Evaluation of Select Condition Indicators for Detecting Damage in Geared Transmissions Using a Finite Element/Contact Mechanics Model

Abstract This study analyzes the ability of several practical rotorcraft condition indicators for detecting damage in a two-stage rotor-craft transmission utilizing a finite element/contact mechanics (FE/CM) model. The transmission includes an input spiral-bevel stage and an output planetary stage. The vibrations of the transmission are calculated at typical operating torque and speed using a full dynamic solution of the finite element transmission model. In this solution, the contact conditions at all meshes are calculated at each time instant accounting for changes due to the kinematic motion of the individual gears; the elastic deformations of all teeth, blanks, shafts, and other components; and vibration. The elastic vibrations produced from the housing contain frequency content that appears in clusters near the mesh frequency harmonics of each gear stage. The planetary gear mesh frequency clusters have prominent upper and lower sidebands that occur at the planet-pass frequency. When damage (i.e., tooth root cracks or tooth surface pits) occurs on the individual gears of the planetary stage, additional transient vibrations occur that meaningfully alter the resulting vibrations and their spectrum. The frequency components near planetary gear mesh frequency clusters are not sensitive to damage, even when the damage is large. The use of condition indicators (CIs) for detecting damage is explored. The ability of the CIs to detect damage is shown to be sensitive to the input vibration signal used in the CI calculation. Time synchronous averaging is shown to improve the performance of CIs, provided the averaging does not artificially modify the underlying vibratory signal. We demonstrate that averaging over the planet-pass period effectively eliminates non-planetary gear vibration components and improves CI sensitivity. Multiple constructions of the difference signal are analyzed, and their impact on CI performance is quantified. Tooth root cracks with small lengths less than 50 percent through the tooth thickness generally do not lead to substantial increases in the individual CIs. It was observed that most CIs increased when more sidebands were removed for crack lengths above 50 percent. The results from this work will enable reliable damage detection techniques for geared transmissions, thus improving safety and reliability in rotorcraft propulsion.