Holistic approach for fatigue life design of vehicle transmissions

Compared to many other types of gearboxes, the loading of vehicle transmissions is quite unsteady in terms of torque and speed. Consequently, the applicability of standardized calculation methods regarding the load carrying capacity like ISO 6336-series is limited as it is not fully capable of calculations with load spectra. State of the art vehicle transmissions are usually designed with particular regard to their ancestors regarding specific design parameters. If a lot of knowledge from the field is available, an efficient design of refined gearboxes satisfying the modern demands regarding efficiency, power-to-weight ratio and load carrying capacity is possible. Apart from that, designing a new kind of gearbox for vehicles from white sheet of paper is quite challenging if all previously stated requirements have to be fulfilled. This scenario might become true if companies are entering the market of electric cars, if gearboxes are designed for motorsports applications or if the user behavior is massively changing. This paper describes a holistic approach for the fatigue life design of vehicle transmissions using the load carrying capacity of the gearing as an example. The approach takes into account all parts of a typical transmission design process, beginning with the evaluation of decisive load spectra, passing the basic design process of the gearing and concluding with the final testing for approval of the gearbox system. The basic calculation of the load carrying capacity is based on the standardized calculation methods. Further on, methods of a local damage accumulation are described. Finally, the shown methods are summarized in a holistic procedure model.