Concept Design Improvement of Shift Fork for New Dog Clutch Actuator : Simulation driven product development approach

Abstract: Kongsberg Automotive is developing a brand-new actuator for engaging and disengaging a clutch for different driveline applications. This master thesis research improves the concept design of the shift fork for the new Dog-Clutch Actuator using Design for Manufacturability (DFM). Initially, the knowledge about the mechanism of the product is gained with the aid of the design team and the proper boundary conditions for the boundary value problem are obtained. The conventional die-cast materials are investigated, and appropriate material is selected to create the material model. Most of the traditional HPDC aluminum alloys are aluminum-silicon system; therefore, a detailed study on the nucleation of Silicon in the melt and how it influences the mechanical properties of the alloy is conducted. During gear engagement, the two rotating gears of the dog-clutch collide and synchronize the angular velocity of the hub and the input gear. The synchronization force is dynamic; therefore, explicit time integration is used to capture the system's response with the assistance of FEM software. As the shift fork undergoes cyclic load during the gear shift, the fatigue analysis is performed to evaluate the life (Nf) of the component using Wohler's curve. The value of the maximum principal stress at the critical spots like notch and its direction are determined using the 3D Mohr's circle. In this analysis, the endurance limit correction factors and notch factor (Kf) are used for the S-N curve correction, and Goodman's criteria are used to incorporate the mean stress effect. Fatigue analysis requires a very fine mesh to estimate the precise stress magnitude at the critical locations and, the structural optimization algorithm requires many iterations to determine the optimal layout of the shift fork. Therefore, the explicit integration scheme is not efficient as it will be computationally expensive and time-consuming to solve the problem. Hence, the equivalent static load is determined for the gear shift force at the peak load and used for calculations and product development. As the initial concept design of the shift fork is asymmetrical, it requires varying stiffness in its structure to transfer the force efficiently to the shift sleeve. The FEA results state that one prong of the shift fork experience up to 75% of the total load, which increases the overall stress of the component (up to 0.9Sy). The shift fork also doesn't have adequate torsional stiffness, and as a result, stress concentration has occurred in one of the fillets in the shift fork. The iterative design is set up to improve the design of the shift fork by optimizing the stiffness of the two prongs which provided the key observations that describe the design changes which improved the design. In this phase, the overall stress of the component is reduced by 20% and minimizes the difference in the load between the two prongs by 27.5% compared to the initial design. The shift fork needs to be light to achieve the necessary acceleration during the gear shift. Therefore, topology optimization using the projected subgradient method is implemented to optimize the mass and compliance of the improved design in the iterative design phase. Then the design realization phase is set up to implement the results obtained from the topology optimization to conceptualize the viable product. The optimized result decreased the overall stress and maximum deflection by 20%. It also reduced the load difference in the two prongs of the shift fork by 35% by maintaining the same mass as the initial concept design.