Fracture of a Helicopter Main Transmission Spiral Bevel Gear Caused by Residual Stress and Caustic Embrittlement

A gear tooth from a helicopter main transmission spiral bevel gear fractured as a result of fatigue that originated from the drive side root radius. However, this fatigue crack was secondary. The primary fatigue crack originated behind the fractured tooth at a bolt hole location that attached the spiral bevel gear to the gearshaft. Intergranular features were observed at the primary fatigue origin, which resulted from residual stress that was induced by rough machining and a subsequent black oxide treatment.