Gear Teeth Parameter Identification in Helicopter Planetary Gearbox Using Tachometer and Vibration Signals

Accurate identification of gear tooth counts in planetary gearboxes is essential for condition-based maintenance and health monitoring in gearbox systems. However, direct inspection of internal gear components—especially planet gears—is often infeasible due to the gearbox's enclosed structure and lack of documentation in aging or mixed fleets. This paper presents a methodology to estimate the number of gear teeth in bevel, sun, planet, and ring gears with fixed ring gear planetary gearboxes using vibration and tachometer signals. By applying Time Synchronous Averaging (TSA) and Fast Fourier Transform (FFT), we isolate gear mesh frequencies from noise and harmonics. Three case studies— UH60 Black Hawk, AS350, and Bell 407GXi main gearboxes—demonstrate the application of the technique. For bevel gear estimation, TSA is performed using the input shaft rate, allowing dominant frequency peaks to reveal the gear mesh. In planetary gears, TSA is applied using the output shaft rate. Candidate mesh frequencies are identified, with harmonics and known gear interactions used to eliminate false positives. The most plausible planetary mesh frequency is used to estimate the ring gear tooth count, from which the sun gear tooth count is inferred using the total gear ratio.