Shaft coupling model-based prognostics enhanced by vibration diagnostics

Abstract

Flexible power transmission couplings are a vital part in the drive train of rotorcraft, conventional aircraft and other land, sea and marine propulsion systems. These components are generally subject to high torque, rotational speeds and temperatures, which result in high static and dynamic loads that make them susceptible to degradation and failure. Additionally, couplings can experience accelerated failures, mainly driven by high cyclic stresses induced by extreme shaft misalignment and secondary causes such as improper assembly, fastener damage and lubricant loss. Damage to other drive train components can also lead to increased vibration and misalignment which, in turn, induces coupling failure. As with many complex systems, the service life of a shaft is randomly distributed and can be modelled by a stochastic deterioration process that allows approximation of remaining useful life (RUL). However, in such an approach, knowledge of actual shaft damage (ie current crack size) is still required. In practical health monitoring applications, actual damage is estimated via inference, as it is nearly impossible to be measured directly. To address this critical need, a shaft and shaft coupling prognostics and health management (PHM) system was developed that integrates vibration diagnostics, finite element stress analysis and material fatigue modelling to predict RUL of helicopter drive shafts and flexible shaft couplings. This modelling and prediction approach was developed and validated against actual sub-scale failure testing that included the requisite measurement of the ‘ground truth’ damage level. Data from a scaled drive shaft/ coupling test stand was used to validate the approach. The results indicated a cumulative prognostic accuracy of 80% over the last 25 minutes of the coupling life. Vibration-based diagnostics helped to improve the accuracy to over 95% once a crack had initiated and been detected over the last five minutes of coupling life. The application of these technologies will establish an economical, safe and effective maintenance-scheduling regime for helicopter shafts and couplings by scheduling maintenance towards the end of actual component life, increasing availability by preventing unscheduled maintenance and reducing inspections.