Fatigue Failure of a Spiral Bevel Gear in a Typical Low-Bypass Turbofan Engine

Abstract

In a twin spool turbofan engine, power gets transmitted from the HP (high pressure) rotor to the engine accessories gearbox (EAGB) to drive various accessories through a pair of spiral bevel gears. Over the years, straight bevel gear assembly has been modified to spiral bevel gear configuration for providing a smooth operation. This case study deals with failure analysis of a spiral bevel gear having 1.5 module, 28 numbers of teeth, rotating at approximately 16,000 RPM and transmitting a power of approximately 350 KW. The gear has logged 1341.19 h after installation against the specified life of 1400 h. Failure of the gear has resulted in excessive generation of metallic particles leading to the seizure of high-pressure rotors during flying. This gear is manufactured from a low-alloy steel 12X2HBФA-BД of Russian origin by forging route with surface hardened by carburizing with a case depth of 0.6–1.0 mm. The spiral bevel gear has broken into three fragments approximately 120° apart. Stereomicroscopic examination reveals fatigue striations which could be observed distinctly with the help of Scanning Electron Microscope at a magnification of 3000×. The fatigue failure has progressed at the root of the gear across the tooth thickness and with initiation at an under-filling having an approximate size of 10 μm by 30 μm on the working side. No metallurgical abnormality is attributed to the failure of gear. As the subjected gear failed during flag end of its specified technical life in a progressive mode with a flaw near surface, magnetic particle inspection of the gear has been introduced during overhaul.