Fault Localization for Gear Tooth Failure in Coupled Mechanical Systems;Fillet stress related to non-uniformly loaded; Friction Cancellation in Flexible Systems Using Extended Kalman-Bucy Filtering; Gear Tooth Crack Signals and their Detection via the FM4 Measure in Application for a Helicopter HUMS (Health Usage and Management System);Gear tooth stiffness reduction measurement using modal analysis and its use in wear fault severity assessment of spur gears