Growth of Subsurface Fatigue Fractures in Large‐Size Bevel Gears

Large-size bevel gears are commonly used in marine industry for the applications of propulsion, such as azimuth thrusters. The gears are typically designed for specific use, as they are directly transferring power from one component to another. The gear design needs to take into account external factors, such as deformation of transmission components, lubrication conditions, and long lifetime up to 25 years. In long lifetime applications, failure modes characteristic of very high cycle fatigue regimes are critical. Recently, several researchers have focused on the topic of subsurface fatigue failure modes. To better understand the initiation and growth of subsurface fractures, this work studies crack growth during systematic fatigue tests and ultrasonic inspection is applied to monitor the growth of cracks. The collected crack and fracture data is finally used to evaluate the usability of the ultrasonic inspection method for fault detection, and the effects of case hardening depth in a gear on fatigue life. Also, the accuracy of the state-of-the-art finite element prediction is compared to the experimental results.