Investigation of Spline-Induced Excitation Forces in Electric Drive Units via Flexible Multibody Dynamic Simulation

This study investigates the NVH characteristics of the spline coupling that connects the motor and reducer shafts in an electric drive unit, using flexible multibody dynamic simulations. Focusing on the source stage of the NVH analysis process, the excitation force magnitude and spline trajectory are examined under various spline design conditions. The study compares spline fit types (side fit vs. major fit), clearance vs. interference conditions, and variations in tooth number and module size. All spline configurations are designed to meet AGMA safety factor requirements. Side fit splines exhibit lower first-order excitation forces compared to major fit splines, but significantly higher excitation forces at higher orders. This leads to increased spline trajectory amplitude and amplified whirling of the input shaft. Since the input gear is directly coupled to the input shaft, this whirling behavior induces eccentricity in the gear itself. In particular, clearance fit conditions result in greater higher-order excitation forces and gear eccentricity than interference fits. Major fit splines, on the other hand, show more stable trajectories and lower higher-order excitation forces, minimizing their impact on the overall system. The analysis of tooth number variation reveals that increasing the number of teeth reduces first-order excitation forces while increasing higher-order components, indicating that force distribution within the spline can be tuned. Tooth number adjustment does not affect the rest of the system, suggesting it is a practical strategy for achieving desired NVH characteristics. In conclusion, spline design parameters such as fit type and tooth number significantly influence the magnitude and directional behavior of excitation forces, which lead to system-level effects such as input shaft whirling and gear eccentricity. Major fit splines demonstrated more stable trajectories and lower excitation forces, while tooth number adjustment offers a practical design strategy for tuning excitation force distribution without affecting input shaft whirling.