyclic monitoring capability through the current HUMS. AH-64A, AH-64D, and OH-58D script files did not include sun or planet STAs. Incorrect STAs were incorporated into the MH-47 and CH-47 HUMS script files. The sun and planet gear ratios were incorrect, causing the STAs to track gear rotation relative to the transmission housing, not the planetary carrier.

RMCI supplied the Army with formulas to calculate epicyclic gear monitoring ratios on all HUMS-equipped aircraft. RMCI verified the formulas using AH-64D and UH-60A time-domain data, demonstrating that gear mesh harmonics were dominant within their respective STAs using the new method. Platform specific ratios were provided in a format that could readily be implemented for the new AH-64D Block III main transmission as well as the MH-47G and CH-47D forward and aft transmissions. The supplied formulas also allow for future epicyclic monitoring on platforms monitored by various HUMS vendors.