Gear tooth fracture caused by subcase fatigue failure below the carburized layer.

Purpose

In the recent years there have been an increased number of gear fatigue failures caused by tooth fracture on propulsion gears, especially on large azimuth thrusters. An unacceptable high numbers of these failures occured on DNV certified azimuth thrusters. Approximately 50 – 60 of these type thruster units are classed with DNV each year (150 – 170 tunnel thrusters). The crack origin of these failures was identified subcase beneath the hardening layer. This type of fatigue failure has been rather rare, therefore no standards or methods for prediction of this kind of failures exist.

Benefits

The outcome from the project is unique knowledge of the fatigue properties of the gear material. The results from the material testing imply that the Classification Note for marine gears probably needs revising. The project delivery is a technical report where the method to predict the subcase fatigue failure is described, and also a proposal for a method to predict the risk of this failures in marine gears. The project results subsequently increase the competence in complex fatigue analysis for machinery components.

Features

This project is a joint industry research project together with participants from the marine gear industry. The project work has involved the investigation and analysis of the contact pressure on gear tooth flanks that cause complex stress distribution into the gear tooth. Detailed analysis of crack nucleation and crack growth by means of modern fatigue analysis methods (critical plane approach) has been necessary. This method requires extensive laboratory investigation of the gear material properties, i.e. by fatigue testing. The objective of this project has been to develop a method to predict this kind of fatigue failure on marine gears. The project work also includes study of reports and theories of similar research of fatigue problems.